# Lesson 20 Abnormal Pressure

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

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

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

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

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

* Fracture Gradients Casing Setting Depths

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

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...

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

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

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

Well “A” found only Normal Pressure ...

sob p sz sOB = p + sZ

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

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

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

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

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)

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

. .

What is d-exponent? Decreasing ROP

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

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

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

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

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

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

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

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

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

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

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.

In normally pressured shales, shale compaction increases with depth

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.

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

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

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