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Drilling Fluids Self Learning Package

Sugar Land Learning Center

SUGAR LAND LEARNING CENTER

Drilling Fluids SELF- LEARNING COURSE

USEFUL PRE-REQUISITES Completion of Introduction to MWD Self-Learning Package Completion of Stuck Pipe Self-Learning Package Completion of Well Bore Stability Self-Learning Package Completion of Density Neutron Theory Self-Learning Package Completion of Invasion Self-Learning Package Completion of Ideal Real Time Initialization for RTTC Self-Learning Package Completion of Resistivity Measurement Principles Self-Learning Package

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Drilling Fluids Self Learning Package

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INTRODUCTION TO SELF-LEARNING Self-learning enables you to learn at your pace, in your time and in your way. This course book provides the content, structure and organization of your learning, which would otherwise be managed by an instructor in a class. It even asks you questions to confirm your understanding – as they probably would! So you need to work through the document and here is some information to help you. Layout: The document is laid out consistently as shown opposite. You will see each time you turn a page: -

The text to follow on the right hand page A header on the page so you know where you are. Text headings, side heading and in-text-sub headings as shown. The diagrams on the left hand page- referred to in the text. The self-test questions follow at the end of each section. The answers to the questions on the last page.

How to use the course book -

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Try to do the course in a maximum of two sessions, close together: (e.g. Work session: Break: Work session) Set yourself up in a suitable environment: no noise, no interruptions, etc. Use the self-test questions to confirm that you have understood. They are yours: they are not assessed or marked. If you get an answer wrong, just go back through the material. Self-test questions will be drawn only from the pages covered by that section. It is useful, but not essential for you to have met the pre-course requirements stated on the course book cover.

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Table of Contents OBJECTIVES…………………………………….………..………..…………………….5 INTRODUCTION………………………………………………………..………………………………7 TYPES OF DRILLING FLUIDS………………………………………………………..….……….11 WATER BASED FLUID SYSTEMS…………………………………………………………….….…..11 OIL BASED FLUID SYSTEMS………………………………………………………………....……..13 SYNTHETIC BASED SYSTEMS..………………….………………………………………….………15

FUNCTIONS OF DRILLING FLUIDS…………………………………………………19 HOLE CLEANING…………………………………………………………………………….……..19 MAINTAINING HYDROSTATIC PRESSURE & FORMATION PRESSURE CONTROL……………….…...21 SEALING OFF PERMEABLE FORMATIONS..…….……………………………………………….…..23 FORMATION DAMAGE MINIMIZATION……………………………………………………………..23 LUBRICATION & COOLING…………………….…………………………………………….……..23 HYDRAULIC ENERGY TRANSMISSION………..…………………………………………………….23 FORMATION EVALUATION………………………………………………………………………….25 CORROSION CONTROL, CEMENTING & COMPLETION…………………………………….………..25 STUCK PIPE………………………………………………………………………………..……….27

MUD RHEOLOGY & PHYSICAL PROPERTIES……………………………………..29 HYDRAULICS & FLUID FLOW DYNAMICS ……………………………………………..…………..30 CUTTINGS SETTLING MECHANISMS……………….………………………………………….……31 FILTRATION CONTROL & WALL CAKE…………………………………………………….………33 CUTTING SUSPENSION……………………………………………………………………….……..33 SOLIDS CONTROL……………………………………………………………..……………………35 SPECIAL FLUIDS………………….………………………………………………………….……..37

TESTING EQUIPMENT………………………………………………………..……….39 MEASUREMENTS WHILE DRILLING……….………………………………..……..45 PARAMERTERS EFFECTING MWD OPERATION…………..……………………………….45

LOGGING WHILE DRILLING……………………………………………………...….51 PARAMETERS EFFECTING LWD MEASUREMENTS……………………………………….51

HSE………………………………………………………………………………..……..55

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Objectives Upon completion of the training module, you should be able to: • •

Describe different types of Drilling Fluids and their application. Describe the different functions of the mud and discuss the functions in considerable details. • Describe the factors associated with hole cleaning. • Describe the factors that can lead to stuck pipe. • Describe the common laws for the drilling fluid hydraulic modeling. • Describe various stages of flow and cutting settling mechanisms. • Describe how cuttings are suspended and important parameters associated with it. • Discuss different methods of solids control and their applications. • Discuss testing procedures for measuring and monitoring mud properties. • Discuss factors effecting signal strength related to mud properties including rig equipment and drilling conditions. • Discuss the effect of mud additives on LWD measurements. • Discuss Health, Safety, and environmental aspects associated with the drilling fluids, and how to access relevant documentation.

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Introduction The drilling fluid not only makes it possible to drill, complete and produce oil & gas wells but also aids in formation evaluation and provides knowledge about downhole conditions and parameters. . The nature and type of the fluid may change depending on the application. The following chapters will discuss in detail the applications, characteristics, properties & types of fluids commonly used in drilling process. In addition to that, elements effecting the Measurements while Drilling and Logging while Drilling process will also be discussed. Most of the new terms used will be explained briefly before or during their use. Some of the unexplained terms are discussed here in this section. Important terms used and their definitions: Viscosity: Resistance of a fluid to flow. Some of the common viscosity measurements are: Funnel viscosity (sec / l) it is the relative indicator of the fluid viscosity and it is used to monitor the relative changes in fluid rheology. By definition viscosity can be given as Viscosity = shear stress / shear rate The shear stress and shear rate come into play when analyzing and discussing the fluid dynamics. When the fluid is flowing, a force exists in the opposite direction to the flow, which is referred to as the shearing stress acting on a certain surface area; the rate at which one layer is moving past the other is the shear rate. Plastic viscosity: It is the resistance to flow due to mechanical friction caused by solid size, density and shape. Apparent viscosity: Apparent viscosity is the viscosity of a fluid (subject to compressibility effects) under certain conditions of temperature & pressure also referred to as effective viscosity (cP), Yield Point: Yield Point is a measurement of the attractive forces (Electro-chemical) in the fluid medium, these forces are due to the opposite charged particles that attract each other implying that their viscosity decrease as they flow or are sheared. Thixotropy: Thixotropy is the property of a fluid to form a gel like structure when static and when sheared the fluid goes back to its fluid properties. Thixotropic fluids are Non-Newtonian shear thinning fluids. Gel strength: Gel strength is the resistance of a fluid to shear when in gel form or static conditions. Excessive gel strengths can pose problems such as solid or gas entrapment, excessive pressure while breaking circulation, reduction in efficiency of solid control equipment, swabbing or surging during trips, difficulty with logging tools going to bottom and pressure surges. Some of the common additives described later in this document are discussed below along with the reasons of their use in the mud. Bentonite: These are fine clay particles when added to mud they are hydrated and act as effective viscosofying agents. It is also commonly used for fluid loss control and forming an effective filter cake. Caustic Soda: It is used to control the acidity or basicity of the mud better referred to as the pH level. pH is an indicator having a numeric value. A pH level between 1-6 indicates acidic medium, a pH of 8-14 indicates basic behavior. And a pH of 7 shows a neutral characteristic. Lignosulfonate: These are thinning agents commonly used for reducing viscosity. They also provide inhibition, fluid loss control and temperature stabilization in the system.

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Barite & Hematite: Both of them are used as weighing materials in the mud to balance the downhole porepressures. Barite is used for weighing up to 20 pounds per gallon and Hematite is used for weighing up to 25 ppg. Additional types of additives are used to weight up if more weight is required to balance the hydrostatic pressure.

Questions Part 1: Q1.

What is plastic viscosity?

Q2.

Define yield point?

Q3.

Is Bentonite used as a weighing material?

Q4.

Is Barite heavier than Hematite?

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Hydration of Bentonite particles at different time intervals

Figure 3.1

Bentonite Particle under Electron Microscope

Figure 3.2

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Types of Drilling Fluids There are various types of drilling fluids used within the oilfield mainly depending on the type of application for the particular type of fluid system. These systems have their advantages and disadvantages in terms of physical & chemical properties, cost and environmental factors associated with their use. Selecting any one of the main system or their sub system as part of the drilling fluid program for a particular well is primarily based on a detailed analysis of the drilling program, drilling conditions & parameters. The drilling fluids can be generally categorized into three main systems. 1. 2. 3.

Water- Base Fluid systems Oil- Base Fluid Systems Synthetic Base Fluid Systems

Water- Base Fluid systems The most common drilling mud system used is the Water Base Mud system mainly due to the fact that it is more economical and environmentally friendly. Within the WBM there are many types of sub-systems. Basic systems are changed to more complex system with varying conditions and requirements such as temperature & pressure gradient increases with increase in well bore depth. Multiple mud systems may be used in a typical well. Water based fluid systems can usually be placed into one of the following classifications. Un-weighted Clay-Water System: The basic system is essentially water and bentonite (natural clay); solids are incorporated into the system as drilling continues. Some of the native clays may be benonitic in nature and may increase the viscosity of the system. Usually this type of system is used to spud a well. Therefore, sometimes these types of fluid systems are refereed to as Spud Mud or native mud systems. The surface solid removal equipment removes most of the formation solids. The system is often quite shear thinning (decrease in viscosity with fluid movement or flow). Since the system is not weighted so it has a low buoyancy effect for lifting the cuttings. Thus the entire process of hole cleaning depends largely on flow rate and viscosity. Since the plastic viscosity is low if the solid content is low due to the fact that to increase the viscosity solid clay particles need to be added to the system, therefore the cutting carrying capacity can only be increase by achieving higher yield point. Advantages of this system include low cost & high ROP. Deflocculated, Weighted Clay Water System: These systems are basically designed to control solid and chemical contamination. The systems are usually converted from un-weighted clay-water systems. Dispersants and thinners are used to disperse discrete unwanted clay particles and reduce yield point & gel strength. Since the yield point and gel strength reduction helps control & maintain the fluid properties within the desired limits and avoid unwanted changes with the infiltration of outside solids and chemicals. The most common agents used are Lignosulfonates. Since the Lignosulfonates are acidic in nature therefore pH levels need to be maintained using caustic soda and similar chemicals. The temperature limitation of the system is around 320-450 degree F. Calcium Treated Drilling Fluid Systems: The need for this kind of system arises for controlling the properties of the mud by being able to increase the yield point and gel strength. This is achieved with the ionic exchange of the Na (sodium) in the system with Ca (calcium) resulting in flocculation resulting from partial dehydration of the clay contents reducing the water envelope around them thus bringing them closer to each other resulting in an increase in YP and Gel strength.

