Multi-Porosity, Multi-Permeability Models for Shale Gas Reservoirs

Abstract

Abstract Numerical simulations are commonly used for studying and forecasting the production from conventional reservoirs. However, limitations of the Darcy’s equation have been noted for shale gas reservoirs due to the complexities associated with nano-scale pores, organic-rich heterogeneous rocks, and extremely-low permeability. In addition, the classical dual porosity/permeability models do not adequately describe the complex physics and dynamics in shales. This paper utilizes a novel multi-porosity, multi-permeability model in a commercial simulator, incorporating the structural details of shale reservoirs. The generated models account for the storage capacity of shales by mapping three different porosity systems (i.e., organic, inorganic, and natural fractures) with guidance from recent results of microscopic petrophysical studies such as SEM, TEM, and SANS/USANS. Desorption is set to be active only in organic matter. To capture different transport controls in reservoirs with an overwhelming number of fine pore throats, the models include three different permeability conduit networks including organic and inorganic matrix, and natural fractures. Physical properties contributing to flow capacity such as formation wettability are set to be optionally active in corresponding zones. We also consider the diffusion effect in shale matrix grids based on pore size distribution. Three configurations are used to investigate the connectivity among the three porous systems and the impact of each configuration on ultimate recovery of gas is evaluated. In Configuration 1, an interconnected organic pore system (which feeds the natural fracture system) is largely independent of the inter-granular matrix pore system. However, Configuration 2 assumes that the organic pores feed the inorganic matrix pore system which in turn feeds the fracture system. In Configuration 3, the organic and inorganic pore systems feed the natural fractures in parallel. The results of this study demonstrate that the selected pore connectivity configuration has a significant impact on shale gas production, underscoring the need to carefully choose the configuration to match pore-scale observations.