Factors controlling large-scale hydrodynamic convection in the Taupo Volcanic Zone (TVZ), New Zealand

Abstract

A review of the Taupo Volcanic Zone (TVZ) geothermal fields setting shows that most of these systems share the same general characteristics including a highly permeable or fractured subsurface, a capping structure, close proximity to a river or lake, and low topographical terrain. In a number of these fields, the presence of a surface water body is associated with the separation of the plume into two entities on both sides of the water body (e.g. Wairakei-Tauhara, Rotokawa, Ohaaki, Ngatamariki, Orakeikorako, Atiamuri, Mangakino, Kawerau and others). The aim of this work is to model a typical TVZ geothermal system to natural state using a single large-scale reservoir model. 2D numerical models were developed to include the total extent of the hydrothermal system from the surface down to the ductile impermeable crust. This is to explore the impact of the system’s internal (caprock vertical permeability and fault network) and external (surface water body and topography) settings on the size, shape and location of the convective geothermal plume. Simulations were conducted using the Super Critical-TOUGH2.2 numerical simulation code. Fault networks and an overlying caprock are found to have a critical role in the development of the geothermal reservoir providing a preferential pathway for the rising of geothermal fluid and trapping it within the subsurface. Surface water bodies such as lakes and rivers have only a limited shallow impact on the natural thermal output whereas the topography strongly influences the geometry of the thermal plume.