Reserves Estimation

Chapter 3 Reserve Estimation

Lecture notes for PET 370 Spring 2010 Prepared by: Thomas W. Engler, Ph.D., P.E.

Reserve Estimation Volumetric oil N

in

place,

7758A n h (1 S ) i i wi B oi i 1

where, N = oil in place, stb A = drainage area, acres Boi = initial formation volume factor,rb/stb hi = individual zone thickness, ft i = porosity, fraction Swi = water saturation, fraction Reserves are defined by: R = N * (Er) Er = recovery factor

Depth merge & correlate

Measured Water Sample

Sonic

Published Water Tables

Density Lithology

Rw = Ro/F

Neutron Gas/Liquid

SP log

others

a

F

m

Sw

Vsh SP

F * Rw Rt

Induction log

GR

Laterologs

So = 1 - S w

Porosity logs

Volumetric oil N

in

place,

7758A n h i i (1 B oi i 1

Reserves, R = N * RF Flowchart for Well Log Interpretation

S wi )

Reserve Estimation

Definition of Reservoir Intervals

Reserve Estimation

Cutoff Values

1. Shale content (Vsh) – eliminate the portion of the formation which contains large quantities of shale. Vshcutoff ≈ 20 to 30 % 2. Porosity – eliminate the portion of the formation which is low porosity (and low permeability) and therefore would be non-productive. Sandstones cutoff ≈ 7% gas cutoff ≈ 8% oil Carbonates cutoff ≈4% 3. Water saturation – eliminate the portion of the formation which contains large volumes of water in the pore space. Sandstones Swcutoff ≈ 60% Carbonates Swcutoff ≈ 50%

Reserve Estimation

Cutoff Values

Specific Correlation

• Applicable to U.S. gulf coast sands • Correlation between k, and Sw • Assumes oil/water viscosity ratio = 2.0

Reserve Estimation

Averaging

Porosity – thickness weighted average n h i i

i 1 n i 1

h1

1

h2

2

h3

3

h i

Water saturation – volume weighted average n S S

w

i 1 n

φ h wi i i

i 1

φ h i i

Reserve Estimation

V

b

h A 3 n

A n 1

or V b

h A 2 n

A n 1

Reservoir Volume

A *A n n 1

Reserve Estimation

Recovery Factor

Determined from: 1. displacement efficiency studies 2. correlations based on statistical studies of particular types of reservoir mechanisms

OIL volumetric reservoirs

1 S E

r

1

w 1 S

S

g Boi B w o

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

1 S E

r

w

1 S

S

or

w

Estimate Sor from the remaining oil in the cores after flushed by mud filtrate and expansion of gas…displacement process

Reserve Estimation

Recovery Factor

GAS Volumetric Reservoirs

B E

r

1

B

gi

ga

Hydraulic Reservoirs

B 1 S E

r

w

S

gr

1 S

gi B

w

ga

Reserve Estimation

Recovery Factor

From displacement of reservoir fluids (invasion) using well logs

Reserve Estimation

Recovery Factor

From displacement of reservoir fluids (invasion) using well logs Difference between initial oil saturation, Soi and the residual oil saturation, Sor, that remains after the formation is invaded by water: Som = Soi - Sor Inferring mud filtrate invasion as an efficient displacement mechanism, then Som = Sxo - Sw Recovery factor for water drive reservoirs,

S E

r wd

S xo w 1 S w

....for depletion drive, use rule of thumb of 1/2 of (RF)wd

Reserve Estimation

Recovery Factor

Statistical Performance Soln Gor

60

200

600

1000

2000

Oil gravity

maximum

Sandstones average

15 30 50 15 30 50 15 30 50 15 30 50 15 30 50

12.8 21.3 34.2 13.3 22.2 37.4 18.0 24.3 35.6 34.4 33.7 40.7

8.6 15.2 24.8 8.8 15.2 26.4 11.3 15.1 23.0 21.2 20.2 24.8

minimum

2.6 8.7 16.9 3.3 8.4 17.6 6.0 8.4 13.8 12.6 11.6 15.6

maximum

Carbonates average

minimum

28.0 32.8 39.0 27.5 32.3 39.8 26.6 30.0 36.1 32.6 31.8 32.8

4.4 9.9 18.6 4.5 9.8 19.3 6.9 9.6 15.1 13.2 12.0 14.5

0.6 2.9 8.0 0.9 2.6 7.4 1.9 2.5 4.3 4.0 3.1 5.0

Solution Gas Drive Reservoirs (Arps, 1962) Drive Water drive Solution gas drive without supplemental drives Solution gas drive with supplemental drives Gas cap drive Gravity drainage Gas depletion Gas water drive

minimum 27.8 9.5

Sandstones average 51.1 21.3

13.1

28.4

57.9

15.8 16.0 75.0 50.0

32.5 57.2 85.0 70.0

67.0 63.8 95.0 80.0

maximum minimum 86.7 6.3 46.0 15.5

9.0

Carbonates average maximum 43.6 80.5 17.6 20.7

21.8

48.1

Combined with sandstone Data not available

Recovery factor for different drive mechanisms

Reserve Estimation

Todilto

Entrada Ss

OWC

Example

Reserve Estimation

Example

Percent 0

20

40

60

80 100

5160

5170

Pay Todilto

OWC

depth,ft

Entrada Ss

5180

5190

OWC?

5200

5210 porosity

SW

5220

Porosity cutoff = 20% Water Saturation cutoff = 65%

Electrical Properties of Rocks

Parameters Area = 40 acres Boi = 1.2 rb/stb Calculated from log h = 18 ft ave = 27.5% Sw ave = 62% hSo = 1.889 Recovery Factor Sxo = 75 %

Example

OOIP = 489 Mstb

(Er)wd = 34 %

R = 167 Mstb

Actual cumulative production = 126 Mstb

Electrical Properties of Rocks

References

Chapter 11, Sec 11.4-11.7, Bassiouni, Z: Theory, Measurement, and Interpretation of Well Logs, SPE Textbook Series, Vol. 4, (1994)