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PETE 411
Well Drilling
Lesson 21
Prediction ofAbnormal Pore Pressure
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Prediction ofAbnormal 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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HW #11
Slip Velocity
Due 10-28-02
Read:
Applied Drilling Engineering, Ch. 6
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Shale Resistivityvs. Depth
1. Establish trend
line in normally
pressured shale
2. Look fordeviations from
this trend line
(semi-log)
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EXAMPLE
Shale Resistivity
vs. Depth
1. Establish normal
trend line
2. Look for
deviations
(semi-log)
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Shale Resistivityvs. Depth
1. Establish normaltrend line
2. Look fordeviations
3. Use OVERLAYto quantify
pore pressure
(use with caution)
Pore Pressure
(lb/gal equivalent)16 14 12 10
9 ppg
(normal)
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Shale Density , g/cc
Depth,
ft
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Mud Temperature in flowline, deg F
Depth,
ft
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Example
8.2 X
Why?
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Example
8.8 X
Thermal conductivity, heat capacity, pore pressure...
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PHYD - PPORE , psi
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P = (P2 - P1)1,000
Effect of Differential Pressure
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Typical Drill ing Rate Profiles - Shale
The drilling rate in a normally
pressured, solid shalesection will generallygenerate a very steady and
smooth drilling rate curve.
The penetration rate will be
steady and not erratic(normally pressured, clean
shale).
Shale
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Typical Drill ing Rate Profiles - Sand
The drilling rate in a sand willprobably generate an erraticdrill ing rate curve.
Sands in the Gulf Coast area
are generally veryunconsolidated. This maycause sloughing, accompanied
by erratic torque, andtemporarily, erratic drill ingrates.
Sand
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Typical Drill ing Rate Profiles - Shaley Sands
This is generally the mosttroublesome type drilling rate curve
to interpret.
Many times this curve will looksimilar to a solid shale curve thatis moving into a transition zone.
Shaley Sands
Note:This is a prime example why you should not base
your decision on only one drilling parameter, eventhough the drilling rate parameter is one of the betterparameters.
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Typical Drill ing Rate Profiles
If you are drilling close to
balanced, there will probablybe a very smooth, (gradual)
increase in the drilling rate.
This is due to the differencebetween the hydrostatichead and the pore pressurebecoming smaller. p
Transition Zone
Shale
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Typical Drilling Rate Profiles
Transition Zone
Shale
As the pressure becomesvery small, the gas in the pores
has a tendency to expand whichcauses the shale particles to popfrom the wall. This is calledsloughing shale.
The transition zone generallyhas a higher porosity, makingdrilling rates higher. In a clean
shale the ROP will increase in asmooth manner.
p
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Typical Drilling Rate Profiles
Note:
If you are drilling overbalanced in a transition itwill be very difficult to pick up the
transition zone initially.
This will allow you to move well into the
transition zone before detecting the problem.
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Typical Drilling Rate Profiles
This could cause you to move into a permeable
zone which would probably result in a kick.
The conditions you create with overbalanced
hydrostatic head will so disguise the pendingdanger that you may not notice the smalleffect of the drilling rate curve change. This
will allow you to move well into that transitionzone without realizing it.
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Determination of Abnormal Pore
Pressure Using the dc - exponent
From Ben Eaton:
2.1
cn
c
n dd
DP
DS
DS
DP
=
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Where
trendnormalthefromonentexpdd
plotfromentexpondactuald
psi/ftgradient,stressoverburdenD
S
psi/ft0.465,or0.433e.g.,
areaingradientwaternormalD
P
psi/ftgradient,pressureformation
D
P
ccn
cc
n
=
=
=
=
=
2.1
cn
c
n d
d
D
P
D
S
D
S
D
P
=
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Example
Calculate the porepressure at depth X using
the data in this graph.
Assume:
West Texas location withnormal overburden of
1.0 psi/ft.X = 12,000 ft.
