Nonlinear Elastic Response of Partially Saturated Gas Shales in Uniaxial Compression

Elastic properties of gas shales are fundamental for the geomechanical characterization of unconventional reservoirs. Due to the presence of microcracks, a nonlinear elastic behaviour with hysteresis is usually exhibited when gas shales are subjected to unloading/reloading paths in laboratory tests for the determination of the elastic moduli. When gas shales are tested in partially saturated conditions with total suction control to reproduce in situ hydraulic conditions, the hysteretic behaviour results to be significantly affected by the wetting or drying processes imposed to change the water saturation of the material; an opening of the loop is observed; and different secant elastic moduli are measured on the unloading and reloading parts. Experimental evidence from a uniaxial compression test performed on a gas shale specimen is presented in this paper, where several unloading–reloading paths are carried out at different total suction values. This study demonstrates that the swelling (or shrinkage) behaviour experienced during total suction variations imposed before performing the axial stress variation influences significantly the sliding cracks mechanism during the unloading phase. In particular, when a wetting process is applied, the nonlinearity of the response decreases significantly. On the other hand, the reloading part exhibits always a nonlinear trend. Although the discrepancy between the unloading and reloading curves of the performed paths, a systematic impact of total suction on the elastic moduli is highlighted. The presented analysis demonstrates that not considering these mechanisms may lead to misleading interpretation of the experimental results.