TAMU Pemex Well Control

1
TAMU - PemexWell Control

• Lesson 10
• Logging While Drilling (LWD)

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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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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.

4
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
gp, psi/ft
Dto Dtn, msec/ft
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Matthews and Kelly
Normal
gp, psi/ft
Dto Dtn, msec/ft
8
Relationships vary from area to area and from age
to age
But, the trends are the same.
gp, psi/ft
Dto Dtn, msec/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

10
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.

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Porosity, f

• 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
Depth, ft
Ro , W.m
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Example 2.20 Rw estimated from nearby
well. Estimate the pore pressure at 14,188 ft
using Foster and Whalens techinque. So, at
14,188 ft, FR 28.24
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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
8,720 pne 0.4658,720 4,055 pp ppe (sob
- sobe) 4,055(13,816-8,171) 9,703 psig
13.16 ppg
Transition at 11,800
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Fig. 2.65 -Hottman Johnsons upper Gulf Coast
Relationship between shale resistivity and pore
pressure
Gp, psi/ft
Rn/Ro
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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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Example 2.21 Fig. 2.67
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)
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Fig. 2.66
gp 0.81 psi/ft rp 15.6 ppg pp 9,315 psig
4.36
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Eatons Equations
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Eatons 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 Eatons technique
• Using Hottman and Johnsons

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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 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
Fig. 2.68
In normally pressured shales the cps increases
with depth
Any departure from this trend may signal a
transition into abnormal pressure
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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 Zoellers
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. 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.