The Influence of Carbon Dioxide on Reservoir Fluids at Utah FORGE

Abstract

A key component of the Utah FORGE field program has been chemical monitoring of flowback and produced fluids. In this regard, the ability to confirm and quantify the presence of gaseous species in circulated fluids dominated by carbon dioxide was first achieved in 2024. However, the effects of carbon dioxide are interpreted to have been present since the earliest phases of reservoir stimulation in April 2022 when flowback waters showed sharp increases in solutes accompanied by significant enrichments in oxygen and hydrogen. The possibility that two-phase conditions had developed in response to boiling was inconsistent with injection pressures that greatly exceeded vapor saturation. Later, when halite was identified along fractures in granitic rocks, it was evident that dissolution of soluble salts was most likely responsible for the increases in total dissolved solutes. The next significant observation followed the completion of 16B(78)-32 in July 2023 when during the first short term circulation test there was evidence of slugging flow and effervescing water indicating gas in the produced fluid. As a result, the flow lines for the Aug-Sept 2024 circulation test were designed to accommodate temporary installation of a mini-separator via which steam and gas could be collected and analyzed. The resulting produced water contained 0.07 to 0.13 wt. % CO2, inducing weak acidification, and the total carbon dioxide production was ~220 metric tons. Monitoring of well waters in 2025 continued to show infiltration of carbon dioxide and isotopic evidence of a deep upper mantle origin. Speculatively and drawing on experience from carbon dioxide injection for enhanced oil recovery and sequestration, carbon dioxide infiltration is linked to the drying out of waters, accounting for the stable isotope trends and salt deposition in fractures. This appears to be an active process that is associated with reservoir development, and one that also occurred naturally in the geologic past.