Frictional behaviour of altered rhyolitic volcanic rocks under in situ conditions in the Wairakei Geothermal Field, New Zealand

Abstract

The utilisation of geothermal resources disturbs the reservoir conditions due to extraction and reinjection, which may have consequences for the behaviour of faults, including triggering earthquakes and permeability enhancement. The frictional properties of the fault gouge are needed for simulation models of faults and earthquakes. The frictional properties of volcanic rhyolitic fault gouges under in situ conditions have been investigated in this study in an attempt to better comprehend the conditions in which faults slip. The frictional behaviour and seismic potential was investigated for altered rhyolitic tuffs, ignimbrites and lava fault gouges in this study. The samples used were taken from cores and cuttings originate originating from 100 m to 2 km depths from drilling in the Wairakei geothermal field in New Zealand. The frictional behaviour was tested on a ring shear apparatus by velocity stepping and slide-hold-slide experiments. The loading velocities varied between 10-6-10-4 m/s. The applied conditions were 50 MPa effective normal stress and 20 MPa pore fluid pressure corresponding to a depth of 2-2.5 km at Wairakei, at variable temperatures of 24, 100, 200 and 300 °C consistent with reservoir temperatures. The obtained results show coefficients of friction under the applied conditions for the different lithologies. A velocity weakening is observed under all temperatures indicating, in the framework of rate-and-state friction, that all fault gouges show the potential for unstable slip, i.e. for the nucleation of earthquakes. A sampled ignimbrite from a well in Karapiti South shows a significant weaker frictional behaviour compared to samples from the Poihipi West area. I did not observe effects of welding on the frictional strength of the simulated fault gouges. A fault re-strengthens after slip by strengthening of contacts, the increases of area contact and gouge compaction and loglinear relationship that is a time-dependent process. The healing rate of the fault gouge increases strongly with temperature. The increase in shear strength ranges from 0.004ln(t) to 0.018ln(t). The frictional properties of rocks in hydrothermal altered rocks and the loglinear re-strengthening of faults presented in this work will support future numerical models for fault behaviour and help in evaluating reservoir permeability.