Geothermal Prospecting of Ko'olau and Wai'anae Volcanoes, O'ahu, Hawai'i

摘要

Energy demand on the Island of O'ahu is greater than all the other islands in the state combined, such that the viability of developing a (currently unproven) geothermal resource on this island would be high. Although the State of Hawai'i's geothermal resource is largely uncharacterized, subsurface heat that may constitute a geothermal resource is most likely to exist within the caldera region of the state's large shield-type volcanoes. Two shield volcanoes comprise the Island of O'ahu: Ko'olau and Wai'anae. The Hawai'i Groundwater and Geothermal Resources Center is in the process of completing an over 5 year-long statewide geothermal resource assessment funded by the U.S. Department of Energy (Hawai'i Play Fairway grant # DE-EE0006729). This project 1) identified and compiled existing data relevant to subsurface heat, fluid, and permeability across the state; 2) identified priority sites for additional geothermal prospecting; 3) outlined and pursued a cost-effective strategy for such prospecting in priority locations, and 4) deepened an existing well proximal to the caldera of Lana'i volcano to approximately 1 km (～3500 feet) and found an elevated temperature gradient there. In general, we suggest a step-wise collection of field data within locations of interest as follows: ⅰ) collect and analyze groundwater samples from existing wells, ⅱ) perform a magnetotelluric geophysical survey to image the electrical conductivity of the subsurface, and ⅲ) drill at least 1 slim hole to a depth of 1.8 to 2 km (～6000 feet) to define the deep thermal gradient. A dearth of groundwater wells, proximity to the ocean, and large amounts of electrical infrastructure within the caldera region of the Wai'anae and Ko'olau volcanoes render groundwater and magnetotelluric data collection infeasible. As such, the next prudent geothermal exploration step for this region is the drilling of a slim hole. Limited groundwater data that exist within or in the vicinity of both calderas, as well as the preliminary results from the drilling on Lana'i (in 2019) suggest that the presence of elevated groundwater temperatures within the two shield volcanoes on O'ahu is entirely plausible. Note that drilling to depths sufficient to define the deep temperature gradient is the only way to uniquely validate the presence of subsurface heat. We have estimated that funding in the amount of $2.5 to 3M to the University of Hawai'i would enable us to: 1) obtain needed permits from the state, city and county; 2) drill a continuously-cored slim hole from surface to a depth of approximately 1.8 to2 km (～6000 feet); and 3) conduct downhole testing on water parameters, including temperature, for up to a year following wellbore completion.