Triple-continuum modeling of shale gas reservoirs considering the effect of kerogen

Abstract

Shale gas storage and transport mechanisms are notably different in kerogen systems and inorganic matrix systems. Based on these complex shale gas transport mechanisms, including viscous flow, Knudsen diffusion, surface diffusion and gas adsorption/desorption on the internal kerogen grain surfaces, a kerogen – inorganic matrix – fracture triple-continuum model is established. There are two transfer terms in the triple-continuum model in this paper. The kerogen – inorganic matrix transfer flow is simulated by the Warren-Root pseudo-steady state (PSS) transfer model, while the inorganic matrix – fracture transfer flow is simulated by the Vermeulen transient transfer model.To investigate the impact of the kerogen continuum on shale gas reservoir performance, a comparison between the matrix – fracture dual-continuum model and kerogen – inorganic matrix – fracture triple-continuum model is conducted. The matrix – fracture transfer flow in the dual-continuum model is also simulated by the Vermeulen transient transfer model. In addition, two triple-continuum models with different inorganic matrix – fracture transfer models are compared to investigate the impact of the transfer model. One triple-continuum model uses the Warren-Root PSS transfer model. The other uses the Vermeulen transient transfer model. The mathematical model is solved by the PDE module of COMSOL Multiphysics. A sensitivity analysis of parameters affecting shale gas production, including kerogen pore volume, kerogen permeability, inorganic matrix permeability, fracture permeability and Langmuir parameters, is conducted.The results indicate that dividing the matrix system into a kerogen continuum and inorganic matrix continuum significantly influences shale gas reservoir performance. Not considering the kerogen continuum could lead to an overestimate in cumulative gas production of approximately 8%. The triple-continuum model that uses the Vermeulen transient transfer model yields a higher recovery than that which uses the Warren-Root PSS transfer model. Moreover, natural fractures are the main permeable channels in shale gas reservoirs and play a more important role than the kerogen and inorganic matrix in shale gas recovery. Langmuir pressure and Langmuir volume also have significant effects on the cumulative production of desorbed gas, free gas and total gas, with the effect of Langmuir volume being relatively larger. In conclusion, a triple-continuum model with a transient transfer term should be incorporated into the numerical simulators of shale gas reservoirs to predict shale gas production more accurately.