On the effects of fluid pressure variations on rock-mass and fault mechanical behaviour

In the past 20 years, the growing interest in deep geo-reservoirs for purposes such as carbon storage, waste water disposal, or geothermal energy exploitation have led to large-volume fluid injections into the upper continental crust. These fluid injections have caused a massive increase in the seismicity rate in some normally “quiet” regions. Recent observations suggested that different injection strategies could produce different mechanical responses in these reservoirs. In this vein, cyclic fluid injections have been proposed as an alternative to conventional monotonic injections to mitigate induced earthquakes. Additionally, recent studies have suggested that injection into deeper geo-reservoirs, at the brittle-ductile transition, could reduce (or suppress) induced earthquakes. In this context, this research has aimed at bettering our understanding of reservoir deformation during different fluid injection strategies. Laboratory experiments were performed on porous reservoir rock under different reservoir conditions and fluid injection strategies. In the first section, it is demonstrated that water-saturated conditions, compared to dry conditions, cause a reduction in uniaxial compression strength in five sandstones. Additional experiments suggest that this water weakening of sandstones is due to the reduction of the fracture toughness and of the static friction of the materials. In the second section, it is shown that under drained conditions, pore fluid pressure oscillations affect the long-term mechanical behaviour of intact porous rocks. More than the amplitude of the oscillations, the period controls the time-to-failure, failure strength and dilatancy rate of the sample. Additionally, the pore fluid pressure oscillations control the AE events (a proxy for the seismicity): the AE events rate oscillates in-phase with the pore fluid pressure variations. Increasing the differential stress, the amplitude and the period of the oscillations accentuates this behaviour. In the third section, it is demonstrated that under drained conditions, pore fluid pressure oscillations strongly affect the mechanical behaviour of faults. The pore fluid pressure signal directly controls the instabilities’ (i.e., stick-slip and AE events) distribution, with an increase of events at elevated pore fluid pressure. For initially stable sliding faults, the pore fluid pressure oscillation signal controls the onset of unstable slip: the higher the pore fluid pressure oscillation’s amplitude, the lower the stress and displacement at the onset of a seismic event. In the fourth section, it is shown that under quasi-drained conditions, a pore fluid pressure increase within a rock undergoing ductile deformation causes instantaneous dilation of the system. Further increasing the pore fluid pressure leads to the development of localized deformations and the shear fracturing of the rock. The macroscopic shear fracturing is not instantaneous when the ductile to brittle transition is passed. Prior to this, a creeping phase is necessary. The time and strain length of this creeping phase are controlled by the rate of pore fluid pressure increase: the faster the injection, the longer the creeping phase. This work provides new insight into the mechanical behaviour of rock masses and faults under various crustal conditions during pore fluid pressure variations. It helps to constrain the mechanisms involved in deep geo-reservoirs submitted to different injection strategies.