Pressure Solution Compaction During Creep Deformation of Tournemire Shale: Implications for Temporal Sealing in Shales

The temporal evolution of gouge compaction determines fluid transfer and rock rupture dynamics. Thus, studies on the time-dependent creep compaction processes of shale materials may elucidate the chemo-mechanical behavior of shallow clay-rich zones. We investigated this problem by combining creep experiments conducted in triaxial compression under upper crustal conditions with modeled pressure solution processes in Tournemire shale. The shale samples were deformed parallel and perpendicular to the bedding at low (10 MPa, 26 degrees C, this study) and high (80 MPa, 26 degrees C, published by Geng et al., 2018, ) pressures. We monitored the deformation during stepping creep experiments until sample failure. Our results differ from those of traditional creep experiments and show that the creep failure strength of Tournemire shale samples increased significantly (by similar to 64%) at both pressures. Our experiments suggest that at appropriate temperatures, the pressure solution is highly active and is the dominant temporal sealing mechanism in the shale. Using our experimental data and the statistical rock physics method, we modeled the temporal reduction of effective porosity in terms of depth and temperature. Our thermal-stress coupled modeling results suggest that the pressure solution induced sealing is the most active at middle-shallow depths (similar to 3.8 km). We believe that the sealing capacity and creep failure strength of dolomite-rich shales may change significantly at middle-shallow depths, indicating an important influence on reservoir fluids transfer and fault gouge strength.