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Another important effect of Calcium in the mud is well bore inhibition, which is avoiding hydration of solids. A good example of inhibition would be in the case of sloughing shales present in the drilled formations. Solubility of calcium is inversely proportional to the pH of the system and directly proportional to the salinity signifying that pH & Chloride content need to be regulated in order to achieve the desired results. Some other chemicals such as lime maybe used to reduce the presence of unwanted gases like Carbon dioxide and Hydrogen Sulfide in the system. The system can also be based on seawater simply due to the reason that in offshore applications there is an abundance of seawater supply. The other advantage for the use of seawater would be lesser degree of hydration of drilled clays then using fresh water due to presence of salts (Cl contents) in the seawater. The advantages of the system are drilling in water sensitive formation, which react to hydration by swelling. Potassium being a salt as Potassium Chloride can also be used to avoid shale hydration in systems called Inhibitive Fluid Systems, since Potassium also helps stabilize water sensitive shales. Other types of salts can also aid to control the same type of problems. Saturated Salt Water Systems: The application of this system is generally in areas having large quantity of formation containing salts. If regular water systems are used the well bore would dissolve into the system not only effecting the properties of the drilling fluid but also causing hole stability problems which may result in stuck pipe situations. This is simply achieved by adding salt to the system, which would commonly be Sodium chloride (NaCl). Salt is added to the system until the system is saturated with salt at the operational temperature conditions. The temperature limitation of this system is less then 300 degree F. In addition to these salts other chemicals are used to control the chemical and physical property of the fluid system. Also other chemical combination systems are also used in WBM systems for various special applications such as Chrome free, environmentally safe fluids and HTHP applications. Oil- Base Fluid systems In Oil Base fluid systems oil functions as the continuous external phase. Hence it is commonly referred to as the ‘Base Oil’. Base oil is usually diesel or mineral oils. One of the big advantages of this kind of system is extremely low fluid loss which can be beneficial against formations sensitive to water, also the fluid has the ability to be highly stable and inherit controlled HTHP properties. The system carrying the capacity to control the fluid loss with the addition of some fluid loss control additives is also called the conventional system. And systems not having fluid loss control capabilities are called relaxed-filtrate systems; these systems are relatively inexpensive and are designed for higher ROP. A relaxed system can be converted to a conventional system by adding few additives for fluid loss control but the opposite cannot be achieved. Some of the important features and terms associated with OBM are given below: Emulsion: This type of fluid system is also known as the invert-emulsified fluid system. Emulsification is basically mixing of water with the oil (types of systems which are normally insoluble as a mixture) with oil in the continuous phase meaning that the dominant constituent fluid of the system. The ratio referred to as the Oil to Water Ratio ranges from 60% to 80 % Oil and the remaining being water, a typical ratio is 70 % oil and 30 % water. The degree of emulsification generally affects the water droplet size in the system. So the better the emulsification the smaller the size of the water droplet and this can be achieved by adding certain additives with temperature and pressure variations. The water droplets are suspended evenly in the non-aqueous or oil phase in order to ensure homogeneity of the system that would be important for controlling and maintaining various physical and chemical properties of the fluid system. Therefore, coalescing or combining of the droplets has to be prohibited for the above reasons and this is achieved by introducing surfactants in the system, which act between the two phase of the system.

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Electrical Stability: It is a relative indication of the emulsion stability. It can be defined, as the measure of electrical voltage required to breakdown the emulsion and allow the water droplets to connect thus causing electrical current to flow through the medium. This is recorded in volts; the higher the electrical stability the better the emulsification and more stable the fluid system at different temperatures & pressures. The Electrical stability measurement is always at a specified temperature. Water Content: The ratio of water content affects the electrical stability of the system. More water content results in lesser distance between the individual water droplets making the system less stable. There are solids present in the system, which have a film of water surrounding them. These are called the water-wet solids, these solids also aid in conducting electrical current consequently reducing the electrical stability of the fluid. Other factors affecting these properties will be the different types of other solids in the system. Electrical stability is constantly monitored to ensure the stability of the system is within the required range. The other common component of the OBM can be Brine (CaCl2), which is primarily used for shale inhibition. Higher salt (Chloride) content will again promote low electrical stability, which is an undesired condition. In addition to brine some other common additives for OBM systems are Lime (Ca(OH)2) & Quick Lime (CaO), for pH control. The other fluid properties such as weight, viscosity, yield point and gel strength are controlled according to the specific applications. For example plastic viscosity should be maintained at minimum values to optimize bit hydraulics and the penetration rates, if the plastic viscosity trends upwards without an increase in mud weight over a period of time this indicates that fine solids are building up in the mud. Decrease in the oil to water ratio will increase the plastic viscosity. Yield point and gel strength are governed by two requirements, first being able to maintain good gel structure (thixotropy) to suspend the cutting and weighing material. Secondly minimizing annular pressure losses. All these properties and other characteristics mentioned are case specific and can be manipulated to suit the application. The HTHP temperature capacity of this type of system can be up to 500 degree F. Lost circulation with Oil Base mud system can become intolerable due to the cost of the fluid system. Thus viscosity control can be considered one of the most significant factors of this type of system. Similar scenarios can occur when increasing the flow rates too fast. Oil base mud also has better lubricity, which helps to reduce torque, drag and friction on the BHA and the Bit respectively. Synthetic Base Fluid Systems: The need for synthetic based mud was recognized in the early 80s when environmental awareness was gaining popularity. Efforts were made to reduce the environmental impact of oil base mud by replacing more refined petroleum products such as mineral oils instead of diesel oil. Fluid systems designed from these oils were less toxic but still contained sufficient aromatic compounds, which were considered environmentally harmful. More and more efforts were focussed on synthetic based fluids such as ester and experiments were performed and more synthetic materials followed as a consequence. By the 90s compounds such as Linear Paraffin, Linear alpha Olefins & Acetal were synthesized in the manufacturing industry to make sure that no trace pollutants were present in the system. Today use of synthetic base fluid systems in drilling operation is a fairly common practice mainly in North Sea, Gulf of Mexico, Far East, Australia and South America. Due to the evolution of the industry, today’s synthetic fluids are believed to be 2nd generations synthetic fluid systems, this distinction is mainly due to the fact that the new systems are cheaper and their viscosity is lesser as base fluids.

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The general definition of a synthetic material is something that is produced by chemical synthesis. In order to use the synthetic based fluid system in an area two conditions have to be met, 1st the system has to meet the local environmental standards & regulations for discharge of drilled formation cuttings into the ocean. Since if the cuttings have to be transported to land the advantage is lost and the operation becomes expensive. Secondly the synthetic material must be a base fluid for a stable system with inhibitive properties of an OBM system or Invert-emulsion fluid system. In this kind of mud system synthetic fluids are used as a continuous phase of the invert-emulsion mud system similar to the OBM system. The fluid system must behave closely to an Oil Base Fluid system. The products used to make and maintain SBM are similar or sometimes are the same as those used in the standard OBM systems. The products used in SBM can be used in OBM but generally the opposite is not applicable since a lot of products in OBM are refined oils, which would contaminate the SBM system. Two basic factors for deciding on the use & applications of synthetic system are kinetic viscosity and thermal or chemical stability. Thinner (Paraffin & Olefin) base fluids are good for deep water applications where density is a bigger concern and ester based fluids are not recommended for high temperature applications since thermal degradation of these system occurs. The properties of the system such as viscosity, density, yield point, electrical stability, gel strength, salinity etc are controlled in similar fashion to a typical OBM system. And various chemical compound combinations are used for each type of application weather it is shale inhibition, fluid loss control, salinity, high temperature applications or formation contamination. Chemically synthetics contain oxygen, hydrogen and carbon in their structure. Chemically balanced chains of CH (Carbon and Hydrogen) form the structure of the synthetics. One of the important concerns when building a synthetic system is avoiding contamination from diesel and other environmentally unacceptable additives. Environmental issues with the SBM have a lot of significance. Toxicity tests are carried out for the system to determine their effects on living organisms. The chemistry of SBM such as the molecular weight has direct impact on the toxicity of the system. Biodegradation tests are also performed for these systems. At the same time these systems are tested against local standards and regulations prior to their use. In addition to these factors other Health & Safety related precautionary measures are required for handling of the substances. The rig equipment used for this system should be the same as used for OBM system to ensure HSE standard compliance. Some of the applications of the SBM system are in drilling development wells, deep water operations, extended reach wells, horizontal wells, rigs with limited torque capacity for rotating the pipe, replacement of OBM etc. Similarly lost circulation issues can be addressed in same way as OBM systems. Questions Part 2: Q5. What is the most commonly used mud system and why? Q6.

Name the four sub-systems in water based drilling fluids?

Q7.

What is the application of Salt-Saturated water based mud systems?

Q8.

What is in the continuous phase of Oil Base Fluid System?

Q9.

What are water-wet solids?

Q10.

What effect will an increase in water content have in OBM?

Q11.

