Helium expansion revisited: Effects of accessible volume on excess adsorption in kerogen matrices

Abstract

Enhancing our understanding of the excess adsorption capacity in shale gas reservoirs is paramount for accurately predicting production capabilities and refining extraction processes. A significant factor in this calculation is the accessible volume, which can only be measured indirectly using helium as a proxy. In this study, hybrid grand canonical Monte Carlo/molecular dynamics (GCMC/MD) simulations were employed to scrutinize the accessible volume and quantify the excess adsorption capacities of various gases in kerogen matrices, characterized by diverse micropore distributions at 363.15 K (90 °C) and pressures up to 50 MPa. We evaluated multiple approaches to determine accessible volume in simulations, emphasizing the importance of selecting a probe size that reflects the actual size of the adsorbate. The simulation outcomes reveal that accessible volumes derived from the helium expansion method, mimicking the traditional experimental techniques, tend to be overestimated by around a factor of two. This finding challenges the reliability of such measurements and suggests a need for their recalibration based on computer simulation models. Furthermore, when our simulations were compared with various theoretical adsorption isotherm models, the more advanced Supercritical Dubinin-Radushkevich and Supercritical Brunauer-Emmett-Teller models demonstrated better accuracy in predicting absolute adsorption values compared to the more conventional Langmuir model. However, neither model accurately predicted the absolute adsorption quantities, indicating room for improvement. Finally, the simulations underscore the significant adsorption capacity of CO2 compared to other gases, highlighting its promising role in facilitating enhanced gas recovery and geological sequestration within shale formations.