The thermal structure and temporal evolution of high-enthalpy geothermal systems

Abstract

a b s t r a c tNumerical modeling is a powerful tool to investigate the response of high-enthalpy geothermal systems to production, yet few studies have examined the long-term evolution and thermal structure of these systems. Here we report a series of numerical simulations of fluid flow and heat transfer around magmatic intrusions which reveal key features of the natural thermal and hydraulic structures of high-enthalpy geothermal systems. We explore the effect of key geologic controls, such as host rock permeability, the emplacement depth and geometry of the intrusion, and temperature-dependent permeability near the intrusion, on 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 (≥10 −14 m 2 ) show shallow boiling zones restricted to ≤1 km depth, while intermediate permeability (∼10 −15 m 2 ) 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. If a cooling intrusion becomes permeable at temperatures significantly in excess of the critical temperature of water, the enthalpy of the upflow becomes high enough that systems with high permeability show vertically extensive boiling zones, and intermediate permeability systems spatially extensive zones of supercritical water near the intrusion. The development of multiple, spatially separated upflow plumes above a single intrusive body is characteristic of systems with high permeability and deep emplacement depth. Depending on the primary geologic controls, 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.

Boiling and condensation of saline geothermal fluids above magmatic intrusions

“…The simulations describe the development of a saline geothermal system within a two‐dimensional vertical section above a magmatic body intruded into host rocks with a permeability of 10 −15 m 2 initially containing pore waters with seawater salinity. We choose this permeability value because previous studies have shown that it is common in natural systems [ Björnsson and Bödvarsson , ; Manning and Ingebritsen , ] and close to optimal for the formation of spatially extensive supercritical geothermal resources in pure water systems [ Scott et al ., , ]. We performed simulations testing possible conditions for the Reykjanes system, in which intrusion depth and brittle‐ductile transition temperature are treated as variables.…”

Utilizing supercritical geothermal systems: a review of past ventures and ongoing research activities

“…Scott et al (2015a, b) applied this modeling approach to evaluate processes in the supercritical zone in the IDDP-1 well. Scott et al (2016) modeled the temporal evolution of high enthalpy geothermal systems associated with shallow intrusions, and observed that host rock permeability and composition, intrusion depth, intrusion geometry, and strain rate all play important roles in the thermal structure. Scott et al (2017) noted that, for saline hydrothermal systems, the depth of the magmatic intrusions powering geothermal systems impacts the efficiency of heat transfer.…”

The Effect of Boiling on Seismic Properties of Water‐Saturated Fractured Rock

“…Here we will refer to this state as a young system. The temperature-depth profile of such a young system is reproduced from Scott et al (2016) in blue in Figure 8a. At later times, fluid convection involves larger and larger parts of the system, up to the point where the upflowing fluid is no longer hindered by the cold and dense overlying liquid groundwater and a boiling zone is established from large depths up to the surface.…”