
Application of the Law of Conservation of Mass to
oil and gas reservoirs
(Original hydrocarbon mass)-(produced
hydrocarbon mass)=(remaining hydrocarbon
mass)
Gp
GBgi
Initial Conditions
P=Pi
(G-Gp)Bg
Later Conditions
P<Pi

Understand reservoir performance
◦ Identify drive mechanism

Predict future performance
◦ Estimate OGIP
◦ Predict productions rates/pressure decline
◦ Estimate ultimate recovery
Measured
pressure
Cumulative gas
Production @ P
p pi  G p 

 1 
z zi 
G 
4000
(p/z)i
3500
Last measured
data point
Gas property, f(T,P,g)
p/z, psia
3000
2500
2000
extrapolate
1500
1000
(p/z)a
500
G=10 Bscf
Gpa=8.5 Bscf
0
0
1
2
3
4
5
6
7
8
9
10
11
12
Cumulative gas produced,Bscf
GRM-Engler-09
1.
Collect data:
a)
b)
c)
2.
3.
4.
5.
6.
7.
8.
Initial reservoir pressure
Various reservoir pressures throughout the life of the well
Associated cumulative production at each pressure
Determine z-factor for each pressure
Calculate P/z
Plot P/z versus GP
Draw a straight line through the data points.
Extrapolate the straight line to P/z = 0
Obtain estimate of OGIP @ P/z=0: Gp must equal G
Estimate ultimate recovery @ abandonment pressure
P (psia)
Gp (MMscf)
z
P/z
4000
0
0.8
5000
3500
2.46
0.73
4795
3000
4.92
0.66
4545
2500
7.88
0.6
4167
2000
11.2
0.55
3636
Step 1
Step 2 Step 3
Step 6
4
5
Step 8
Step 7





Water drive reservoir vs. gas expansion
Abnormally pressured reservoirs (formation
compressibility)
Pressure measurements/calculations
Low permeability
Retrograde gas reservoir
Geopressured
component

Gas in solution

p
1  c e( p ) ( p i  p ) 
z
5.615
 p  p / z i 
Wp B w  WinjB w  We
G p  G inj  Wp R sw 
  
G 
Bg
 z i

Gas injection




Water drive
component
GRM-Engler-09
Cumulative water influx, rcf


G p Bg  We  5.615B w Wp
G
Bg  Bgi
Cumulative water
production, stb
strength
(p/z)i
(p/z)a
water drive
p/z
RF (water drive) < RF (depletion)
45 to 75%
>75%
(p/z)a
Depletion drive
Gp
GRM-Engler-09
p  G 
i 1  p 
z 
G 
p
i


z 
 p  p 
1

c




e i
Gas expansion
+
Formation compaction
+
Water expansion
(p/z)i
p/z
Where,
c S  c 




w
wi
f
c 
1 S
e
wi
Gas expansion
Overestimate of G
Gp
GRM-Engler-09