| Abstract |
In the Taupo Volcanic Zone (TVZ) of New Zealand geothermal circulation is facilitated high porosity quaternary volcanic deposits, and by fracture permeability associated with widespread normal faulting. High temperature fluids (>250°C) exit the crust at more than 20 locations, many of which are associated with outcropping faults or fault tips, e.g., Te Kopia, Orakeikorako, Waiotapu-Waimangu. Frequent seismicity associated with these structures suggests that fracture permeability is not a static property of the system, but may oscillate or evolve in response to the applied tectonic extension, perhaps modulated by fluctuation in pore fluid pressure. Further, epithermal mineralization in the near surface can have a detrimental effect on permeability, creating a low porosity alteration shroud that influences surface outflow. In this work we report on a 3-D computational model that considers two perturbations to steady state convection in the vicinity of a fault. First, the rate of mineral deposition, as a function of mass flux and temperature gradient, is used to iteratively update porosity and permeability distributions with time, producing a low permeability capzone and sealing the system. Second, the effects of fault rupture are approximated by a transient permeability increase along a fault plane, permitting renewed circulation to the surface. Results are discussed in the context of geothermal field evolution over geological time scales and effects on the reservoir considered. |