10. Logging While Drilling

TAMU - Pemex

 Well Control Lesson 10 Logging While Drilling (LWD)

Logging While Drilling 

Sonic Travel Time



Resistivity and Conductivity



Eaton’s Equations (R, C, ∆ t, dc)



Natural Gamma Ray



Other… 2

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. 3

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.

4

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.

5

Pore Pressure Gradient vs. difference between actual and normal sonic travel time From Hottman and Johnson LA Upper TX Gulf Coast tf /i

s p , g

p

to –

tn,

sec/ft

6

Matthews and Kelly

Normal

tf /i

s p , g

p

to –

tn,

sec/ft

7

Relationships vary from area to area and from age to age But, the trends are the same. tf /i

s p , g

p

to –

tn,

sec/ft

8

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 9

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.

10

Resistivity and Conductivity Under

normal compaction, R increases with depth.

Deviation

from the normal trend suggests abnormal pressure

11

Resistivity and Conductivity  FR

= Ro/Rw



FR = formation resistivity factor 



Ro = resistivity of watersaturated formation



Rw = resistivity of pore water  12

Resistivity of formation water  Rw

reflects the dissolved salt content of  the water, and is dependant upon temperature. Rw2

=R

 T + 6.77        T + 6 . 77     1

w1

2

o

where T1 and T2 are in F

Equation

shows that Rw decreases with increasing temperature, and consequently, decreases with depth.

13

Porosity, 

Porosity of water-saturated rock,

1/ m

aFR 

-0.5 R



If a = 1, and m = 2, then φ = F



So, φ = (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. 14

Fig. 2.63 – Normal Compaction

t f ht p e D

,

R

15

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 

R o

0.96

R w

0.034

FR = 28.24

16

Using Eaton’s Gulf  Coast correlations, σ ob = 0.974 psi/ft or  13,819 psig at 14,188’ Eq. Depth = 8,720’ σ obe = 0.937 psi/ft or  8,170 psig at 8,720’

pne = 0.465*8,720 Transition at ~11,800’

= 4,055 pp = ppe + (σ

ob

-σ

obe

)

= 4,055+(13,816-8,17 4,055+(13,816-8,171) 1) = 9,703 psig 17

Fig. 2.65 -Hottman & Johnson’s upper  Gulf Coast Relationship between shale resistivity and pore pressure

G p,  psi/ft

R /R  R  /R 

18

Example 2.21 Matthews and Kelly Determine the transition depth and estimate the pore pressure at 11,500’

19

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)

20

Fig. 2.66 gp = 0.81 psi/ft ρ

 p

= 15.6 ppg

pp = 9,315 psig

4.36 21

Eaton’s Equations 3

gp

g ob

g ob

gn

tn

Eq. 2.34

to 1. 2

gp

g ob

g ob

gn

R o

Eq. 2.35

R n 1. 2

gp

g ob

g ob

gn

Cn

Eq. 2.36

Co 1.2

gp

g ob

g ob

gn

d co d cn 22

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 23

Example 2.22 In

an offshore Louisiana well, (R o/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 24

Solution Eaton From

gp

Eq. 2.35, gp = gob - (gob - gn)(Ro/Rn)1.2

= 0.920 - (0.920 - 0.465)(0.264)

1.2

gp = 0.827 psi/ft 25

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

26

Discussion Actual

pressure gradient was determined to be 0.818 psi/ft!

In

this example the Eaton method came c ame within 104 psi or 0.17 ppg p pg equivalent mud density of measured values

This

lends some credibility to the Eaton method. 27

Discussion In

older sediments, exponent may be lowered to 1.0 for resistivities.

Service

companies may have more accurate numbers for exponents.

28

Natural Gamma Ray Tools

measure the natural radioactive emissions of rock, especially from: 

Potassium



Uranium



Thorium

29

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 30

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.

31

Fig. 2.68

Natural Gamma Ray In normally pressured shales the cps increases with depth Any departure from this trend may signal a transition into abnormal pressure

32

Pore pressure gradient prediction from observed and normal Gamma Ray counts

33

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.

34

At 11,100’ NGRn / NGRo 57/42

= 1.36

From below, gp = 0.61 psi/ft or 11.7 ppg ppg

35

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 36

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. 37