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

Mineral carbon storage in basalt has been proven as an effective means of durable and verifiable geologic carbon sequestration. Here we describe and investigate 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 simulate WAG injections into various basalt reservoirs to investigate injection parameters and reservoir characteristics that accelerate mineralization during WAG injections. Results indicate that optimized WAG injections can double mineralization compared to traditional scCO₂-only using half as much water as an aqueous-phase approach. 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.