Play fairway analysis of geothermal resources across the state of Hawai‘i: 5. Slim hole drilling on Lāna‘i island

Abstract

This paper builds on four earlier papers detailing the results of the Hawai‘i Play Fairway (PF) project's first two phases. In Phase 3 the project deepened a water well on the rim of the caldera of Lāna‘i Volcano to a maximum depth of just over 1 km. Due to funding constraints, the project deepened an already existing water well that was proximal to our area of highest resource probability within the caldera. Drilling preparation included: completion of an Environmental Assessment, lowering a camera down the well, deviation logging, coordinating site preparation, procure and shipping, and participating in multiple community meetings. Drilling occurred 24/7 the entire month of June 2019. The project deepened Lāna‘i Well 10 from 427 m to 1057 m, while collecting continuous core, and measured a roughly linear temperature gradient averaging 42 °C/km and a maximum bottom hole temperature of 66 °C. This gradient is more than twice the background for Hawai‘i and within a range of gradients measured in this depth range for some exploration wells within the volcanically active East Rift Zone of Kīlauea Volcano. Geothermal gradient determinations and computed chemical geothermometer temperatures indicate that accessible temperatures within Lāna‘i Well 10 are 130–200 °C between 2 and 3 km depth. This paper includes a summary of detailed core logging, which found pervasive hydrothermal alteration. We recommend drilling a slim hole within Lāna‘i's caldera to ∼2 km, where considerably higher temperatures may be encountered. The positive implications this project's results have for the island of O‘ahu are substantial. The shield stage of O‘ahu's volcanoes ended 1–2 Million years earlier, however O'ahu uses more electricity than the rest of the islands combined.