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.

<p>The development of Carbon Capture Utilization and Storage (CCUS) technology paired with existing energy systems will facilitate a successful transition to a carbon-neutral economy that offers efficient and sustainable energy. It will also enable the survival of multiple and vital economic sectors of high-energy industries that possess few other options to decarbonize. Nowadays, just about one-ten-thousandth of the global annual emissions are being captured and geologically-stored, and therefore with today’s emission panorama, CCS large-scale deployment is more pressing than ever. In this study, a 3D model that represents the key reservoir uncertainties for a CCUS pilot was constructed to investigate the feasibility of CO2 storage in the Unayzah Formation in Saudi Arabia. The study site covers the area of the city of Riyadh and the Hawtah and Nuayyim Trends, which contain one of the most prolific petroleum-producing systems in the country. The Unayzah reservoir is highly stratified and it is subdivided into three compartments: the Unayzah C (Ghazal Member), the Unayzah B (Jawb Member), and the Unayzah A (Wudayhi and Tinat Members). This formation was deposited under a variety of environments, such as glaciofluvial, fluvial, eolian, and coastal plain. Facies probability trend maps and well log data were used to generate a facies model that accounted for the architecture, facies distribution, and lateral and vertical heterogeneity of this high complexity reservoir. Porosity and predicted permeability logs were used with Sequential Gaussian Simulation and co-kriging methods to construct the porosity and permeability models. The static model was then used for CO2 injection simulation purposes to understand the impact of the flow conduits, barriers, and baffles in CO2 flow in all dimensions. Similarly, the CO2 simulations allowed us to better understand the CO2 entrapment process and to estimate a more realistic and reliable CO2 storage capacity of the Unayzah reservoir in the area. To test the robustness of the model predictions, geological uncertainty quantification and a sensitivity analysis were run. Parameters such as porosity, permeability, pay thickness, anisotropy, and connectivity were analyzed as well as how various combinations between them affected the CO2 storage capacity, injectivity, and containment. This approach could improve the storage efficiency of CO2 exceeding 60%. The analyzed reservoir was found to be a promising storage site. The proposed workflow and findings of the static and dynamic modeling described in this publication could serve as a guideline methodology to test the feasibility of the imminent upcoming pilots and facilitate the large-scale deployment of this very promising technology.</p>