Studies into confined methane adsorption in shale nanopores using a simplified local density model

Abstract

The large number of nanopores in shale reservoirs provide an effective storage space for gases, which can assist in accurately characterizing the confined adsorption characteristics, including the adsorption amount of fluid in nanopores that is important for a detailed understanding of shale gas storage mechanisms and production prediction. Although the simplified local density (SLD) model has been widely used to describe the behavior of confined adsorption in slit-like pores, its application in relation to cylindrical pores has not been studied in detail. In this study, the importance of fluid–fluid interaction and fluid–solid interaction has been analyzed, with the expressions for the distribution of confined attractive parameter in cylindrical pores are derived by combining with the (3/8)σff assumption to establish a new SLD model. The reliability of the new model was verified by the publicly available experimental methane excess adsorption data for shale at different temperatures and the simulation results from the Grand Canonical Monte Carlo (GCMC). Moreover, by comparing with the SLD model with the slit pore structure, it was found that the adsorption isotherms and the confined fluid density distribution pattern characterized by the SLD model with the cylindrical pores were closer to the actual reservoir conditions of shale. In addition, the densities characterized by SLD models with a (3/8)σff assumption and with a (1/2)σff assumption at different Λb have been discussed and analyzed, and it is found that the selection of a suitable Λb would help to improve the accuracy of SLD model. The results might provide new theoretical insights into confined methane adsorption, with the new SLD model providing better engineering applications in shale gas development, gas storage, and pore structure characterization.