Natural Subsidence at the Rotokawa Field and Implications for Permeability Development

Abstract

Levelling surveys of the Waikato River, running through the Rotokawa Geothermal Field and dating back to 1950 show that the field underwent significant subsidence prior to development in 1997. This natural state subsidence can be related to mass removal from the reservoir by hot spring discharge. Rock mass discharged by the Parariki stream, which drains the main thermal area in the field at Lake Rotokawa is calculated to represent 0.01% to 0.03% of the rock volume of the geothermal system per millennia, depending upon assumptions. Robey (2010) shows that altered Rotokawa reservoir andesite is depleted in silica by as much as 15% compared to unaltered rock. This depletion is manifested as porosity enhancement due to plagioclase feldspar phenocrysts dissolution, similar to that found by Stimac et al (2002 for reservoir rock at the Tiwi field, Philippines.

Long term subsidence within the reservoir is expected to create strain similar to that experienced in producing oil field reservoirs. Well damage in oil reservoirs undergoing subsidence is correlated with horizontal thrust strain along hard-soft bedding plane boundaries and due to reactivation of fault structures. Similar processes within the Rotokawa geothermal reservoir, coupled with incremental strain due to local tectonic stress and periodic hydraulic pressure changes, provides a mechanism for continued fracturing needed to maintain interconnected permeability in what would otherwise be a self-sealing geochemical system.