Natural Subsidence at the Rotokawa Geothermal 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 exploitation in 1997. This natural state subsidence can be related to mass removal from the reservoir by hydrothermal alteration. Pochee (2010) shows that altered Rotokawa reservoir andesite is depleted in silica by as much as 15% relative to unaltered reservoir andesite. This depletion is manifested as porosity enhancement due mostly to plagioclase feldspar phenocryst dissolution, similar to that found by Stimac et al (2004) for reservoir rock at the Tiwi field, Philippines. The natural subsidence at Rotokawa is inferred to be due to the gradual collapse of this pore space. Long term subsidence within the reservoir is expected to create strain similar to that experienced in producing oil field reservoirs. Well damage due to subsidence commonly occurs due to horizontal thrust strain along hard-soft bedding plane boundaries and due to reactivation of fault structures. Similar processes may occur within the Rotokawa geothermal reservoir due to alteration-induced compaction, although a lesser strain rate coupled with softer, altered rock material, suggest that the effect may be more subtle. The rocks most susceptible to fracturing due to alteration-induced compaction would be networks of relatively hard rocks (e.g. quartz veins and zones of silicification) surrounding softer altering reservoir rock. In this way, progressive alteration compaction would lead to fracturing of encompassing vein networks, leading to a positive feedback between alteration and permeability. At a minimum, alteration compaction would play a role in providing incremental rock stress to fracturing due to local tectonic stress and periodic hydraulic pressure changes. Alteration induced subsidence, such as inferred for Rotokawa, suggests that hidden geothermal resources might be expressed at the surface by geologic evidence of long term subsidence or be observed by modern measurements of surface deformation, such as historic geodetic measurements, precision GPS and satellite interferometry.