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 behaviour 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 MPa 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 delocalises 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 behaviour with alteration is sustained at high temperatures, but the overall frictional stability is reduced which increases the potential for induced seismicity at higher temperatures.

Mature fault cores are comprised of extremely fine, low permeability, clay-bearing gouges. Saturated granular fault materials are known to dilate in response to increases in sliding velocity, resulting in significant pore pressure drops that can suppress instability. Up to now, dilatancy has been measured only in clay-poor gouges. Clay minerals have low frictional strengths and, in previous experiments, even small proportions of clay minerals were shown to affect the frictional properties of a fault. It is important, therefore, to document in detail the impact of the proportion of clay on the frictional behaviour and dilatancy of fault rocks. In this work, a suite of triaxial deformation experiments elucidated the frictional behaviour of saturated, synthetic quartz-clay (kaolinite) fault gouges at effective normal stresses of 60 MPa, 25 MPa and 10 MPa. Upon a 10-fold velocity increase, gouges of all clay-quartz contents displayed measurable dilatancy with clay-poor samples yielding comparable changes to previous studies. Peak dilation did not occur in the pure quartz gouges, but rather in gouges containing 10 to 20 wt% clay. The clay content of the simulated gouges was found to control the gouge frictional strength and the stability of slip. A transition occurred at ~40 wt% clay from strong, unstably sliding quartz-dominated gouges to weak but stably sliding clay-dominated gouges. These results indicate that in a low permeability, clay-rich fault zone, the increases in pore volume could generate pore-fluid pressure transients, contributing to the arrest of earthquake nucleation or potentially the promotion of sustained slow slip.

Deformation bands are the main structural element of fault damage zones within sandstone reservoirs. The prediction of band occurrence and their petrophysical impacts is based largely on the understanding that the yield and deformation mechanism of sandstones is primarily controlled by porosity and mean grain size. Whilst this is supported by field observations within aeolian successions, where bands are predictably favoured within coarse-grained, high-porosity sandstones, the prediction of deformation bands within texturally complex mixed aeolian-fluvial reservoirs on the basis of porosity and grain size alone, may be unreliable. The effect of grain sorting on the mechanical behaviour of sandstones is not well understood, although it is generally regarded that deformation band formation is inhibited in texturally immature sandstones with a poor level of sorting. We examine the effect of sorting on both the inelastic yield of sandstones, the dominant deformation mechanism by which yield occurs, and the textural and microstructural changes with deformation, using a series of triaxial experiments on unconsolidated quartz sands. Hydrostatic experiments were conducted on over-consolidated samples of very well- to moderately-sorted sands with a range of mean grain sizes from 128-700µm. We report accurate prediction of P* using porosity x grain radius, with P* reduced with decreased sorting. Constant displacement rate triaxial experiments are performed at up to 10% axial strain to explore yield behaviour in both the brittle dilatant regime and shear-enhanced compactive regime. Experiments were repeated with systematically varied grain sorting whilst mean grain size and porosity was maintained. The textural and petrophysical changes are observed and quantified using pore volumometry, back scattered electron microscopy, digital image analysis and point counting. Results show that in well-sorted sands, localised cataclasis and deformation band formation is the dominant deformation mechanism. In poorly-sorted sands deformation occurs through a combination of grain boundary sliding and randomly distributed pockets of cataclasis. Using grain size analysis we identify greater levels of cataclasis and production of fines in well-sorted sands, resulting in permeability reduction up to one order of magnitude more than that of poorly-sorted sands deformed at the same conditions. We hypothesise that band formation within poorly sorted sandstones may be promoted by the formation and propagation of bands in adjacent well sorted sandstones where band formation is favoured. These results give insight into the deformation, textural changes, and permeability impact of both unconsolidated and consolidated siliciclastic reservoirs.

Determining conditions for earthquake slip on faults is a key goal of fault mechanics highly relevant to seismic hazard. Previous studies have demonstrated that enhanced dynamic weakening (EDW) can lead to dynamic rupture of faults with much lower shear stress than required for rupture nucleation. We study the stress conditions before earthquake ruptures of different sizes that spontaneously evolve in numerical simulations of earthquake sequences on rate-and-state faults with EDW due to thermal pressurization of pore fluids. We find that average shear stress right before dynamic rupture (aka shear prestress) systematically varies with the rupture size. The smallest ruptures have prestress comparable to the local shear stress required for nucleation. Larger ruptures weaken the fault more, propagate over increasingly under-stressed areas due to dynamic stress concentration, and result in progressively lower average prestress over the entire rupture. The effect is more significant in fault models with more efficient EDW. We find that, as a result, fault models with more efficient weakening produce fewer small events and result in systematically lower b-values of the frequency-magnitude event distributions. The findings 1) illustrate that large earthquakes can occur on faults that appear not to be critically stressed compared to stresses required for slip nucleation; 2) highlight the importance of finite-fault modeling in relating the local friction behavior determined in the lab to the field scale; and 3) suggest that paucity of small events or seismic quiescence may be the observational indication of mature faults that operate under low shear stress due to EDW.