Direct shear experiments to investigate the effect of chemical alteration on fault frictional behaviour in granitic geothermal systems

Enhanced temperature gradients related to locally elevated heat production in granitic plutons offer the potential for low carbon geothermal energy production. Cornwall in SW England hosts several granitic plutons that are the subject of current geothermal projects (United Downs Deep Geothermal Power [UDDGP] Project and Eden Project). These projects target fault zones in crystalline rock that provide pre-existing pathways for fluid flow. Reinjection of cooler fluids into the reservoir after heat extraction may result in chemical disequilibrium with the host rock, potentially driving precipitation or chemical alteration. Such changes could influence the frictional properties of the fault zones, and hence require modifications to numerical risk-based calculations of the likelihood, or not, of induced seismicity.

In order to study the effects of such alterations, we have conducted a series of direct shear experiments under representative in-situ conditions on Cornish Carnmenellis granite samples which have undergone varying degrees of natural chemical alteration. The direct shear experiments were conducted on gouges (grain size < 125 μm) and at effective normal stresses of 80-105 MPa, pore fluid pressures of 25-50 MPa and temperatures of 16-180 °C. These conditions are relevant for the depths where the UDDGP project injection and production boreholes intercept the Porthtowan Fault zone, the assumed main conduit for fluid flow. In each test, load point velocity was stepped between 0.3 μm/s, 1 μm/s and 3 μm/s, and shear resistance of the sample was measured to determine the stability of sliding and thus the likelihood of induced seismicity as a function of degree of alteration. Initial shear tests at room temperature suggest little difference in the frictional response of altered and unaltered samples.