Poroelastic Dual-Porosity Dual-Permeability Simulation of Pressure Transmission Test on Chemically Active Shale

Abstract

This paper presents the poroelastic dual-porosity dual-permeability analytical solutions simulating the pressure transmission test on chemically active shale, taking into account shale anisotropy and electrokinetic effects. Laboratory data from pressure transmission tests on a shale sample with high clay content were also simulated by using both single-porosity and dual-porosity dual-permeability analytical solutions. The matching provides estimates of crucial shale parameters, including hydraulic permeabilities, membrane coefficient, ions diffusion coefficients, and electroosmotic permeability. The matches between analytical solutions and laboratory data included not only pore pressure, which had been the focus of previous studies, but also axial strain. Such double matches provided additional confidence in the estimations compared with conventional simulations of only pore-pressure measurements. It was found that the single-porosity simulation could not match pore pressure and axial strain simultaneously. In particular, if all measured properties were honored, the simulation yielded higher pore-pressure and lower axial strain predictions than laboratory data. To obtain a good match in both pore pressure and axial strain by using the single-porosity simulation, some parameters such as Young’s moduli and cation exchange capacity (CEC) had to be changed substantially from measured values. Therefore, it was postulated that most of the volume change occurred within the clay grains and in between the clay layers, which cannot be captured by the single-porosity model. This hypothesis was further supported by the fact that the dual-porosity dual-permeability simulation was able to model the pressure transmission test very well for both pore pressure and axial strain. Sensitivity analysis was also conducted to identify parameters with the most influence on the outcomes of the pressure transmission test. This paper highlights the importance of accounting for shale dual-porosity dual-permeability and chemically active natures when simulating shale responses.