Depletion-Induced Permeability Changes in Naturally-Fractured Gas-Sorbing Formations: A Double-Porosity Fluid Flow and Poromechanical Model

Reservoir stress changes associated with pore-pressure depletion are well-known to have significant impact on permeability in reservoirs with abundant natural fractures. In most fractured reservoirs depletion induces increases of effective stresses and therefore a decrease in permeability. However, some reservoirs that accommodate significant amounts of gas adsorbed in organic micropores in the rock matrix can manifest opposite trends with increases in permeability during depletion. This permeability enhancement occurs due to stress relaxation and opening of fractures induced by rock matrix shrinkage during gas desorption. Additionally, pore pressure drawdown results in increased stress anisotropy potentially leading to reactivation of critically-oriented fractures and shear failure. Fracture and rock matrix depletion have different time scales which depend on fracture permeability, rock matrix permeability, surface area of fractures in contact with the rock matrix, and fracture connectivity among others. The objective of this paper is to present a FEM solution of a double-porosity coupled poromechanical model that discriminates fracture pressure and rock matrix pressure and accounts for desorption-induced stresses. The results of the study show that implications from desorption such as gas production and stress relaxation can be significantly delayed with respect to fracture depletion, and therefore, wellhead pressure. We highlight the importance of multiple-porosity approach in calculating the timing of sorption stresses, which is crucial when modeling production in stress-sensitive unconventional reservoirs.