Mineral Reactions Between Carbon Dioxide And Reservoir Rock. Natural Analogues For Co2 Subsurface Storage And Disposal, Otway Basin, Australia.
Honours Degree, 2000
University of Adelaide
The carbon dioxide (CO2) enriched reservoirs of the Pretty Hill Formation and Waarre Sandstone in the Otway Basin are excellent analogues for subsurface disposal of CO2. The source of CO2 in these formations is interpreted to be the magmas of the Quaternary Mount Gambier volcanic chain.
The study consists of petrological core examination and water analysis of CO2-enriched reservoirs compared to nearby CO2-deficient wells. Samples were selected above and below the gas-water contact, in coarse-grained lithic rock, in order to gain a greater understanding of diagenetic history. ` CO2 is observed to have reacted with most of the volcanic fragments and sodium-rich plagioclase minerals that are typical of the Otway Basin reservoir lithologies. Early authigenic calcite appears to have been dissolved from most of the rock. The reaction rate is heightened in the Pretty Hill Formation due to high percentages of non-quartz mineralogy and larger grain surface areas.
Water analyses in the Pretty Hill Formation and Waarre Sandstone show elevated levels of sodium, magnesium and bicarbonate in solution. The source for the sodium ions in the waters appears to be the dissolution of sodium-rich plagioclase. Bicarbonate and carbonate levels correlate directly to CO2 reacting into the water. This characteristic sodium and bicarbonate signature for CO2-enriched water is easily recognised when presented on Stiff Diagrams. Hypothetical mineral combinations suggest that sodium carbonates are likely precipitates from formation waters in CO2-enriched reservoirs.
Kaolinite clays and authigenic quartz result from the interaction of CO2 with reservoir lithologies. A co-genetic development of these minerals has been observed through the analysis of samples using scanning electron microscope (SEM) techniques. Some later iron and magnesium rich carbonates have been identified through cathodoluminescence (CL), as a result of to CO2 affecting reservoir chemistry.
The presence of CO2 in the formation prevents precipitation of the soluble carbonates in the rock. Production of chlorite also appears to have ceased since the influx of CO2 into the reservoir. No hydrous forms of carbonate have precipitated in the reservoir due to high permeability and low pH conditions.
The result of this study will aid in the identification of reservoirs that are suitable for the sequestration of CO2. This study has shown that the magnitude of mineral sequestration of CO2 into reservoirs is dependent on two factors: The percentage of lithic and felsic compositions in the rock for CO2 to react with, and the overall grainsize of reservoir rocks that has been shown to influence the reaction rates.