PETE 411 Well Drilling

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PETE 411Well Drilling
Lesson 20Abnormal Pressure
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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

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HW 11 Slip VelocityDue 10-28-02
ReadApplied Drilling Engineering, Ch. 6
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Normal and Abnormal Pore Pressure
Normal Pressure Gradients West Texas 0.433
psi/ft Gulf Coast 0.465 psi/ft
Abnormal Pressure Gradients
Depth, ft
10,000
? ?
Pore Pressure, psig
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0.433 psi/ft 8.33 lb/gal
0.465 psi/ft 9.00 lb/gal
Normal
Abormal
Density of mud required to control this pore
pressure
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Fracture Gradient
Pore Pressure Gradient
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• Pore Pressure Gradients
• Fracture Gradients
• Casing Setting Depths

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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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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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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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HIGH PRESSURE
NORMAL PRESSURE
Hydrostatic pressure gradient is lower in gas or
oil than in water.
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When crossing faults it is possible to go from
normal pressure to abnormally high pressure in a
short interval.
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Well A found only Normal Pressure ...
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sob
p sz
sOB p sZ
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?
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Abnormal Pressure contd

• Detection of Abnormal Pore Pressures
• Prediction of Abnormal Pore Pressures
• D-Exponent
• DC-Exponent
• Example
• Importance of Shale Density

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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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Indications of Abnormal Pore Pressures
Methods, contd 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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Prediction and Detection of Abnormal Pressure
Zones

• 1. Before drilling
• Shallow seismic surveys
• Deep seismic surveys
• Comparison with nearby wells

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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)

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Prediction and Detection of Abnormal Pressure
Zones

• 3. After drilling
• Resistivity log
• Conductivity log
• Sonic log
• Density log

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.

.
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What is d-exponent?
Decreasing ROP
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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
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D - Exponent

• If we assume that K 1
• and E 1
• Then

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D - Exponent

• A modified version of this equation follows

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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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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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Example

• d may be Corrected for density as follows

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Example 2

• Calculate d if R 20 ft/hr
• N 100 RPM
• W 25,000 lbf
• DB 9 7/8 in

d 1.63
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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 dc?

dc 1.21
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Procedure for Determining Pore Pressure From dc -
Exponent

• Calculate dc over 10-30 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

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

• If possible, quantify the magnitude of the
  abnormal pore pressure using overlays, or Ben
  Eatons Method

Pore Pressure Grad.
Normal Pore Pressure Grad.
Overburden Stress Grad.
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In normally pressured shales, shale compaction
increases with depth
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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.
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Alternate Method Use variable density column.
See p. 270 in text
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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. Depth10,000 ft
0.2 0.5 1 2 3
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EATON

• From plot, Rn 1.55 ohms
• Robs 0.80 ohms
• From Eaton

0.7307 psi/ft 14.05 lb/gal
P 0.7307 10,000 7,307 psi