Control of pore fluid pressure on rupture propagation: implications for induced seismicity

A causal correlation has been established between industrial activity involving fluid injection (e.g. deep geothermal and hydraulic fracturing operations) and the induced seismicity caused by reactivation of pre-existing faults at depth.

According to existing empirical models, the maximum magnitude of injection-induced earthquakes can be estimated from the known net injected volume of fluids. However, such empirical approaches do not take into account the control exerted by pore fluid pressure on rupture nucleation and propagation.

Here we investigate the role played by pore fluid pressure to potentially develop runaway ruptures beyond the injected rock volume. We conducted slide-hold-slide tests (hold time 1­-3 10³ s) on Westerly granite saw-cut samples, at reservoirs pressure up to 70 MPa confining pressure, for a range of pore pressure between 2 MPa and 15 MPa, under triaxial loading conditions. Linear dynamic strain gauges and piezoelectric sensors were positioned along the fault plane to acquire strain and acoustic data in the low (Hz) and high frequency (MHz) domains. Two triaxial failure tests setups were utilized to saturate the sample by 1) diffuse fluid percolation and 2) direct injection into the fault by borehole.

Our results show that, for the same effective pressure, higher values of pore pressure result in larger fault healing and produce stick-slip events with larger slip (up to 50 microns) and stress drop (8 MPa), and shorter rise time. Surprisingly, the mode of fluid injection seems to play a role too, as saturation by diffuse percolation produces large stick-slip events for modest fault healing, with larger slip (up to 100 microns) and stress drop (16 MPa).

Our results help addressing the controls that in-situ pore pressure conditions exert on the critical transition between contained self-arrested and large run-away induced ruptures during geothermal and hydraulic fracturing operations.