CO2 Sequestration and Enhanced Oil Recovery via the Water Alternating Gas Scheme in a Mixed Transgressive Sandstone-Carbonate Reservoir: Case Study of a Large Middle East Oilfield

The knowledge on CO2 sequestration and CO2 enhanced oil recovery (CO2-EOR) in mature mixed and interbedded hydrocarbon reservoirs are limited. In this vein, the feasibility of CO2-water alternating gas (CO2-WAG) for coupling CO2 sequestration and CO2-EOR in a mature mixed sandstone-carbonate reservoir was investigated using the S1A reservoir. First, core sample analysis and scanning electron microscopy (SEM) were conducted to evaluate the reservoir characterization. Next, a geological model with dual porosity and permeability was developed and transferred to a reservoir simulation model, and 21-year field production data were utilized for history matching and constraining of the reservoir model. Then, continuous CO2 and CO2-WAG injection methods were simulated using newly developed and history-matched geo-models and compared to assess their CO2-EOR and CO2 storage mechanisms and determine their potential in a mixed sandstone-carbonate reservoir. The effect of the anisotropic permeability ratio and hysteresis on CO2 storage mechanisms was addressed in this work. The results indicate that the CO2-WAG scheme can yield a +3% oil recovery factor than the continuous CO2 injection method, and CO2-WAG injection can utilize up to 14 and 12% of the total CO2 injected for residual and solubility trappings, respectively, while a minimal 2.9 and 0.03% was utilized from continuous CO2 injection for residual and solubility trappings, respectively. The WAG ratio of 2:1 could yield a higher recovery factor and greater CO2 utilization for solubility and residual trappings in a mixed reservoir. In mixed and interbedded reservoirs, geological anisotropy can also strongly influence reservoir performance during the CO2-EOR process, in which higher values of the anisotropy ratio (Kv/Kh) in CO2-WAG can yield greater oil recovery and more CO2 storage; also, hysteresis has great impact on residual trapping. This study provides valuable insights into the potential of CO2-WAG for CO2 sequestration and CO2-EOR in mature mixed and interbedded reservoirs.