Influence of 3D forced water-based working fluid imbibition on dynamic responses of deep anisotropic shale reservoir

Drilling and hydraulic fracturing are water-rich processes where shale reservoirs around wellbores and fractures often suffer forced water-based working fluid imbibition. In this study, we focus on increasing dynamic loads in deep shale gas extraction and report dynamic compression on thermally treated (25–200 °C) Longmaxi shale samples with five bedding orientations (0°, 30°, 45°, 60°, and 90°) after 3D forced water imbibition with four driving pressures (0, 3, 6, and 9 MPa). Results show that increasing water driving pressure increases the additional pore space inside shale samples non-linearly, and higher reservoir temperatures promote this effect. Increasing the water driving pressure obviously reduces dynamic compressive strengths of samples except those with the bedding orientation of 90° when the temperature exceeds 100 °C. Interestingly, approximate V-shaped curves between the dynamic strength and bedding orientation are neither affected by water driving pressure nor reservoir temperature. However, the anisotropy magnitude of dynamic strength is dependent on reservoir temperature and water driving pressure. Bedding activation for shale samples with bedding orientations of 30°, 45° and 60° is promising under high temperatures by increasing water driving pressure. Compared to pure thermal treatment, forced water imbibition limits the number of activated bedding planes in thermally treated shale samples with bedding orientations of 45° and 60°. Micro-mechanisms involving microstructural degradation, capillary force, poroelasticity, structural effective stress, dynamic viscous resistance, and dynamic fracture response explain the responses of strength magnitude, and the nature of stress wave propagation in bedded shale explains the responses of strength anisotropy. This study provides a basic understanding on dynamic responses of deep anisotropic shale reservoir considering reservoir temperature and forced water-based working fluid imbibition, which is applicable for improving drilling and hydraulic fracturing programs.