Review of micro seepage mechanisms in shale gas reservoirs

Abstract Accurate understanding of gas micro seepage mechanisms in shale plays is of great importance for numerical simulation and productivity prediction. Classical seepage theory fails to build the constitutive relation of fluid flow in shale matrix, while current models show discrepancies from one to another. In this review, current bulk phase gas transporting models in literatures are classified into eight types, where consistency and diversity among them are revealed, with three recommended for shale gas seepage modeling on different purposes. The unification contributes to understand the role of different seepage mechanisms and helps to eliminate the confusion of different roles that each mechanism may play during mathematical modeling and gas production. Different approaches of handling geo-mechanical effects, ad-/de-sorption, real gas effects and rarefaction effects are introduced and compared, with recommended methods given after comparison. Based on practical pore size distribution, in-situ pressure and temperature, five ways of calculating gas mean free paths are compared, after which the possible flow regimes in practical shale plays are divided according to Knudsen number. Various empirical Klinkenberg correction methods and slip boundary conditions are summarized and compared, showing that some are inappropriate to describe rarefaction effects. Different ways of modeling Knudsen and surface diffusion are reviewed and analyzed. Considering the role of viscous flow, slippage effect, Knudsen diffusion and surface diffusion, gas transporting models are categorized into three types. The review provides the recent development and a systematic summarization on analytical modeling of gas transporting as well as the related unique phenomena in shale nanopores, which promotes the understanding of the complex and special micro seepage mechanisms in shale gas reservoirs.