Water-alternating-gas injections for optimized mineral carbon storage in basalt

Abstract

Mineral carbon storage in basalt has been proven as an effective means of durable and verifiable geologic carbon sequestration. This study investigates a novel technology aimed at optimizing subsurface mineralization: water-alternating-gas (WAG), or cycled injections of free-phase CO₂ (e.g., supercritical) and water. Incorporating injection of supercritical CO₂ (scCO₂) into basalt can minimize water demand, increase per-well injection capacity, and expand the feasible range of basalt carbon storage. Cycling water between injection of scCO₂ can accelerate geochemical reactions and shorten mineralization timeframes. We model aqueous-phase, scCO₂-only, and WAG injections into subsea and onshore basalt sites using the STOMP-CO₂ simulator. We first simulate WAG injection into mid-ocean ridge basalt at the Juan de Fuca plate in the Northeast Pacific Ocean to investigate injection parameters and reservoir characteristics that accelerate mineralization during WAG injections. Results indicate that WAG injections can be optimized to mineralize 100 % of a 1 Mt CO₂ injection within 40 years. Optimized WAG injections can double mineralization compared to traditional scCO₂-only using half as much water as an aqueous-phase approach. We then compare the efficiency of WAG injections by simulating scenarios at two additional sites: offshore basalt in the Louisville Seamount, a subocean volcano in the Southwest Pacific, and continental flood basalt along the Columbia River in Washington State. We observe faster mineralization at the Louisville seamount than the Juan de Fuca site, likely due to variations in injection-zone mineralogy. At the Columbia River site, WAG scenarios improve mineralization the most relative to the scCO₂-only injection and increase feasible per-well injection rates relative to aqueous-phase approaches. Our results indicate that WAG has the potential to optimize carbon mineralization in basalt and substantially advance the scalability of this technology.