| Abstrakt: |
The geochemical alteration of host rocks might affect the productivity and the potential for induced seismicity of geothermal systems. In addition to natural alteration, following production and heat extraction, re‐injected fluids at lower temperatures and different pressures may be in chemical disequilibrium with the rock, impacting mineral solubility and dissolution/precipitation processes. In this study, we investigate the effect of geochemical alteration on the frictional behavior of granites, and their seismogenic potential, by conducting direct shear experiments using samples with varying degrees of alteration. The samples originate from the Carnmenellis granite in Cornwall, SW England, and represent the formation used in the United Downs Deep Geothermal Power Project for heat extraction. Experiments were conducted on granite powders (referred to as gouges) at room temperature and 180°C, at simulated in situ confining and pore pressures of 130 and 50 MPa, respectively (∼5 km depth). With increasing degree of alteration, the frictional strength of the gouges decreases while frictional stability increases. At high temperature, frictional stability is reduced for all samples while maintaining the trend with alteration stage. Microstructural investigation of the sheared gouges shows alteration delocalizes shear by reducing grain size and increasing clay fraction, which promotes the formation of pervasive shear fabrics. Our work suggests that, within the range of tested pressures, more alteration of granite initially causes more stable shearing in a fault. This behavior with alteration is sustained at high temperatures, but the overall frictional stability is reduced which increases the potential for induced seismicity at higher temperatures. Plain Language Summary: Geothermal systems are of high interest as renewable energy source. However, hot fluids in the deep subsurface can chemically react with the host rock they flow through. These reactions then affect the fluid chemistry and the rock mineral composition. With changes in mineral composition, changes in mechanical and hydraulic properties are observed. We test the effects of a specific group of reactions (argillic alteration) on the frictional properties of granite powders by using an example rock that is targeted in geothermal projects in Cornwall, SW England. The frictional properties give information about how likely it is for granitic rocks to produce an earthquake while sliding. Our tests show that the more the granite has chemically changed, the easier the powder is to slide but the more stably it will slide. This means that chemically changed powders are less likely to create seismic waves on sliding. In addition, tests at high temperature (180°C) showed the same stabilizing effect within the reacted rocks but the general sliding stability was reduced over all samples. This implies that the potential for earthquakes is higher at higher temperatures, although the analyzed chemical reactions reduce this potential in granitic geothermal systems. Key Points: Large fault systems are targeted at depth as geothermal reservoirs in high‐heat producing granites in CornwallDirect shear experiments were conducted on a series of successively more naturally altered granites from a fault in Carnmenellis graniteAlteration makes sliding more likely but also more stable, while higher temperatures destabilize shearing [ABSTRACT FROM AUTHOR] |
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