| dc.description.abstract | Due to the mounting concern about climate change, geologic carbon storage (GCS) has become an attractive method of reducing atmospheric carbon. The United States has set ambitious goals for decreasing their CO2 emissions and in order to meet those goals, implementation of GCS is almost certainly necessary. As a result, the Department of Energy has funded the examination of the Arbuckle saline aquifer, as well as other aquifers around the United States, to assess their potential as carbon storage sites. Geologic sequestration of CO2 has been shown to be a feasible option, but research into details of CO2 storage is still needed. The Arbuckle is a deep (approximately 1270-meters below land surface), saline aquifer located in south-central Kansas. This study used geological, geochemical, and microbiological data combined with laboratory experimentation to examine the reservoir connectivity and caprock integrity of the Arbuckle saline aquifer using materials from two cores collected in the Wellington oil field in Sumner County, Kansas. Results from field characterization present strong evidence of hydraulic separation of the Upper and Lower Arbuckle and the likelihood of an extensive fracture network evidenced by essentially homogeneous brines in the Lower Arbuckle. Hydraulic separation of the Upper and Lower Arbuckle could result in decreased storage capacity, however isotopic data also points toward the presence of smaller, less influential baffles in the Upper Arbuckle which could serve as important impediments to buoyant plume behavior, increasing pore space and solubility trapping. Controlled, laboratory batch experiments carried out as part of this study also produced results with interesting implications for injectivity and caprock integrity of the Arbuckle aquifer. These experiments utilized the Chattanooga Shale, the immediate seal for the Arbuckle, the Cherokee Shale, the regional seal, and dolomite, the most abundant mineral in the storage reservoir. Gypsum precipitation occurred when the Chattanooga Shale containing pyrite was exposed to 100% pCO2. Similarly, gypsum precipitation and rhomboclase dissolution occurred when pure pyrite was exposed to 100% pCO2. Dissolution of the dolomite was the predominant reaction for the dolomite experiments. Although XRD did not detect bulk mineralogic changes in Cherokee Shale experiments decreases in iron, increases in magnesium, and decreases in calcium indicate reactions with the Cherokee Shale are occurring. Results indicate that precipitation of secondary gypsum will occur when Arbuckle rocks containing pyrite are exposed to CO2. These results have important implications for GCS in the Arbuckle saline aquifer. Hydraulic separation of the Upper and Lower Arbuckle will positively impact the plume movement in the reservoir, retarding buoyant flow. This is important because the data also show that the Lower Arbuckle brines are relatively homogenous and undergoing rapid mixing due to an extensive fracture network suggesting the plume will travel relatively quickly to the central baffle. Furthermore, the separation impacts storage capacity estimates because the tight nature of the rock that is causing the hydraulic separation is not likely to receive volumes of CO2 equivalent to other parts of the reservoir. Additionally, if CO2 is not able to pass through the baffle to fill the shallower pore space within tens or hundreds of years (however long humans are injecting) then it effectively removes that volume of storage from capacity unless CO2 is injected separately into each zone. The results of the caprock integrity study also have important implications for GCS in the Arbuckle saline aquifer. The precipitation of gypsum caused by the presence of pyrite in the reservoir will positively impact the seal integrity by beneficially filling pore space or fractures in seals resulting in even better sealing of the reservoir. However, the reservoir rocks also contain pyrite, which could also lead to gypsum precipitation, which would detrimentally clog valuable pore space in the CO2 storage reservoir, lowering storage capacity, or decreasing injection capability. | |
