Petroleum Engineering 405 Drilling Engineering

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1.10-1
Abnormal Pressure
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Abnormal Pressure - Definition, Causes -

• Normal Pore Pressures
• Abnormal Pore Pressure Gradients
• Fracture Gradients
• Mud Weights
• Casing Seat Depths
• What Causes Abnormal Pressure?

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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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Pore Pressure vs. Depth
0 5,000 10,000 15,000 20,000
0.433 psi/ft 8.33 lb/gal 0.465
psi/ft 9.0 lb/gal
Depth, ft
Normal
Abormal

• 9 10 11 12 13 14
  15 16
• Pore Pressure Equivalent, lb/gal

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 are
  still helping to 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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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

• d may be Corrected for mud density as follows

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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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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
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Prediction of Abnormal Pore Pressure

• Resistivity of Shale
• Temperature in the Return Mud
• Drilling Rate Increase
• dc - Exponent
• Sonic Travel Time
• Conductivity of Shale

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EXAMPLE Shale Resistivity vs. Depth 1.
Establish normal trend line 2. Look for
deviations (semi-log)
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Pore Pressure (lb/gal equivalent) 16
14 12 10
Shale Resistivity vs. Depth 1. Establish
normal trend line 2. Look for deviations 3.
Use OVERLAY to quantify pore pressure (use
with caution)
9 ppg (normal)
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Example
8.2 X
Why?
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Determination of Abnormal Pore Pressure Using the
dc - exponent

• From Ben Eaton

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• Where

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Example

• Calculate the pore pressure at depth X using the
  data in this graph.
• Assume
• West Texas location with normal overburden of
• 1.0 psi/ft.
• X 12,000 ft.

X
1.2 1.5 dc
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Example

• From Ben Eaton

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Example
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E.S. Pennebaker

• Used seismic field data for the detection of
  abnormal pressures.
• Under normally pressured conditions the sonic
  velocity increases with depth. (i.e. Travel
  time decreases with depth)
• (why?)

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E.S. Pennebaker

• Any departure from this trend is an
  indication of possible abnormal pressures.
• Pennebaker used overlays to estimate abnormal
  pore pressures from the difference between
  normal and actual travel times.

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Depth, ft
Interval Travel Time, msec per ft
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Ben Eaton

• also found a way to determine pore pressure from
  interval travel times.
• Example
• In a Gulf Coast well, the speed of sound is
  10,000 ft/sec at a depth of 13,500 ft. The
  normal speed of sound at this depth, based on
  extrapolated trends, would be 12,000 ft/sec.
  What is the pore pressure at this depth?
• Assume S/D 1.0 psi/ft

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Ben Eaton

• From Ben Eaton,

( Dt a 1/v )
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Ben Eaton

• From Ben Eaton
• Note Exponent is 3.0 this time,
• NOT 1.2!

r (0.6904 / 0.052) 13.28 lb/gal p 0.6904
13,500 9,320 psig
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Equations for Pore Pressure Determination
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Pore Pressure Determination