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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

Gas gravity		(air=1)
Reservoir temperature		°F
Abandonment pressure		psi

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OUTPUT   VARIABLES   &   GRAPHS

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

#	Cumulative gas produced	Reservoir pressure	Z	P/Z
	(MMscf)	(psi)		(psi)
1
2
:
:
19
20

Determination Of Original Oil Place For A Volumetric Gas Reservoir

The original gas in place (OGIP) in a volumetric dry gas resevoir can be estimated from early production data. 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. A volumetric reservoir is one in which is it is assumed that: aquifer impact is negligible and rock, shale and water compressibilities are negligible compared to the compressibility of the gas.

The applicable Material Balance equation is:

where
     p = reservoir pressure, psia
     p_i = initial reservoir pressure, psia
     z = gas compressibility factor (dimensionless)
     z_i = initial gas compressibility factor
     G = original gas in place, MMscf/day
     G_p = cumulative gas produced
     R = gas recovery factor at abandonment pressure

Via the least-squares technique, a straight-line is fitted through the plot of p/z versus G_p. The intercept at [p/z]=0 gives the OGIP, G. Also, the G_p at any pressure and Gas recovery factor can be derived from the 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.

BIBLIOGRAPHY

• Ikoku C.U.; Natural Gas Reservoir Engineering; Krierger Publ. Co., Malabar, Florida, 1992, chap. 1.
• Dake L.P.; Fundamentals Of Reservoir Engineering; Elsevier Scientific Publ. Co., Amsterdam, Netherlands, 1978, chap. 1.
• Walsh M.P., Ansah J & Raghavan R.; The new generalized material balance as an equation of a straight line: Part 1 - Applications to understurated volumetric reservoirs; paper SPE-27684, SPE, Richardson Texas, 1994.