Correction for and Geologic Significance of the Abnormal Adsorption of Methane to Shales

Abnormal adsorption phenomena are widely observed when applying isothermal adsorption to evaluate the adsorptive capacity of shales. Under experimental conditions, the isotherms of moisture-equilibrated samples initially behaved like Type I isotherms, then crossed the abscissa when saturated, indicating a limited adsorption capacity. As the pressure increased, the free gas density increased and the error associated with the adsorbed phase decreased the apparent value. Applying the Gibbs excess adsorption model provides a simple correction for the saturated adsorptive capacity. Compensating for the saturated adsorptive capacity reveals the true adsorptive capacity of shale gas reservoirs, which allows a more accurate estimate of the reservoir resources, as the adsorbed gas content calculated based on the apparent value would underestimate shale gas reserves. Deeply buried shale gas reservoirs will have a higher percentage of free gas, generally >60% if emissions are omitted while adsorbed gas is more easily retained in the reservoir than free gas.