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7/31/2019 8. Logging While Drilling
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Logging While Drilling
Sonic Travel Time
Resistivity and Conductivity
Eatons Equations (R, C, Dt, dc)
Natural Gamma Ray
Other
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Assignments
Homework # 5:
Ch 2, Problems 21-30
due Friday, June 18
Read: All of Chapter 2
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Logging While Drilling
Any log that infers shale porosity
can indicate the compaction state ofthe 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
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 Cmeasurements.
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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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Porosity,
m
RaF/1
Porosity of water-saturated rock,
If a = 1, and m = 2, then f = FR-0.5
So, f = (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
Depth,
ft
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Example 2.20
Rw estimated fromnearby well.
Estimate the pore
pressure at 14,188 ftusing Foster and
Whalens techinque.
So, at 14,188 ft,
FR = 28.24
034.0
96.0
w
o
RR
RF
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Transition at
~11,800
Using Eatons Gulf
Coast correlations,
sob = 0.974 psi/ft or
13,819 psig at 14,188
Eq. Depth = 8,720
sobe = 0.937 psi/ft or
8,170 psig at 8,720pne = 0.465*8,720
= 4,055
pp = ppe + (sob - sobe)= 4,055+(13,816-8,171)
= 9,703 psig
= 13.16 ppg
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Fig. 2.65 -Hottman & Johnsons upperGulf Coast Relationship between
shale resistivity and pore pressure
Rn/Ro
Gp,
psi/ft
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Example 2.21
Matthews andKelly
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
rp= 15.6 ppg
pp = 9,315 psig
Fig. 2.66
4.36
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Eatons 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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Eatons Equations
These equations differ from the earliercorrelations in that they take into
consideration the effect a variable
overburden stress may have on theeffective 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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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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Discussion
Actual pressure gradient was
determined to be 0.818 psi/ft!
In this example the Eaton method camewithin 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
The K40 isotope tends to concentrate in
shale minerals thereby leading to the
traditional use of GR to determine theshaliness 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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Pore pressure gradient prediction fromobserved and normal Gamma Ray counts
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Example 2.23
From table 2.17,determine the porepressure gradient at11,100 ft usingZoellers correlation.
Use the first threedata points to
establish the normaltrend line.
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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. Choes 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.