1. Lesson 20 Abnormal Pressure
PETE 411 Well Drilling
Lesson 20 Abnormal Pressure

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

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

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

5. Density of mud required to control this pore pressure
0.433 psi/ft lb/gal
0.465 psi/ft lb/gal
Normal
Abormal
Density of mud required to control this pore pressure

6. Fracture Gradient
Pore Pressure
Gradient

7. * Pore Pressure Gradients
* Fracture Gradients
Casing Setting Depths

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

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...

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.

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

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

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

14. sob
p sz
sOB = p + sZ

15. ?

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

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

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

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

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

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

22. . – .

24. What is d-exponent?
Decreasing ROP

25. The drilling rate equation:
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
DB = bit diameter, in
D = bit wt. Exponent
or D - exponent

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

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

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

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

30. d may be Corrected for density as follows
Example
d may be Corrected for density as follows

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

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

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

35. Procedure for Determining Pore Pressure From dc - Exponent
Normal Trend
Normal
Depth
Abnormal
dc - Exponent

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

37. In normally pressured shales, shale compaction increases with depth

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 Wtot.
Note: Dry sample carefully with towel. Do not apply heat.

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

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

41. From plot, Rn = 1.55 ohms Robs = 0.80 ohms From Eaton:
= psi/ft = lb/gal
P = * 10,000 = 7,307 psi