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Quartz School for Well Site Supervisors

Schlumberger Private

Module – 11 Drilling Problems & Solutions Section – 2 Stuck Pipe: Free Point & Back Off

Quartz School. Module 11: Drilling Problems & Solutions / Section 2: Stuck Pipe: Free Point & Back Off

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Materials Strength Concepts • Intensity of Stress Is the stress per unit area due to a force F (pounds), producing tension, compression or shear over an area of A, square inches. Schlumberger Private

S= F/A (lbs/in2)

Quartz School. Module 11: Drilling Problems & Solutions / Section 2: Stuck Pipe: Free Point & Back Off

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Materials Strength Concepts • Elastic Limit Below the elastic limit, deformations are directly proportional to the forces producing them (Hooke’s law applies). Schlumberger Private

• Yield Point Is the point above which permanent plastic deformation occurs with little or no increase in stress.

• Ultimate Stress Is the maximum stress which can be produced before rupture occurs.

Quartz School. Module 11: Drilling Problems & Solutions / Section 2: Stuck Pipe: Free Point & Back Off

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Materials Strength Concepts • Strain, ε Is the elongation e, per unit length of a bar or pipe. Strain may be tensile or compressive. L

• Modulus of Elasticity, Ε

=

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(e). (E). (A) (ΔP).(12)

Is the factor of proportionality between stress (σ) and strain (ε) within the elastic limits of the material:

σ ~ ε Æ σ = (Ε) . (ε) E =

Stress Strain

=

P/A e/L

(lbs/in2)

Æ (E) x [e/L] = P/A Æ (E) x (e) x (A) = L x PÆ L = (E) (e) (A) / (P)

Quartz School. Module 11: Drilling Problems & Solutions / Section 2: Stuck Pipe: Free Point & Back Off

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Stretch in Drill String • Hanging vertically from the block a pipe is in tensile stress due to it’s own weight. Schlumberger Private

Quartz School. Module 11: Drilling Problems & Solutions / Section 2: Stuck Pipe: Free Point & Back Off

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Stretch in Drill String • When a pull is applied to a stuck pipe, above the weight of the pipe down to the stuck point, the pipe stretches an amount ΔL at surface proportionally to the additional pull OP (overpull) Schlumberger Private

Quartz School. Module 11: Drilling Problems & Solutions / Section 2: Stuck Pipe: Free Point & Back Off

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Stretch in Drill String • For estimating the length of free pipe, the stretch formula can be re-arranged as follows: Where:

(ΔL).(E).(A)

ΔL = Stretch ( inches )

=

A = Cross sectional Area (inch 2 )

(ΔP).(12)

E = Modulus of elasticity ( lb / inch 2 ) = 30 x 10 6 psi…(for steel) ΔP = •

Drill string overpull ( lbs)

Introducing:

A=

∏ x (OD2 – ID2)

and E = 30 x10 6 psi

4

Where: OD = Outside diameter of tubular to be stretched ( inches) ID = Inside diameter of tubular to be stretched (inches)

(ΔL).(30 X 10 6). Π .[(OD)2 – (ID)2] Lfree

=

ΔP.(12)

Quartz School. Module 11: Drilling Problems & Solutions / Section 2: Stuck Pipe: Free Point & Back Off

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Lfree

Lfree = length of free pipe ( feet )

Stretch in Drill String • Then:

=

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Lfree(ft)

ΔL (in) x [OD (in)2 – ID (in)2] x 1,963.5 ΔP (1,000 lbs)

By using the nominal weight of the pipe body instead of its diameters, the formula can be expressed as follows: Lfree

=

735294 * W (lb/ft)

* ΔL (in)

( in Feet)

ΔP

Quartz School. Module 11: Drilling Problems & Solutions / Section 2: Stuck Pipe: Free Point & Back Off

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Free Point Estimation from Pipe Stretch 1.

4. 5. 6. 7.

Quartz School. Module 11: Drilling Problems & Solutions / Section 2: Stuck Pipe: Free Point & Back Off

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2. 3.

