Alberto Minardi

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.

The Role of Anisotropy on the Volumetric Behaviour of Opalinus Clay upon Suction Change

An experimental investigation to analyse the anisotropic volumetric response of shaly and sandy facies of Opalinus Clay upon suction variations is presented. Obtained results demonstrate the different behaviour of the tested facies to a wetting-drying cycle. The shaly facies exhibits higher water retention capacity and stronger volumetric response than the sandy facies. Anisotropic response is experienced by both facies with the strain perpendicular to bedding higher than in the parallel direction. The sandy facies exhibits a more pronounced anisotropic behaviour in particular during the drying phase. A detailed analysis of the response in the two directions with respect to the bedding orientation proves that the different anisotropic behaviour between the two facies is mainly caused by a different response parallel to bedding rather than perpendicular.

Anisotropic Volumetric Response of Shales upon Suction Changes

Anisotropic fabric and volumetric response to suction changes represent two main features of shales, which are involved in several engineering applications. However, a comprehensive understanding of the interaction between these two aspects is still missing. The present work aims to provide a detailed analysis of the coupling between the water retention capacity of shales and their anisotropic behaviour through an advanced experimental methodology. Among the various tested facies of the Opalinus Clay Shale (Mont Terri, Switzerland), the results from the shaly facies are presented in this study. The behaviour of the material is characterized by swelling and shrinkage respectively upon wetting and drying and by an anisotropic volumetric response upon suction changes, which is higher in the direction perpendicular to bedding plane. As a consequence, a different swelling pressure in the two directions would be expected when the material is wetted in confined conditions. The evolution of the water content is strongly affected by the volumetric strain of the material, where the increase of the volume upon wetting provides more void space for the water to penetrate into the material, enhancing the shale response.

Gas Shale Water Imbibition Tests with Controlled Suction Technique

Water loss during flowback operations represents one of the main challenges related to the use of hydraulic stimulation to exploit shale gas resources. About 20% of the injected fracturing fluids are usually recovered after stimulation. Fluid imbibition is expected to be one of the main mechanisms responsible for the water uptake of shale gas reservoirs. Imbibition tests are typically performed to analyse this issue. This study presents a new experimental methodology based on the control of total suction to quantify the impact of the swelling response of gas shales on the water uptake during imbibition processes. The obtained results demonstrate that a precise quantification of the gas shale water uptake cannot be performed neglecting the volumetric behaviour and the presence of the mechanical stress during the imbibition process.