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Chapter 13 –Gas Bearing Formation Interpretation
Lecture notes for PET 370
Spring 2012
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 rate
resulting in low a.
– Shale effect is opposite to the gas effect, makes detection extremely difficult
• 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
Gas-Bearing Formation Interpretation
• 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
Geo
met
ric
fact
or
0.2
0.4
0.6
0.8 FDC
CNL
Gas-Bearing Formation Interpretation
Idealized example of saturation effects on density and neutron logs. (Helander,1983)
classification
Type I: mirror image, gas crossover (both FDC and CNL investigate same Saturation profile)
Type II: asymmetric gas crossover
Type III: Shaly gas sand
Gas-Bearing Formation Interpretation Example
Density – neutron log illustrating Type I gas effect (Hilchie, 1978) - deep invasion, or - Extremely shallow invasion
Gas-Bearing Formation Interpretation Example
Density – neutron log illustrating the effect of shallow to moderate invasion. (Type II) (Bassiouni, 1994)
Gas-Bearing Formation Interpretation Example
Density – neutron log illustrating a gas-bearing shaly sand. (Type III) (Hilchie, 1978)
Gas-Bearing Formation Interpretation False Gas Effect
Gas-Bearing Formation Interpretation
• 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(mfxoS[ma)1(b rrrr
]g)D)(xoS1(xoS[D
mfma
gmag)D(
rr
rr
(1)
(2)
(3)
(4)
Gas-Bearing Formation Interpretation
* Assume invasion extends beyond the zone of investigation of the neutron tool,
Where,
Hma, Hmf and Hg are hydrogen indices for matrix, filtrate and gas,
respectively,
]gH)xoS1(mfHxoS[maH)1(N
dolomite.or lms, ss, purefor 0maH
gg5.216
g5.249gH
mf)n1(mfH
r
r
r
r
(5)
(6)
(7)
(8)
Porosity Determination
Gas-Bearing Formation Interpretation
• 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
r
rr
N
xoS
(9)
(10)
(11)
Porosity Determination
Gas-Bearing Formation Interpretation
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
2N
2D=
General case
2)]xoS1(n5.1[2)]xoS1(12.1[2
2N
2D=2
2)]xoS1(12.1[2
2N
2D=2
(12)
(13)
(14)
Gas-Bearing Formation Interpretation
• properly calibrated neutron log will respond to hydrogen in water and hydrocarbons.
• 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
Gas-Bearing Formation Interpretation
* Empirical correction,
Where,
Swh is the equivalent saturation based on the hydrogen content of the pore fluids,
When fresh mud and low pressure gas are assumed, then Swh = Sxo
* Add correction to neutron log reading,
Excavation Effect
NexNNc
)whS-)(10.04+whS2k(2=Nex
2
2.65
ma =k
r
gH)xoS1(mfHxoSwhS
(15)
(16)
(17)
(18)
Gas-Bearing Formation Interpretation
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
Gas-Bearing Formation Interpretation
On density logs:
Or
On neutron logs:
Gas effect on crossplot
Nex)gHmfH)(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 Indices Hmf {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
xoSnd
;
2
Mineral Fractions rmaa
Gas-Bearing Formation Interpretation
13.1 A clean sandstone, suspected to be gas bearing, had the following
recorded log readings: a lithology-correct N = 5% and a rb = 2.00
gm/cc. Assuming the gas is low density and the mud is fresh mud,
determine the true porosity and the flushed zone saturation.
Exercises
Gas-Bearing Formation Interpretation
13.2 Repeat Ex. 13.1 but include the excavation effect. Compare with
the answers to Example 13.1.
Exercises
Gas-Bearing Formation Interpretation
13.3 A clean, gas-bearing sandstone exhibited neutron and density
porosity readings of 10 and 20 %, respectively. Assume a fresh
mud 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
Gas-Bearing Formation Interpretation
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