REACTIVITY OF BASALT FROM TWO CONTINENTAL FLOOD BASALT PROVINCES IN THE PRESENCE OF HIGH PRESSURE CO2 SATURATED WATER

One option to mitigate effects of green house gas emissions is to capture and sequester carbon dioxide (CO₂) in deep geologic formations. Basalt formations in Eastern Washington are currently being considered as an injection reservoir for permanent CO₂ sequestration through formation of carbonates. Columbia River Basalt (CRB) samples collected from the pilot well drilled as part of the DOE’s Big Sky Regional Carbon Sequestration Partnership were exposed to water and scCO₂ in high pressure laboratory experiments. These CRB vesicular flowtop samples are rich in vugs coated in naturally occurring secondary minerals (weathering products) such as chlorite, cristobalite, and heulandite. Experiments reacting CRB with aqueous CO₂ were conducted at varying pressures and temperatures to evaluate impacts of depth and secondary minerals on carbonate formation. For comparison, an unaltered basalt (no secondary minerals) representing the Central Atlantic Magmatic Province (CAMP) was tested under identical conditions.

Basalt chips averaging ~1cm in size were exposed to aqueous scCO₂ at six different temperatures (34°, 55°, 75°, 96°, 116°, and 137°C) and pressures (7.5, 12.0, 16.5, 21.0, 25.5, 31.0 MPa) for ~180 days. Non-weathered CAMP samples and weathered CRB flow top samples were reacted in separate vessels at the same temperatures and pressures, with the weathered CRB samples visually appearing to accumulate more surface precipitates. At the lowest pressures and temperatures reaction products were minimal. In higher pressure reaction vessels (96°, 21.0MPa) XRD identified calcite on both the CRB and CAMP samples, though the CRB samples exhibited a much greater amount of precipitation. Increasing pressure and temperature dramatically enhanced the formation of carbonates in the surfaces of the basalts. Tests conducted at 137°C, 31.0 MPa produced carbonates with an XRD pattern resembling Kutnohorite [Ca(Ca,Mn)(CO₃)₂]. The CRB basalts were coated with well-formed crystals, while the CAMP samples displayed only a few areas of mineralization. Similar mineralogies of CAMP and CRB basalts indicate that the greater degree of carbonate mineralization on CRB basalts can likely be credited to the presence of weathering products. Mineral kinetics such as these will be important to accurately model long term CO₂ storage.