Analytical models for gas production in a shale reservoir: A review focusing on pore network system

Abstract

Shale gas reservoirs may contain pores with different origins (; natural or induced) and scales. They can be divided into four groups, inorganic porosity, organic porosity, natural micro-fractures porosity and artificially created fractures porosity. The inorganic porosity is the void spaces within matrix of clay, pyrite, silica and other non-organic minerals. The organic porosity is the void space that remains in organic matter after conversion the kerogen to gas and oil. Organic matter in the form of kerogen is finely dispersed within inorganic matrix and contain void spaces (organic porosity). Micro-fractures network contains void spaces (natural micro-fractures porosity) and pore network system is also formed after creation of hydraulically induced fractures (artificially created fractures porosity). Simulating gas production from shale gas is a complex process due to interaction of fluid with various pore scales. In the current research work, shale gas transport through complex porous network is reviewed. Transport mechanism for free and adsorbed gas in dispersed organic nano-pores is combination of both Darcy and non-Darcy phenomena. Slippage of gas molecules occurs in organic pores and desorption of gas molecules occurs as the reservoir pressure depletes. The combined flux from organic pores is transported into inorganic pores then transported into micro-fractures network which can be exploited if hydraulically induced fractures are created in the vicinity of wellbore. It is a huge challenge to model gas production from shales due to presence of multi-scaled porosities. Slippage effects and desorption further add to the complexity in shale gas reservoirs. Analytical models, presented in the current review paper, incorporate complexities in shale gas reservoirs so that production from shale gas can be modeled precisely.