Methane adsorption on shale under in situ conditions: Gas-in-place estimation considering in situ stress

Abstract

Gas-in-place (GIP) is one of the essential indices for the resource evaluation of shale gas storage capacity. In this study, based on the simplified local density (SLD) theory, the shale adsorption capacity was investigated considering the effect of burial depth on the shale pore structure. Shale isotherm adsorption experiments under triaxial conditions were performed with a homemade adsorption test device, and CH4 adsorption isotherms under different shale reservoir conditions representative of different burial depths were obtained. By establishing the relationship between the SLD model regression parameters and burial depth, a shale GIP estimation model considering the influence of in situ stress was established. In situ stress controls the pore structure of shale and consequently affects the gas storage capacity of shale. The conventional method does not consider the influence of in situ stress, resulting in overestimation of the free gas content, underestimation of the adsorbed gas content, and overall overestimation of the GIP by 1.12–1.26 times. Taking into account the influence of the stress state, the contribution of the free gas content to GIP is weakened. In contrast, the adsorbed gas content plays a key role in GIP. The distribution of the shale reservoir GIP concentrates at depths of 2500–4500 m. These findings will be helpful in the future evaluation of shale gas reserves and improvement in shale gas content evaluation methods.