Pacific Section AAPG, SPE and SEPM Joint Technical Conference

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Geological Carbon Sequestration: Possible Impact On Shallow Freshwater Aquifers Near A Proposed Sequestration Site, Southern San Joaquin Valley, California


Carbon Capture and Sequestration (CCS) of CO2 from high-emission point-source sites such as power plants is one strategy to reduce greenhouse gas emissions and mobilize residual oil in reservoirs for enhanced oil recovery (EOR). Much work has been done on the effects of injected CO2 on brine geochemistry and the mineralogy of proposed geologic storage formations under the high pressures and temperatures specific to injection sites. However, another aspect that needs consideration is the unintentional migration of CO2 into shallow fresh groundwater aquifers through conduits such as fractures and faulty deep well casings. This could have a profound impact on the quality of the groundwater by mobilization of trace metals such as arsenic and needs to be investigated for each proposed CCS site. In this study, a total of twelve aquifer samples from a well in the Kern Water Bank were used to quantify changes in aqueous geochemistry and aquifer mineralogy resulting from the interaction of the freshwater aquifer system with a 100% CO2 atmosphere. Samples are from the Upper Tulare Formation and have been previously identified to contain elevated concentrations of arsenic. Arsenic and other heavy metal mobilizations due to groundwater interaction with CO2 are of special interest in this study. Sediment samples were combined with deionized water and placed in a precision controlled atmospheric glove box where they were exposed to and sampled in a CO2 environment at near surface pressures and temperatures for 31 days. Mineralogical analysis conducted before and after the experiments using a scanning electron microscope with energy- and wavelength-dispersive X-ray spectrometers focused on changes in aquifer mineralogy and texture. Aqueous geochemical analyses included pH, alkalinity titrations, ion chromatography for major ions, and ICP mass spectrometry for trace elements. After CO2 exposure, pH of all samples dropped by two points and then gradually increased back to a new equilibrium value just below the pH 6.5 secondary drinking water standard. Alkalinity increased throughout the experiments indicating mineral dissolution. Geochemical data are modeled with the geochemical speciation model PHREEQC to identify specific dissolution/precipitation reactions.