COALBED METHANE: ORIGINAL GAS IN PLACE BY MODIFIED P/Z* METHOD

CBM: OGIP By Modified P/Z* Method

INPUT   DATA ♥ EXAMPLE Of Input/Output

Title  

#  Cumulative gas
produced  		Reservoir
pressure 
  (MMscf)  (psi) 
1 0
2
3
4
5
6
7
8
9
10  
11
12
13
14
15
16
17
18
19
20

Reservoir temperature °F
Abandonment pressure psi
Langmuir pressure constant psia
Langmuir volume constant scf/ton
Mass fraction ash fraction
Bulk coalbed density gm/cc
Reservoir porosity fraction
Average water saturation fraction
Gas gravity (air = 1)
Average thickness ft

     Reset

OUTPUT   VARIABLES   &   GRAPHS

Original gas in place (G) MMscf
Gas recovery factor (R) %
Drainage Area (A) Acres

#  Cumulative gas
produced  		 Reservoir
pressure 		 ZZ*P/Z*
  (MMscf)  (psi)    
1
2
:
:
19
20
THEORY  &   FORMULAE

Original Gas In Place For A Coalbed Methane Reservoir

The original gas in place (OGIP) for CBM reservoir can be estimated from early production data. The method proposed by Seidel is a modification of an earlier work by King and the classical P/Z method. The method requires a plot of P/Z* versus cumulative gas production, followed by the fitting of a straight line through the plot and extrapolation of this line.

The CBM model is the convention gas material balance equation extended to take account of the adsorption of gas in the coal seams. Gas adsorption is modeled with the Langmuir's adsorption isotherm (the amount of gas in equilibrium with the the rock surface as a function of pressure at a constant temperature). Langmuir volume parameter is the maximum gas capacity of the coal, and the Langmuir pressure parameter is the pressure at which the coal absorbs half of its maximum gas capacity.

The governing equations are given below:

    

where
     p = reservoir pressure, psia
     pi = initial reservoir pressure, psia
     psc = pressure @ standard condition (14.7), psia
     Z = standard gas compressibility factor (dimensionless)
     Zi = initial gas compressibility factor
     Zsc = standard gas compressibility @ standard condition
     Z* = modified gas compressibility factor (dimensionless)
     Z*i = initial modified gas compressibility
     Z*sc = modified gas compressibility @ standard condition
     G = original gas in place, MMscf
     Gp = cumulative gas produced
     Sw = average water saturation, fraction
     T = reservoir temperature, °R
     Tsc = temperature @ standard condition, 520°R
     VL = Langmuir volume constant of the coal, scf/ton
     PL = Langmuir pressure constant, psia
     a = mass fraction ash in coal
     ρ = bulk coalbed density, gm/cc
     φ = porosity of coal, fraction
     h = reservoir thickness, ft.
     A = drainage area, ft2
     R = gas recovery factor at abandonment pressure

Via the least-squares technique, a straight-line is fitted through the plot of p/z* versus Gp. The intercept at [p/z*]=0 gives the OGIP, G. Also, the Gp at any pressure and Gas recovery factor can be derived from the line, including the recovery factor at abandonment, and the drainage area. z-values are determined from Hall & Yarborough correlations based on measured gas gravity, temperature and pressure.

Tips

    ◊ Use link EXAMPLE Of Input/Output  to demo data entry expectations and results; you may edit & use it as starting point
    ◊ Between 3 to 20 measurement points may be entered
    ◊ In the output plot, the blue points represents observed data, yellow the abandonment point and green the OGIP.
    ◊ If the required Java plug-in not installed on your computer, an auto-download of this plug-in will be initiated before the schematic is displayed.

BIBLIOGRAPHY