8. Logging While Drilling

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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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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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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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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Under normal compaction, R increases

with depth.

Deviation from the normal trend

suggests abnormal pressure


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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.

8. Logging While Drilling

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