Permeability Enhancement in Shale Induced by Desorption

Shale reservoirs remain a critical energy resource in the United States and globally as they serve as a reservoir rock for hydrocarbon production as well as a reservoir or caprock for carbon storage. Understanding permeability evolution resulting from adsorption/desorption within these ultralow permeability rocks remains an active challenge in industry and academia. In this work, we summarize the current state of the art of sorption-induced permeability evolution in shales using a strain-based framework. Shale is represented as a composite mineral aggregate with transversely isotropic permeability directed parallel and perpendicular to bedding planes. As organic and clay components are sorptive while carbonate and silicate components are not, local changes to the preferential flow network are complex ensembles of disparate strains. We highlight the field equations that are typically implemented to model adsorption and permeability evolution in shale, then detail the methods by which permeability can be enhanced by desorption. Desorption occurs with decreasing pore gas pressures (Δ P ↓), increasing temperature (Δ T ↑), or changing gas composition (Δ C ) and impacts permeabilities. We demonstrate how to isolate such sorptive strains and sorptive permeability evolution from laboratory data. Finally, we use a case study of nitrogen flooding to demonstrate proof of concept and to show how sorptive permeability evolution can be isolated from other processes in laboratory observations. We discuss emerging concepts in desorption-induced permeability evolution, particularly CO ₂ –N ₂ injection for enhanced gas recovery or geologic carbon sequestration. The impacts of mineral distribution and heterogeneity on initial and evolving permeability anisotropy and the importance of preferential flow paths are emphasized throughout.