Transcript
Abnormal Pressure 10.1- 1
1.10-1
Abnormal Pressure
Abnormal Pressure 10.1- 2
Abnormal Pressure - Definition, Causes -
Normal Pore Pressures
Abnormal Pore Pressure Gradients
Fracture Gradients
Mud Weights
Casing Seat Depths
What Causes Abnormal Pressure?
Abnormal Pressure 10.1- 3
Abnormal Pressure Gradients
Normal Pressure Gradients West Texas: 0.433 psi/ft Gulf Coast: 0.465 psi/ft
Normal and Abnormal Pore Pressure
Pore Pressure, psig
Dep
th,
ft
10,000’ ? ?
Abnormal Pressure 10.1- 4
Pore Pressure vs. Depth
8 9 10 11 12 13 14 15 16Pore Pressure Equivalent, lb/gal
0
5,000
10,000
15,000
20,000
{ Density of mud required to control this pore pressure }
Dep
th,
ft
Normal
Abormal
0.433 psi/ft 8.33 lb/gal0.465 psi/ft 9.0 lb/gal
Abnormal Pressure 10.1- 5
Pore Pressure Gradient
Fracture Gradient
Abnormal Pressure 10.1- 6
* Pore Pressure
Gradients
* Fracture Gradients
* Casing Setting Depths
Abnormal Pressure 10.1- 7
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
Abnormal Pressure 10.1- 8
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 Pressure 10.1- 9
HIGH PRESSURE
NORMAL PRESSURE
Thick, impermeable layers of shale (or salt) restrict the movement of water. Below such layers abnormal pressure may be found.
Abnormal Pressure 10.1- 10
HIGH PRESSURE
NORMAL PRESSURE
Hydrostatic pressure gradient is lower in gas or oil than in water.
Abnormal Pressure 10.1- 11
When crossing faults it is possible to go from normal pressure to abnormally high pressure in a short interval.
Abnormal Pressure 10.1- 12
Well “A” found only Normal Pressure ...
Abnormal Pressure 10.1- 13
OB = p + Z
ob
pz
Abnormal Pressure 10.1- 14
?
Abnormal Pressure 10.1- 15
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
Abnormal Pressure 10.1- 16
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
Abnormal Pressure 10.1- 17
Prediction and Detection of Abnormal Pressure Zones
1. Before drilling
Shallow seismic surveys
Deep seismic surveys
Comparison with nearby wells
Abnormal Pressure 10.1- 18
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)
Abnormal Pressure 10.1- 19
Prediction and Detection of Abnormal Pressure Zones
3. After drilling
Resistivity log
Conductivity log
Sonic log
Density log
Abnormal Pressure 10.1- 20
. – .SD000085.0e41.0
Abnormal Pressure 10.1- 21
Decreasing ROP
What is d-exponent?
Abnormal Pressure 10.1- 22
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
WNKR
Abnormal Pressure 10.1- 23
D - Exponent
If we assume that K = 1
and E = 1
Then
D
B
E
D
WNKR
BDW
log
NR
logD
D
BD
W
N
R
Abnormal Pressure 10.1- 24
D - Exponent
A modified version of this equation follows:
B6 D10W12
log
N60R
log
d
Abnormal Pressure 10.1- 25
Example
d may be Corrected for mud density as follows:
)ppg(useinweightmudactual
)ppg(gradientnormalforweightmudddc
37.112
9*82.1
12
9dd.,g.e c
Abnormal Pressure 10.1- 26
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
Abnormal Pressure 10.1- 27
Procedure for Determining Pore Pressure From dc - Exponent
dc - Exponent
Dep
th Normal
AbnormalN
ormal Trend
Abnormal Pressure 10.1- 28
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 normald
calculatedd
D
P
D
S
D
S
D
P
Abnormal Pressure 10.1- 29
In normally pressured shales, shale
compaction increases with depth
Abnormal Pressure 10.1- 30
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’
Dep
th
Abnormal Pressure 10.1- 31
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
80.0465.095.095.0
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
Abnormal Pressure 10.1- 32
Prediction of Abnormal Pore Pressure
Resistivity of Shale Temperature in the Return Mud Drilling Rate Increase dc - Exponent
Sonic Travel Time Conductivity of Shale
Abnormal Pressure 10.1- 33
EXAMPLE
Shale Resistivity vs. Depth
1. Establish normal trend line
2. Look for deviations
(semi-log)
Abnormal Pressure 10.1- 34
Shale Resistivity vs. Depth
1. Establish normal trend line
2. Look for deviations
3. Use OVERLAYto quantify pore pressure
(use with caution)
Pore Pressure(lb/gal equivalent)
16 14 12 10
9 ppg (normal)
Abnormal Pressure 10.1- 35
Example
8.2 X
Why?
Abnormal Pressure 10.1- 36
Determination of Abnormal Pore Pressure Using the dc - exponent
From Ben Eaton:
2.1
cn
c
n d
d
D
P
D
S
D
S
D
P
Abnormal Pressure 10.1- 37
Where
trendnormal thefrom onentexpd d
plot from entexpond actual d
psi/ft gradient, stress overburden D
S
psi/ft 0.465,or 0.433 e.g.,
areain gradient water normalD
P
psi/ft gradient, pressureformation D
P
ccn
cc
n
2.1
cn
c
n d
d
D
P
D
S
D
S
D
P
Abnormal Pressure 10.1- 38
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
Abnormal Pressure 10.1- 39
Example
From Ben Eaton:
psi/ft 5662.0D
P
5.1
2.1]433.00.1[0.1
d
d
D
P
D
S
D
S
D
P
2.1
2.1
cn
c
n
Abnormal Pressure 10.1- 40
Example
lbm/gal 9.1012,000 x 0.052
6794EMW
psi 6794000,12 x 5662.0P
Abnormal Pressure 10.1- 41
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?)
Abnormal Pressure 10.1- 42
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.
Abnormal Pressure 10.1- 43
Interval Travel Time, sec per ft
Dep
th,
ft
Abnormal Pressure 10.1- 44
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
Abnormal Pressure 10.1- 45
Ben Eaton
From Ben Eaton,
psi/ft 0.6904
12,000
10,0000.465]-[1.0-1.0
t
t
D
P
D
S
D
S
D
P
3
0.3
n
n
( t 1/v )
Abnormal Pressure 10.1- 46
Ben Eaton
From Ben Eaton
Note: Exponent is 3.0 this time,
NOT 1.2!
= (0.6904 / 0.052) = 13.28 lb/gal
p = 0.6904 * 13,500 = 9,320 psig
Abnormal Pressure 10.1- 47
Equations for Pore Pressure Determination
2.1
c
c
n normald
calculatedd
D
P
D
S
D
S
D
P
2.1
n
obs
n R
R
D
P
D
S
D
S
D
P
ACTUAL
NORMAL
B6
C *
D10W12
log
N60R
log
d
2.1
o
n
n C
C
D
P
D
S
D
S
D
P
0.3
o
n
n t
t
D
P
D
S
D
S
D
P
Abnormal Pressure 10.1- 48
Pore Pressure Determination
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