PETE 625 Well Control

1
PETE 625Well Control

• Lesson 8
• Logging While Drilling (LWD)

2
Logging While Drilling

• Sonic Travel Time
• Resistivity and Conductivity
• Eatons Equations (R, C, Dt, dc)
• Natural Gamma Ray
• Other

3
Assignments

• Homework 5
• Ch 2, Problems 21-30
• due Friday, June 18
• Read All of Chapter 2

4
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.

5
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.

6
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.

7
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
8
Matthews and Kelly
Normal
gp, psi/ft
Dto Dtn, msec/ft
9
Relationships vary from area to area and from age
to age
But, the trends are the same.
gp, psi/ft
Dto Dtn, msec/ft
10
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

11
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.

12
Resistivity and Conductivity

• Under normal compaction, R increases with depth.
• Deviation from the normal trend suggests abnormal
  pressure

13
Resistivity and Conductivity

• FR Ro/Rw

• FR formation
• resistivity factor
• Ro resistivity of water-
• saturated formation
• Rw resistivity of pore water

14
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.

15
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.

16
Fig. 2.63 Normal Compaction
Depth, ft
Ro , W.m
17
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
18
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
19
Fig. 2.65 -Hottman Johnsons upper Gulf Coast
Relationship between shale resistivity and pore
pressure
Gp, psi/ft
Rn/Ro
20
Example 2.21 Matthews and Kelly
Determine the transition depth and estimate the
pore pressure at 11,500
21
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)
22
Fig. 2.66
gp 0.81 psi/ft rp 15.6 ppg pp 9,315 psig
4.36
23
Eatons Equations
24
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

25
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

26
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

27
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

28
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.

29
Discussion

• In older sediments, exponent may be lowered to
  1.0 for resistivities.
• Service companies may have more accurate numbers
  for exponents.

30
Natural Gamma Ray

• Tools measure the natural radioactive emissions
  of rock, especially from
• Potassium
• Uranium
• Thorium

31
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

32
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.

33
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
34
Pore pressure gradient prediction from observed
and normal Gamma Ray counts
35
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.
36
At 11,100 NGRn/ NGRo 57/42
1.36 From below, gp 0.61 psi/ft or 11.7 ppg
37
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

38
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.