Abstract:

Reservoir space in unconventional coal-seam gas reservoirs,also known as coalbed methane reservoirs,consists of the primary matrix porosity system(micropores) and the secondary fracture porosity system(macropores).Some of coal-seam gas are stored in the matrix system in an adsorbed state and the rest are in the fracture system as free gas.For saturated coal-seam gas reservoirs whose initial reservoir pressure equals to the critical desorption pressure,King proposed a material balance equation by using the balance fundamental of gas mole numbers in the process of gas recovery.However,the equation was failed to be put into real practice due to its inherent limitations.Based on King's research,Ahmed proposed a linear relationship formula which may be used to determine the gas-bearing area of constant-volume saturated coal-seam gas reservoirs and has already been employed in practice.By combining the studies of King and Ahmed with the basic fundamental of gas reservoir engineering,the article presents a new linear relationship formula that may be to used to determine adsorption gas in-place and free gas in-place simultaneously.In addition,methods for determination of recoverable reserves and recovery ratio are also proposed.The practicability and effectiveness of these methods have been testified during their application.

Key words: adsorbed gas, free gas, OGIP, recoverable reserve, recovery ratio, coal-seam gas reservoir