What are the advantages of using SBM systems?

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Functions of Drilling Fluids The drilling mud is vital for a drilling operation from its very beginning to the end. In the early oil filed years common clays were mixed with water to build the mud. The functions that the drilling fluid performs affect the key aspects of drilling process. General functions of the drilling fluids are listed below. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

Remove cuttings from the borehole by bringing them to surface. Maintain hydrostatic pressure to control the formation pressure. Suspend and release cuttings. Transmit hydraulic power/energy or hydraulic horsepower to the bit. Provide hydraulic energy to the down-hole tools such as Mud Motors, Turbines and MWD tools. Seal permeable formations by forming a wall cake. Minimize damage to the reservoir by isolating it. Maintain well bore stability. Cool and lubricate the bit and down hole tools. Control corrosion of the BHA and the drill string. Support part of the weight of the BHA with buoyancy effect. Facilitate cementing and completion process. Provide formation evaluation with mud logging and MWD/LWD tools.

In addition to these function some other significant roles of drilling fluids will be explained such as lost circulation prevention and stuck pipe prevention. Similar functions can be categorized & merged together and discussed further. Hole Cleaning: As the bit drills the new formation cuttings are generated. These cuttings need to be removed in order to control & maintain the physical & chemical properties of the drilling fluid and keep the borehole stable. This also helps in maintaining the drilling mechanics, ROP, flow rate, weight on bit, ECD and pumping pressures etc. To achieve this, drilling fluid is circulated from the mud pits through the drill string and up the annuls, passing through the mud conditioning equipment to remove the undesired solids and back to the mud pits. Cutting removal from the hole is a function of the ROP, flow rate, string rotation, mud viscosity & density, cutting size & shape, hole deviation, flow profile and annular velocity of the mud. The rheology and physical properties of the drilling fluid determine & influence the cutting removal efficiency. Cuttings fall or settle at faster rate in less viscous fluids, an example would be a piece of rock in a glass of water compared to the same object in a highly viscous fluid like honey. As a rule of thumb, fluids with higher viscosity suspend & remove cuttings better then the fluids with lower viscosity. Most drilling fluids used now a day are thixotropic. This helps in cutting suspension when the fluid is in static conditions or during connections. Shear thinning fluids or thixotropic fluids prove to be better for hole cleaning purposes due the fact that they form a gel structure when they are static. Annular velocities are the relative velocities of solid particles in the annulus when the mud is being circulated. Generally the higher the annular velocities the better the hole cleaning. The annular velocity is measured in ft/min. A good figure for annular velocities would be 100-150 ft/min for optimal hole cleaning depending on the drilling conditions and application. Although hole cleaning best practices suggest annular velocities between 80-150 ft/min for better cutting removal. Higher annular velocities combined with thinner or less viscous drilling fluids cause turbulent flow which aids in hole cleaning but may cause some other problems such as hole wash out, filter cake thickness reduction and erosion. The rate at which the cuttings settle in a fluid is called the slip velocity. So if the annular velocity of the fluid is greater then the slip velocity of the cuttings, more cuttings will be transported to the surface. Therefore the net velocity with which the cuttings move up to the surface is called the Cuttings Transport Velocity (Transport Velocity = annular velocity – slip velocity).

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Cuttings settle at the low side in deviated holes

Figure 4.1

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Cutting transport in vertical wells is different from cutting transport in deviated wells since in deviated wells cuttings tend to settle on the low side of the hole, this occurs much faster than cuttings settling to bottom in vertical hole so cutting build up is faster in deviated holes. Refer to the figure on the previous page. For better hole-cleaning purposes especially in deviated wells thixotropic fluids are used with high Low Shear Rate Viscosity (LSRV), and turbulent flow is preferred. So at low flow conditions the viscosity helps in suspension of cuttings. The alternate option may be a less viscous fluid with high flows causing turbulent flow profile but cuttings settle quickly when the flow is stopped during connections. The application of the later technique is limited. The mud must be able to suspend cuttings, weighing material and other chemicals and also be able to release the cuttings when passed through the solids control equipment. Weighing material normally barite when settles in the mud this phenomenon is called the barite sag, this creates a wide variation in density inside the well bore. Sag occurs in static & dynamic conditions in highly deviated (mud weight > 12ppg and Hole angle > 30 degrees) well with low annular velocities. High concentrations of drilled solids may have a negative effect on the ROP and drilling efficiency. Increase in drilled solids not only increases the mud weight and viscosity but also increases the cost of fluid maintenance not to mention the increase in horsepower required to circulate the fluid. Increase in torque and drag, which poses a potential for differential sticking problems. A reliable indicator of the presence of drilled solids in the system is the sand content and solids percentage in the drilling fluid. The other effective method for cutting removal can be by using ‘sweeps‘ which are smaller volumes of fluid spacers pumped to aid is hole cleaning process. Low viscosity sweeps are used to stir up the cuttings by inducing turbulent flow profile followed by high viscosity sweeps to lift the stirred up cuttings to the surface. The problems associated with sweeps are difficulty in maintaining mud properties due to contamination from sweep fluids. The potential also exits of formation fracture if too many cuttings are picked up at the same time. Higher density fluids aid in hole cleaning simply due to the fact that they have higher buoyancy effect. So in relative terms high-density fluids may clean the hole better with lower annular velocities. But this also has disadvantages due to the reason that too much overbalance with higher mud density affects other drilling parameter and can even cause lost circulation. Similarly drill string rotation also helps in hole cleaning, it creates a stirring actions and introduces a circular component in the annular flow path generating a corkscrew effect on the cuttings moving up the annulus. Drill string rotation is one of the best methods used for hole cleaning in deviated holes. Some of the good practices such as pipe rotation greater than 50 rpm & wiper trips help in cutting removal. Hole inclinations between 30-60 degree are critical for hole cleaning optimization. Maintaining hydrostatic Pressure and formation pressure control: One of the important functions of the mud is to control formation pressures. As we drill deeper the formation pressures increase, drilling fluid density is increased by adding barite or hematite, to balance the formation pressures to achieve borehole stability. This hydrostatic pressure due to the fluid density keeps the formation fluids from flowing into the well bore. The pressure exerted by the drilling fluid on the walls of the borehole while static conditions prevail is called hydrostatic pressure and it is a function of the mud density and true vertical depth of the hole. Maintaining the well bore pressure equal or slightly higher then the formation fluid pressure keeps the well stable and avoids kicks and blowouts. These types of controlled conditions associated with pressure vary from cases where no formation fluid is allowed inside the well bore to cases where high levels of background gasses are tolerated or even in cases when the well is producing commercial quantities of Oil & Gas while the hole is being drilled. Another important role that the drilling fluid plays other then balancing the formation pressure is controlling stresses in the well bore due to geological activities in the formations. Balancing these stresses with hydrostatic pressure can stabilize well bores in tectonically stressed formations. Often formations with sub normal pressures may be drilled with compressible fluids like gas, mist, aerated mud or foam.

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The mud density used to drill any particular hole is optimized so that it is slightly over the formation pressure but does not reach the fracture pressure of the formation. The mud properties should be controlled in order to keep the well bore stable until casing is run and the section is sealed off. Well bore instability is often indicated by sloughing formations, which causes tight hole conditions, bridges and fills on trips. This often leads to excessive reaming to the bottom after the trips; these indicators can also be signs of poor hole cleaning specially in deviated wells. The borehole is most stable when it is in gauge. Once the hole starts to erode number of problems start to appear including lower annular velocities, increased maintenance of the mud, higher cementing costs and poor cementing jobs. For more knowledge on well bore stability refer to Well Bore Stability SLP or use link: Hole Stability In shales generally if the pressures & stresses are completely balanced by the drilling fluid, then good hole stability exists. But with water based drilling fluids a possibility exists that with time the shale formations start interacting with the water content in the mud. This leads to swelling and softening of the shales. Highly fractured, dry and brittle shales especially in deviated holes can pose serious problems. Generally failure of shaly formations is related to mechanical failures rather than the mud associated problems. Sealing off Permeable Formations: Permeability is necessary for the production of hydrocarbons, since it is the ability of any fluid to flow through porous formations. Drilling mud at slightly higher pressure then the formation can flow into the formations and contaminate the reservoir, which would result in a negative impact on the production. Therefore mud should have a capability to seal off the permeable formations this is achieved by the formation of the mud cake on the wall of the borehole. The mud cake is a thin layer of low permeable filter cake that limits invasion of the drilling fluid into the formations. This not only improves well bore satiability but prevents other drilling problems associated with thicker wall cake such as tight hole or sticking, which can result in a stuck pipe situation. In certain formations with larger pore size it is difficult to form the filter cake just with the aid of the drilling mud. Special sealing agents are used to plug up the formations such as cellulose and a wide variety of lost circulation material (LCM). Some of the common additives for building filter cake are bentonite, natural and synthetic polymers, asphalt and natural deflocculating agents. Minimize Formation Damage: Any reduction in the production of the formation due to permeability and porosity problems related to effects of drilling fluids are considered formation damage. This can happen due to plugging of the formation pore spaces by mud or drilled solids through chemical or mechanical interaction. This can occur due to either reactive formations or drill string interactions. Generally formation damage is referred to as skin damage. Cool & Lubricate bit and Tools: Considerable amount of heat is generated due to friction between the bit and the formations and also in the down hole tools due to mechanical work. Drilling fluids carry the heat away from the surface and as a result cool them, this keeps the temperature profile fairly evenly distributed in the borehole instead of being concentrated close of the bit area. In addition to cooling the bit, mud also lubricates the BHA, Bit, down-hole tools and the drill string to reduce drag & wear. The lubricity of any fluid depends on the coefficient of friction for that particular fluid. Some fluids are more efficient in lubrication than the others an example would be oil base mud & synthetic base mud. These fluids have much better lubricating characteristics. But lubricants can also be added to WBM and other type of drilling fluids to enhance the lubricity of the fluid. Also the amount of lubricity depends on the type & quantities of drilled solids, weighing material, chemicals, salinity, pH level and hardness of the fluid. The alteration of mud system to increase the lubricity may not show the desired results on torque or drag simply because of the fact that multiple factors and parameters are involved in the drilling operation & mechanics. Some good indicators of poor lubricity are increased torque and drag, unusual wear and thermal stressing on drill string components like heat checking. Once again these problems can also be associated with other drilling conditions like high doglegs, BHA design, hole cleaning etc.

