Adsorption-Induced Pore Volume Deformation: Implications for Excess Adsorption in Kerogen Matrices

Accurately predicting gas storage in shale requires understanding how adsorption alters the porous structure of kerogen, the main organic component of the rock. Gas adsorption can induce structural deformation in the microporous framework of kerogen, thereby modifying the observed excess adsorption isotherms. However, no comprehensive investigation has addressed how accessible volume evolves during adsorption. To address this gap, we employ a hybrid grand canonical Monte Carlo/molecular dynamics simulation of both immature and overmature amorphous kerogen matrices at 363.15 K and pressures up to 50 MPa, providing molecular-level insights into this adsorption-deformation process. Among the investigated gases, C₂H₆ displays the most pronounced reduction in excess adsorption at low pressures, attributed to its large molecular size. At higher pressures, excess adsorption decreases in the order CO₂ > C₂H₆ > CH₄ > N₂, corresponding to roughly 50, 40, 30, and 20% reductions, respectively. Moreover, kerogen can undergo 2-9% strain at pressures up to 50 MPa, depending on gas type and kerogen structure. The Tóth model, which is a physical model that allows extending the Langmuir adsorption model to heterogeneous systems, further demonstrates that neglecting deformation-induced changes in accessible volume can lead to substantial errors when converting excess adsorption to absolute adsorption. These findings underscore the dynamic and deformable nature of kerogen, challenging the longstanding assumption of a rigid kerogen molecular structure. This has direct implications for accurately estimating gas-in-place in shale gas reservoirs and assessing storage capacity for subsurface carbon sequestration.