PETE 411 Well Drilling
Post on 02-Jan-2016
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2
Prediction of Abnormal Pore Pressure
Resistivity of Shale Temperature in the Return Mud Drilling Rate Increase dc - Exponent
Sonic Travel Time Conductivity of Shale
4
Shale Resistivityvs. Depth
1. Establish trend line in normally pressured shale
2. Look for deviations from this trend line
(semi-log)
6
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)
13
Typical Drilling Rate Profiles - Shale
The drilling rate in a normally pressured, solid shale section will generally generate a very steady and smooth drilling rate curve.
The penetration rate will be steady and not erratic (normally pressured, clean shale).
Shale
14
Typical Drilling Rate Profiles - Sand
The drilling rate in a sand will probably generate an erratic drilling rate curve.
Sands in the Gulf Coast area are generally very unconsolidated. This may cause sloughing, accompanied by erratic torque, and temporarily, erratic drilling rates.
Sand
15
Typical Drilling Rate Profiles - Shaley Sands
This is generally the most troublesome type drilling rate curve to interpret.
Many times this curve will look similar to a solid shale curve that is 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, even though the drilling rate parameter is one of the better parameters.
16
Typical Drilling Rate Profiles
If you are drilling close to balanced, there will probably be a very smooth, (gradual) increase in the drilling rate.
This is due to the difference between the hydrostatic head and the pore pressure becoming smaller. p
Transition Zone Shale
17
Typical Drilling Rate Profiles
Transition Zone Shale
As the pressure becomes very small, the gas in the pores has a tendency to expand which causes the shale particles to pop from the wall. This is called sloughing shale.
The transition zone generally has a higher porosity, making drilling rates higher. In a clean shale the ROP will increase in a smooth manner.
p
18
Typical Drilling Rate Profiles
Note:
If you are drilling overbalanced in a transition it will 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.
19
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 pending danger that you may not notice the small effect of the drilling rate curve change. This will allow you to move well into that transition zone without realizing it.
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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
22
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
25
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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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
30
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 *
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
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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 well kicks from formation at 10,000 ft.
34Pressure, psi
Dep
th,
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 - SolutionThe 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 – cont’d
This additional pressure, at 10,000 ft, is also 525 psi, and would amount to an additional pressure gradient of:
525 psi / 10,000 ft = 0.0525 psi/ft
This represents an equivalent mud weight of0.0525 / 0.052 = 1.01 lb/gal
This is the kick tolerance for a small kick!
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Problem #3 - Alternate Solution
When a well kicks, the well is shut in and the wellbore pressure increases until the new BHP equals the new formation pressure.
At that point influx of formation fluids into the wellbore ceases.
Since the mud gradient in the wellbore has not changed, the pressure increases uniformly everywhere.
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Wellbore Pressure, psi
Dep
th,
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 of525 / (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
Dep
th,
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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