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…

3

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.

5

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.

6

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

7

Matthews and KellyNormal

to – tn, sec/ft

gp,

psi

/ft

8

Relationships vary from area to area and from age to age

But, the trends are the same.

to – tn, sec/ft

gp,

psi

/ft

9

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.

11

Resistivity and Conductivity

Under normal compaction, R increases with depth.

Deviation from the normal trend suggests abnormal pressure

12

Resistivity and Conductivity

FR = Ro/Rw FR = formation

resistivity factor

Ro = resistivity of water-

saturated formation

Rw = resistivity of pore water

13

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

14

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.

15

Fig. 2.63 – Normal Compaction

Ro , m

Dep

th,

ft

16

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

17

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

19

Example 2.21 Matthews and

Kelly

Determine the transition depth and estimate the pore pressure at 11,500’

20

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

21

gp = 0.81 psi/ft

p = 15.6 ppg

pp = 9,315 psig

Fig. 2.66

4.36

22

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

23

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

24

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

27

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.

28

Discussion

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

Service companies may have more accurate numbers for exponents.

29

Natural Gamma Ray

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

Potassium

Uranium

Thorium

30

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

31

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.

32

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

33

Pore pressure gradient prediction from observed and normal Gamma Ray counts

34

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.

35

At 11,100’ NGRn/ NGRo 57/42 = 1.36

From below, gp = 0.61 psi/ft

or 11.7 ppg

36

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

37

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.