Experimental Observations of Gas-sorption-Induced Strain Gradients and their Implications on Permeability Evolution of Shale

Abstract

Gas adsorption/desorption can result in swelling/shrinking of the matrix in fractured shale. The significant contrast in permeability between fractures and matrix results in an extended duration for the equilibration of the gas injection or depletion-created pressure difference. This spatially non-uniform pressure dissipation induces non-uniform deformations inside the matrix. We follow this response with carefully constrained laboratory measurements integrated with numerical modelling to explore the relation between the strain gradients that develop in the matrix adjacent to fractures and the evolution of permeability each under conditions of constant confining pressure. The microstructures of the sample were characterized by X-ray computed tomography, field-emission scanning electron microscopy and mercury injection capillary pressure porosimetry. A distributed array of strain gauges was attached to the matrix to directly measure the evolving strain. Then a 3D multiphysics numerical model was built to model the evolution of strain gradients from initial to ultimate equilibrium. The influence of these strain gradients on the evolution of fracture permeability is evaluated by a non-uniform strain-based permeability model. The results indicate that the swelling of the matrix near fractures can also compress the matrix away from the fracture under constant confining pressure conditions. Under the influence of the matrix–fracture interaction, a transient and complex distribution of strain gradients develops within the matrix.