The thermo-hydraulic structure of high-enthalpy geothermal systems

High-enthalpy geothermal systems are focused zones of fluid flow and heat transfer, important sources of renewable energy and present-day analogues for some types of ore-forming systems. Numerical modeling of fluid flow and heat transfer is a key tool for elucidating the physical structure and dynamics of magma-driven hydrothermal systems. Although previous studies have provided important insights into the factors governing fluid convection, temperature restrictions or other simplifications have limited the applicability of model results to building understanding of natural geothermal systems, and particularly the interface between the magmatic heat source and the overlying hydrothermal system. The objective of this study is to derive a broad fundament for understanding the physical dynamics of high-enthalpy geothermal systems, focusing particularly on thermal structure, hydrodynamics and heat transfer in the vicinity of a magmatic intrusion. We present geologically-constrained numerical simulations exploring the influence of various controlling factors such as host rock permeability, magma chamber geometry and depth, the temperature to which the host rock around the intrusion remains permeable, and fluid salinity. The simulations highlight how mass and heat transfer dynamics in the vicinity of intrusion are controlled by these major geologic factors. The simulations document for the first time how and under what geologic conditions supercritical geothermal resources form around magmatic intrusions, revealing that they are an integral part of magma-driven geothermal systems. Potentially exploitable resources form in rocks with a brittle-ductile transition temperature higher than 450 ◦C, such as basalt. Water temperatures and enthalpies can exceed 400 ◦C and iv 3 MJ kg−1 in systems with intermediate host rock permeability (∼10−15 m). Conventional high-enthalpy resources result from mixing of ascending supercritical and cooler surrounding water, the extent of which occurs to a greater extent in systems with high host rock permeability (∼10−14 m). The models reproduce the measured thermal conditions of the resource discovered at the Krafla geothermal field, where the Iceland Deep Drilling Project drilled into a magmatic intrusion at 2 km depth, and suggest that similar resources may be widespread below conventional high-enthalpy geothermal systems. The key geologic factors control the depth and extent of boiling zones, the number and spatial configuration of upflow plumes, and how these aspects evolve over the systems’ lifetime. Host rock permeability is a primary control on the general structure, temperature distribution and extent of boiling zones, as systems with high permeability show shallow boiling zones restricted to ≤1 km depth, while intermediate permeability systems display vertically extensive boiling zones reaching from the surface to the intrusion. Intrusion emplacement depth is a further control, as intermediate permeability systems driven by an intrusion at ≥3 km depth only show boiling above 1 km. The development of multiple, spatially separated upflow plumes above a single intrusive body is characteristic of systems with high permeability and deep intrusion emplacement depth. Systems exhibit characteristic lateral and vertical gradients in pressure, temperature and enthalpy relative to the intrusive heat source which may aid in geothermal exploration and interpretation of field measurements. While numerical simulations using pure water fluids allow insight into the dynamics of high-enthalpy systems with dilute geothermal fluids, some active high-enthalpy geothermal systems contain saline fluids. Spatially extensive hypersaline brines only develop in systems with vertically extensive boiling zones, which form in host rocks with a intermediate permeability. These brines are underlain by halite-saturated vapor as long as the host rock remains permeable to high enough temperatures. This temperature depends on fluid pressure above the intrusion, and is ≥450 ◦C for shallow (∼2.5 km depth) intrusions and ≥550 ◦C for deeper (∼4.5 km depth) intrusions. Halite precipitation from hypersaline brines above shallow intrusions may lead to v sealing of pore space within zones of halite-saturated vapor. Hypersaline brines and halite-saturated vapor do not form in high permeability systems, which feature boiling zones restricted to <1.5 km depth. As a result, the main processes controlling salinity distribution in high permeability systems are adiabatic boiling and evaporative concentration of NaCl in liquid and dilution of thermal waters by mixing with meteoric waters. In intermediate permeability systems, salinities within the upflow zone are controlled by density-driven vertical segregation of ascending, low-salinity vapor and descending, high-salinity liquid reduces bulk fluid salinity in the near-surface while concurrently increasing salinity above the intrusion and leading to brine accumulation. Since chloride is commonly measured in geothermal fluids, these models may aid in interpretation of field observations. The process of phase separation of saline fluid into high-salinity brine and lowsalinity vapor occurs at temperatures and pressures in excess of the critical point of pure water. Therefore, numerical models of supercritical geothermal resource formation in pure water systems are not applicable to saline systems. Intrusion emplacement depth exercises a primary control on the style of phase separation near the intrusion and consequent patterns of heat and mass transfer. Above shallow intrusions (∼2.5 km depth), phase separation is only carried out by boiling, with vapor bubbles nucleating out of a liquid-like fluid. In contrast, phase separation above intrusions emplaced at 4.5 km depth occurs locally via condensation of hypersaline brine from intermediate or low density fluid. Although dense, low-mobility hypersaline brines reduce heat transfer from the top of the shallow intrusions, the fraction of brine within condensation zones remains small, thereby allowing heat transfer to be maximized. Therefore, in contrast to pure water systems, for which shallow intrusions make better targets for supercritical resource exploitation, the optimal targets in saline systems are located above deeper intrusions.