TAMU - Pemex
Well Control
Lesson 10
Logging While Drilling (LWD)
2
Logging While Drilling
Sonic Travel Time
Resistivity and Conductivity
Eaton’s Equations (R, C, t, dc)
Natural Gamma Ray
Other…
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Logging While Drilling (LWD)
The parameters obtained with LWD lag penetration by 3’ to 60’, depending on the location of the tool. Some tools have the ability to “see” ahead of the bit.
These are most commonly used for Geo-steering, but can be used in detection of abnormal pressure.
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Logging While Drilling
Any log that infers shale porosity
can indicate the compaction state of the rock,
and hence any abnormal pressure associated with undercompaction.
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Logging While Drilling
Most of the published correlations are based on sonic and electric log
data.
Density logs can also be used if sufficient data are available.
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Pore Pressure Gradient vs. difference between actual and normal sonic travel time
From Hottman and Johnson
LA Upper TX Gulf Coast
to – tn, sec/ft
gp,
psi
/ft
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Matthews and KellyNormal
to – tn, sec/ft
gp,
psi
/ft
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Relationships vary from area to area and from age to age
But, the trends are the same.
to – tn, sec/ft
gp,
psi
/ft
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Resistivity and Conductivity
The ability of rock to conduct electric current can be used to infer porosity.
Resistivity -- ohm-m2/m or ohm-m
Conductivity -- 10-3m/ohm-m2 or millimhos/m
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Resistivity and Conductivity
Rock grains, in general, are very poor conductors.
Saline water in the pores conducts electricity and this fact forms the basis for inferring porosity from bulk R or C measurements.
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Resistivity and Conductivity
Under normal compaction, R increases with depth.
Deviation from the normal trend suggests abnormal pressure
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Resistivity and Conductivity
FR = Ro/Rw FR = formation
resistivity factor
Ro = resistivity of water-
saturated formation
Rw = resistivity of pore water
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Resistivity of formation water
Rw reflects the dissolved salt content of the water, and is dependant upon temperature.
Equation shows that Rw decreases with increasing temperature, and consequently, decreases with depth.
77.6T
77.6TRR
2
11w2w
F in are T and T where o21
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Porosity, m
RaF /1 Porosity of water-saturated rock,
If a = 1, and m = 2, then = FR-0.5
So, = (Ro/Rw)-0.5
Rw in shales cannot be measured directly so Rw in a nearby sand is used instead.
Ro would tend to increase with increasing depth under normally pressured conditions. See Fig. 2.63.
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Fig. 2.63 – Normal Compaction
Ro , m
Dep
th,
ft
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Example 2.20
Rw estimated from nearby well.
Estimate the pore pressure at 14,188 ft using Foster and Whalen’s techinque.
So, at 14,188 ft,
FR = 28.24
034.0
96.0
w
oR R
RF
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Transition at ~11,800’
Using Eaton’s Gulf Coast correlations, ob = 0.974 psi/ft or 13,819 psig at 14,188’
Eq. Depth = 8,720’
obe = 0.937 psi/ft or 8,170 psig at 8,720’
pne = 0.465*8,720
= 4,055
pp = ppe + (ob - obe)
= 4,055+(13,816-8,171)
= 9,703 psig
= 13.16 ppg
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Fig. 2.65 -Hottman & Johnson’s upper Gulf Coast Relationship between shale resistivity and pore pressure
Rn/Ro
Gp, psi/ft
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Example 2.21 Matthews and
Kelly
Determine the transition depth and estimate the pore pressure at 11,500’
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Transition is at ~9,600 ft.
At 11,500 ft:
Co = 1,920, and Cn = 440
Co/Cn = 1,920 / 440 = 4.36
gp = 0.81 psi/ft (Fig 2.66)
Example 2.21
Fig. 2.67
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gp = 0.81 psi/ft
p = 15.6 ppg
pp = 9,315 psig
Fig. 2.66
4.36
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Eaton’s Equations
2.1
2.1
2.1
3
cn
conobobp
o
nnobobp
n
onobobp
o
nnobobp
d
dgggg
C
Cgggg
R
Rgggg
t
tgggg 34.2.Eq
35.2.Eq
36.2.Eq
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Eaton’s Equations
These equations differ from the earlier correlations in that they take into consideration the effect a variable overburden stress may have on the effective stress and the pore pressure.
Probably the most widely used of the log-derived methods
Have been used over 20 years
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Example 2.22
In an offshore Louisiana well, (Ro/Rn) = 0.264 in a Miocene shale at 11,494’. An integrated density log indicates an overburden stress gradient of 0.920 psi/ft. Estimate the pore pressure.
Using Eaton’s technique
Using Hottman and Johnson’s
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Solution
Eaton
From Eq. 2.35, gp = gob - (gob - gn)(Ro/Rn)1.2
gp = 0.920 - (0.920 - 0.465)(0.264) 1.2
gp = 0.827 psi/ft
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Solution Hottman & Johnson
Rn/Ro = 1/(0.264) = 3.79
From Fig 2.65, we then get
gp = 0.894 psi/ft
Difference = 0.894 – 0.827 = 0.067 psi/ft
Answers differ by 770 psi or 1.3 ppg
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DiscussionActual pressure gradient was
determined to be 0.818 psi/ft!
In this example the Eaton method came within 104 psi or 0.17 ppg equivalent mud density of measured values
This lends some credibility to the Eaton method.
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Discussion
In older sediments, exponent may be lowered to 1.0 for resistivities.
Service companies may have more accurate numbers for exponents.
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Natural Gamma Ray
Tools measure the natural radioactive emissions of rock, especially from:
Potassium
Uranium
Thorium
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Natural Gamma Ray
The K40 isotope tends to concentrate in shale minerals thereby leading to the traditional use of GR to determine the shaliness of a rock stratum.
It follows that GR intensity may be used to infer the porosity in shales of consistent minerology
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Natural Gamma Ray
Pore pressure prediction using MWD is now possible (Fig. 2.68).
Lower cps (counts per second) may indicate higher porosity and perhaps abnormal pressure.
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Natural Gamma Ray
In normally pressured shales the cps increases with depth
Any departure from this trend may signal a transition into abnormal pressure
Fig. 2.68
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Pore pressure gradient prediction from observed and normal Gamma Ray counts
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Example 2.23
From table 2.17, determine the pore pressure gradient at 11,100 ft using Zoeller’s correlation.
Use the first three data points to establish the normal trend line.
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At 11,100’ NGRn/ NGRo 57/42 = 1.36
From below, gp = 0.61 psi/ft
or 11.7 ppg
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Effective Stress Models
Use data from MWD/LWD
Rely on the effective-stress principle as the basis for empirical or analytical prediction
Apply log-derived petrophysical parameters of the rock to a compaction model to quantify effective stress
Knowing the overburden pressure, the pore pressure can then be determined
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Dr. Choe’s Kick Simulator
Take a kick
Circulate the kick out of the hole
Plot casing seat pressure vs. time
Plot surface pressure vs. time
Plot kick size vs. time
etc.
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