The Transport Mechanisms and Dynamics of Shale via Multiscale Multiphysics Pore Network Modeling

Abstract

Source rocks such as shale are highly heterogeneous, consisting of organic matter and various inorganic minerals. Microscopic images suggest that microcracks serve as conduits for the gas released from organic nanopores. The permeability of the shale matrix is primarily attributed to stress-sensitive microcracks that are highly influenced by changes in fluid pressure. As the microcracks are depleted, more gas molecules desorb from the organic nanopores; this, in turn, affects the fluid pressure in the microcracks. Linking the local properties of the organic nanopores to the microcracks allows for a better understanding of the coupling between them, which is necessary for improved modeling. In this research, a multiscale pore network modeling approach is presented to describe the organic material and microcrack system and investigate the large-scale features of gas transport in shale. A multiscale pore network model consisting of clusters of organic pore networks and microcracks was built to examine shale gas transport on a microscopic scale. The organic part of the network model consisted of nano-capillaries interconnected at nanopores. The network accounted for the adsorptive–convective–diffusive transport mechanisms recently derived for a single capillary. This organic nanopore network was hydraulically connected to a single microcrack. Then, the mass balance at each node in the new domain was solved, along with the assumed boundary conditions. Using the information at the nodes, the total flowrate and pressure distribution in the system were obtained as a function of time. The results show that the fluid pressure in the microcrack was primarily sensitive to the content of the organic material and its permeability. Then, the microcracks–organic materials interactions are studied and empirically quantified at larger macroscopic scale of gridblocks. This relationship can be investigated in the laboratory and used in theoretical models to predict shale gas production.