Detection and Prediction of the
Abnormal Pressure value Before,
While, During Drilling
By
Dr. Eissa Mohamed Shokir
Abnormal Pressure
- Definition, Causes • Normal Pore Pressures
• Abnormal Pore Pressure Gradients
• Fracture Gradients
• Mud Weights
• Casing Seat Depths
• What Causes Abnormal Pressure?
Depth, ft
Normal and Abnormal Pore Pressure
10,000’
Normal Pressure Gradients
West Texas: 0.433 psi/ft
Gulf Coast: 0.465 psi/ft
Abnormal
Pressure
Gradients
Pore Pressure, psig
??
Pore Pressure vs. Depth
Depth, ft
0
0.433 psi/ft
0.465 psi/ft
8.33 lb/gal
9.0 lb/gal
5,000
Normal
10,000
Abormal
15,000
20,000
8
9
10
11 12
13
14
15
Pore Pressure Equivalent, lb/gal
16
{ Density of mud required to control this pore pressure }
Fracture Gradient
Pore Pressure
Gradient
* Pore
Pressure
Gradients
* Fracture
Gradients
* Casing
Setting
Depths
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
4 Uplift
4 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...
• Abnormal Formations do not have the
freedom of pressure communications. If
they did, the high pressures would rapidly
dissipate revert to normal pressure.
• Assuming that a pressure seal is present, the
causes or origin of pressure depend on some
items as lithology, minerology, tectonic
action, and rate of sedimentation
1- Compaction of sediments.
Db
Normal Pressure
Abnormal Pressure
Ab normal pore pressure are generated under
compacted region because the shale matrix
can’t support the overburden stress
P = 0.465 psi/ft * Db + 1.0 psi/ft *(Dt-Db)
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 ...
σob
p
σz
σOB = p + σZ
?
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
4 Shallow seismic surveys
4 Deep seismic surveys
4 Comparison with nearby wells
Prediction and Detection of Abnormal
Pressure Zones
2. While drilling
4 Drilling rate, gas in mud, etc. etc.
4 D - Exponent
4 DC - Exponent
4 MWD - LWD
4 Density of shale (cuttings)
Prediction and Detection of Abnormal
Pressure Zones
3. After drilling
4 Resistivity log
4 Conductivity log
4 Sonic log
4 Density log
φ = 0..41 e
−0.000085 DS
– .
What is dexponent?
Decreasing ROP
D - Exponent
The
drilling rate
equation:
⎛W⎞
⎟⎟
R = K N ⎜⎜
⎝ DB ⎠
E
D
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
D - Exponent
⎛W⎞
⎟⎟
R = K N ⎜⎜
⎝ DB ⎠
E
If we assume that K = 1
and E = 1
Then
R ⎛W⎞
⎟⎟
= ⎜⎜
N ⎝ DB ⎠
D
⎛R⎞
log ⎜ ⎟
N⎠
⎝
D=
⎛W⎞
⎟⎟
log ⎜⎜
⎝ DB ⎠
D
D - Exponent
A modified version of this equation follows:
⎛ R ⎞
⎟⎟
log ⎜⎜
60 N ⎠
⎝
d=
⎛ 12 W ⎞
⎟⎟
log ⎜⎜ 6
⎝ 10 DB ⎠
Example
d may be Corrected for mud density as
follows:
⎛ mud weight for normal gradient (ppg) ⎞
⎟⎟
dc = d ⎜⎜
actual mud weight in use(ppg)
⎠
⎝
⎛ 9 ⎞
⎛ 9 ⎞
e.g., dc = d ⎜ ⎟ = 1.82 * ⎜ ⎟ = 1.37
⎝ 12 ⎠
⎝ 12 ⎠
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
vs. depth plot.
• Establish where dc deviates from the
normal line to determine abnormal
pressure zone
dc
Procedure for Determining Pore Pressure
From dc - Exponent
n
Tre
al
d
Depth
rm
No
Normal
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
P
S ⎛S ⎛P ⎞ ⎞
=
− ⎜⎜ − ⎜ ⎟ ⎟⎟
D D ⎝ D ⎝ D ⎠n ⎠
Pore
Pressure
Grad.
Overburden
Stress Grad.
⎛ d c calculated
⎜
⎜ d normal
c
⎝
⎞
⎟
⎟
⎠
1 .2
Normal Pore
Pressure Grad.
In normally pressured
shales, shale
compaction increases
with depth
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
Depth
Pore Pressure from
Resistivity
10,000’
0.2
0.5
1
2 3
EATON
From plot, Rn = 1.55 ohms
Robs = 0.80 ohms
P
S ⎛S ⎛P ⎞ ⎞
=
− ⎜⎜ − ⎜ ⎟ ⎟⎟
D D ⎝ D ⎝ D ⎠n ⎠
⎛ R obs
⎜⎜
⎝ Rn
⎞
⎟⎟
⎠
Depth
From Eaton:
1 .2
P
⎛ 0 . 80 ⎞
= 0 . 95 − (0 . 95 − 0 . 465 ) ⎜
⎟
D
⎝ 1 . 55 ⎠
1 .2
10,000’
= 0.7307 psi/ft = 14.05 lb/gal
P = 0.7307 * 10,000 = 7,307 psi
0.2
0.5
1
2 3
Prediction of
Abnormal Pore Pressure
•
•
•
•
•
•
Resistivity of Shale
Temperature in the Return Mud
Drilling Rate Increase
dc - Exponent
Sonic Travel Time
Conductivity of Shale
EXAMPLE
Shale Resistivity
vs. Depth
1. Establish normal
trend line
2. Look for
deviations
(semi-log)
Shale Resistivity
vs. Depth
Pore Pressure
(lb/gal equivalent)
16 14 12 10
1. Establish normal
trend line
2. Look for
deviations
3. Use OVERLAY
to quantify
pore pressure
(use with caution)
9 ppg
(normal)
Example
8.2 X
Why?
