Modeling of multi-scale transport phenomena in shale gas production — A critical review

Shale gas, although unconventional, is a prospective clean energy source. Shale gas production is a complex multi-scale process with its spatial size ranging from the nanoscale to kilometer-scale. During shale production, the gas transport process involves the diffusion of dissolved gas molecules into the matrix bulk, desorption of adsorbed gas from the micropore surface, Knudsen diffusion and slip flow of free gas in the pore, and Darcy flow or even high-speed non-Darcy flow of free gas in the fracture network. Accordingly, understanding the shale gas transport process in the shale reservoirs poses a long-standing problem to researchers and engineers. Computational modeling offers an opportunity to effectively reveal the gas multi-scale transport mechanisms and accurately predict the amount of shale production. In this review, the shale gas transport process during shale gas production is firstly introduced. Thereafter, the multi-scale transport phenomena involving shale gas molecule desorption from the shale matrix at the atomic and molecular level, diffusion in the nanopore, diffusion and seepage into the micropore, and convection and mass flow in the mesoscopic pores and macropore are elucidated. Moreover, the corresponding multi-scale simulation models that describe the above phenomena and shale production are explained. The shale gas production genome model, which provides insights into the entire process of the shale gas production model, is proposed and clarified according to the multi-scale simulation models used in the shale gas production prediction. The shale gas production genome model is convenient for elucidating shale transport mechanisms and guiding shale gas reservoir exploitation.