Influence of triaxial compaction on the mechanical behaviour and porosity evolution of reservoir rocks.

Mineralogical composition and porosity significantly influence the mechanical behavior of hydrocarbon reservoir rocks in crustal conditions. To understand the mechanical failure behavior and dynamic porosity changes with axial loading, triaxial compression experiments were conducted, in three distinct reservoir rock types: KG Basin Sandstone, Bombay High limestone, and Boise Sandstone. Initial porosities of collected samples varied; with KG sandstone at 30%-32%, Bombay limestone at 9.5%-12.7%, and Boise sandstone at 20.6%-25%, determined under dry testing conditions. A range of effective pressures (P_eff) was applied to investigate brittle faulting and cataclastic flow under dynamic axial loading. Both Boise and KG sandstones transitioned from a brittle dilatant regime to a compactive cataclastic flow regime, with Boise sandstone displaying brittle behavior up to 5 MPa P_eff and KG sandstone up to 50 MPa P_eff. Conversely, the Bombay limestone consistently exhibited compactive cataclastic flow at all tested experimental conditions. KG sandstone demonstrated highest peak strength relative to the other rock types due to its lower porosity. Additionally, Boise sandstone showed higher peak strength than Bombay limestone, primarily due to the presence of quartz within the sandstone matrix, which is more resistant relative to the carbonate minerals of limestone formations. In KG sandstone, there was a significant increase in porosity prior to sample failure, particularly up to 70 MPa P_eff, and the final porosity observed post-failure surpassed the initial porosity levels recorded, upto 50 MPa P_eff. In contrast, both Boise sandstone and Bombay limestone showed a continuous decrease in porosity with increasing differential loading throughout the experiment at all tested conditions. Deflection patterns of triaxial curves indicated a shift from shear-induced dilation to shear-enhanced compaction in both Boise sandstone (dilation up to 5 MPa P_eff) and KG sandstone (dilation up to 50 MPa P_eff), while the Bombay limestone predominantly displayed shear-enhanced compaction across all tested P_eff beyond a critical stress threshold. KG sandstone exhibited a transition from pre-failure dilatant to post-failure compactive inelastic strain with increasing P_eff, which contrasts with the post-failure compactive inelastic strain that was consistently observed in the case of the Bombay limestone.

    By integrating these petrophysical and mechanical parameters into reservoir management, hydrocarbon extraction can be made more effective and safer. These insights are essential for creating accurate geomechanical models, which are vital for strategic field operation planning, optimizing production rates, and maintaining reservoir stability.