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Chapter 13 –Gas Bearing

Formation Interpretation

Lecture notes for PET 370

Spring 2011

Prepared by: Thomas W. Engler, Ph.D., P.E.



Gas-Bearing Formation Interpretation

• Effect of gas on neutron log response

– lower hydrogen content than calibrated value, thus higher count rateresulting in low a.

– Shale effect is opposite to the gas effect, makes detection extremelydifficult

• Effect of gas on density log response

– presence of gas reduces bulk density, resulting in a high apparent porosity.

– shale effect can increase or decrease bulk density, dependent on shale’s

bulk density.

• Effect of gas on sonic log response

– increase in sonic log porosity in poorly-consolidated sands.

– not quantitative or predictable

Impact




• Log response is function of different depths of investigation of the FDC –

CNL tools and the degree of invasion.

Background

2 4 6 8 10

Distance from borehole wall, in

G e o m e t r i c

f a c t o r

0.2

0.4

0.6

0.8FDC

CNL



Gas-Bearing Formation Interpretation Example

Density – neutron logillustrating Type I gas effect

(Hilchie, 1978)

- deep invasion, or

- Extremely shallow invasion




Density – neutron log illustrating

the effect of shallow to moderate

invasion. (Type II)

(Bassiouni, 1994)




Density – neutron log illustrating

a gas-bearing shaly sand. (Type III)

(Hilchie, 1978)



Gas-Bearing Formation Interpretation False Gas Effect




• Assume invasion extends beyond the density tool,

where

rg is “apparent gas density” seen by the density log

In terms of porosity, Eq (1) can be written as

where

* gas density is f(P,T,g)* Mud filtrate density is f(salinity)

Where n is fractional salinity (Cppmx10-6)

Porosity Determination

n73.01mf r

]g)xoS1(mf xoS[ma)1( b rrrr

]g)D)(xoS1(xoS[D

mf ma

gmag)D(

rr

rr

(1)

(2)

(3)

(4)




•Consider the simple case of:

1. fresh mud rmf = 1, Hmf = 1

2. Low pressure rg 0, Hg 0

solve for porosity,

solve for flushed zone saturation,

Case I: Fresh mud, low pressure reservoir

ma

ND

ma

1ma=

or ,ma

N)

bma(

=

r

r

r

rrr

NxoS

(9)

(10)

(11)

Porosity Determination




Empirical derivation, applicable for any rg.

If fresh mud,

Further reduce by assuming Sxo is large, such that ,12(1-Sxo) 0,

1/2

2

2 N

2D=

General case

2)]xoS1(n5.1[2)]xoS1(12.1[2

2 N

2D=2

2)]xoS1(12.1[2

2 N

2D=2

(12)

(13)

(14)




• properly calibrated neutron log will respond to hydrogen in water andhydrocarbons.

• Due to low H2 content of gas,

the neutron log responds to

the water fraction, only.

• Difference between two

formations is the “Excavation”of 15% by volume of matrix

material and replaced by gas.

• Magnitude of excavation effect

dependent on hydrocarbon

saturation and fluid HI.

Excavation Effect




Example

Swh = Sxo = 0.5, fresh mud, Hgas = 0

Measured Neutron porosity =

24%

Excavation effect, Nex

= 6%

Corrected neutron porosity =

24 + 6 = 30%

Excavation Effect

Typical excavation effect curve:

Dolomite, = 30%, Hgas = 0




On density logs:

Or

On neutron logs:

Gas effect on crossplot

Nex)gHmf H)(xoS1( Ng

)Dg1)(xoS1(Dg

)gmf )(xoS1(g rrr

(19)

(20)

(21)



Gas-Bearing Formation Interpretation Flowchart

INPUT DATA{rma, N, rb or D,

Cppm, P,T,g}

INITIAL GUESS Sxo

{crossplot} {Eq.11}

GAS DENSITY

{EOS}Hydrogen IndicesHmf {Eq.6} Hg {Eq.7}

r g or Dg{Eq 18} {Eq. 19}

SwH {Eq.17}

Excavation Effect{Eq.15}

Ng {Eq.20}

Update and Sxo

< TOL?

STOP

END

Y

N

n

xoS

nd

;2

Mineral Fractions

rmaa




13.1 A clean sandstone, suspected to be gas bearing, had the following

recorded log readings: a lithology-correct N = 5% and a r b = 2.00gm/cc. Assuming the gas is low density and the mud is fresh mud,

determine the true porosity and the flushed zone saturation.

Exercises




13.2 Repeat Ex. 13.1 but include the excavation effect. Compare with

the answers to Example 13.1.

Exercises




13.3 A clean, gas-bearing sandstone exhibited neutron and density

porosity readings of 10 and 20 %, respectively. Assume a freshmud filtrate. Investigate the effect of gas density on the resulting

true porosity and flushed zone saturation by considering two

separate cases: (1) with a gas density assumed to be zero, and (2) a

gas density = 0.25 gm/cc. Ignore excavation effect.

Exercises




13.4 In Example 13.3, consider the porosity readings are on a limestone

matrix . Determine the true porosity and flushed zone saturation.What is the effect in change of matrix type?

Exercises



Gas-Bearing Formation Interpretation References

Theory, Measurement, and Interpretation of Well Logs, Bassiouni, SPE

Textbook Series, Vol. 4, (1994)

Chapter 16 – Evaluation of Gas-Bearing Formations

density neutron crossover

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