GEOCHEMICAL RESPONSE TO THERMAL ENERGY STORAGE IN THE COLUMBIA RIVER BASALT AQUIFER SYSTEM BENEATH THE PORTLAND BASIN, OREGON

As part of a study to determine the efficacy of Deep Direct-Use Thermal Energy Storage (TES), we examined the geochemical and resulting hydrogeological implications of injecting and extracting heated water into the Columbia River Basalt Group beneath downtown Portland, OR. TES entails seasonally repeated cycles of hot or cold water injection, storage, and extraction from slow-moving groundwater-flow zones within deep layers of the aquifer system. Extracted water can then be used to heat and cool buildings. Though a promising technology, DDU-TES cycles may trigger or accelerate mineral dissolution and precipitation reactions, particularly at elevated temperatures. This may alter aquifer porosity and permeability and result in scale formation in heat exchange systems that reduces the thermal storage-and-release efficiency. Geochemical reaction modeling of native waters at both ambient and elevated temperatures suggests that calcite, witherite, siderite, and rhodochrosite may be key carbonate precipitates, while kaolinite and smectite may be significant clay precipitates. Batch reaction experiments simulating thermal energy storage were conducted by heating representative samples of basalt and groundwater to 35°C and 72°C for a period of 30 - 60 days, then cooling sub-samples of reacted water. Experimental results indicate that water-rock reaction rates are surface controlled. The mineral assemblage in interflow zones appears more reactive than that of basalt flow interiors. As expected, dissolved Si and Al concentrations increased with temperature, while Ca levels decreased. Calcium and silicon mass transfers, resulting from batch reactions at 72°C, were used to calculate the maximum potential mass of calcite precipitated and amorphous silica dissolved in different zones of the aquifer. Upon heating, basalt flow interior samples precipitated the equivalent of 24 mg/L CaCO₃ and dissolved 54 mg/L SiO₂, while interflow zone samples precipitated the equivalent of 74 mg/L CaCO₃ and dissolved 100 mg/L SiO₂. Calcite scale may form in pipes at elevated temperatures, and silica dissolved at higher temperatures is likely to form silica deposits when the water is again cooled.