EXPERIMENTAL DETERMINATION OF FRICTIONAL PROPERTIES AND FAULT STABILITY AT THE HOTTEST ONSHORE GEOTHERMAL PROJECT IN THE UK, THE PORTHOWAN FAULT (Invited Presentation)

Fluid-rock interactions can induce mineral reactions that significantly affect the physical properties of fault rocks. These changes have notable effects in the permeability, pore pressure and frictional strength of the system, all of which contribute to the state of stress of faults and therefore are determinant in the seismic cycle. In this study, we analyse the case of the Porthowan Fault Zone (PFZ), the target structure of the first deep geothermal power project in the UK, which aims to utilize the fault damage zone as fluid paths in a thermal exchange loop. We combine photogrammetry, XRD characterization and triaxial friction experiments on samples extracted during the drilling campaign to understand how the physical properties of the fault have been affected by fluid-rock interaction. We then use fault stability analysis to assess their effect on potential reactivation of the fault.

The surface outcrop is dominated by two subvertical fracture families, striking 75°-255° and N-S. At 2 km depth, the 75°-255° fracture family remains constant, while a 150°-330° striking fracture set becomes predominant. XRD analysis of cuttings show diffractograms mainly differing from the fresh granite by the presence of kaolinite and Mg-cronstedtite. Frictional strength experiments on cuttings (µ=0.28) show significantly lower coefficients than those of intact granite (µ=0.52). Cronstedtite is a Fe/Mg-rich, low temperature hydrothermal mineral from the serpentine group, revealing deep-reaching fluid-rock interaction that facilitates Fe and Mg mobilization to deep areas of an otherwise Fe-Mg-depleted rock. Both cronstedtite and kaolinite can account for the relatively lower friction coefficient of cuttings with respect to the fresh surface granite due to their sheet-like structure. Our calculations suggest that the PFZ is bound to remain locked under the current considered stress regime and measured frictional strength, even at pore pressures up to 10 MPa. Meanwhile the 75°-255° fracture system is more favorably oriented to slip. These results suggest that the frictional properties of the PFZ have been significantly affected by fluid-rock interaction, promoting phyllosilicate accumulation, and likely changing its deformation mechanisms.