Fluid Flux in Fractured Rock of the Alpine Fault Hanging‐Wall Determined from Temperature Logs in the DFDP‐2B Borehole, New Zealand

Abstract

Abstract Sixteen temperature logs were acquired during breaks in drilling of the 893m‐deep DFDP‐2B borehole, which is in the Alpine Fault hanging‐wall. The logs record various states of temperature recovery after thermal disturbances induced by mud circulation. The long‐wavelength temperature signal in each log was estimated using a sixth‐order polynomial, and residual (reduced) temperature logs were analyzed by fitting discrete template wavelets defined by depth, amplitude, and width parameters. Almost two hundred wavelets are correlated between multiple logs. Anomalies generally have amplitudes <1°C, and downhole widths <20m. The largest amplitudes are found in the first day after mud circulation stops, but many anomalies persist with similar amplitude for up to 15 days. Our models show that thermal and hydraulic diffusive processes are dominant during the first few days of re‐equilibration after mud circulation stops, and fluid advection of heat in the surrounding rock produces temperature anomalies that may persist for several weeks. Models indicate that the fluid flux normal to the borehole within fractured zones is of order 10−7 to 10−6 m s−1, which is 2–3 orders of magnitude higher than the regional flux. Our approach could be applied more widely to boreholes, as it uses the thermal re‐equilibration phase to derive useful information about the surrounding rock mass and its fluid flow regime.

Keywords

• permeability
• groundwater
• hydrogeology
• geothermal
• aquifer