Interfacial Adsorption Kinetics of Methane in Microporous Kerogen

Abstract

Rapid declines in unconventional shale production arise from the poorly understood interplay between gas transport and adsorption processes in microporous organic rock. Here, we use highfidelity molecular dynamics (MD) simulations to resolve the timevarying adsorption of methane gas in realistic organic rock samples, known as kerogen. The kerogen samples derive from various geological shale fields with porosities ranging between 20% and 50%. We propose a kinetics sorption model based on a generalized solution of diffusive transport inside a nanopore to describe the adsorption kinetics in kerogen, which gives excellent fits with all our MD results, and we demonstrate it scales with the square of the length of kerogen. The MD adsorption time constants for all samples are compared with a simplified theoretical model, which we derive from the Langmuir isotherm for adsorption capacitance and the freevolume theory for steady, highly confined bulk transport. While the agreement with the MD results is qualitatively very good, it reveals that, in the limit of low porosity, the diffusive transport term dominates the characteristic time scale of adsorption, while the adsorption capacitance becomes important for higher pressures. This work provides the first data set for adsorption kinetics of methane in kerogen, a validated model to accurately describe this process, and a qualitative model that links adsorption capacitance and transport with the adsorption kinetics. Furthermore, this work paves the way to upscale interfacial adsorption processes to the next scale of gas transport simulations in mesopores and macropores of shale reservoirs.

Occurrence characteristics and controlling factors of transitional shale gas: a case study from Shanxi Formation, Eastern Ordos Basin, China

“…Compared to Type I kerogen, Types II2 and III have stronger adsorption capacity for methane (Chalmers and Bustin, 2008). The more aromatic structures in Type III kerogen enhance the binding ability between kerogen and methane molecules, thereby enhancing the adsorption capacity of shale (Tian et al, 2017;Wang et al, 2023b). This may be an important reason for the high gas adsorption capacity of transitional shale.…”