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Chapter 3 Reserve Estimation
Lecture notes for PET 370
Spring 2012Prepared by: Thomas W. Engler,
Ph.D., P.E.
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Reserve Estimation
where,N = oil in place, stb
A = drainage area, acresBoi = initial formation volume factor,rb/stb
h i = individual zone thickness, ft
f i = porosity, fractionSwi = water saturation, fraction
Reserves are defined by:
R = N * (Er)
Er = recovery factor
n
1i)wiS(1iih
oiB7758A
N
place,inoilVolumetric
f
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t
ww R
R F *S
Measured Water Sample
Published Water Tables
R w = R o/F
SP log
Depth merge & correlate
Sonic
Density
Neutron
others
Lithology
Gas/Liquid
ma
f F
Induction log
Laterologs
So = 1 - S w
n
1iwiii
oi)S(1hB
7758A N
place,inoilVolumetric
f
Reserves,R = N * RF
Flowchart for Well Log Interpretation
VshSP
GR
Porosity logs
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Reserve Estimation
Definition of Reservoir Intervals
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Reserve Estimation
1. Shale content (Vsh) – eliminate the portion of theformation which contains large quantities of shale.Vshcutoff ≈ 20 to 30 % (Note: shale plays up to 40 to 50%) GROSS SAND
2. Porosity – eliminate the portion of the formation which is
low porosity (and low permeability) and therefore wouldbe non-productive.Sandstones
f cutoff ≈ 5% to 15% consolidated → friable, unconsolidated
Carbonates
f cutoff ≈4% (Note: shale plays ~ 3 to 4%) NET SAND
3. Water saturation – eliminate the portion of the formationwhich contains large volumes of water in the pore space.Sandstones
Swcutoff ≈ 60% Carbonates
Swcutoff ≈ 50%
NET PAY
Cutoff Values
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Reserve Estimation
Specific Correlation
• Applicable to U.S. gulf coast sands• Correlation between k, f, and Sw• Assumes oil/water viscosity ratio = 2.0
Cutoff Values
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Reserve Estimation
Porosity – thickness weighted average
Averaging
h1
h2
h3
f 1
f 2
3 n
i ih
n
i i
hi
1
1f
f
n
i
n
iw
S
1 ih
iφ
1 i
hiφwi
S
Water saturation – volume weighted average
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Reserve Estimation Reservoir Volume
1*
13 n A
n A
n A
n A
h
bV
12 n An Ah
bV
or
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Reserve Estimation
Determined from:
1. displacement efficiency studies2. correlations based on statistical studies of particular
types of reservoir mechanisms
OILvolumetric reservoirs
Total free gas saturation at abandonment by core analysis. Assumes core fluids (oil and water)are displaced by expansion of the liberated gas while removing the core to the surface…similar
process to depletion.
hydraulic reservoirs
Estimate S or from the remaining oil in the cores after flushed by mud filtrate and expansion ofgas…displacement process
Recovery Factor
o Boi B
wS
g S wS
r E 1
11
wS
or S wS
r E
1
1
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Reserve Estimation
GASVolumetric Reservoirs
Hydraulic Reservoirs
Recovery Factor
ga B
gi B
r E 1
wS
ga B gi B
gr S
wS
r E
1
1
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Reserve Estimation
From displacement of reservoir fluids (invasion) using welllogs
Recovery Factor
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Reserve Estimation
From displacement of reservoir fluids (invasion) using welllogs
Difference between initial oil saturation, S oi and theresidual oil saturation, S or, that remains after theformation is invaded by water:
Som = Soi - Sor
Inferring mud filtrate invasion as an efficientdisplacement mechanism, then
Som
= Sxo
- Sw
Recovery factor for water drive reservoirs,
....for depletion drive, use rule of thumb of 1/2 of (RF) wd
Recovery Factor
w
S wS xoS
wd r E 1
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Reserve Estimation Recovery Factor
Driveminimum
Sandstonesaverage maximum minimum
Carbonatesaverage maximum
Water drive 27.8 51.1 86.7 6.3 43.6 80.5Solution gasdrive withoutsupplementaldrives
9.5 21.3 46.0 15.5 17.6 20.7
Solution gasdrive withsupplementaldrives
13.1 28.4 57.9 9.0 21.8 48.1
Gas cap drive 15.8 32.5 67.0 Combined with sandstoneData not availableGravity drainage 16.0 57.2 63.8
Gas depletion 75.0 85.0 95.0Gas water drive 50.0 70.0 80.0
Solution Gas Drive Reservoirs (Arps, 1962)
Statistical Performance
Recovery factor for different drive mechanisms
Soln Gor Oilgravity maximum
Sandstonesaverage minimum maximum
Carbonatesaverage minimum
60 153050
12.821.334.2
8.615.224.8
2.68.7
16.9
28.032.839.0
4.49.9
18.6
0.62.98.0
200 153050
13.322.237.4
8.815.226.4
3.38.4
17.6
27.532.339.8
4.59.8
19.3
0.92.67.4
600 15
3050
18.0
24.335.6
11.3
15.123.0
6.0
8.413.8
26.6
30.036.1
6.9
9.615.1
1.9
2.54.31000 15
3050
-34.433.7
-21.220.2
-12.611.6
-32.631.8
-13.212.0
-4.03.1
2000 153050
--
40.7
--
24.8
--
15.6
--
32.8
--
14.5
--
5.0
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Electrical Properties of Rocks
Chapter 11, Sec 11.4-11.7 , Bassiouni, Z: Theory,Measurement, and Interpretation of Well Logs, SPETextbook Series, Vol. 4, (1994)
Corelab, Fundamentals of Core Analysis, Houston, TX(1983), Chapter 7
References