ALTERACTION

The project focuses on (1) the feedback between alteration and tectonic activity of active faults in the upper crust, which enhances the process of fault maturation of from chemical interactions, and (2) how this process impacts the hydromechanical properties of enhanced geothermal systems (EGS). The main research hypothesis is that fault activity induces new fractures that enhance fluid-rock interactions and associated fault zone alteration. This feedback forcing would accelerate the maturation of active faults, by creating clay gouges that weaken the fault and decrease its core permeability over time. For EGS reservoirs, when located in active tectonic zones, tectonic stresses may accelerate alteration, which clogs the created fractures and may weaken them, increasing the risk of induced seismicity during production. Our focus is on granitoid rocks, a representative lithology of the upper crust, which is also the lithology of both the Rhine Valley geothermal reservoirs and that of the wall rocks of the Nojima fault responsible for the 1995 Nanbu-Kobe earthquake in Japan. Dynamic loading pulverizes granitoid rocks, which enhances their permeability. This facilitates laboratory alteration of centimetric samples, as demonstrated by preliminary experiments. We will use core samples from the GSJ Hirabayashi scientific borehole intersecting the Nojima fault to (1) obtain a continuous alteration profile through the fault by combining core and downhole geophysical data and (2) perform flow-through laboratory alteration experiments at various differential stresses to assess the intertwined effects of tectonic loading and fault alteration.