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Hydraulic optimization through bit nozzle selection for achieving required hydraulic impact forces

Figure 4.2

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Transmission of Hydraulic Energy: Hydraulic energy is the force generated by the mud after it is jetted out of the drill bit. It is also used in supplying power to the down hole tools since hydraulic energy can be changed to mechanical energy with turbines and motors. Power Pulse* turbine is a good example. Hydraulics is optimized by sizing the bit nozzles according to the specific application and need. See figure on the last page. The rig pumping capacity puts a limit on the hydraulics. Nozzle sizes are selected to maximize the fluid impact force on the formation since its helps in removing cuttings faster and increases the penetration rates. Drill string pressure losses are higher when using fluids of higher density, viscosity and higher solid content. The use of small drill pipe and BHA increases the pressure losses and decreases the annular velocity. Also down hole tools reduce the amount of hydraulic power available at the bit. Fluids containing lesser solids, fluids that are shear thinning or fluids having drag reducing characteristics, for example polymers are more effective in hydraulic power transmission to the bit. As the well deepens the annular pressure losses increase thus reducing the available hydraulic horsepower at the bit. So by controlling the mud properties the hydraulics can be optimized for deeper drilling. Formation Evaluation: Drilling fluid serves as a vital source for information related to formation evaluation. During the drilling operation especially for exploration wells any changes in properties of the mud are monitored and any signs of external fluid influx such as oil, gas or formation water are detected by mud loggers. They also examine the cuttings coming out of the hole for mineral composition, paleontology and visual signs of hydrocarbon. All this information is presented on a mug log, which can comprise of information about lithology, ROP, surface gas and presence of oil reflected in the cuttings. Other forms of formation evaluation are by using logging tools in the drill string for Real Time formation evaluation called MWD & LWD (Measurement While Drilling/Logging While Drilling). Wireline tools maybe run and formation samples may be taken in form of formation fluid samples or cores (sections of rock/formation). All of these formation evaluation procedures are made possible by the drilling fluid. If the cuttings are not properly brought to surface then mud logging is not very effective. For the LWD & MWD tools the chemical compositions of the mud may effect the measurements taken by the tools consequently providing invalid or insufficient data. Similar case may occur for Wireline logging operation. Excessive filtrate can flush oil or gas for the immediate vicinity of the borehole adversely effecting logging process. Mud contents or additives can also affect density, porosity and electrical measurements; these additives can be salt content, weighing mater, polymers chains etc. in the fluid system. For coring operation the mud is selected based on the type of analysis being performed. If the cores are taken only for mineral or lithology evaluation then mud is not a major concern but if the cores are taken for more complicated studies such as wetablity studies, pH studies (Studies of system without the presence of thinners) then the use of surfactants and minimal additives in the mud is recommended. Corrosion Control, Cementing and Completion: The drill string is essentially all-metal and in one way or the other it is susceptible to corrosion. Dissolved gases such as hydrogen sulfide, oxygen and carbon dioxide can pose serious corrosion problems throughout the system. Generally the phenomenon of corrosion is associated with the acidity of the drilling fluid or the pH (pH < 7) of the mud. In addition to providing corrosion protection to the metal components the drilling fluid must also protect rubber and other materials used in the system from wear or corrosion. There are various types of corrosion such as general corrosion on a metal surface, pitting corrosion which is localized form of corrosion, stress corrosion cracking, sulfide stress cracking associated with H2S, erosion corrosion etc. To prevent corrosion, chemical inhibitors are used in the mud system and by constantly checking the properties associated with corrosive nature of the mud corrective actions can be taken if the system is not working efficiently. Hydrogen sulfide is responsible for majority of the corrosion-related failures in the drill sting. When drilling in H2S area constant pH checks should be performed, and chemicals scavenging the sulfides (like zinc) should be used in the system.

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Another important role of the drilling fluid is to facilitate the process of setting casing and cementing. Cementing is crucial for zone isolation and further drilling activity. The drilling fluid should have properties aiding in the process of cementing such as proper cutting removal, non fractured formation structure, gauge hole (stable hole) etc. Generally prior to cementing, fluid spacers are pumped and specially treated fluids are circulated to ensure good bonding for the cementing process. Similarly completion operations also require a stable, in gauge hole in order to successfully complete the process. Stuck Pipe Prevention: if the cutting are not removed from the well bore properly then they accumulate in the well resulting in hole pack off, this can often occur in over gauged sections of deviated wells especially around the BHA components. Thus when the bigger elements of BHA such as stabilizers are pulled back they sweep up the built up cuttings and as a result a pack off or stuck pipe situation occurs. This situation relating to cutting build up can arise from higher ROP & inadequate hole cleaning, low cutting suspension capacity of the drilling fluid & poor annular hydraulics, formation swelling and blind drilling (no mud returns at the surface). Major indicators of poor hole cleaning can be low cutting return on the shakers, increase in low gravity solids in the mud, and increase in torque and over pull while tripping out. To counter these problems mud rheology should be maintained according to the ROP and hole angle. Frequent sweeps should be pumped, short trips, hydraulic optimization, controlled drilling etc. should be considered. Formation instability issues should be considered in the planing phase of a well, it should not be left completely as a reactive response. The other type of sticking problem directly related to the mud is the differential sticking mechanism. Generally it is caused by higher hydrostatic over balance, fractured formation, thick filter cake and with drilling fluids containing high density or high solid content. To learn more about stuck pipe refer to Stuck Pipe Self-Learning Package or click on the link: Self-Learning-Package or for Driller’s Stuck Pipe Hand Book Self-Learning Package click on the link: Drillers Handbook Self-Learning-Package. A lot of indicators can be noticed and interpreted by keeping a close eye on the mud system. Majority of the drilling related problems can be linked to these indicators or signs. Constantly monitoring the properties and sufficient knowledge on how to interpret the changes can save costs, reduce environmental impact and avoid potential hazards. Basically selecting the type of fluid to satisfy all the drilling conditions is like any other engineering solution, it is not a total solution but an it can only to be an optimal solution. And if the priorities are set right and the conditions and parameters are properly defined then mud system selection becomes quite a scientific and logical process.

Questions Part 3: Q12.

Name the functions of the drilling mud?

Q13.

What are the common factors impacting hole cleaning?

Q14.

Define annular velocity and cutting transport velocity?

Q15.

What types of borehole forces are balanced by the drilling mud?

Q16.

What is the most common type of evaluations done by mud loggers?

Q17.

What can cause a corrosive environment in the mud?

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Different flow conditions in the well bore

Well bore

Figure 4.3

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Mud Rheology & Physical Properties Rheology is the property of a fluid to deform and flow. It falls in the category of physical properties of a fluid. The fluid properties effect various aspects of the drilling operation as discussed in the earlier sections such as pressure control, hydraulics, well bore stability, suspension & removal of cuttings and separation of solids & gas at the surface. To drill a particular well efficiently a balance must be achieved between the various mud properties. Rheology and hydraulics are interrelated studies of fluid behavior. Rheology is primarily concerned with the stresses (shear stress and shear rates) related to the fluid flow. Hydraulics is concerned with how the fluid flow creates and utilizes forces & pressure. Essentially rheology and fluid hydraulics can be discussed at the same time. Hydraulics and Fluid Flow Dynamics: Measuring and analyzing flow characteristics under different temperature, shear and pressure conditions can help predict and plot the flow behavior for a particular fluid. The most common models used for the analysis of the rheological properties of the drilling fluids are discussed below. Bingham Plastic Model: According to this fluid model a finite amount of force is required to reach the yield point or the flow condition. The fluid viscosity remains constant with increasing shear rate. Most of the drilling fluids do not obey this law. This model closely approximates most flocculated clay water based fluids in which yield point and initial gel strengths are equal. Refer to Figure 5.2 & 5.3 on next page. Power Law: This model is a little more complicated then the previous one since it does not consider a linear relationship between shear rate and shear stress. This law describes a fluid in which shear stress increases as a function of shear rate mathematically raised to some power. API has chosen Power Law as the standard model. An extended version of power law known as the Modified Power Law takes care of the discrepancies concerning yield stress and underestimation of LSRV (low shear rate values). Modified Power Law is also called Hercheal-Bulkley model, which also accounts for stresses acting to initiate fluid motion. Refer to figure 5.4. Types and Stages of flow: The drilling fluid flow can be modeled by dividing it in different stages and then explaining those individual stages. Please see the figure describing the flow stages on the previous page. No Flow: The first stage is no flow when the fluid is static and gradual pressure is being applied to initiate flow and overcome the gel strength. Plug Flow: As the pressure is increased gradually with increase in flow rate the yield stress is reached and a flow is initiated but at this stage the fluid moves as a solid mass or otherwise referred to as a plug until the next stage of flow is reached with the shear rate being increased. Laminar Flow: The layers within the fluid start to break and consequently the size of the plug is reduced and the fluid flow rate & fluid velocity are increased. The fluid flow is such that the fluid velocity is highest in the middle of the flow profile and zero at the boundaries resembling the shape of a parabola. Transitional Flow: This is a transition of flow from laminar to turbulent flow regime. The flow profiles changes form streamlines or flowlines parallel to each other to random streamlines as the flow rate is increased. Turbulent Flow: As the flow rate is increased further the constant and parallel streamlines start to disappear and the fluid flow profiles become random and swirly. Also the direction of flow movements is random at this stage. After the turbulent state is reached any increase in flow rate or velocity will increase the degree of turbulence in the flow regime. All of these flow profiles have different implications in terms of pumping pressure, hole cleaning and hole washout.

