Flow modeling in shale gas reservoirs: A comprehensive review

Shale gas reservoirs (SGRs) have been exploited for commercial natural gas production for the past decade and has become a major source of energy in today's world. Due to their rising importance, description and characterization of these types of reservoirs are inevitable, especially from the flow modeling point of view. Considering their unique features such as ultra-tight porous structure, depth, and large lateral extent, their petrophysical and reservoir properties are different and distinctive compared to conventional reservoirs. This difference enforces particular mechanisms of flow for the gas through these low-permeable beds, which has made scientists develop a variety of models to represent the fluid flow. In this article, a comprehensive review of existing flow models for SGRs is presented while their specific features including phase behavior of confined gas in tight pores, adsorption/desorption of the gas, flow mechanisms (slip flow, Knudsen diffusion, and surface diffusion), and stress sensitivity is covered. In addition, to declare the effect of porous structure of shale rocks on gas flow, pore network models (PNMs) as representatives of realistic shale porous structure are introduced and most outstanding studies in this area are reviewed as well. All models that have been reviewed here are the most recent literature, while its attempted that earlier ones that are still mainstream are not ignored. Also, the authors made comparisons against experimental data in most cases and implemented sensitivity analysis between models in order to make sure the audience get a better understanding of which one to use in their studies. Ultimately, it was the goal to make this review article a guidebook for future researches in SGR flow and pressure/rate transient analysis and modeling.