Numerical modeling and comparative analysis of strategies for enhancing oil recovery and geological storage of CO₂ in a depleted oil reservoir

The object of the study is the processes of enhancing oil recovery and geological storage of CO₂ in a depleted, highly waterflooded oil reservoir, modeled using a three-dimensional compositional reservoir simulation model. The key problem addressed in CCUS projects is the internal contradiction between maximizing oil production and optimizing the volume and safety of long-term CO₂ storage. The study examined the choice of an operational strategy that would balance these objectives under conditions of high geological heterogeneity and the risk of early gas breakthrough. It was established that the “injection – depletion” strategy provides the highest cumulative oil production (about 1.8 million m³) but is inefficient due to early gas breakthrough (after ~ 2 years). The pressure-maintenance strategy proved to be the most balanced: gas breakthrough was delayed by 1.5 years, ensuring high CO₂ storage efficiency, but cumulative oil production was lower (about 1.5 million m³). The water-alternating-gas (WAG) technology, for the geological conditions of this reservoir, proved detrimental, causing abnormal pressure build-up (up to 824 bar) and blockage of oil reserves. The obtained results are explained by the physics of the process. The early gas breakthrough in the first scenario is due to CO₂ gravitational segregation and the formation of a gravity override (“gravity tongue”). The efficiency of the second scenario is associated with the creation of a more stable displacement front through pressure maintenance. The complete inefficiency of WAG is explained by the presence of high-permeability channels in the geologically heterogeneous formation, through which water moved, bypassing the oil. The results can be practically applied by operators of mature fields to justify the choice of a CCUS strategy. They provide a quantitative basis for assessing the trade-off between short-term economic benefits (production) and long-term environmental objectives (storage). The study confirms the critical importance of conducting detailed geological modeling before applying WAG, in order to avoid substantial financial losses.