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Cutting Settling Mechanisms

Figure 5.1 Newtonian Fluid

Bingham Plastic

Figure 5.2

Figure 5.3

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Once the flow models are in place hydraulic calculations are performed to find out the pressure losses in the system, ECD and hydraulic horsepower values are also optimized. The maximum allowable circulation pressure is often limited by the rig pump and surface equipment specifications. Therefore the fluid rheological properties are often the only variables available in the optimization process. API refers to the fluid velocity in the annulus or pipe as the bulk fluid velocity in feet per minute. This assumes laminar flow and formulates equation for calculating the bulk velocity. The equatuions for bulk velocity as formulated by API is: Vp = (24.48 x Q) / (D2 2 - D1 2 ) & Va = (24.48 x Q) / (D2 2 - D1 2 ) Where Vp is the Bulk velocity in the Pipe in ft/min. Va is the Bulk velocity in the Pipe in ft/min. Q is the flow rate in gpm. D2 is the outside diameter of the pipe in inches. D1 is the inside diameter of the pipe in inches. Another important parameter in the hydraulic calculation is the Reynolds’s Number (NRe ) and it has no units. This number is used to distinguish between the fluid flow profiles (Laminar & Turbulent) based on value or ranges e.g. a NRe > 2100 indicates a turbulent flow. NRe is given by a general formula: NRe = (V D ρ ) / µ Where V is the velocity is ft/min D is the diameter of the pipe or annulus (D2 - D1 ) in inches. ρ is the density of the fluid. µ is the viscosity of the fluid. The term critical velocity is used to describe the flow in the transition region of the flow dynamics (between Laminar and turbulent flow regime). Calculating the pressure losses is another major consideration and these calculations are based on the total pumping pressure available minus the pressure losses in the system such as surface equipment, drill pipe, annulus, down hole tools, bit etc. These losses are mechanical frictional losses calculated by using different coefficients of friction for each component or equipment involved in the circulatory system. When circulating the mud through the system the pressure on the formation increase due to the pressure losses in the annular, bit and the surface system resisting the fluid flow. This pressure is in addition to the hydrostatic pressure exerted by the fluid column, this increased pressure due to circulating is interpreted as increased fluid density and it is called Equivalent Circulating Density (ECD) of the system. The bit hydraulic calculations involve calculating the hydraulic horsepower, pressure drop and bit nozzle selection. Also from these calculations bit hydraulic impact force in pounds per square inch is calculated. As a conclusion the drilling performance, efficiency and mechanics depend greatly on the rheological properties, optimizing them within the mechanical specifications of the rig can result in higher ROP, less pressure losses, better hole cleaning and stabilization & optimal hydraulics which help in reducing cost of the operation. Cutting Settling Mechanisms: The hole cleaning process must counteract gravitational forces acting on the cuttings and reduce their effect during dynamic and static periods. Three basic settling mechanisms can apply. Please refer to the figure on the previous page for graphic description of these mechanisms.

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Power Law

Figure 5.4

Building process of the Wall Cake by deposition of solids on the walls of the hole

Figure 5.5

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Free Settling: Free settling occurs when a single particle falls through a fluid without interference from any other particles or the walls of the hole. The larger the difference between the density of the cutting and of the liquid, the faster the particle will settle. The larger the particle size the faster it will settle and the lower the fluid viscosity, the faster the settling rate. Refer to Figure 5.1 two pages earlier in the section. Hindered Settling: Hindered setting is more realistic for near vertical and near horizontal sections. This type of settling occurs when fluid displaced by falling particles creates upward flows (forces) on the adjacent particles slowing them down and improving the cutting removal. The net effect is still a downward movement of the drilled solids thus the mechanism still is not very effective for proper hole cleaning. This settling mechanism is most effective in vertical hole, coupled with the long settling distances it helps in explaining why hole cleaning in vertical holes is less problematic. Boycott Settling: This settling mechanism is an accelerated settling process occurring in deviated holes. This is a result of fast settling occurring adjacent to high (up) and low (down) sides of the deviated holes. This causes a pressure imbalance so that the upper fluid is lighter and moves up, the bottom fluid becomes heavier and moves down with the cutting beds. At low flow condition the mud flows mostly on the high side of the well bore and increases the Boycott effect. The cuttings tend to settle at the low side of the hole, which can result in a pack off. High flow rates and pipe rotation help prevent this pressure imbalance. Settling in deviated holes is 3-5 times faster then in vertical holes. Angles from 40-60 degrees are generally troublesome for hole cleaning purposes. Mud rheology such as better or optimum viscosity & gel strength parameters can help improve the situation. The gel strength of the fluid provides suspension under both static and dynamic conditions. The ideal situation would be for the mud to have high, fragile gels that develop quickly and are also easily broken. Excessive gel strengths should be avoided since they cause high surge pressures. Filtration Control & Wall Cake: A drilling fluid will deposit a filter cake on the wall of the well bore. This cake helps protect the formation by blocking the passage of mud filtrate into it the formation matrix. Formations with higher permeability can absorb large volumes of mud filtrate. The quality and the amount of solids in the mud determine the type of the wall cake. Refer to the figure on the last page. Ideally the wall cake thickness should be as thin as possible to avoid other hole problems. A drilling mud with a low filtrate water loss will form a thin & tough filter cake. Specially formulated bentonite products can provide filtrate control and deposit a thin & tough filter cake. Contaminates that flocculate the bentonite particles should be treated out immediately in order to optimize the process of wall cake build up. If excessive reaming and circulation is carried out the cake can erode causing fluid loss, which may result in a stuck pipe situation. Bentonite, polymers and starch are used to control the water loss. It is essential that these products be mixed in the correct order. Some products have an adverse effect on other products when they are mixed in an incorrect order. For example, if PHPA polymers are mixed before the bentonite then the bentonite will not yield properly. Cutting Suspension: Cutting size depends on the bit cutter size, extent of under-balanced drilling and high penetration rates. The largest particles or cavings are created by over pressurized shale formations. Cuttings can be physically altered by reacting with the mud (dispersion), reaction with themselves (segregation) or mechanical degradation. Larger cuttings are not easily transported out of the annulus so these maybe recirculated until they are reduced in size. If not properly supported the cuttings can settle to the bottom or low side of the hole until they build up to a point where an avalanche effect is initiated resulting in a pack off and stuck pipe. Mud cutting suspension properties at low flow rates are important to support the cuttings under static conditions. As discussed previously cutting transport efficiency is largely a function of the annular velocity and annular velocity profile. In a completely concentric hole the velocity profile is evenly distributed around the drill string. However in practical scenarios the drill string sags on the lower side of the hole disrupting the velocity profile and disturbing the energy profile used in cutting suspension process.

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Shale Shaker

Figure 5.6

Table A

Figure 5.7

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Different drilling muds provide similar transport capabilities if their physical properties are also similar. Mud weight helps in providing buoyancy to the cuttings and also slows down their settling rate. Mud viscosity helps determine the cutting carrying capacity, more recent analysis have concluded that Fann3-6 vales are better in deterring the carrying capacity rather then the yield point. Refer to Testing Equipment for details on Fann readings. These values are more representative of LSRV, which helps in hole cleaning related issues. Better gel strengths provide suspension under both static and low shear rate conditions. Another important parameter for cutting suspension is the CTR or the Cutting Transport Rate. CTR is mathematically given as. CTR = 100 x ((VAnn – VSlip) / VAnn) Where CTR is the cuttings Transport Rate in % VAnn is Annular velocity in ft/min VSlip is Slip velocity in ft/min CTR values greater then 50% are generally suitable for vertical wells. CTR is an indication of hole cleaning efficiency. Solids Control: The presence of solids in the drilling fluids effects almost all the physical and rheological properties such as density, filter cake, viscosity and even gel structure of the drilling fluid. Solids their size and quantity influences the well and its cost. Solids also affect the drilling parameters like ROP, hole stability, pump and surface equipment wear. Due to all these factors solids control is the most important aspect of mud conditioning in order to achieve the desired drilling efficiency. Solid control related maintenance of the mud makes a significant percentage of the total cost for drilling a particular well. Solids control is generally achieved by: • • • •

Screening (Shale Shakers) Settling (Sand Traps) Hydro-cyclones (Deciliters & Desanders) Centrifuges

Depending on the size of the solids invading the mud, the application of each of the above equipment is optimized and selected. The solids particles are classifieds in microns according to their size. Refer to Table A for the solid size classification. Screening: Most of the shale shakers will remove larger particles form the mud with great efficiency. It is one of the most significant solids control devices used and also the most effective as well. The shale shakers have vibrating screens sometime inclined to increase the solid removal efficiency. The sizes of the screens can be changed based on the size of the solids being generated in the system and mud properties such as viscosity, density and the flow rate. The shale shakers have a capability of removing any particle over the size of 74 microns with certain flow rate limitations. Which essentially eliminates the use of hydro-cyclones. Settling: Use of sand traps in another method used for solid removal mostly in the older days of oilfield industry. The rate of settling of the sand or solids mainly depends on the size, shape and the weight of the individual particles. It also depends on the fluid properties like density, viscosity, flow rate & flow profiles and the total time spent in the pits. According to Stokes Law optimum settling with this method of solids control is achieved when the fluid is in laminar flow regime. Hydro-cyclones: Centrifugal pumps jet the drilling mud through a tangential opening into a larger end of a funnel shaped structure, the hydro-cyclone. With the proper pressure applied a whirling motion of the fluid results following the laws of fluid dynamics. This motion then results in expelling heavier solids from the open bottom and the clean liquid returning from the top of the funnel. Different sizes of hydro-cyclones are designed for the removal of different sizes of solids.

