29543572 10 Logging While Drilling

7/29/2019 29543572 10 Logging While Drilling

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TAMU - PemexWell Control

Lesson 10

Logging While Drilling

(LWD)


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Sonic Travel Time

Resistivity and Conductivity

Eatons Equations (R, C, t, 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 indetection of abnormal pressure.


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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/ f

t


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Matthews and KellyNormal

to tn, sec/ft

gp

,psi/ f

t


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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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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


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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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Resistivity of formation water

Rw reflects the dissolved salt content ofthe 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

FinareTandTwhereo

21


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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.


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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,

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


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Fig. 2.65 -Hottman & Johnsons upper

Gulf 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

p= 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

co

nobobp

o

n

nobobp

n

onobobp

o

n

nobobp

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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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 0.920psi/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 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

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 theshaliness of a rock stratum.

It follows that GR intensity may be usedto 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.

Fi 2 68


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Natural Gamma Ray

In normally pressuredshales the cps

increases with depth

Any departure from this

trend may signal a

transition into abnormalpressure

Fig. 2.68


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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 normaltrend 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.

29543572 10 Logging While Drilling

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