Gas storage capacity estimates of shales are routinely assessed using laboratory data from unconfined methane sorption and porosity measurements. In this study, the stress dependence of the methane excess sorption capacity and specific pore volume are investigated simultaneously. Experiments were performed on dry core plugs (Cambrian–Ordovician Alum, Jurassic Bossier, Late Cretaceous Eagle Ford, and Jurassic Kimmeridge shales) at 30°C under controlled confining stress up to 40 MPa and gas pressures up to 20 MPa.
Increasing overburden stress results in a significant decrease of both specific pore volume and excess sorption capacity. The stress sensitivity of the specific pore volume was most prominent for the total organic carbon (TOC)–rich Kimmeridge sample (45% TOC) and further decreased in the order of Bossier, Eagle Ford, and Alum. Stress dependence of the methane excess sorption capacity, expressed as percentage reduction at 40-MPa overburden as compared to unconfined conditions, decreases in the order Eagle Ford (∼56%), Bossier (∼30%), Kimmeridge (∼14%), and Alum (∼5%). Although the decrease of specific pore volume is definitely caused by poroelastic compression, the mechanism(s) leading to the reduction of excess sorption capacity with stress require further investigation.
Gas storage calculations show that routine methods based on unconfined data may grossly overestimate the total storage capacity. In this scenario, at 2500-m depth, the total gas storage capacity will be overestimated by 5% for the Alum, 28% for the Bossier, 18% for the Eagle Ford, and 28% for the Kimmeridge if the stress dependent reduction of volume and sorptive storage capacity is not considered.