The Importance of
Water Influx in Gas Reservoirs
SPE 1244 example
Introduction
• The presence of water drive mechanism is one of the main concerns when
dealing with gas reservoirs
• Gas recovery under water drive highly depends on the following factors:
– Production Rate and the manner of production
– The Residual Gas Saturation (Sgr)
– Aquifer Properties
– Volumetric displacement efficiency of water invading the gas reservoir
• The General Gas Material Balance Equation:
must be coupled with the end-point condition:
2
Worked Example (SPE 1244)
• The Reservoir and aquifer properties are as follows:
–
–
–
–
–
–
–
–
–
–
–
–
Reservoir Area:
5000 Acres
Porosity:
Net thickness:
Initial Water Saturation:
Formation Temperature:
Gas Gravity:
Reservoir radius:
Water Formation Volume Factor:
Water Viscosity:
Formation Permeability:
Initial Pressure:
Residual Gas Saturation:
15%
100 ft
25%
211 F
0.65
8340 ft
1.02
0.35 cp
5 md
7000 psia
35%
• The objective of the exercise is to estimate the Gas RF in the case of a large
aquifer (RD=10) as a function of Volumetric Seep Efficiency (Ep=65%, 75%
and 85%) and gas production rates
3
Water Influx in Gas Reservoirs
Sgr =35%
Eff =100%
MBAL RUNS up to Sgr
NO WATER PRODUCTION ALLOWED
Depletion drive
4
The End Point Condition
Effect of Volumetric Sweep efficiency (Ep)
GAS WATER DRIVE
SPE1244
8000
Limit Sgr=35% Eff= 65%
7000
Limit Sgr=35% Eff=75%
Limit Sgr=35% Eff=85%
NO AQUIFER
6000
RD=10 QG=50
RD=10 QG=30
5000
P/Z
RD=10 QG=10
4000
RD
3000
Sgr
%
Ep
%
QG
MMSCFD
10
30
50
70
10
30
50
70
10
30
50
70
65%
2000
10
35%
75%
1000
85%
Gas RF
%
40%
48%
54%
59%
46%
54%
60%
65%
50%
59%
65%
72%
0
0
100
200
300
400
500
600
700
800
Cumulative Gas
5