Effect of fluid saturation and salinity on sandstone rock weakening: experimental investigations and interpretations from physicochemical perspective

Abstract

Fluid saturation plays an important role in rock weakening. However, few researches reveal the controlling factor(s) behind the rock weakening particularly from geochemical perspective. We performed rock failure/fracture reactivation tests on dry and wet samples (saturated with high/low salinity brine). We examined the effect of fluid saturation and salinity on rock mechanical properties through a unique triaxial compression system with combination of micro-seismicity monitoring. Besides, the induced time-dependent micro-seismicity events were located to characterize the fracture orientation together with medical CT imaging. Moreover, we performed geochemical modelling to link the surface energy to rock mechanics from geochemical perspective.Our results show that the peak stress of saturated samples is 11.7% to 15.0% less than unsaturated samples, confirming that fluid saturation can weaken rock. Lowering salinity appears to play a negligible role in the reduction of elastic modulus and peak stress of intact sandstones. We also did not observe the expansion of micro-seismicity cloud along main fracture plane, suggesting that lowering salinity may not trigger micro-fracture generation under confining condition. However, sample saturated with low salinity brine shows a much slower increase rate of differential stress during the plastic deformation to reach yield point during fracture reactivation process compared to the sample saturated with high salinity brine. This work contributes to the existing knowledge of rock weakening by providing quantitative geochemical characterization with implications in water-assisted enhancing hydrocarbon recovery and CO2 geological storage.