At Utah FORGE, a US Department of Energy sponsored EGS demonstration site, a three stage hydraulic stimulation in the toe section of highly deviated injection well 16A(78)-32 was completed in April 2022. Two types of fluids were used, slickwater and a crosslinked polymer fluid. In addition, a distinct naphthalene sulfonate compound was added to the injected fluids at each stage to act as a non-reactive tracer. Samples of flowback waters were collected and analyzed to monitor tracer concentrations and water chemistry. Of the 10,062 bbl injected, 6,240 bbl, or 62% of the fluid was recovered. The tracer return data are interpreted to show no cross contamination of injected fluids between stages due to either communication between stimulated volumes, or along the exterior of the casing string through the cement. These results also demonstrate the tracer’s effectiveness under EGS reservoir conditions, and its utility to confirm zonal isolation in multistage hydraulic stimulations. The chemical composition of the returned fluids shows unexpected, and drastic changes over relatively short periods of time ( less than 33½ hours). The dominant trend is for the concentrations of dissolved solids to increase with time. The viscous fluids appear to have limited the ability of elements to dissolve and mobilize into the aqueous phase. Changes become more pronounced the longer the fluids were in contact with the reservoir rocks. The transient chemical effects are interpreted to result from water-rock interaction in the reservoir.

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