CO2-water-rock predictions from aquifer and oil field drill core data: The Precipice Sandstone-Evergreen Formation CO2 storage reservoir-seal pair

Summary The Surat Basin is one of the most prospective onshore basins in Australia for CO2 storage. The Precipice Sandstone and Evergreen Formation have been appraised for their feasibility as a future CO2 storage reservoir-seal pair. Here we will focus on predicted CO2-water-rock reactions. These predictions rely on mineral and porosity data from drill core. Data were obtained from northern and two southern regions of the Basin. The northern region was more data rich. The southern region is more well core and data sparse with the exception of the Moonie oil Field. Additional drill core samples were collected from archived well core of Moonie and other parts of the basin. The core samples were characterised for porosity, mineral, and metal content to build geochemical models to predict local CO2-water-rock reactions and their potential effect on reservoir scaling, changes to porosity and mineral trapping of CO2. For the northern region, our work has predicted low reactivity of the Precipice Sandstone, with mineral trapping in the Evergreen Formation. The Precipice Sandstone sampled in the Moonie field has different mineralogical characteristics to wells in the Northern region. Here, CO2-water-rock predictions indicate minor alteration of plagioclase and K-feldspar to kaolinite, chalcedony and ankerite in cleaner Moonie sandstones, with additionally precipitation of smectite in clay rich sands. Formation water pH was buffered between 5 and 6 by dissolution of calcite or siderite cements. Sampled core has also shown evidence of previous natural CO2 and hydrothermal fluid alteration, fractured quartz grains, and fracture fills with mineral trapping as carbonates. This type of natural analogue data is vital to validate long term predictions. New drill core and data are still required in future for the southern and central Surat Basin region which is most prospective for CO2 injection and storage.