Shale Dual-Porosity Dual-Permeability Poromechanical and Chemical Properties Extracted from Experimental Pressure Transmission Tests

Abstract

So far, no effective methods have been found to measure shale dual-porosity dual-permeability poromechanical and chemical parameters. This paper presents a practical method to determine such parameters by matching the newly derived two-dimensional transversely isotropic dual-poro-chemo-electro-elastic analytical solutions with laboratory pressure transmission test data of a clay-rich shale. The matching provides estimations of crucial shale parameters, including dual permeabilities, membrane coefficient, ions’ diffusion coefficients, and electro-osmotic permeability. Moreover, this matching includes not only pore pressure but also axial strain and radial strain. Such triple matches provide more confidence in the estimations compared to conventional simulations of only pore pressure measurements. Both the single porosity poromechanics solution and the newly derived dual-poro-chemo-electro-elastic solution are applied to analyze the test. It is shown that the single poro-chemo-electro-elastic simulation could not reproduce the experimental pore pressure or strain responses while the dual-poro-chemo-electro-elastic simulation could capture simultaneously pore pressure, axial strain, and radial strain responses very well. Compared to the single-porosity simulation, the dual-porosity simulation indicates that most of the shale volume change occurred within the clay grains and in between clay layers, pore pressure in between clay layers was higher than that outside of clay grains due to chemical osmosis. Sensitive analysis is implemented to show the confidence of the estimations and to identify parameters with the most influence on the results of the pressure transmission test. This paper also highlights the importance of accounting for shale dual-porosity nature when simulating its responses.