Note the following information: a) Drill string data (diameters and weights) b) Buoyancy Factor c) Traveling Block weight (includes T. Block, Bails, Kelly or Top Drive) d) Drill string weight before stuck Calculate the weight of the string (drill string, tubing or casing) in mud. Calculate the maximum overpull margin on the string (maximum allowable pull – weight of string in mud) Pull on the string until the weight indicator shows a pull of T1=string weight + 10% of the maximum safe overpull Draw a mark at the rotary table / kelly bushing level Increase the pull to string weight + 20% of maximum safe overpull. Return to pulling string weight + 10% of the maximum overpull.

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Free Point Estimation from Pipe Stretch •

1st OPERATION 1st Mark String Weight

Rotary Table

T1 = SW + 10% MOP Schlumberger Private

Fig 1

Fig 2 1st Mark Datum line A

Increase SW+20% MOP

T1 = SW+ 10% MOP

Fig 3

2nd Mark

Fig 4

Quartz School. Module 11: Drilling Problems & Solutions / Section 2: Stuck Pipe: Free Point & Back Off

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Free Point Estimation from Pipe Stretch 8. 9. 10.

ΔL (in) x [OD (in)2 – ID (in)2] x 1,963.5 Lfree(ft)

=

Lfree

=

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11. 12. 13. 14. 15. 16.

Draw a second mark at kelly bushing. This second mark should be different from the first due to the stretch of the string in the hole. Draw a datum line A, midway between the two marks. Proceed as before applying a pull T2=string weight+30% of the allowable pull margin. Draw a mark at kelly bushing level Increase the pull to string weight + 40% of maximum overpull. Return to pulling string weight + 30% of allowable pull margin Draw a datum line B, midway between the two marks Measure the distance between the two datum lines Apply the any of the free point estimation formulas discussed previously : 735294 * W (lb/ft) * ΔL (in)

F (1,000 lbs) Quartz School. Module 11: Drilling Problems & Solutions / Section 2: Stuck Pipe: Free Point & Back Off

ΔP 12/33

Tubular Free Point Calculation •

2nd OPERATION Datum line A T1=SW+ 30% MOP

3rd Mark

Datum line A T1=SW+ 40% MOP

T1=SW+ 30% MOP

Fig 6 Datum line A

Datum line A

3rd Mark

3rd Mark

DL

Datum line B

4rd Mark

Fig 7

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Fig 5

3rd Mark

4rd Mark

Fig 8

Quartz School. Module 11: Drilling Problems & Solutions / Section 2: Stuck Pipe: Free Point & Back Off

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Torsion in Pipe Strings Pipe in Torsion If torque is applied at surface to a stuck pipe string with a constant cross section, the angular displacement or twist will vary linearly from a maximum at surface to zero at the stuck point or above it Schlumberger Private

Quartz School. Module 11: Drilling Problems & Solutions / Section 2: Stuck Pipe: Free Point & Back Off

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Free Point Indicator Tool

Quartz School. Module 11: Drilling Problems & Solutions / Section 2: Stuck Pipe: Free Point & Back Off

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Reading Stretch • The FPIT measures pipe elongation over the distance between two anchoring points. • To determine if the pipe is stuck or free at a particular depth, the string is pull to its “normal weight” (buoyed weight of the entire string in the hole) • The FPIT is then positioned at a given depth (anchored) inside the string and an additional pull at surface is applied to the pipe. • The tool measures the strain between its anchoring points which gives and indication of how “free” is the pipe at that particular depth Reading Torque • A right hand torque is applied to the string at surface and worked down • The FPIT measures the twist strain between the two anchoring points, proportional to the applied torque that also indicates the free or stuck condition of the string at that point • Stretch and Torque readings are both considered to determine the free point of the string, below which the string is considered stuck

Free Point Indicator Tool Principle of Measurement •

Stretch Sensor •

Quartz School. Module 11: Drilling Problems & Solutions / Section 2: Stuck Pipe: Free Point & Back Off