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Hydro-cyclone Design

Figure 5.8

Centrifuge

Figure 5.9

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According to these design specifications they are categorized as Desanders and Desilters, silt being smaller in size then sand particles. These Desanders and Desilters are normally arranged and combined together into what is called the Mud Cleaner. See figure on the last page. Centrifuges: This device increases the efficiency of solid removal by using centrifugal force. Fluid is injected into a rotating cylinder-shape chamber. The rotation at high rpm causes the solids to move against the inside of the wall of the cylinder and a conveyer system pushes them to the end for discharge. Centrifuges are very effective devices especially in conditions when the mud is weighted to higher degrees and weighing material needs to be recovered. The discharged liquid volume is replaced by new or diluted fluid. The problem with these devices is low capacity, and the fact that most of the rigs do not have them so these devices are rented mostly from sub-contractors. Due to their size and capacity limitation a small volume of the mud is passed through them and cost is specially a concern when more then one device is rigged up for better handling of the fluid volume. Please refer to the figure of a centrifuge on the previous page. Special Fluids: Fluids used in completions and work-over are examples of the special type of fluids. Completion fluids are used in pay zones after the well has been drilled prior to starting the production of the well. Work-over fluids are used in remedial work unusually attempting to enhance the well production. Some of their functions are: • • • • •

Formation damage control. Formation Stimulation. Transportation and removal of solids. Pressure control. Control losses and retain fluid properties.

Some of these fluids can be brines with or without the weighing material and other chemical additives or converted drilling fluids systems. The primary criterion for the selection of either of the two fluids is basically the density of the fluid.

Questions Part 4: Q18.

Name the laws used for hydraulic modeling of the fluids?

Q19.

Define laminar flow?

Q20.

What are the cutting settling mechanisms?

Q21.

What is the most common equipment used for removal of drilled solids sized around 74 microns?

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Mud Balance

Figure 6.1

V-G Viscometer or Fann Viscometer

Figure 6.3

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Marsh Funnel

Figure 6.2

API Filter Press

Figure 6.4

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Testing Procedures & Equipment Mud properties need to be measured and analyzed to ensure that they stay within the required specifications and they need to be controlled if necessary. Rheological properties, solid content, fluid loss characteristics, density, viscosity, yield pint and gel strengths are a few of the important properties that need to be measured. These tests can vary slightly for different types of mud systems namely OBM, WBM and SBM systems but the general procedures for conducting these tests are quite similar for different fluid systems. Each of the mud types may have certain tests specific to them like OMB has an electrical stability test which is not applicable for the case of WBM systems. The following section discusses some of the tests carried out to determine the desired mud properties and the equipment used in performing those tests. Please refer to the relevant figures of the testing equipment on the previous & the next page. Density: The Mud Balance is used to accurately determine the density of the drilling fluid. The accuracy is of this equipment is within 0.1 ppg. The weight of the mud is responsible for providing hydrostatic pressure for downhole pressure control. The mud balance consists of a graduated arm and cup sitting on a base with a knife-edge on it. The arm has a counter balance, which slides on the graduated arm. For testing the density the cup is filled with mud and then the counter balanced is shifted until the apparatus is completely balanced. Then the value is read from the graduated arm, which would be the desired density of the fluid. The scale on this equipment is in pounds per gallon (ppg) and pounds per square inch (PSI). Relative Viscosity: The viscosity measurement in the field is taken by using the apparatus called the Marsh Funnel. This funnel is 6 inches in diameter and 12 inches in length. The bottom of the funnel is a 2-inch long tube to direct & control the flow while the mud is being tested, the inside diameter of the end tube is 3/16 inch. On the topside of the funnel, half the opening is covered with a wire screen. The funnel holds a volume up to the screen level of 1500 ml in total. The testing procedure for measuring the relative viscosity is, first the funnel is filled with the mud sample up to the wire mesh level and then the fluid is allowed to flow out of the funnel and the time taken for the funnel to completely drain all the fluid is noted down. This time in seconds is the relative viscosity of the fluid. This equipment is a qualitative measurement of the property and is also dependent on temperature of the fluid. Plastic Viscosity, Apparent Viscosity, Gel Strength & Yield Point: A rotating V-G viscometer is used to take the plastic viscosity and yield point measurements of the drilling mud. Direct indicating V-G meters are rotating devices, which are powered by an electric motor or a hand crank. The mud is contained between the outer rotating cylinder and the inner sleeve of the equipment. The inner sleeve has a torque gauge with a needle on the upper side of the apparatus. The rotation of the outer cylinder produces a torque, which is then transferred through the fluid due to frictional forces to the inside sleeve showing a torque reading on the needle gauge. The apparatus is calibrated such that the 300-rpm reading & 600-rpm values are used to obtain the plastic viscosity and yield point values of the mud. The rotor normally has 600, 300, 200, 100, 6 & 3 rpm speed settings. The 3-rpm setting is used to determine the gel strength of the mud. The mud is heated to 120 degree F before it is placed in cylinder. The measurement of Yield Point is in the units of pounds per 100 feet square (lb/100 ft2) and viscosity is in Centipoise (cp) units. The plastic viscosity is the 600-rpm reading minus the 300-rpm readings. The yield point is the 300-rpm value minus the plastic viscosity value. Apparent viscosity measurement is the 600-rpm reading divided by 2. Gel strengths are measured by stirring up the mud at 600 rpm for 15 seconds and then the apparatus is left with the motor shut off for 10 seconds. Then the switch is put in the low speed position and the reading on the dial is taken. Another value of gel strength is taken at 10 minutes and temperatures are also noted down.

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Orion pH Meter

Electrical Stability Meter

Figure 6.5

Figure 6.6

Retrot

Figure 6.7

Sand Content Set

Figure 6.8

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pH Indicator Sticks

Figure 6.9

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Fluid Loss, Resistivity of Mud Filtrate & Filter Cake: The device used to measure the filter loss, cake thickness and wall building capability of the mud is called the API Filter Press. The test is performed under certain conditions of temperature, pressure and time. The fluid loss measurement is taken at the surface temperature at 100 psi of pressure and for 30 minutes time duration, the fluid deposited in the graduated cylinder after the set time is measured in millimeters. This instrument consists of a mud chamber and a pressure regulator & a gauge. A filter paper is placed in the modular chamber and then pressure is applied by using a pressurized carbon dioxide gas cartridge. Once the pressure is applied the fluid starts to filter through the filter paper slowly and is collected in the graduated cylinder on the bottom of the apparatus. At the end of the experiment the thickness of the mud cake deposited on the filter paper is also measured in units of 1/32 of an inch. High Temperature Hi Pressure (HTHP) fluid loss measurements are carried out in a similar apparatus with a heating jacket to heat the fluid to simulate the bottom hole conditions. Generally the standard HTHP test readings are taken at 300 degree F and 500 psi pressure value for 30 minutes time. Once these tests are performed by the mud engineer the filter cake and the mud filtrate is available to the LWD engineers for evaluation. LWD or Wireline logging engineers take those samples and measure the resistivity of the filtrate and mud cake. These readings along with the resistively of the mud are used to apply environmental corrections to the logs & relevant measurements. Charts such as GEN9 etc. are available in the Schlumberger Log Interpretation Chart Book for applying these corrections. Sand Content: A Sand Content Set is used to measure the sand content present in the mud. This procedure is only an estimation of the sand content present in the mud. This apparatus to carry out the test consists of a 74-micron mesh sieve, which is 2.5 inches in diameter. A funnel that fits the sieve and a graduated glass tube. The glass tube also has a scale on the bottom section from 0-20% scale. Once the fluid is kept stationary for some time the solids settle to the bottom of the cylinder and their level can be matched on the cylinder scale to get the required percentage of solids in a specified volume of the drilling fluid. Liquid and Solid Content: The amount of Liquid and solids in the drilling mud is determined by an apparatus called the Mud Retort. This device has a built in oven to heat the mud samples. Mud sample normally of the volume of 30-50 ml is placed in a cup and a lid is placed on it. This lid allows for evaporation of the fluid outside of the cup. The sample is then heated till the liquid completely evaporates. The liquid vapors pass through a condenser and are collected in a graduated cylinder. The fluid portion in the cylinder is directly read in percentage of volume using the scale on the cylinder. The solid content is the fluid percentage minus 100. pH or Hydrogen Ion Concentration: The pH is measured for the fresh water mud systems using either the indicator strips or the pH meter. The later method is more accurate and commonly used in the industry. According to the pH level of the fluid different colored charts are provide for comparison. The sticks are immersed in the fluid sample and they indicate a color change based on the pH level of the fluid. This color is then compared to the charts to indicate the pH level of the sample. The more reliable method is by using the glass electrode electronic pH meter, which uses a probe to measure the pH of a fluid under specific temperature conditions. Corrosion Analysis: There are a few tests used to analyze the corrosiveness of a drilling fluid, chemical agents in the system look for oxygen, hydrogen sulfide and carbon dioxide contents. Besides these methods drill pipe corrosion ring coupons are used to note the effect of corrosion on the drill string. These coupons are placed inside the drill pipe and are removed approximately after 40-100 hours of exposure to the fluid and then analyzed for corrosion. This ring coupon is circular in shape and is designed to fit in tool joint of the drill pipe. The material of the ring should be the same as the pipe material, which is commonly steel. The ring after being exposed to the fluid environment is cleaned and sent to the laboratory in town for analysis and once the results are obtained remedial actions are planned. The laboratory analysis consists of visual inspection of the coupon for signs of corrosion also some calculations are carried to measure the rate of corrosion.

