5. 5
Pore Pressure vs. Depth
0
5,000
10,000
15,000
5 10 15 20
Pore Pressure, lb/gal equivalent
Depth,
ft
Normal Abormal
Density of mud required to control this pore pressure
0.433 psi/ft 8.33 lb/gal
0.465 psi/ft 9.00 lb/gal
8. 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. 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. 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.
16. 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. 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. 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. 19
Prediction and Detection of Abnormal
Pressure Zones
1. Before drilling
Shallow seismic surveys
Deep seismic surveys
Comparison with nearby wells
20. 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. 21
Prediction and Detection of Abnormal
Pressure Zones
3. After drilling
Resistivity log
Conductivity log
Sonic log
Density log
25. 25
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
D
B
E
D
W
N
K
R
26. 26
D - Exponent
If we assume that K = 1
and E = 1
Then
D
B
E
D
W
N
K
R
B
D
W
log
N
R
log
D
D
B
D
W
N
R
27. 27
D - Exponent
A modified version of this equation
follows:
B
6
D
10
W
12
log
N
60
R
log
d
28. 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.
B
6
D
10
W
12
log
N
60
R
log
d
2308
.
1
2341
.
2
25
.
12
10
000
,
60
*
12
log
100
*
60
35
log
6
d = 1.82
29. 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
57
.
1
2308
.
1
9331
.
1
25
.
12
10
000
,
60
*
12
log
100
*
60
70
log
d
6
30. 30
Example
d may be Corrected for density as
follows
)
ppg
(
use
in
weight
mud
actual
)
ppg
(
gradient
normal
for
weight
mud
d
dc
37
.
1
12
9
*
82
.
1
12
9
d
d
.,
g
.
e c
31. 31
Example 2
Calculate “d” if: R = 20 ft/hr
N = 100 RPM
W = 25,000 lbf
DB = 9 7/8 in
875
.
9
*
10
000
,
25
*
12
log
100
*
60
20
log
6
B
6
D
10
W
12
log
N
60
R
log
d
d = 1.63
32. 32
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”?
2
.
11
33
.
8
*
63
.
1
)
ppg
(
weight
mud
actual
)
ppg
(
gradient
normal
d
dc
dc = 1.21
34. 34
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
36. 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.
Overburden
Stress Grad.
Normal Pore
Pressure Grad.
2
.
1
c
c
n normal
d
calculated
d
D
P
D
S
D
S
D
P
38. 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.
tot
W
66
.
16
33
.
8
Gravity
.
Spec
40. 40
Pore Pressure from
Resistivity
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
0.2 0.5 1 2 3
10,000’
41. 41
From plot, Rn = 1.55 ohms
Robs = 0.80 ohms
From Eaton:
EATON
2
.
1
n
obs
n R
R
D
P
D
S
D
S
D
P
2
.
1
55
.
1
80
.
0
465
.
0
95
.
0
95
.
0
D
P
= 0.7307 psi/ft = 14.05 lb/gal
P = 0.7307 * 10,000 = 7,307 psi
0.2 0.5 1 2 3
10,000’
Dep
th
