A Multiphase and Multicomponent Model Considering Molecular Diffusion for Simulating Shale Oil Reservoirs With Complex Fracture Networks

Abstract

Molecular diffusion is a critical mechanism of enhance oil recovery (EOR) in developing shale oil reservoir by huff “n” puff. The ultra-low permeability lead to the accumulation of injected gas within the complex fracture networks (CFNs), thereby, enhancing concentration gradient and mass transfer by molecular diffusion between the CFNs and matrix. A proper understanding of CFNs influence on molecular diffusive mass transfer becomes critical for predicting oil recovery and remaining oil distribution in shale oil reservoirs. Therefore, a multiphase and multicomponent mathematical model with molecular diffusion was developed for describing mass transfer of molecular diffusion. Molecular diffusion coefficients were predicted by the extend Sigmund method. The finite volume method (FVM) and two-point flux approximation (TPFA) were applied to discretize and approximate mass transfer equations. The embedded discrete fracture model (EDFM) was utilized to explicitly simulate CFNs and extended to couple mass transfer by molecular diffusion in the fracture to that in the matrix. Model validation clearly demonstrates that the proposed numerical model is capable of effectively and accurately simulating diffusion mass transfer in shale oil reservoirs with CFNs. By applying the proposed numerical compositional model, a series of synthetical models with molecular diffusion were developed by CO₂ huff “n” puff. The simulation results indicated that molecular diffusion is crucial to EOR in developing shale oil reservoirs with CFNs by huff “n” puff, particularly in the injection and soaking stage. However, molecular diffusion contributes to an increased gas production during the production phase. In addition, larger fracture density benefits diffusive mass transfer to EOR by increasing contact areas. And higher diffusion coefficients improve diffusive mobility, which boosts diffusive mass transfer. Meanwhile, greater injection rate additionally makes concentration difference of injected component between fractures and matrix system to rise, resulting in more injected component transferring into matrix. This paper provides a better understanding of molecular diffusion mechanism for EOR in development shale oil reservoirs with CFNs by huff “n” puff.