Modeling Complex Reservoir Dynamics in Multi-Porosity Fractured Formations: Conventional Carbonates and Unconventional Shale Reservoirs

Abstract The subsurface porous media in hydrocarbon formations and aquifers consist of a complex porosity system classified as interparticle porosity, vugs, and fractures. In carbonate reservoirs such as the Yates Field and in aquifers, namely the Avon Park Formation within Floridan aquifer system, these porosities create highly heterogeneous flow behavior. Similarly, hydraulically stimulated shale reservoirs with ultra-low matrix permeabilities develop effective fracture networks and large surface areas, exhibiting a distinct three-porosity system, that create new transport dynamics that enhance production. Furthermore, because of the recent interest in subsurface energy resources and associated transport problems, in this paper we revisit the global mathematical modeling of mass transport in triple-porosity, triple-permeability reservoirs as well as a more pertinent issue regarding the local mass transfer across fracture-matrix interface. The mathematical model governing equations incorporates multiphase, multi-component flow, including capillarity, gravity, diffusion, and fracture-matrix transfer. Within this framework we address realistic simulation of mass transport in fractured reservoirs across fracture-matrix interface and its impact on hydraulically fractured shale production and enhanced oil recovery. Similarly, we will look at CO2 molecular diffusion across caprock in CO2 stored aquifers. In essence, we will address the impact of hydraulic connectivity, wettability, and diffusion-dominated transport on oil recovery, and diffusion of CO2 and H2 in aquifers. To further elucidate, both analytical and numerical analyses indicate that hydrocarbon production from stimulated shale formations is overwhelmingly dominated by man-made macro- and micro-fractures, and capillary forces, and molecular diffusion plays major roles in enhanced oil recovery and mass transport across caprock in aquifers; thus, the significance of capturing multi-porosity flow in reservoirs for reservoir management. Such findings are compelling reasons to integrate multi-porosity and multi-permeability dynamics into decision models for enhanced oil recovery strategies, improving well performance, and ensuring the integrity and safety of subsurface storage projects. Finally, the modeling methods presented in this paper remind us of the beauty of analytical formulations and solutions, and that addressing fundamental concepts will continue to be the foundation of good engineering.