title: Structurally controlled fluid flow in geothermal reservoirs: Insights from the field and laboratory
Creators: name: Beynon, Steven
Publisher: name: University of Liverpool Repository
Year: 2022

description: The understanding of fluid flow within the Earth’s crust is of great importance to our knowledge of geohazards (such as earthquakes) and use of georesources (such as geothermal energy). Fluid flow in crystalline rocks is largely controlled by the permeability of faults and their associated fracture networks, the physical properties and temporal history of which can control to what degree they act as flow conduits or barriers. Better constraints are required as to how these fracture networks form, how fluids are transported within them, how they respond to deformation and how permeability may evolve with time. In an applied context, this insight into the physiochemical processes ongoing in deep, high temperature, fractured hydrothermal systems has the potential to increase the long-term productivity and sustainability of geothermal energy production. Since fault properties and behaviours are difficult to observe at depth, an approach using field analogues and laboratory experiments is key to understanding such complex systems. In the field, an exhumed duplex-hosted hydrothermal system was studied (using statistical analyses of mechanical damage and mineralogical data from fractures sealed by epithermal precipitation) to investigate the fault properties that control the spatial and temporal heterogeneity of structurally controlled fluid flow within fault damage zones. In the laboratory, a range of thin section analyses and experimental rock deformation techniques are employed to investigate how hydrothermal fluid flow affects the composition, texture and mechanical behaviour of altered host rock and epithermal veins within it. The frictional strength and stability of argillic alteration (i.e. clay ‘gouges’ more typical of fault cores) is also investigated experimentally, with a focus on the effect of fluids on deformation processes. Field data show that the density and connectivity of structural damage across the studied fault duplex is predictably heterogeneous at a range of scales, conforming to previous observations suggesting that such variation may be caused by the amount of fault displacement coupled with regional fault density. Analysis of epithermal precipitation within fractures reveals that in general fluid flow is greatest in areas of high structural density, with most sealing by precipitation having occurred in thin fractures favourably oriented perpendicular to the minimum principal stress (σ3). Laboratory data indicate that the nature of precipitation (i.e. vein composition and texture) evolves both spatially and temporally. Furthermore, the degree of propylitic alteration in the fractured host rock appears to control where subsequent creation of structural permeability takes place, rather than the presence of a vein itself as a planar discontinuity. With regards to argillic alteration, the frictional strength of clay fault gouges increases as water is progressively removed, whilst their frictional stability decreases, suggesting that the saturation state of clays may play a part in controlling permeability and fluid flow through seismicity. The presence of water appears to be key in promoting time- and slip-dependent frictional changes, constraining operative grain-scale deformation mechanisms to those that are fluid assisted. The data presented here have important implications for geothermal exploration, as well as elements of production and stimulation by fluid injection within Enhanced Geothermal Systems. It is understood that further insight could be gained through a more quantitative understanding of fracture sealing processes that commonly cause issues with flow rates in geothermal wells, namely the influence of changing fluid pressures, temperatures and chemistries on the nature, rate and volume of precipitation. With this goal in mind, a new high-pressure, high temperature triaxial deformation apparatus has been designed and built to simulate a range of upper crustal geothermal gradients whilst under confining pressure. Here, the new apparatus is described in detail, and a suite of experiments is proposed based on a comprehensive literature review, whereby fluid containing minerals in solution can be flowed through a fractured rock core at a range of carefully controlled physical conditions. It is hoped that this experimental setup may soon provide crucial data to help improve models of fluid flow and permeability evolution in structurally controlled geothermal reservoirs, and ultimately the long-term productivity and sustainability of geothermal energy production.