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Electrical Stability Test for OBM: This test is carried out to measure the relative stability of water in oil emulsion. The measurement is made using a pair of an equally spaced electrode setup. The electrode plates are placed in the fluid and an AC voltage from a DC source is applied to electrodes with a constant rate of voltage increase. When the emulsion becomes conductive corresponding voltage reading is noted down. This voltage is indicated by a current flow through the fluid sample completing the circuit and illuminating an indicator bulb. The average of two measurements is taken as the final reading for the test. In addition to the testing methods discussed above some other tests are performed to determine chemical characteristics such as chloride content, potassium content etc. As discussed earlier some of the testing methods for the same measurement differ for each type of mud system but the generality of the testing procedure remains the same. Questions Part 5: Q22.

Is the mud balance used to measure relative viscosity?

Q23.

How is the plastic viscosity measured?

Q24.

What is a corrosion ring?

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Measurements While Drilling (MWD) MWD tools have revolutionized the drilling process in many ways. Real Time information through the MWD tools makes decision making possible while the well is being drilled. Directional wells are easier, faster and economical with the help of the real time information providing drift, direction and down hole orientation of the toolface. Also more complicated well designs are possible because of this information. At the same time MWD tools integrated with Logging while Drilling tools provide formation evaluation in real time. As a result wells are more cost efficient due to prompt identification of the zones of interest and zones with potential problems. Also down hole drilling parameters such as torque, pressure, weight on bit, vibrations etc. are received in real time which help in improving drilling efficiency and avoiding potential problems such as sticking etc. The MWD tool receives information from its down hole sensors & LWD tools and sends it through the drilling fluid up hole where it is interpreted and used. The information is sent by creating pressure pulses caused by a modulator or restrictor assembly, which can restrict the fluid flow through the tool and create a pulse. This modulator is computer controlled to generate the desired pulse sequence for the down hole information. In addition turbines are present in the MWD tools to supply power to them and some additional logging tools. On the surface, sensors are placed in the Standpipe assembly, which detect these small pressure pulses sent by the MWD tool. These sensors in turn send the received signal to the logging unit where the information is interpreted and displayed to the client in real time. The term Measurements While Drilling is generally related to the down hole information pertaining to the Directional (Angle & Azimuth) or the drilling part (Weight on Bit, Pressure, Torque, vibrations etc.) only. And it is separated with the LWD based on the definition that MWD is drilling and Directional information and LWD is formation evaluation. Further information on MWD operation can be found in Introduction to MWD SelfLearning Package or by clinking on this link: Self-Learning-Package The fluid column inside the drill pipe acts as channel for the transmission of the MWD signal up the hole. Therefore the parameters associated with the BHA and primarily the mud has a huge effect on the quality & transmission of the signal. Parameters Effecting MWD Signal: The MWD signal travels through the mud and it is in the form of a pressure pulse with a certain frequency and strength in pounds per square inches. The factors affecting the signal can be placed in two categories. These categories can be Mud related factors and drilling parameters related factors. The factors related to the fluid are basically the type of fluid, rheology and physical properties that can affect the signal by either dampening it or increasing its effect. The factors associated with drilling are hole depth, surface noise from pumps, string vibrations, string or BHA inner diameter changes, BHA material, down hole noise due to presence of other tools such as motors or bits, noise due to drilling through certain types of formations and other drilling parameters like torque, flow rate, ROP, WOB etc. Drilling Fluid Related Factors: The following factors and fluid properties affect the strength of the signal from the MWD tools. 1.

Mud Type: Oil Base Mud and Synthetic Base Muds behave as compressible fluids in drilling conditions compared to WBM that is why when using OBM or SBM the signal strength is reduced due the dampening effect caused by this compression. The fluid column of OBM will have more reduction is signal strength compared to a WBM fluid column of same depth.

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Generation of MWD Signal through the fluid column inside the drill pipe

Surface Sensors Logging Unit

MWD Tool

Signal Path

Figure 7.1

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2.

Viscosity: Increase is viscosity results in decrease in the signal strength. As viscosity increases the resistance of the fluid to flow increases, this increase the bonding forces between the individual particles of the fluid. This increase in bonding forces tend to smooth out any pressure surges or in other words attenuates the signal to a greater extent. The fluid particle set themselves in chains similar to a net allowing less and less objects to penetrate and the whole net absorbs the effect of a pressure pulse. As the viscosity increase the fluid acts similar to a rubber dampner, which reduces pressure surges created by the MWD modulator.

3.

Density: Increase in density means increase in number of solid particles in the fluid. More solid particle means more interactions of the particles between each other. As a result of these increased collisions or interactions the effect of any force travelling through the fluid will result in an increase. Thus increase in mud density will increase the signal strength. This effect is similar to the fact that sound & heat travel faster in solids then liquids due to the fact that the energy is transferred more efficiently in solids due the presence of more particles and less void space which cause the dampening effect.

4.

Gel Strength: The effect of gel strength is similar to the effect of plastic viscosity. Increase in the gel properties of the mud will decrease the signal strength because of the rubber type behavior of the mud.

5.

Temperature: The effect of increase in down hole temperature will increase the signal strength for OBM and WBM. As the temperature increases the fluid viscosity decreases, increasing the signal strength. For some of the SBM systems this effect may be opposite due to the fact the chemical chains in the fluid may be more compressible at higher temperatures thus casing a damping effect due to the compressibility factor. The temperature effect is always associated with viscosity so theoretically there will be an observed change in viscosity associated with the temperature change. Generally this effect is so small that it is almost negligible compared to the effects from other fluid properties.

Drilling Parameters & Equipment Related Factors: Other than the mud a lot of factors play important role in signal transmission and signal strength. 1.

Hole Depth: The signal strength decreases with increase in hole depth, since a larger fluid column exits and signal is attenuated more, which reduces the signal strength.

2.

Pump Noise: One of the types of surface noise affecting the signal is the noise produced by the mud pumps. Most of the rigs have duplex or triplex pumps in which pistons are oscillating at certain frequencies according to the flow rates of the circulatory system. This frequency pulse originating from the pumps causes an interference with the MWD signal. This effect is mostly on higher frequency mud telemetry. The noise frequency harmonics are calculated & pump strokes per minute can be manipulated to get the noise peeks out of the signal bandwidth. Also filtering the signal improves the strength and quality of the signal.

3.

Electrical Noise: Another common surface noise is the electrical noise coming from ground problems and electrical wires close to the surface sensors, wiring and equipment. This noise can be identified in the SPM and filtered out or the source can be identified and eliminated.

4.

Downhole Noise: Unlike pump-noise drilling noise occurs at low frequencies. This noise can come form downhole tools like motors when they stall or some times some noise problems arise for motors with newly lined stator rubber. Pressure changes at the bit while drilling certain formations can cause noise as well. Excessive vibrations or even pipe rotation can produce signals that affect the MWD signal. Excessive ROP and torque problems can do the same to the signal. The inside diameter of the BHA also effects the signal, every change in the inside diameter effects the signal. Any big change from one diameter to the other reduces the signal slightly.

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The other parameters affecting the signal strength are the modulator gap & the pump flow rates. Modulator gap is set in the shop for each application & flow rates can be varied according to hydraulics requirements and rig capacities. Questions Part 6: Q25.

How will the signal strength be affected by increasing mud density, mud viscosity & hole depth?

Q26.

What are some of the sources of down hole noise?

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IDEAL Real Time Environmental Correction Page

Figure 8.1

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Logging While Drilling (LWD) LWD formation evaluation tools always have mud as the background environment. This environment effects any measurements taken by these tools. The evolution of drilling fluids and the development of new types of chemical poses a challenge for LWD engineers in order to define these effects and be able to correct for them so that the formation evaluation is performed as accurately as possible. This section will discuss each measurement and how it is affected by the mud properties and the mud constituents and the possible corrections applied to these measurements. Resistivity Measurement: Laterolog devices and Induction devices are two types of resistivity measurements used by Schlumberger. Their examples would be RAB and ARC tools respectively. The corrections are applied for RAB tool in RT & RM and can also be applicable for ARC tool. For details on IDEAL real time corrections and inputs refer to the Ideal Real Time Initialization for RTTC Jobs SelfLearning Package or click on the link: Self-Learning-Package Induction resistivity measurements are conductivity seeking tools. Therefore their measurement is effected by the most conductive element in its volume of the investigation. Conductivity of the mud system largely depends on the amount of chlorides in the system. •

Chlorides (Cl): Chlorides in the mud add to the conductivity of the system therefore, effecting the resistivity measurements. In salt saturated mud systems most of the signal is generated inside the mud around the tool. The environmental corrections would involve Rm (resistivity of mud), Rmf (resistivity of mud filtrate) & Rmc (resistivity of mud cake). Here the component Rm is used in applying the environmental correction for resistivity measurement. The ARC5 tool environmental corrections are the hole size or borehole compensation. For the case of the RAB tool the mud resistivity correction both in real time and recorded mode has to be applied.

Gamma Ray Measurement: Gamma ray measurements are corrected for •

Potassium (K): Potassium is radioactive and it emits gamma rays so its presence in the mud needs to be corrected in order to present the true measurements to the client. Potassium in the mud is normally in the form of KCL (Potassium Chloride). The inputs to apply this correction in IDEAL are Potassium percentage in the mud, mud density and borehole size. All of these inputs need to be there for the algorithm to function properly in order to correct for Potassium in the system.