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The stretch sensor is a transformer that consists of axial coils. One primary coil as a transmitter exists in the lower part of a movable assembly while the upper part carries two coils forming a receiver

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Free Point Indicator Tool Principle of Measurement • Torque Sensor •

The torque sensor is also a transformer that consists of coils, but in this case, of radial coils Schlumberger Private

Quartz School. Module 11: Drilling Problems & Solutions / Section 2: Stuck Pipe: Free Point & Back Off

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Free Point Indicator Tool

Quartz School. Module 11: Drilling Problems & Solutions / Section 2: Stuck Pipe: Free Point & Back Off

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Taking Data • Tie in CCL depths with the driller first by checking a prominent feature such as the jars, HWDP or top of the collars. Then record CCL log up to 1000’ above estimated stuck point. This log will be valuable later in selecting settings and the backoff point. • Take readings of both stretch and torque every 90 ’ (every 3 connections) starting about 500’ above the suspected stuck point (use the free point estimated with the stretch test). This will establish a free pipe meter reading. • Once the stuck point has been established, reduce the measurement intervals to 30 or every connection, and take a few readings on either side of stuck point. Interpretation • The free point selected should have torque reading above 85% of free pipe values The free point is the deepest point at which the pipe can be backed-off and • recovered. • To cover all possible manners in which the pipe can be stuck requires both torque and stretch readings. 19/33

Free Point Indicator Tool Interpretation Four “typical cases” of stuck pipe are given on the following slides. Schlumberger Private

One may not be able to determine which cases exactly fit, however, if the torque and stretch reading are > 80% of free pipe, the pipe will most likely backoff.

Quartz School. Module 11: Drilling Problems & Solutions / Section 2: Stuck Pipe: Free Point & Back Off

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Free Point Indicator Tool Example of FP Interpretation Straight hole, straight stuck in drill collars

pipe,

•

Both stretch and torque readings show the stuck point in the same DC

•

Where there is a little wall friction, a sharp drop off in both readings observed below the stuck point

•

The pipe is considered recoverable when torque and stretch indicate 80-85% of free readings.

Quartz School. Module 11: Drilling Problems & Solutions / Section 2: Stuck Pipe: Free Point & Back Off

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1.

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Free Point Indicator Tool Example of FP Interpretation Differential or packed stuck pipe

•

In this case, transmission of pull and torque becomes more difficult

•

The readings decreases below collar B, pipe should be backed-off either at collar A or collar B

•

Pipe should be backed-off at the collar nearest 80 to 85% of the reading in free pipe

Quartz School. Module 11: Drilling Problems & Solutions / Section 2: Stuck Pipe: Free Point & Back Off

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2.

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Free Point Indicator Tool Example of FP Interpretation Straight hole, extremely crooked pipe

•

This case illustrates the inability to obtain normal torque in a badly bent string Normal stretch and torque for this string are read above collar B Below collar B, stretch reading is normal but torque reading is decreasing with increasing depth Normal torque reading can often be obtained below collar B by applying an overpull on the pipe The Backoff should be performed where the torque readings is still high

• • • •

Quartz School. Module 11: Drilling Problems & Solutions / Section 2: Stuck Pipe: Free Point & Back Off

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3.

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Free Point Indicator Tool Example of FP Interpretation Crooked Deviated Wells

•

In deviated wells it is normally possible to transmit torque deeper than stretch The torque reading is often a function of the pull on pipe. Generally, best torque transmission is obtained at relatively low values of pull Some judgment must be exercised in what can be backed-off. While a Backoff may at times be made with less than 25 % stretch reading, it is not recommended to attempt a backoff without a torque reading of 50 % free pipe

•

•

Quartz School. Module 11: Drilling Problems & Solutions / Section 2: Stuck Pipe: Free Point & Back Off

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4.

