Heat and Fluid Transport Induced by Convective Fluid Circulation Within a Fracture or Fault

Abstract

AbstractNatural water convection in subvertical fractures, fracture zones, or faults can perturb the temperature field around the fracture and enhance and focus vertical heat flow within. We investigate, by means of numerical simulation, the effects of convection in a deeply buried vertical fracture zone. Fracture zone transmissivity, defined as permeability times thickness of the permeable region, is found to be the primary control on convection style rather than fracture zone thickness or permeability alone. In an impermeable host rock, the convection‐induced thermal anomaly propagates solely via conduction, diminishing away from the fracture. Convective heat flow increases with fracture transmissivity up to ~10−8 m3, when a plateau in convective heat flow is reached, constrained by fracture size and the host rock's thermal conductivity. Permeable host rocks modify these results significantly. In a moderately permeable host rock (10−14 m2), convection in the fracture induces non‐Rayleigh fluid convection, while thermal effects and fluid exchange between host rock and fracture remain moderate. If the host rock is sufficiently permeable to allow porous medium Rayleigh convection to occur (10−13 m2), the convection patterns within the fracture are overprinted by the host's convective patterns. Fluid exchange between the fracture and the rock will be significant. Our findings provide insight into how thermal anomalies in the uppermost crust may relate to locally enhanced heat flow from convection in nonoutcropping fractures below. Furthermore, the results for permeable host rocks provide evidence for previously inferred hydrologic scenarios for the formation of certain hydrothermal, vein‐type mineral deposits.