A Methodological Determination of Sorptive Poromechanical Constitutive Coefficients for Double Porosity Shales

Double porosity constitutive models are necessary components in mechanical analysis of fractured rocks. To solve the field constitutive equations, values of constitutive coefficients are required. Several theoretical expressions have been proposed to calculate the double porosity constitutive coefficients as their physical estimation is challenging due to a lack of efficiently designed experiments. Yet experimental determination of constitutive coefficients provides confidence in the accuracy of the models. Here, we present a methodological framework to determine new constitutive coefficients arising by coupling of gas sorption kinetics in the sorptive poromechanical model developed by the authors for double porosity media (Algazlan et al. 2022a). To physically characterise the new constitutive coefficients, we designed an innovative methodological process consisting of (i) obtaining gas diffusivities and Langmuir parameters by measuring gas uptake on different shales, (ii) computing the time evolutions of the free and adsorbed matrix gas concentrations, and (iii) optical imaging of temporal changes in the porosity of fractures. A novel shear-microscopy cell was used allowing stress application to the samples whilst being exposed to surficial fluid flow coupled with optical monitoring. The methodological framework resulted in the efficient determination of the new sorptive constitutive coefficients paving way for further advanced validation studies of multiphysics constitutive models.