Coal reservoir saturation: Impact of temperature and pressure

Abstract

Methane adsorption isotherms measured for a series of coals with varying rank at a wide range of temperatures and pressures allows the prediction of the change in sorption capacity as a function of tectonic history. Changes in sorption capacity in response to declining pressure and temperature associated with uplift may increase or decrease the capacity of the coal and, if the coal is initially saturated, result in excess gas or a deficiency of gas (undersaturation). Assuming reasonable geothermal and pressure gradients, our data indicate that the sorption capacity will generally decrease with uplift and associated exhumation, suggesting that an initially gas-saturated coal will desorb gas during uplift of the reservoir. The desorbed gas would be available for migration and/or, potentially, resaturation of an undersaturated coal. Our results argue against the generally accepted theory that undersaturation of coal reservoirs results from an increase in the sorption capacity with uplift except for coals at such high pressures that the isotherms are essentially flat or for very high pressure and geothermal gradients.