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INPUT   DATA

♥ EXAMPLE Of Input/Output

Title

Initial reservoir pressure		psia
Reservoir temperature		°F
Abandonment pressure		psia
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)

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OUTPUT   VARIABLES   &   GRAPHS

PLOT OF RESERVOIR PRESSURE Vs GAS RECOVERY

Original Gas In Place For A Coalbed Methane Reservoir

The Material Balance Equation can be used for estimating future gross reservoir performance, given a set of initial conditions including the initial gas in place. The result is usually in the form of average pressure response to gas withdrawal over time.

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 version of CBM MBE proposed by by Seidle is a modification of an earlier work by King . A simplified version of the CBM MBE was also developed recently by Cooper.

Seidel-King equation can be re-arranged as follows:

And Cooper's equation can be re-arranged as follows:

where
     p = reservoir pressure, psia
     p_i = initial reservoir pressure, psia
     p_sc = pressure @ standard condition (14.7), psia
     Z = standard gas compressibility factor (dimensionless)
     Z_i = initial gas compressibility factor
     Z_sc = 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
     G_p = cumulative gas produced
     S_w = average water saturation, fraction
     T = reservoir temperature, °R
     T_sc = temperature @ standard condition, 520°R
     V_L = Langmuir volume constant of the coal, scf/ton
     P_L = Langmuir pressure constant, psia
     a = mass fraction ash in coal
     ρ = bulk coalbed density, gm/cc
     φ = porosity of coal, fraction
     R = gas recovery factor at abandonment pressure

Tips

    ◊ Use link EXAMPLE Of Input/Output  to demo data entry expectations and results; you may edit & use it as starting point
    ◊ In the output plot, the yellow line the abandonment pressure whose intersections with the blue and red curves lead to the corresponding recovery factors at abandonment.
    ◊ 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

• Seidle J.P.; A modified p/Z method for coal wells; paper SPE-55605, SPE, Richardson, Texas, 1999.
• King G.R.; Material-balance techniques for coal-seam and Devonian shale gas reservoirs with limited water influx; SPE Reservoir Eng., Feb. 1993, p 67-72.
• Cooper J.E.; Simplified prediction of reservoir pressure in coalbed methane wells; paper SPE-78693, SPE, Richardson, Texas, 2002.
• Saghafi A.; Coal seam gas reservoir characterisation; Gas from Coal Symposium, Brisbane, Australia, 27-Mar-2001, 14 pages.