Experimental determination of porosity and methane sorption capacity of organic-rich shales as a function of effective stress: Implications for gas storage capacity

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 degrees 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 (similar to 56%), Bossier (similar to 30%), Kimmeridge (similar to 14%), and Alum (similar to 5%). Although the decrease of specific pore volume is definitely caused by poroelastic compression, themechanism(s) leading to the reduction of excess sorption capacity with stress require further investigation.

Automated summary

Experimental results showed that increasing overburden stress significantly reduced both specific pore volume and excess sorption capacity. The stress sensitivity was most prominent in the total organic carbon (TOC)-rich Kimmeridge sample and decreased in the order of Bossier, Eagle Ford, and Alum. The stress dependence of the methane excess sorption capacity decreased in the order of Eagle Ford, Bossier, Kimmeridge, and Alum.