| Title | ASSESSING FLUID FLOW IN LOW-ENTHALPY GEOTHERMAL FIELDS — AN EXAMPLE FROM THE WHATAROA VALLEY, SOUTH WESTLAND, NEW ZEALAND |
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| Authors | L. Janku-Capova, J. Townend, R. Sutherland |
| Year | 2019 |
| Conference | New Zealand Geothermal Workshop |
| Keywords | heat flow, thermal conductivity, thermal diffusivity, radiogenic heat productivity, geothermal gradient, fluid flux, permeable fractures, Alpine Fault |
| Abstract | Geothermal production in New Zealand is confined to high-enthalpy fields in the central North Island. The rest of the country would benefit from increased attention to low-enthalpy fields for direct use. The Whataroa Valley in South Westland is an example of a non-volcanic area with a high geothermal gradient (125 ± 55°C km–1) and heat flux (up to 460 mW m–2), produced by uplift along the Alpine Fault and the combined effects of rock and groundwater advection. Laboratory measurements show that the thermal properties of the schist bedrock in the hanging-wall of the Alpine Fault cannot by themselves account for the subsurface temperatures in the Whataroa Valley. Rather, the anomalously high heat flow is likely caused by fluid circulation. A new wavelet-based method of identifying anomalies in temperature logs and correlating them between logs acquired at different times during drilling of the 893 m-deep DFDP-2B borehole in the Whataroa Valley allows the identification of fractured zones and implies fluid fluxes of order of 10–7 to 10–6 m s–1. A fibre-optic cable in DFDP-2B has also been used to study post-drilling thermal equilibration. Variations about the steady-state temperature profile reflect large-scale flow patterns. Indications of temperature changes of ~2°C following regional seismicity imply that fractured reservoirs are sensitive to large-magnitude (>M6) earthquakes. The approaches to determining the hydraulic properties of fracture zones that we have developed in the Whataroa Valley could be used to assess the thermal state of other low-enthalpy geothermal fields. Laboratory measurements of thermal conductivity provide the baseline for assessing the conductive component of heat flow. Filtering signals from downhole temperature measurements discriminates long-wavelength anomalies (101 to 102 m) as aquifers and short-wavelength anomalies (100 m) as individual fractures. By analysing measurements acquired following a thermal disturbance, the fluid flux in these fractures can be quantified. |