Worldwide Inventory of Dissolved CO2 Sequestration Potential in Geothermal Systems

Abstract

Geothermal electricity generation has relatively low carbon emissions (average ~100 gCO2/kWh) compared to hydrocarbon alternatives (e.g., coal, ~1000 gCO2/kWh; natural gas, ~400 gCO2/kWh). Nevertheless, recent attention on emissions of magmatic CO2 and other non-condensable gas (NCG) from geothermal plants has prompted interest in their capture and reinjection. Geothermal fields are uniquely placed to effect emissions sequestration because the reinjection infrastructure is mostly in place (e.g., wells, pumps), and there is likely to be some understanding of subsurface interactions between NCGs and the reservoir.

Dissolving CO2 into reinjected fluid, either in the surface line or using a downhole bubbler, is one way to dispatch emissions without creating a buoyant free phase in the reservoir. The CO2 can be sourced as NCG capture from the produced geothermal fluid, by direct air capture (e.g., Iceland’s Orca project) or from a hybrid fuel energy scheme like bioenergy carbon capture and sequestration (BECCS). The latter two options allow the possibility of increasing CO2 concentration beyond the reservoir baseline, turning the geothermal system into a carbon sink. Depending on the local context, spillover benefits could include increased conversion efficiency (for hybrid schemes) and additional revenue where emissions are priced and traded.

In this study, we develop an inventory of dissolved CO2 sequestration potential in producing geothermal systems around the world. We estimate bounds on the potential sequestration rate (kt CO2/y) using fluid throughput rates inferred from plant capacity, and the likely range of rejection temperature and pressure. For select cases, we estimate bounds on the potential sequestration volume (Mt CO2) using volumetric estimates of resource size like heat in-place. Spillover benefits, including reduced fluid production through hybridization (geothermal-BECCS) and potential revenues from trading CO2 are also calculated.