Temperature memory gauge survey and estimation of formation temperature of the USDP-4 conduit hole at Unzen Volcano, Japan

The Unzen Scientific Drilling Project was initiated in 1999 to investigate a magma conduit that had recently fed a volcanic eruption. The conduit hole, USDP-4, was drilled to 810 m in 2003. In 2004, the hole was extended with intention of reaching the conduit at a drilled depth of about 2000 m. This objective was achieved, and the final measured depth of USDP-4 was 1995.75 m. A new temperature memory gauge for rotary drilling application was designed and developed for the project in order to measure borehole temperatures as often as possible in the expected high temperature environment near the conduit. Temperature ratings of the tool are up to 250 °C with dewar. A total of 10 surveys were conducted at depths of 1555-1995.75 m. This tool was used in six surveys, and borehole temperature data was successfully obtained from all surveys. Based on the measured temperatures, it is apparent that the thermal energy potential of the conduit regime (i.e. thermal supply from conduit) is not large compared to that of most known geothermal systems. Formation temperatures along the conduit hole were estimated from measured temperatures by using a borehole temperature simulator, which we developed and subsequently modified. According to the simulation, the maximum in the formation temperature profile is located at a depth of about 1970 m, and its temperature was about 170 °C before the drilling operation started. This maximum approximately coincides with the center of the conduit, but is surprisingly low for a system that ceased eruption only 9 years before the temperature survey.