Determination of Abnormal Pore
Pressure Using the dc - exponent
From Ben Eaton:
P S ⎡ S ⎛ P ⎞ ⎤⎛ d c
= − ⎢ − ⎜ ⎟ ⎥ ⎜⎜
D D ⎣ D ⎝ D ⎠ n ⎦ ⎝ d cn
⎞
⎟⎟
⎠
1 .2
P S ⎡ S ⎛ P ⎞ ⎤⎛ d c
= − ⎢ − ⎜ ⎟ ⎥ ⎜⎜
D D ⎣ D ⎝ D ⎠ n ⎦ ⎝ d cn
Where
P
D
⎞
⎟⎟
⎠
1 .2
= formation pressure gradient, psi/ft
⎛P⎞
⎜ ⎟ = normal water gradient in area
⎝ D ⎠n
e.g., 0.433 or 0.465, psi/ft
S
D
= overburden stress gradient, psi/ft
dc
= actual d c − expon ent from plot
d cn
= d c − exp onent from the normal trend
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
Example
From Ben Eaton:
P
S ⎡ S ⎛ P ⎞ ⎤⎛ d c
=
− ⎢ − ⎜ ⎟ ⎥ ⎜⎜
D D ⎣ D ⎝ D ⎠ n ⎦ ⎝ d cn
⎞
⎟⎟
⎠
1 .2
⎛ 1 .2 ⎞
= 1 . 0 − [1 . 0 − 0 . 433 ]⎜
⎟
⎝ 1 .5 ⎠
P
= 0 . 5662 psi/ft
D
1 .2
Example
∴ P = 0.5662 x 12,000 = 6794 psi
6794
EMW =
= 10.9 lbm/gal
0.052 x 12,000
E.S. Pennebaker
8 Used seismic field data for the
detection of abnormal pressures.
8 Under normally pressured conditions the sonic
velocity increases with depth.
(i.e. Travel
time decreases with depth)
(why?)
E.S. Pennebaker
4 Any departure from this trend is an
indication of possible abnormal
pressures.
4 Pennebaker used overlays to estimate
abnormal pore pressures from the
difference between normal and actual
travel times.
Depth, ft
Interval Travel Time, µsec per ft
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
Ben Eaton
From Ben Eaton,
P S ⎡ S ⎛ P ⎞ ⎤ ⎛ ∆t n ⎞
= − ⎢ − ⎜ ⎟ ⎥⎜
⎟
D D ⎣ D ⎝ D ⎠ n ⎦ ⎝ ∆t ⎠
3 .0
⎛ 10,000 ⎞
= 1.0 - [1.0 - 0.465] ⎜
⎟
⎝ 12,000 ⎠
= 0.6904 psi/ft
3
( ∆t α 1/v )
Ben Eaton
From Ben Eaton
ρ = (0.6904 / 0.052) = 13.28 lb/gal
p = 0.6904 * 13,500 = 9,320 psig
Note: Exponent is 3.0 this time,
NOT 1.2!
Equations for Pore Pressure Determination
log
d
C
=
log
P
D
P
D
P
D
P
D
⎛
⎜⎜
⎝
⎛
R
⎜⎜
⎝ 60 N
12 W
10 6 D
⎞
⎟⎟
⎠
B
⎛ ρ NORMAL
* ⎜⎜
⎞
⎝ ρ ACTUAL
⎟⎟
⎠
⎛ S
S
⎛ P ⎞
=
− ⎜⎜
− ⎜
⎟
D
⎝ D ⎠n
⎝ D
=
=
=
S
D
⎛ S
− ⎜⎜
⎝ D
⎛ S
⎜⎜
⎝ D
S
D
−
S
D
⎛ S
− ⎜⎜
⎝ D
⎞
⎟⎟
⎠
−
⎞
⎟
⎟
⎠
⎛ d c calculated
⎜
⎜
d c normal
⎝
⎛ P ⎞
− ⎜
⎟
⎝ D ⎠
⎛ P
⎜
⎝ D
⎞
⎟⎟
⎠
⎞
⎟⎟
⎠
n
⎞
⎟
⎠
⎛ P ⎞
− ⎜
⎟
⎝ D ⎠
n
n
⎞
⎟⎟
⎠
⎞
⎟⎟
⎠
1 .2
⎛ R obs
⎜⎜
⎝ R n
⎞
⎟⎟
⎠
⎛ C
⎜⎜
⎝ C
⎞
⎟⎟
⎠
1 .2
⎞
⎟⎟
⎠
3 .0
⎛ ∆ t
⎜⎜
⎝ ∆ t
n
o
n
o
1 .2
Pore Pressure Determination