A 3-D numerical model for geothermal circulation in a 'TVZ-like' setting

Abstract

In the TVZ heat is transported through the brittle crust by the up-flow of hot fluid through more than 20 distinct convective plumes, the locations of which depend upon prevailing permeability and temperature conditions. Complete knowledge of such conditions would greatly assist modelling studies investigating deep circulation or field evolution over geological time; processes that are typically impenetrable to direct surface observation. In practice, data is available only for the near surface and at depths greater than 2 km estimates of temperature and permeability are largely inferred or reasoned.

In this paper we detail a 3-D numerical model for geothermal circulation in a rifted setting, in which heat is transported through a 50 x 80 km region by numerous convection cells. The model attempts to describe the TVZ in a statistical sense by matching average values or distributions of geophysical observables, e.g., the average heat output across the rift, the average dimensions and properties of geothermal fields, etc. This approach permits permeability properties and temperature boundary conditions to be specified stochastically, i.e., by an average value about which some degree of fluctuation and spatial correlation is permitted.

The resulting model reproduces the average surface heat output for the TVZ of 0.6-0.8 W m-2, an across strike bimodal heat distribution, and individual field heat outputs and dimensions consistent with TVZ fields. Fluid flow paths indicate that recharge is sourced from an area 4-5 km distant from the field boundary, in which fluids slowly percolate to depths of 3-7 km before rapidly ascending to the surface. Total residence time varies widely both within and between fields but averages 10-30 kyr, of which the final 20% represents buoyant upwelling of the fluid.