PETROLEUM DEVELOPMENT GEOLOGY RESERVES ESTIMATE

• The most important role of a DG is to:
– estimate the oil and gas reserves that may
be discovered in a particular venture.
– keep track of the reserves in all past
ventures.


1. Educated Guess and/or Comparison
with nearby production.
2. Static Reserves Estimates

Ü

Volumetric Calculations

3. Dynamic Reserves Estimates

Ü
Ü
Ü

Decline Curve Analysis
Material balance calculations
Reservoir Simulation


• Consider a region where production is from a
highly fractured tight formation or where
poroperm heterogeneity is unpredictable.

• Volumetric calculations are largely
meaningless.
• A way to estimate potential production from
a well is to consider those nearby.
• Generally, such a wildcat well will not
perform better than the nearest wells: best to
estimate cautiously


• Most accurate and widely used methods of reserves
estimation.
• Carried out by geologists as they are based on
geological structure and isopach maps.
• Rock volumes are established that are assumed to
contain hydrocarbons (e.g. seismic bright spot).
• Can be a simple volume calculation or a complex net
gas or net oil isopach approach, determined by
structure contours modified by fluid contacts and net
isopachs (net reservoir thickness map).
• Accuracy of volumetrics depends on data for porosity,
saturation, net thickness, areal extent, formation
volume factor, integrity of those data within a reservoir.


Volumetric Method
• RR = 7758 x A.t x φ(1 – Sw) x FVF x RF
•

Amount of oil in reservoir

̇ RR
̇ 7758

=
=

̇
̇
̇
̇
̇

=
=
=
=
=

A
t
φ
(1-Sw)
FVF
̇ Bo/Bg

̇ RF

•

Amount of recoverable oil

Recoverable Reserves
conversion from acreft to barrels (if vol. in
m3. this conversion number is eliminated)
area of porous rock, acre
thickness in feet
porosity,%
water saturation of reservoir
Formation Volume Factor (1/Bo & 1/Bg)

å reservoir volume / surface volume (vr / vs )

=

Recovery Factor


• Most rock volumes established through use of
net gas and net oil isopachs (net pay map).
• Constructed from superimposing of net isopach
map and structure contour maps then cut
(reduced) it with well defined OWC and/or GOC.
• Calculate the volume of net pay map by
planimeter (or digitizer table) and/or grid square
counting


HORIZON MAP
(Superimposed Structure and Net Isopach Maps)
0m

5m
10 m

15 m

10 m
5m
0m


NET PAY MAP


Rock Volume Calculations
2 methods :
1. PYRAMID

2. TRAPEZOIDS

A
h
n
t

:
:
:
:

area, m2 or acre
isopach/contour interval, m or ft
contour number (0 å n)
avg. thickness above the top of max. thickness



RF
Recovery Factor

• Usually RF determination is carried out by
Reservoir Engineer.
• Mainly based on the reservoir drive, rock
properties and fluid properties.
• For oil with effective water drive the
primary recoveries are in 25 – 40 % range
(max. 75%).
• For gas with gravity drainage, water drive
and depletion drive can provide RF > 80%.


Drive Mechanism

Solution-gas drive
Gas-cap drive
Water drive
Gravity-drainage
drive
Drive Mechanism

Volumetric reservoir
(Gas expansion drive)
Water drive

Average Oil Recovery
Factors,
% of OOIP
Range
Average
5 - 30
15
15 - 50
30
30 - 60
40
16 - 85
50
Average Gas Recovery
Factors,
% of OGIP
Range
Average
70 - 90
80
35 - 65

50

Decline Curve Analysis
(Reservoir Engineer’s jobs)

• After wells have been producing for a while:
– The rate of production is graphed
– Generally 6 months – 1 year after start of
production
• Good reserves estimates can be derived.
Often compared with volumetric technique
results.
• Can be done by well, by a group of well, by
block, by reservoir, by field


Decline Analysis Results
• Determine remaining recoverable reserves
under natural depletion rate.
• To forecast production under existing
conditions
• Limitation:
– The degree of the accuracy is depend on the
reliability of the production data.



Production Plots
1. A plot of log(q) vs t is
p
p

Linear if decline is exponential
Concave upward if decline is hyperbolic (n>0) or harmonic

2. A plot of q vs Np is
p
p

Linear if decline is exponential
Concave upward if decline is hyperbolic(n>0) or harmonic

3. A plot of log(q) vs Np is
p
p
p

Linear if decline is harmonic
Concave downward if decline is hyperbolic (n<1) or exponential
Concave upward if decline is hyperbolic with n>1.

4. A plot of 1/q vs t is
p
p
p

Linear if decline is harmonic
Concave downward if decline is hyperbolic (n<1) or exponential
Concave upward if decline is hyperbolic with n>1.

Example. Exponential decline

Rate, stb/d

.

10000

q = 6049.1e

-0.0524 t

1000

Slope=-D 1/quarter year
100
0

10

20

30

40

time (quarter year)

50

60


Example. Rate decline with production

7000
6000

q = -0.4301Np + 5768.7

q stb/d

5000
4000

Reserves

3000
2000

q abondonment

1000
0
0

2000

4000

6000

8000

10000

Cum. prod, MSTB

12000

14000


12000

40

Rate (stb/d)

30
8000
25
6000

20
15

4000
10
2000

5

0

0
0

2

4

6

8

Time (years)

10

12

14

16

Cum. Production (MMstb)

35

10000


Hyperbolic Decline curve
10000
9000
8000

q STB/D

7000
6000
5000
4000
3000
2000
1000
0
0

50

100

150

200
days

250

300

350


MATERIAL BALANCE
of a Petroleum Reservoir
(Mostly carried out by Reservoir Engineer)
General Concept of Material Balance.

a. Initial reservoir conditions.

From: Petroleum Reservoir Engineering
— Amyx, Bass, and Whiting (1960).

b. Conditions after producing Np STB of oil,
and Gp SCF of gas, and Wp STB of water.

Material Balance: Key Issues
̈
̈
̈
̈

Must have accurate production measurements (oil, water, gas).
Estimates of average reservoir pressure (from pressure tests).
Suites of PVT data (oil, gas, water).
Reservoir properties: saturations, formation compressibility, etc.