ABSTRACT
Methane production from coal seams has been actively pursued in the U.S. since the mid-1970's. This resource is regarded as an unconventional natural gas supply due to several unique characteristics of the reservoir, including: two-phase (water and gas) flow, low effective permeability to gas, and the majority of the methane being held in an adsorbed state on the internal surfaces of the microporous coal structure rather than as compressed or dissolved gas within the pores.
In a virgin coal reservoir with ideal conditions the adsorbed methane concentration is in equilibrium with the initial reservoir pressure. Production of gas from the coal bed depends on the release of this adsorbed methane to a free state, which then flows through the natural fracture system of the coal towards the wellbore. Release of the adsorbed methane is accomplished by dewatering the reservoir which decreases the local pressure and thereby upsets the equilibria.
In the past, researchers have utilized laboratory-measured adsorption isotherms to model the gas concentration/reservoir pressure equilibrium relationship. In the current study both adsorption (pressure increasing) and desorption (pressure decreasing) isotherms were determined for several coal samples. It was found that there are hysteresis effects which cause the two sorption modes to be different. The desorption isotherms were more non-linear than the adsorption isotherms. In terms of reservoir performance this means that coalbed reservoirs must be drawn down to pressures lower than those indicated by adsorption isotherms to achieve the same degree of methane release.
This paper presents a review of methane/coal adsorption phenomena and compares results of the present work with those of previous investigators. Experimental procedures are given along with error analyses and substantiation of repeatability. Variability of gas content within the coal seam is determined for a deep western coal from the Piceance Basin in Colorado. The effect of moisture content on methane sorptive capacity is investigated for an eastern coal from the Black Warrior Basin in Alabama. All samples and funding were provided from Gas Research Institute's R&D field sites.
