EXPERIMENTAL PHYSIOCHEMICAL INVESTIGATION OF HIGH-TEMPERATURE BRINE-SHALE INTERACTIONS

When water is injected into the deep surface, mineral dissolution and precipitation reactions influence the host rock's porosity and permeability. These changes either enhance or retard the fluid flow, influencing the success of activities such as hydrofracking, deep geothermal operations, and the disposal of contaminated waters. To improve understanding of these crucial reactions in shale-rich strata, this study uses high-temperature batch reaction experiments to evaluate the effects of water-rock interaction over a range of pH and salinity conditions. Specifically, we investigated fluids at pHs of 2, 4, and 6, and at salinities of 12% TDS, 3.2% TDS, and negligible TDS, and solutions characterized by Na-Cl and Na-Ca-Mg-K-Cl chemical compositions.

Analysis of the changes to pH and alkalinity over time identifies distinct reaction pathways related to the different fluid conditions. Low salinity solutions with initial pH values of 2 maintain an acidic pH for the first few hours before rising to circumneutral pH for the experiments' duration. High salinity solutions with an initial pH of 2 reach circumneutral pH within the first few minutes. Dilute fluids, and saline fluids with only a Na-Cl composition, with initial pH values of 4 and 6 increase in pH to values as high as 9 during the first 24 hours before decreasing to a circumneutral pH range over the following weeks. The Na-Ca-Mg-K-Cl solutions with initial pH values of 4 and 6 vacillate around circumneutral conditions for the experiments' duration. Changes in alkalinity combined with some initial analysis of bulk elemental concentrations of experimental solutions identify a number of mineral dissolution, oxidation, and precipitation reactions. Carbonate dissolution is marked by increasing alkalinity and the release of Ca, Mg, and Sr. Silicate dissolution is marked by increases of Si, Al, Na, and K, and subsequent precipitation of the same phases is marked by the loss of the same elements. Sulfide oxidation and oxide precipitation is marked by a rapid release of dissolved Fe and trace metals followed by their disappearance. These results suggest a series of distinct chemical reactions that tend to converge on a single fluid chemistry within two months. The next step will be to understand how the pathways impact rock properties such as porosity and permeability.