Can Fault Leakage Occur Before or Without Reactivation? Results from an In Situ Fault Reactivation Experiment at Mont Terri

We conducted fluid injection tests in four packed-off borehole intervals across the Mont Terri clay-rich fault zone which is an analogue to a minor fault that would hardly be detectable from surface seismic surveys during the initial design of a sequestration site. While pressurizing the intervals at maximum injection pressures of 5 to 6.3 MPa, we simultaneously monitored fault displacement, pore pressure and injection flowrate in two boreholes. A large (factor of 100 to 1000) transmissivity increase occurred in the fractured damage zone intervals close to the interface with the fault core. No significant transmissivity variation was measured in the fault core. The transmissivity variations in the fault damage zone occurred at pressures above 3.9 to 4.4 MPa, corresponding to the hydraulic opening of natural shear planes oriented N039 to N058 and dipping 59 to 69 degrees E. They are associated with slip on the order of 0.01 mm to larger than 0.1 mm on the activated planes, the highest value being observed at the interface between the low permeable fault core and the damage zone. Contrasted fault movements were observed, mainly dilatant in the fault core, highly dilatant-normal slip at the fault core-damage zone interface, and low dilatant-strike-slip-reverse in the damage-tointact zones. These results suggest an important role of local alteration of "regional" stress at the core-damage zone interface on fault hydraulic property variations with limited slip, an important finding when evaluating the loss of natural seal integrity of CO2 sequestration sites. Published by Elservier Ltd.