A multi-scale quadruple-continuum model for production evaluation of shale gas reservoirs considering complex gas transfer mechanisms and geomechanics

Shale gas reservoirs are uniquely characterized by extremely low values of permeability, porosity, and complex fracture network which adequately hinders gas transfer and production evaluation. At the reservoir scale, the analysis of complex gas flow and transfer mechanisms from one domain to another is still a challenge. In this regard, a fully coupled multi-scale quadruple-continuum model is proposed, where the gas transfer mechanisms across the four domains (Kerogen, inorganic matrix, natural fractures, and hydraulic fractures) are adopted and the flow terms are corrected to suit real gas and account for shale deformation considering dynamic effective stress. The complexity of modeling gas transfers from the kerogen to the inorganic matrix to fracture system (natural and hydraulic fractures) and the production well is overcome by using the Warren-Root and Vermeulen transfer terms, respectively. The current model matches field data from typical shale reservoirs and accurately predicts gas production. Numerical simulation results show that increasing the kerogen pore volume and stress sensitivity coefficient decreases cumulative gas production. Increasing domain permeability, porosity, number of natural fractures, and the width and length of natural fractures improve the flow of gas, leading to higher cumulative gas production. Moreover, crisscrossing natural fractures and larger stimulated reservoir domain (SRD) could extend the area of contact with the unstimulated reservoir domain (USRD), thus allowing more gas to flow to the production well. Overall, more gas is produced when the four domains are considered than when either of them is ignored. This study introduces a comprehensive understanding of the four domains' primary flow and transfer mechanisms. It provides a practical baseline for evaluating gas production in fractured shale reservoirs.