Transport mechanisms and dynamics of shales via multiscale, multiphysics pore network modeling

Source rocks such as shale are highly heterogeneous consisting of organic matter and various inorganic minerals. Microscopic images suggest that microcracks serve as conduits of the released gas from organic nanopores. The permeability of the shale matrix is primarily attributed to the stress sensitive microcracks which would be highly influenced by the changes in fluid pressure. As the microcracks are depleted, more gas molecules desorb from the organic nanopores, which, in turn, could affect the fluid pressure in the microcracks. Linking the local properties in the organic nanopores to the microcracks for a better understanding of the coupling between them is necessary for an improved modeling. In this paper, a multiscale pore-network modeling approach is presented to describe the organic material-microcrack system and to investigate large-scale features of gas transport in shale. A multiscale pore network model consisting of clusters of organic pore network and microcrack is built to investigate the shale gas transport at macroscopic scale. The organic part of the network model consists of nano-capillaries interconnected at nanopores and the network accounts for the adsorptive-convective-diffusive transport mechanisms that have been derived recently for a single capillary. This organic nanopore-network is hydraulically connected to a microcrack. Then, mass balance at each node in the new domain is solved along with the assumed boundary conditions. Using the information at the nodes, the total flow rate, and the pressure distribution in the system are obtained as a function of time. The results show that the fluid pressure in the microcrack is sensitive primarily to the content of the organic material and its permeability. An empirical formulation is developed to quantify this sensitivity. This relationship can be investigated in the laboratory and used in theoretical models in predicting the shale gas production.