An In-situ Experiment of Fault Reactivation in a Shaly Formation, Massif Central, France

Fluids & Faults (F&F) is a research project involving Total and several French universities and research institutes. The main objective of F&F is to improve our understanding of the hydromechanical behavior of a fault when it is reactivated by an increase in fluid pressure. To reach the aforementioned objective, a fault zone hosted in Toarcian shales was drilled and stimulated by water injection. Eight inclined boreholes were drilled across the fault zone from a tunnel gallery in the Tournemire Underground Research Laboratory (URL). Several water injection experiments were undertaken while a dense network of monitoring devices recorded rock deformation, fluid pressure, resistivity and micro seismicity.

This experiment shows a complex coupling between fault kinematic and its stress dependent permeability, highlighting that permeability increase may mainly occur off the main slip plane. During test 1, where a limited 40L fluid volume was injected during a relatively short time (6 minutes) it is possible that the fluid pressure did not equilibrate within the fault zone, limiting shear failure. The analysis of additional tests carried out during this in-situ experiment in different fault zone facies and with different durations should give additional insight into the fault hydromechanical behaviour in shaly formations.