Abstract
Carbon dioxide capture and geological storage has been recognised as an essential technique used to reduce CO2 emissions into the atmosphere. The potential storage basin should consist of enough vacant space in the form of pore-spaces, and supercritical CO2 must remain stable within the reservoir rock for storage to be feasible. This study considers the porosity and CO2 adsorption capacity of potential seal or reservoir rock types with a depth of > 800 m, taken from the Willowvale area, South-Eastern Karoo Basin, South Africa. Samples were extracted from a borehole core drilled as part of the Karin project. The samples were selected at depths between ~ 800 – 2100 m, and the borehole intercepted most of the Main Karoo Basin stratigraphy. The samples consisted of typical rocks identified throughout the Karoo Basin: sandstones, shales, and dolerites. A variety of analytical techniques were used to predict storage or seal potential, including microscopy, X-Ray Diffraction (XRD), Brunauer-Emmet-Teller (BET), and High Pressure Volumetric Analysis (HPVA).
The mineralogy was determined using thin section petrography and confirmed by XRD. The sandstone samples consisted of: quartz; albite; chlorite; orthoclase, and muscovite; one sandstone sample (KWV-6) contained carbonates (dolomite). Typically, the shale samples consisted of: quartz; albite; chlorite; muscovite. The carbonaceous samples (KWV-14 and KWV-16) have clay (illite) and carbonate (calcite) compositions. The dolerites samples generally consist of: albite; anorthite, quartz, and diopside, dominated by plagioclase.
Porosity was determined both in thin section, through the microscopic assessment of dyed epoxy resin, and by BET. Microscopically, visible porosity was only found in sandstone samples KWV-2 and KWV-6 as isolated spaces within the rock. BET porosity was found to be very low, ranging between 0.995 – 1.5 % (vol. %) and 0.0032 – 0.0.0045 cm3/g pore volumes.
The CO2 adsorption assessment by a High Pressure Volumetric Analyser conducted at the Illinois State Geological Survey, was problematic. Quantities of CO2 adsorbed onto the rock samples were exaggerated, but the isothermal trends generated were usable. The isothermal trends and shapes of the graphs are comparable to literature in relation to their corresponding lithotypes. Excess adsorption ranges between 1.7 – 0.001 mol/g. Inflection points on isotherms were found with pressure ranges of ~5 – 10 MPa, where maximum adsorption was observed.
The results show that the rock samples of the South-Eastern Main Karoo Basin have the potential to adsorb CO2, some more so than others. Dolerites were found to be the least adsorptive and the carbonaceous shale of the Whitehill Formation was found to adsorb the most CO2. Hence, the dolerite could be considered as a suitable seal, and the Whitehill Formation as both a storage reservoir or seal.
M.Sc. (Geology)