•

Mud Density: Mud density correction is also related to the presence of potassium. The effect of potassium on the gamma ray measurements increases with increase in mud density and hole size. The inputs related to the density effect linked to the potassium presence in IDEAL are the mud density and borehole size.

Density Measurement: The density measurement is made by emitting gamma rays and if barite is present in the mud it captures gamma rays. Therefore the correction for the LWD density measurement is for the density of the drilling fluid. Generally in the LWD tools density has very few environmental corrections. Probably the only place where any mud-related property is used in this measurement algorithm is the leakage correction in ADN6 and Dead Time Correction in ADN4, which are associated with the calculation of RT & RM density values. Porosity Measurement: This measurement has the most of the environmental corrections applied. The corrections for porosity are discussed below. •

Chloride Content (Mud Salinity): Chlorine in the mud is mostly coming from KCL or NaCl (Sodium Chloride). This captures the thermal neurons being emitted from the logging tools taking formation porosity measurements. So in order to correct for the effect on porosity by the presence of chlorides, the amount of the chloride content needs to be know so that it can be placed as an input in the environmental correction panels for both real time and recorded mode data processing.

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For the case of WBM the mud resistivity and mud sample temperature are used to calculate or estimate the chloride content using specific charts and formulas. But for the OBM systems the Rm and temperature are invalid due to the fact that it is a non-conductive medium. Separate fields for the case of OBM are provided in IDEAL to input the correction for borehole salinity. This value is measured in parts per thousand, PPK of NaCl and it is provided in mud report. This may be given in the mud report in different forms as discussed in the following. 1.

Chlorides or NaCl in PPM (parts per million). This needs to be converted to PPK of NaCl by simply dividing by the number 1000. And the result is used as the input in IDEAL.

2.

NaCl in grams per liter. To convert from g/l to PPK of NaCl two things can be done. Firstly at lower salinity values 1 g/l equals 1 PPK of NaCl. Secondly the correct way of doing it, which is also better suited for higher salt content is first by converting grams per liter to grains per liter (1 g/l = 58.40273 x grain/l). After this conversion GEN9 chart on the ‘Log Interpretation Manual ‘ can be used to get the value in PPM and this can be converted to PPK of NaCl to input in IDEAL.

3.

Chlorides in gram/liter. Instead of providing NaCl in g/l if the mud report shows values of Chlorides in g/l then it can be converted by the use of the formula: 1 g/l of Cl = 1.65 g/l of NaCl. These can be converted to PPK of NaCl using the above method.

4.

Water Phase Salinity. In OBM normally percentages of oil and water phases are also provided if the values are given in WPS in PPK. Then for this case the WPS value in PPK is multiplied by the fraction of water phase (30 % would be 30/100 = 0.3) and this would give the PPK of NaCl. Similarly further conversions may be needed if the numbers in the mud report are given in g/l or grains/l in the similar fashion as discussed above.

A spreadsheet is also available to automatically perform the conversions and obtain the value in PPK of NaCl to be put in the environmental corrections in IDEAL. •

Mud Hydrogen Index: Presence of hydrogen in the mud effects the porosity measurement due to the fact that hydrogen captures neutrons, which are emitted from the tool in order to make the porosity measurement. This effect of hydrogen needs to be nullified to get the corrected measurement. Mud weight is used along with TVD of the hole to calculate the downhole pressure (Psi = 0.052 x TVD x Density (ppg)). The inputs of surface temperature and bottom hole temperature in IDEAL are used together with the TVD of the hole to calculate the temperature gradient. This temperature gradient and the bottom hole pressure value are then used to calculate the mud hydrogen index.

These corrections related to mud properties take care of environmental corrections applied to the Logging While Drilling measurements. It is important that the concepts of these and other corrections are known and the correct practices are followed to ensure that the data quality delivered to the client is excellent and as accurate as possible. Since a lot of calculations related to the reservoir volume and production are performed by the perto-physicists based on information about environmental parameters provided by the LWD engineers, this makes it absolutely necessary to ensure that correct information is provided and no parameters are missing in the data & logs given to the Clients. Questions Part 7: Q27.

Which LWD measurements are effected by Potassium in the mud?

Q28.

Which of the measurements are affected by chloride-content or salinity of the mud?

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HSE Drilling fluids have Health, Safety and Environmental concerns associated with them due to the presence of various chemical compounds in them. Presence of diesel oil in OBM is a risk to health of the personnel handling it, their safety due to potential slip hazards and environmental concerns related to spills and contamination of the ecosystem. Special equipment and safety precautions are used for the use and handling of each type of drilling fluids and chemicals. API has certain standards and standard practices for handling of these materials. Most hazardous materials require that a Material Safety Data Sheet (MSDS) be provided with the material containers at all times. And prior to their use the MSDS needs to be referred to. In addition, training courses are conducted on hazard awareness and chemical compound handling procedures. As part of the safety standards, safety equipment like spill kits, eye wash stations, hand wash stations and Emergency Response Plans are required to be in place at all times. As a general rule it is not recommended to handle any chemical compounds associated with the mud or just the mud itself without checking with the concerned authority such as the mud engineer on site or without referring to the MSDS for safe handling and other information on the health & environmental risks associated with these substances. That also includes handling of the mud and its filtrate by the LWD engineers for taking resistivity measurements, the mud engineer should be asked to identify any HSE concerns prior to the physical handling of the fluid or any other substances.

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End of Drilling Fluids Self-Learning Package Congratulations you have successfully completed the Drilling Fluids self-learning package. Take the time now to go back to the beginning of the document and review the objectives.

Answers: A1.

The plastic velocity is a resistance to flow due to mechanical friction caused by solid size, density and shape.

A2.

Yield Point is a measurement of the attractive forces (Electro-chemical) in the fluid; these forces are due to oppositely charged particles that attract each other.

A3.

No, Bentonite is used to increase viscosity.

A4.

Hematite is heavier then barite.

A5.

WBM is more common because of cost, abundance of water especially for offshore drilling and its environmentally friendly nature.

A6.

Un-weighted Clay-Water System, Weighted Clay-Water System, Calcium Treated Systems & Salt Saturated Mud Systems.

A7.

The application of this system is in areas having large quantity of formation containing salts. If regular water systems are used the well bore would dissolve into the system not only effecting the properties of the drilling fluid itself but causing hole stability problems, which may result in stuck pipe situations.

A8.

Base Oil.

A9.

Solids present in the system, which have a film of water surrounding them referred to as the waterwet solids.

A10.

Yes, it will increase.

A11.

Environmentally safe, Special applications like HTHP, less toxic etc.

A12.

Functions of the mud are: 1. Remove cuttings from the borehole by bringing them to surface. 2. Maintain hydrostatic pressure to control the formation pressure down-hole. 3. Suspend and release the cuttings. 4. Transmit hydraulic energy or hydraulic horsepower to the bit. 5. Provide hydraulic energy to the down-hole tools such as Mud Motors, Turbines and MWD tools 6. Seal permeable formations by forming wall cake. 7. Minimize damage to the reservoir by isolating it. 8. Maintain well bore stability. 9. Cool and lubricate the bit and down hole tools. 10. Control corrosion of the BHA and the drill string. 11. Supporting part of the weight of the BHA with buoyancy. 12. Facilitate cementing and completion process. 13. Provide formation evaluation with mud logging and MWD/LWD tools.

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A13.

Hole cleaning is a function of the ROP, flow rate, string rotation, mud viscosity & density, cutting Size & shape, hole deviation, flow profile and annular velocity of the mud.

A14.

Annular velocity is the relative velocity of solid particles in the annulus when the fluid is being circulated. The net velocity with which the cuttings move up to the surface is called the Cuttings Transport Velocity (Transport Velocity = annular velocity – slip velocity)

A15.

Formation Pore Pressure & Tectonic Stress in the formations.

A16.

Mud loggers detect any signs of external fluids like oil, gas or formation water. They also examine the cuttings coming out of the hole for mineral composition, paleontology and visual signs of hydrocarbon. All this information is presented on a mug log such as, lithology, ROP, surface gas and presence of oil reflected in the cuttings.

A17.

The presence of Sulfides mostly from Hydrogen Sulfide gas, Presence of Carbon Dioxide or Oxygen gases. Usually acidic environment causes corrosion.

A18.

Bingham Plastic Law, Power Law & Modified Power Law or Hercheal-Bulkly Law.

A19.

The layers within the fluid start to break and effectively the size of the plug is reduced and the fluid flow rate and velocity increase. The fluid flow is such that the fluid velocity is highest in the middle and zero at the boundaries resembling the shape of a parabola.

A20.

Free Settling, Hindered Settling & Boycott Settling.

A21.

Shale Shakers.

A22.

No, it is used to measure density.

A23.

Using a rotating V-G viscometer. The plastic viscosity is the value of 600-rpm minus the 300-rpm readings.

A24.

This ring is circular in shape and is designed to fit in tool joint. The material of the ring should be the same as the pipe, commonly steel.

A25.

Mud density will increase the signal strength. Increase in viscosity and hole depth will decrease signal strength.

A26.

This noise can come form downhole tools like motors when they stall or some times some noise problems arise for newly rubber relined motor stators. Pressure changes at the bit while drilling certain formations can cause noise as well. Excessive vibrations or even pipe rotation can produce signals that affect the MWD signal. Excessive ROP and torque problems can do the same to the signal. The inside diameter of the BHA also effects the signal, every change in the inside diameter effects the signal. Any big change from one diameter to the other reduces the signal slightly.

A27.

Potassium is radioactive and emits gamma rays so natural Gamma Ray measurements are effected by its presence in the mud.

A28.

For the purpose of environmental corrections, porosity measurements are effected. Also resistivity is affected by the conductive nature of the mud with certain LWD tools.

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