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1 PETE 411 Well Drilling Lesson 20 Abnormal Pressure

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2 Abnormal Pressure u Normal Pore Pressures u Abnormal Pore Pressure Gradients u Fracture Gradients u Mud Weights u Casing Seat Depths u What Causes Abnormal Pressure? u Detection of Abnormal Pressure u Quantification of Abnormal Pressure

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3 HW #11 Slip Velocity Due 10-28-02 Read: Applied Drilling Engineering, Ch. 6

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4 Abnormal Pressure Gradients Normal Pressure Gradients West Texas: 0.433 psi/ft Gulf Coast: 0.465 psi/ft Normal and Abnormal Pore Pressure Pore Pressure, psig Depth, ft 10,000’ ? ?? ?

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5 NormalAbormal Density of mud required to control this pore pressure 0.433 psi/ft 8.33 lb/gal 0.465 psi/ft 9.00 lb/gal

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6 Pore Pressure Gradient Fracture Gradient

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7 * Pore Pressure Gradients * Fracture Gradients Casing Setting Depths

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8 Some Causes of Abnormal Pressure 1. Incomplete compaction of sediments Fluids in sediments have not escaped and help support the overburden. 2. Tectonic movements Uplift Faulting

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9 Some Causes of Abnormal Pressure 3. Aquifers in Mountainous Regions Aquifer recharge is at higher elevation than drilling rig location. 4. Charged shallow reservoirs due to nearby underground blowout. 5. Large structures...

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10 HIGH PRESSURE NORMAL PRESSURE Thick, impermeable layers of shale (or salt) restrict the movement of water. Below such layers abnormal pressure may be found.

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11 HIGH PRESSURE NORMAL PRESSURE Hydrostatic pressure gradient is lower in gas or oil than in water.

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12 When crossing faults it is possible to go from normal pressure to abnormally high pressure in a short interval.

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13 Well “A” found only Normal Pressure...

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14 OB = p + Z ob p z

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15 ?

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16 Abnormal Pressure cont’d u Detection of Abnormal Pore Pressures u Prediction of Abnormal Pore Pressures u D-Exponent u D C -Exponent u Example u Importance of Shale Density

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17 Indications of Abnormal Pore Pressures Methods: 1. Seismic data 2. Drilling rate 3. Sloughing shale 4. Gas units in mud 5. Shale density 6. Chloride content

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18 Indications of Abnormal Pore Pressures Methods, cont’d: 7. Change in Mud properties 8. Temperature of Mud Returns 9. Bentonite content in shale 10. Paleo information 11. Wire-line logs 12. MWD-LWD

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19 Prediction and Detection of Abnormal Pressure Zones 1. Before drilling Shallow seismic surveys Deep seismic surveys Comparison with nearby wells

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20 Prediction and Detection of Abnormal Pressure Zones 2. While drilling Drilling rate, gas in mud, etc. etc. D - Exponent D C - Exponent MWD - LWD Density of shale (cuttings)

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21 Prediction and Detection of Abnormal Pressure Zones 3. After drilling Resistivity log Conductivity log Sonic log Density log

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22. –.

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24 Decreasing ROP What is d- exponent?

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25 D - Exponent The drilling rate equation: Where R = drilling rate, ft/hr K = drillability constant N = rotary speed, RPM E = rotary speed expon. W = bit weight, lbs D B = bit diameter, in D = bit wt. Exponent or D - exponent

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26 D - Exponent If we assume that K = 1 and E = 1 Then

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27 D - Exponent A modified version of this equation follows:

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28 Example Calculate the value of the d - exponent if the drilling rate is 35 ft/hr, the rotary RPM is 100, and the weight on the 12 1/4” bit is 60,000 lbs. d = 1.82

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29 Example What happens to d if R doubles to 70 ft/hr? Note that an increase in R resulted in a decrease in d. Doubling R decreased d from 1.82 to 1.57

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30 Example d may be Corrected for density as follows

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31 Example 2 Calculate “d” if: R = 20 ft/hr N = 100 RPM W = 25,000 lbf D B = 9 7/8 in d = 1.63

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32 Example 2 If the normal pore pressure gradient in the area is 0.433 psi/ft, and the actual mud weight is 11.2 #/gal, what is “d c ”? d c = 1.21

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34 Procedure for Determining Pore Pressure From d c - Exponent u Calculate d c over 10-30 ft intervals u Plot d c vs depth (use only date from Clean shale sections) u Determine the normal line for the d c vs. depth plot. u Establish where d c deviates from the normal line to determine abnormal pressure zone

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35 Procedure for Determining Pore Pressure From d c - Exponent d c - Exponent Depth Normal Abnormal Normal Trend

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36 Procedure for Determining Pore Pressure From d c - Exponent u If possible, quantify the magnitude of the abnormal pore pressure using overlays, or Ben Eaton’s Method Pore Pressure Grad. Overburden Stress Grad. Normal Pore Pressure Grad.

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37 In normally pressured shales, shale compaction increases with depth

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38 Shale Density - Mud Cup Method 1. Fill mud cup with shale until the weight is 8.33. 2. Fill to top with water, and record the reading W tot. Note: Dry sample carefully with towel. Do not apply heat.

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39 Alternate Method: Use variable density column. See p. 270 in text

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40 Pore Pressure from Resistivity Shale resistivity plots may be developed from (i) logs or (ii) cuttings What is the pore pressure at the point indicated on the plot? [Assume Gulf Coast]. Depth=10,000 ft 0.2 0.5 1 2 3 10,000’ Depth

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41 From plot, R n = 1.55 ohms R obs = 0.80 ohms From Eaton: EATON = 0.7307 psi/ft = 14.05 lb/gal P = 0.7307 * 10,000 = 7,307 psi

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