Methane Adsorption Characteristics Under In Situ Reservoir Conditions of the Wufeng-Longmaxi Shale in Southern Sichuan Basin, China: Implications for Gas Content Evaluation

The accuracy of adsorbed gas content under actual in situ reservoir conditions is crucial for the evaluation of shale gas reserves. In this study, the characteristics of methane adsorption on the Wufeng-Longmaxi shale were investigated under a wide range of pressure (0-51 MPa) and actual in situ water saturation. Methane-shale adsorption exhibits the Gibbs excess adsorption phenomenon at high pressure. The excess adsorption amount needs be corrected to absolute adsorption amount, otherwise it will be seriously underestimated. The optimal supercritical methane adsorption model was determined by the corrected Akaike's Information Criterion method. The methane adsorption amount of shale samples ranged from 1.521 to 4.079 m(3)/t. Adsorption capacity was dominated by the total organic carbon content as well as micropore volume and total specific surface area. Additionally, pore volume and specific surface area were contributed mainly by abundant micropores associated with organic matter. Thermodynamic parameters revealed that the adsorption of methane on shale was an exothermic process. As the temperature increased from 40 to 80 degrees C, the methane adsorption capacity decreased from 4.27 to 2.99 m(3)/t, a 30% decrease. The actual in situ water saturation correlated primarily with clay content, regardless of clay types, and increased with clay content. The formation of an adsorbed water film and the blockage of pores for gas adsorption by clusters of water molecules significantly lowered the adsorption capacity. The relative difference in the adsorption capacity increased with water saturation, with the adsorption capacity of water-bearing shales decreasing by 21-84% at a water saturation of 30-71% compared to dry shales.