Laboratory Characterization of Geothermal Fractures using Chloride Concentration

This research continued an investigation into the viability of using chloride concentration to determine the mass inflow rate of feed zone fluid in a geothermal well. Specifically, chloride concentration is measured across the vertical depth of a wellbore, which can then be used in an analytical calculation from a chemical balance to infer the feed zone inflow rate. This project holds direct application for the characterization and quantification of the artificial fractures that distinguish the wells of Enhanced Geothermal Systems. This report focuses on laboratory experiments that replicated the fluid flow behavior in geothermal wells to test the chloride tool’s accuracy and sensitivity in quantifying the feed zone inflow rates. Initial efforts focused on modifying the Artificial Well System in the Stanford Geothermal Laboratory and calibrating the chloride tool. The modifications to the Artificial Well System enabled continuous freshwater experimentation and reduced possibilities of experimental error. The calibration process determined an empirical relationship between the voltage readings of the two ion selective electrodes (ISE) housed in the chloride tool and the chloride concentration of the surrounding fluid. Subsequently, static experiments were conducted to assess the sensitivity of the chloride tool to key independent variables. These experiments granted insight into the effects of varying the chloride concentration of the feed zone fluid, the vertical position of the tool relative to the feed zone, the feed zone inflow rate, and the presence of additional chemical species in the feed zone fluid on the voltage readings and inferred chloride molarity. Additionally, dynamic experiments investigated the effects of these variables, and the wellbore flow rate, on the ability of the chloride tool to locate and quantify the feed zone inflow rate. These experiments indicated that higher chloride concentrations, upward motion, closer proximity to the feed zone jet, and higher feed zone inflow rates resulted in more accurate characterization of the feed zone. Additionally, the presence of additional anions in the feed zone fluid was shown to both increase the magnitude of and reduce the noise in the voltage readings from the ISE probes. When additional anions were present, the absolute chloride concentration could not be determined accurately, but relative changes to apparent chloride concentration still provided accurate estimates for the quantification of the feed zone inflow rate. The findings of this project suggest that the chloride tool could accurately characterize the mass inflow rate of the artificial geothermal feed zone using relative changes in chloride concentration.