This article will demonstrate a material balance equation in gas cap drive mechanism. First, we start with a full material balance equation.
- No water production
- No water injection
- No gas injection
- No water influx
- Neglect formation and connate water compressibility (Cf and Cw have little effect for a gas cap drive mechanism.)
The material balance shown in Figure 2 can be simplified like this:
For a gas cap drive, oil and gas production is equal to oil + dissolved gas expansion pus gas cap expansion.
This can be described in a simple equation (Havlena + Odeh (1963)) like this:
F = N (Eo + m Eg)
F = Net fluid production
N = Oil in place
Eo = oil expansion term
m = initial gas cap volume / initial oil volume (rb/rb)
Eg = gas expansion term
The plot is constructed between F and (Eo+mEg).
If a plot shows a straight line slope, it means that gas cap volume is correct. Any deviation from a straight line indicates either a too small or too big gas cap size as shown in Figure 3.
N = oil initially in place (STOIIP) in reservoir (stb)
Np = cumulative oil production (stb)
Boi = oil volume factor at initial reservoir pressure (rb/stb)
Bo = oil volume factor at current reservoir pressure (rb/stb)
Rsi = solution GOR at initial reservoir pressure (scf/stb)
Rs = solution GOR at current reservoir pressure (scf/stb)
Rp = cumulative produced gas oil ratio (scf/stb)
G = gas volume initially in place (GIIP) in reservoir (scf)
m = ratio of initial gas cap volume to initial oil volume (rb/rb)
Bgi = gas volume factor at initial reservoir pressure (rb/scf)
Bg = gas volume factor at current reservoir pressure (rb/scf)
Swc = connate water saturation (fraction or %)
Cw = water compressibility (1/psi)
Cf = formation (rock) compressibility (1/psi)
Wp = cumulative water production (stb)
We= cumulative water influx from aquifer (rb)
Bw = water volume factor at initial reservoir pressure (rb/stb)
Wi = cumulative water injection (stb)
Gi = cumulative gas injection (scf)
Gp = cumulative gas production (scf)
Eg = gas expansion term (rb/stb)
Eo = oil expansion term (rb/stb)
Efw = formation and connate water expansion term (rb/stb)
Abhijit Y. Dandekar, 2013. Petroleum Reservoir Rock and Fluid Properties, Second Edition. 2 Edition. CRC Press.
L.P. Dake, 1983. Fundamentals of Reservoir Engineering, Volume 8 (Developments in Petroleum Science). New impression Edition. Elsevier Science.
Tarek Ahmed PhD PE, 2011. Advanced Reservoir Management and Engineering, Second Edition. 2 Edition. Gulf Professional Publishing.