X
1.2 1.5
dc
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Example
From Ben Eaton:
psi/ft5662.0D
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
=
=
=
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Example
lbm/gal9.1012,000x0.052
6794EMW
psi6794000,12x5662.0P
==
==
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E.S. Pennebaker
Used seismic field data for thedetection ofabnormal pressures.
Under normally pressured conditions thesonic 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 abnormalpressures.
Pennebaker used overlays to estimateabnormal pore pressures from the
difference between normal and actualtravel times.
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Interval Travel Time, sec per ft
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Ben Eaton
also found a way to determine pore pressurefrom interval travel times.
Example:In a Gulf Coast well, the speed of sound is 10,000ft/sec at a depth of13,500 ft. The normal speed
of sound at this depth, based on extrapolatedtrends, would be 12,000 ft/sec. What is the porepressure at this depth?
Assume: S/D = 1.0 psi/ft
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Ben Eaton
From Ben Eaton,
psi/ft0.6904
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 )
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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
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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 *
D10
W12log
N60
Rlog
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
=
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Pore Pressure Determination
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EXAMPLE 3 - An Application...
Mud Weight = 10 lb/gal. (0.52 psi/ft)
Surface csg. Set at 2,500 ft.Fracture gradient below surf. Csg = 0.73 psi/ftDrilling at 10,000 ft in pressure transition zone
* Mud weight may be less than pore pressure!
DETERMINE Maximum safe underbalance
between mud weight and pore pressure if wellkicks from formation at 10,000 ft.
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D
epth,
ft
Casing Seat
10,000
Mud Wt. Grad
= 0.52 psi/ft
FractureGradient= 0.73 psi/ft
0.73 0.52 = 0.21 (psi/ft)
5,200
2,500
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Example 3 - Solution
The danger here is fracturing the formation near
the casing seat at 2,500 ft.
The fracture gradient at this depth is 0.73 psi/ft,
and the mud weight gradient is 0.52 psi/ft.
So, the additional permissible pressure gradient
is 0.73 0.52 = 0.21 psi/ft, at the casing seat.
This corresponds to an additional pressure of
P = 0.21 psi/ft * 2,500 ft = 525 psi
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Example 3 Solution contd
This additional pressure, at 10,000 ft, is also
525 psi, and would amount to an additionalpressure gradient of:
525 psi / 10,000 ft = 0.0525 psi/ft
This represents an equivalent mud weight of
0.0525 / 0.052 = 1.01 lb/gal
This is the kick tolerance for a small kick!
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Problem #3 - Alternate SolutionWhen a well kicks, the well is shut in
and the wellbore pressure increasesuntil the new BHP equals the newformation pressure.
At that point influx of formation fluidsinto the wellbore ceases.
Since the mud gradient in the wellborehas not changed, the pressureincreases uniformly everywhere.
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Wellbore Pressure, psi
D
epth,
ft
P
Casing Seat at 2,500 ft
Kick at 10,000 ft
Before Kick
After Kick and
Stabilization
525
525
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At 2,500 ftInitial mud pressure = 0.52 psi/ft * 2,500 ft = 1,300 psiFracture pressure = 0.73 psi/ft * 2,500 ft = 1,825 psi
Maximum allowable increase in pressure = 525 psi
At 10,000 ft
Maximum allowable increase in pressure = 525 psi(since the pressure increases uniformly everywhere).
This corresponds to an increase in mud weight of
525 / (0.052 * 10,000) = 1.01 lb/gal= maximum increase in EMW= kick tolarance for a small kick size.
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Wellbore Pressure, psi
D
epth,
ft
P
Casing Seat at 2,500 ft
Kick at 10,000 ft
1,300 psi
1,825 psi
5,725 psi
5,200 psi
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Wellbore Pressure, psi
D
epth,
ft
P
Casing Seat at 2,500 ft
Kick at 10,000 ft
Before Kick
After Small Kick
and Stabilization
After Large Kick and Stabilization