Carbon Capture Storage (CCS) and Carbon Capture Utilization and Storage (CCUS) have recently gained global attention as promising techniques to mitigate net CO2 emissions. Within this framework, the Saudi Arabian 2030 vision targets the large-scale deployment of CCS and CCUS projects to promote its circular carbon economy. This study evaluates the potential for underground sequestration of CO2 emitted from industrial sources near Riyadh, Saudi Arabia, which emit 46 Mton/year.

A deterministic geologic model corresponding to the Unayzah Formation was constructed using published data incorporating sedimentary facies distribution, porosity, permeability, and connectivity. Compositional simulations were performed to assess the CO2 plume flow in the presence of conduits, barriers, and baffles. Similarly, injectivity and injection rate effects on solubility and residual trapping were evaluated. A sensitivity analysis and an uncertainty quantification study were carried out to obtain a probabilistic assessment of the total storage capacity and trapping contributions.

The geological evaluation indicates that the area under Riyadh is unsuitable because the Triassic sandstones are too shallow, and the Paleozoic section was entirely removed by erosion during the Carboniferous. Alternatively, the Hawtah area, at 150 km south of Riyadh, is deemed suitable for CO2 sequestration. These sandstones are porous, permeable, tightly sealed, and correspond to hydrocarbon reservoirs in anticlinal structures along the Hawtah, Nuayyim, and Dilam trends. They are favorable for CO2 disposal outside oil and gas fields due to lateral and vertical permeability barriers and up-dip pinch-out against the Batin arch. Simulation results, fifty years after CO2 injection and two hundred fifty years of monitoring, show that the Unayzah Formation satisfies the conditions of capacity, injectivity, and seal efficiency required for technical feasibility.

Furthermore, lower injection rates promote higher solubility and residual trapping due to gravity-controlled flow exceeding viscous and capillary forces. Residual trapping contributes ~ 50% to the storage, while solubility adds 10%. The variables that have a higher impact on secure trapping are residual gas saturation, water salinity, and permeability. The current CO2 storage capacity in the area evaluated exceeds 300 Megatons (Mt), and the assessment is still ongoing, with no vertical leakage through the caprocks of the Khuff and Sudair Formations.

Overall, the novelty in this research focuses on the unprecedented use of public domain data to construct a detailed geological model of the Unayzah Formation in the Hawtah and Nuayyim area that allowed a better understanding of CO2 flow mechanisms in the reservoir and its capacity to store CO2. This study concludes that the Unayzah reservoir in the Kharj-Hawtah area is a viable candidate for secure CO2 disposal from industrial sources in Riyadh.

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