Using Carbon and Oxygen Isotopes to Monitor Water-Rock Interaction Under Simulated Geothermal Conditions

Engineered or Enhanced Geothermal Systems (EGS) have to potential to unlock vast high-temperature energy resources that lack the natural permeability and fluid flow required for power generation. In order to ensure sustained fracture connectivity in EGS, water-rock interactions must be understood in high temperature and pressure systems. Using cores from Brady's Field Well BCH-03, investigations were conducted to test the feasibility of using carbon and oxygen isotopes to monitor water-rock interactions under simulated geothermal conditions. Reactions were examined in a series of high-temperature pressure vessels and a high-temperature flow-through system in which a calcium bicarbonate solution was exposed to meta-volcanic rocks from up to three target depths (1358, 1396, and 1459 m). Experiments were conducted for 14--20 days at 90--144°C and 103--120 bar. A comprehensive characterization of the cores was conducted prior to experiments to assess the isotopic compositions of the rock samples. Water samples were collected before and after each experiment for isotopic and geochemical analyses. Samples were collected more frequently in the high-temperature flow-through system. Throughout the experiments, the isotopic results revealed significant enrichments of delta13CDIC in the solution while delta18OH2O remained unaffected by water-rock interaction. Geochemical data exhibited a decline in calcium and alkalinity and an increase in sodium and potassium concentrations in solution. The results indicate that the calcium bicarbonate solution, undersaturated with calcite, drove initial calcite dissolution in the rock. As the temperature increased and the solubility of CO2 declined, the solution became oversaturated with calcite, leading to CO2 degassing and subsequently calcite precipitation. The lack of measurable delta18O shifts can be attributed to the abundance of oxygen in the solution that constrained the impact of water-rock interaction. The 13CDIC results indicate the presence of significant calcite dissolution in all experiments with carbon isotope mixing models from the static reactor experiments suggesting a 37.8 to 54.6% contribution of delta13C from the rock to the post-reaction fluid. Although early-stage calcite dissolution may have temporarily increased the calcium and alkalinity concentrations in solution, the effects of CO 2 degassing and subsequent calcite precipitation dominate the geochemical results. The geochemical data also suggests the presence of silicate mineral reactions not observed in the isotopic results.