• Author / Uploaded
• Usman Hasan

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Exercise: ‘d’ and ‘dc’ Exponent a. Whilst drilling the 12 1/4" hole section of a well the mud loggers were recording the data as shown in the table below. Plot the d and dc exponent and determine whether there are any indications of an overpressured zone. b. If an overpressured zone exists, what is the depth of the top of the transition zone? c. Use the Eaton equation to estimate the formation pressure at 8600 ft. Assume a normal formation pressure of 0.465 psi/ft. an overburden gradient of 1.0 psi/ft and a normal mud weight for this area of 9.5 ppg. DEPTH (ft.)

ROP (ft/hr)

RPM

WOB (,000 lbs)

7500 7600 7700 7800 7900 8000 8100 8200 8300 8400 8500 8600 8700

125 103 77 66 45 37 40 42 41 44 34 33 32

120 120 110 110 110 110 110 110 100 100 100 100 110

38 38 38 38 35 37 35 33 33 38 38 40 42

Solution:

MUD WEIGHT PPG 9.5 9.5 9.5 9.6 9.6 9.8 9.8 9.9 10.0 10.25 10.25 11 11

Whilst drilling this section of 12 1/4” hole the mudloggers were also recording data which would allow them to plot the d and dc exponents for this shale section. This data is compiled and the d and dc exponents calculated as shown in Table 2.1. A plot of the d and dc exponents in Figure 2.1 and 2.2 confirms that the top of the overpressured zone is at 8000 ft.

Example The following sonic log data were taken from a well in PED, NED. Plot the data on 3- cycle semi log paper. Calculate the formation pressure at 11,900 ft. Overburden stress is 1.12 psi/ft and normal pore pressure gradient is 0.465 psi/ft. (Ans. 17.5 ppg) Travel Time μ Sec/ft 190 160 140 120 122 105 110 99 99 98 100 100 110 100 110 101 101 105 100 110 100

Depth, ft 3,400 5,000 6,600 7,300 7,900 8,200 8,600 9,000 9,200 9,400 9,600 9,800 10,000 10,200 10,400 10,600 10,800 11,100 11,400 11,600 11,900