Methane/ethane adsorption behavior in shale nanopore systems with mesopores and micropores: Evaluating micropore contribution

Abstract

Shale gas has garnered significant attention as a clean and high-quality fuel resource. Shale formations exhibit broad pore size distributions, with micropores (< 2 nm) and mesopores (2–50 nm), showing different gas sorption behaviors. The sorption behavior in kerogen nanopore systems with interconnected micropores and mesopores remains poorly understood. This study introduces three kerogen nanopore systems—low-density, middle-density, and high-density—each featuring a 7.5-nm mesopore and numerous micropores. Using Grand Canonical Monte Carlo (GCMC) simulations, the sorption behaviors of pure CH₄, C₂H₆, and their mixture (9:1) across a range of pressures (1 MPa to 13 MPa) and temperatures (313.15 K, 323.15 K, and 333.15 K) were investigated. The study identified three Zones: Zone I for the free gas phase, Zone II for adsorption in mesopores, and Zone III for absorption in micropores. The sorption isotherms were calculated by integrating the adsorption amounts, normalized by measured pore volume in the mesopore domain, and absorption amounts, normalized by total organic content. The calculated excess sorption isotherms across different kerogen nanopore systems aligned with experimental results, allowing us to estimate the micropore contribution. We calculated the actual density profiles and estimated the adsorption density in micropores and those on mesopore walls, which can be used for field applications. The selectivity in three zones was compared across three kerogen nanopore systems, showing that it was not so significantly influenced by the pore geometry at all temperatures and pressures. The absolute absorption in micropores and the micropore contribution to the total absolute sorption (in percentage) align consistently with micropore volume across different kerogen nanopore systems, revealing a linear relationship with micropore volume. This research provides recommendations for laboratory experiments and offers valuable insights into the microscopic distribution of shale gas in nanopore systems, emphasizing the significance of micropores in addition to mesopores.