The potential for residual oil zones to maximise CO2 storage capacity in Australian petroleum provinces

Residual oil zones (ROZs), with or without CO2-enhanced oil recovery, could potentially increase the storage resources associated with depleted field CO2 storage projects while capitalizing on existing infrastructure, local geological knowledge and a ready supply of CO2. Under the Australian Government’s former Exploring for the Future program (2016-2024), we identified and characterized ROZs in strategic hydrocarbon provinces in central-eastern Australia using a new integrated workflow that primarily relies on petrophysical analysis. ROZs were found to occur below the main oil column in fields across the study area, mainly in the Jurassic reservoirs of the Cooper and Eromanga basins, and particularly within the Hutton-Birkhead and Namur-McKinlay oil plays of the Eromanga Basin. Most are thin (less than a few tens of meters), with low residual oil saturation (<10%). Multiphase compositional flow modelling conducted on a ‘typical’ Australian ROZ, based on the real ROZs identified in the Hutton Sandstone, was used to estimate CO2 storage efficiency and additional oil production potential under a range of CO2 injection scenarios. Modelling shows that CO2 injection under fully miscible conditions can efficiently mobilize and recover significant amounts of residual oil, and that CO2-oil interactions increase the density and viscosity of CO2, enhance CO2 sweep efficiency and lateral flow, and improve overall storage efficiency. We found that the maximum amount of CO2 storage can be achieved with a gravity stable flood resulting from injection of CO2 at the top of the reservoir. The extent of these effects depends on the quantity and spatial distribution of residual oil in place, the miscibility of CO2 at reservoir conditions, and well configuration. Our study shows that while additional oil recovery potential is relatively low, and not comparable to the billion-barrel ROZs that are being produced in other parts of the world, the good reservoir quality rock below the deepest occurring conventional oil in many of the fields, particularly in the Hutton and Namur Sandstones of the Eromanga Basin, appears to be a promising target for CO2 storage.

Presented at the 17th International Conference on Greenhouse Gas Control Technologies, GHGT-17 2024