ORIGINAL GAS IN PLACE: ABNORMALY PRESSURED GAS RESERVOIR

OGIP for Overpressured & Compressible Reservoir

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
Aquifer-to-reservoir size ratio (reservoir size = 1)
Rock compressibility 10-6psi-1
Water compressibility 10-6psi-1
Shale bulk compressibility 10-6psi-1
Shale fraction fraction
Reservoir porosity fraction
Interstitial water saturation fraction
Gas gravity (air = 1)

     Reset

OUTPUT   VARIABLES   &   GRAPHS

Original gas in place (G) MMscf
Gas recovery factor (R) %

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

Original Gas In Place For Geopressured Gas Reservoir

The original gas in place (OGIP) of a non-volumetric and overpressured dry gas reservoir can be estimated from early production data. An abnormally pressured gas reservoir is defined as one where the reservoir pressure is greater than the normal pressure gradient (0.5 psi/ft), usually > 6.5 psi/ft. A non-volumetric reservoir is one in which in which one or more of the following [aquifer impact, rock, shale and water compressibilities] is considered signficant.

The method proposed by Bourgoyne requires a plot of a "function of P/Z", Y{P/Z}, versus cumulative gas production , followed by the fitting of a straight line through the plot. The straight line should ideally pass through the origin. Extrapolation of this line to an abscissa value of Pi/Zi (ie. effectively P=0) will provide a corresponding x-axis value that is the estimated initial gas in place.

The governing equation is given below:

    

    

where
     P = reservoir pressure, psia
     Pi = initial reservoir pressure, psia
     Z = gas compressibility factor (dimensionless)
     Zi = initial gas compressibility factor
     Y = a function of (P/Z) as defined by Bourgoyne
     G = original gas in place, MMscf
     Gp = cumulative gas produced
     F = aquifer size relative to gas reservoir size
     Cr = rock compressibility, psi-1
     Cw = water compressibility, psi-1
     Cs = bulk shale compressibility, psi-1
     Ce = effective compressibility, psi-1
     Sw = rock compressibility, psi-1
     R = gas recovery factor at abandonment pressure

The least-squares technique is employed in the straight-line fitting process. The intercept should ideally pass close to zero (origin), i.e neglible compared to the estimated gas in place. Otherwise the assumptions regarding the input data, particularly aquifer size, need to be verified.

The Gp at any pressure and Gas recovery factor can be derived from the straight line. z-values are determined from 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 5 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