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FPIT Example

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Quartz School. Module 11: Drilling Problems & Solutions / Section 2: Stuck Pipe: Free Point & Back Off

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Free Point Indicator Tool Operational Hints As a starting point, the weight of the pipe down to the suspected stuck point should be suspended by the blocks. It is recommended to have slightly higher tension such as the buoyed weight of the entire string + 10 %. ( depends upon depth and overall string weight)

•

In certain cases, due to age or condition of the pipe or pulling equipment, the desired weight cannot be picked up. In these cases, pick up on the pipe an amount equal to the maximum allowable pull less the additional pull.

•

In holes with more than 15o deviation, the base weight is usually less than the weight of the pipe. Some experimentation may be necessary to determine a base weight such that the pipe will have uniform stretch when subsequent stretch readings are taken.

•

Back off should be performed immediately after FPIT measurements. Quartz School. Module 11: Drilling Problems & Solutions / Section 2: Stuck Pipe: Free Point & Back Off

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Schlumberger Private

•

Backoff Procedure Recommendations to assure successful Backoff 1. 2.

• 4.

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3.

Tighten up the entire pipe string with torque 30% higher than the value that will be used for backing off, and “work this torque down”. As pipe is tightened to the right count how many turns come back. If less turns are coming back the pipe is getting tightened. When the same amount of right hand turns come back after pipe is worked, pipe is ready to back off. Apply proper pull on the pipe to support buoyed weight of the free string so the connection to unscrew is slightly in tension The FPIT tool can be an excellent indicator of torque / tension being transmitted to the connection to be backed off – Use It !

Apply sufficient left hand torque to the connection to be unscrewed •

First, apply LH torque and gradually work down hole to the connection that has been chosen to back-off. Apply ½ to 1 LH turns per 1000’of drill pipe.

•

Apply as much left-hand torque as can be safely transmitted to the joint without risking a mechanical backoff somewhere else in the drill string (“blind” back-off)

•

Place the buoyed string weight at the neutral point plus a slight overpull ( 2,000 to 10,000 lb ). Quartz School. Module 11: Drilling Problems & Solutions / Section 2: Stuck Pipe: Free Point & Back Off

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Backoff Procedure Backoff Torque recommendations •

Backoff depth (ft)

Rounds per 1,000’

0 – 4,000

½-¾

4,000 – 9,000

½ -1

Over 9,000

¾-1

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•

Turns to the left put in the string to perform the back-off:

Pipe in unknown conditions requires discretion as low grade, badly worn, or fatigued pipe may not accept the average values given in the table above

Quartz School. Module 11: Drilling Problems & Solutions / Section 2: Stuck Pipe: Free Point & Back Off

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Backoff Procedure 5.

Accurately position and fire a “string shot” of adequate strength at the Backoff point • The size of the shot must be strong enough to Backoff tool joint without splitting the pipe body. This can be calculated by using WL tables for the diameter of the pipe.

Backoff point prior to firing the explosive charge of the string shot is often the most critical and difficult part of the operation.

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Applying sufficient left torque to the string and properly working it down to the

Backoff Procedure Guides to determine the proper string weight for Back-Off Theoretically the connection of Back-off should be in neutral condition

2.

However, experience shows that is better to have the joint slightly in tension rather than in compression.

3.

Three different approaches may be used to determine the exact pull at surface: 1.

Calculate buoyed weight of pipe above the back off point. Pull this amount plus 10 %.

1.

Obtain tension, when moving up before sticking from the drillers log. Subtract the weight of the fish to be left in the hole and add 10% of buoyant weight above stuck point

2.

Use FPIT measurements to determine the over pull that puts the string in tension at the backed off point. Use this value in the weight indicator, plus an over pull of 5 Klbs.

Quartz School. Module 11: Drilling Problems & Solutions / Section 2: Stuck Pipe: Free Point & Back Off

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1.

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Backoff Procedure Determination of the proper Backoff weight – Corrections Note whether or not the weight of the block was included when the weight indicator was zeroed

•

Check if the weight of kelly in the string is included in the readings

•

The mud pumps should be turned off while taking the load readings

Quartz School. Module 11: Drilling Problems & Solutions / Section 2: Stuck Pipe: Free Point & Back Off

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•

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Backoff Preparation

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Backoff Execution Excessive primacord used

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