Geothermal Prospecting of Koolau and Waianae Volcanoes, Oahu, Hawaii

Energy demand on the Island of Oahu 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 Hawaii’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. 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 ~6000 feet. A dearth of groundwater wells, proximity to the ocean, and large amounts of electrical infrastructure within the caldera region of the Waianae and Koolau 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 Lanai (in 2019) suggest that the presence of elevated groundwater temperatures within the two shield volcanoes on Oahu is highly 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. Funding in the amount of $2.5 to 3M to the University of Hawaii enables 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 ~6000 feet, and 3) conduct downhole testing on water parameters, including temperature, for up to a year following wellbore completion.