Rationale for Targeting Fault Versus Formation-Hosted Permeability in High-Temperature Geothermal Systems of the Taupo Volcanic Zone, New Zealand

Abstract

Finding permeability is an essential aspect of production or injection well targeting during exploration, resource delineation and production stages of geothermal field development. In the past, production well targeting at New Zealand’s Wairakei and Kawerau geothermal fields was often “fault-biased”, with major faults (determined by mapping, inferred from stratigraphic displacement in wells, and more recently by microseismic techniques) assumed to provide the primary fluid pathways/well inflows. In many cases, we have found that this approach is warranted, but success in New Zealand targeting fault structures has varied within fields, and from field to field. PTS logs of many recently drilled production and injection wells in Ohaaki, Mokai, and Rotokawa geothermal systems, demonstrate primary formation-hosted permeability, rather than being a secondary target, actually provides the major producing (permeable) inflow zones. In these fields, permeable zones are commonly hosted by ignimbrite and fractured lava (rhyolite or andesite) units, even though basement faults are likely to provide deep structural control on the primary fluid upflow, with lateral and/or vertical permeability controlled by contrasts in rock density and porosity, and spacing and inter-connection of fractures providing excellent production permeability (and in some developments, injection targets). In our paper, we highlight production drilling successes at Ohaaki, Mokai, Wairakei and elsewhere, and suggest for geothermal systems where fault-zone permeability is clearly demonstrated that a structure-based rationale for geothermal well targeting should be followed. However, if a fault-zone feed-zone link is equivocal (as is more often the case), a facies approach targeting auto-brecciated or jointed lava or welded pyroclastic units, or formations with intrinsic primary permeability, should be given equal consideration.