CO₂Sequestration 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 CO₂sequestration and CO₂enhanced 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 CO₂sequestration 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 CO₂and CO₂-WAG injection methods were simulated using newly developed and history-matched geo-models and compared to assess their CO₂-EOR and CO₂storage mechanisms and determine their potential in a mixed sandstone-carbonate reservoir. The effect of the anisotropic permeability ratio and hysteresis on CO₂storage mechanisms was addressed in this work. The results indicate that the CO₂-WAG scheme can yield a +3% oil recovery factor than the continuous CO₂injection method, and CO₂-WAG injection can utilize up to 14 and 12% of the total CO₂injected for residual and solubility trappings, respectively, while a minimal 2.9 and 0.03% was utilized from continuous CO₂injection for residual and solubility trappings, respectively. The WAG ratio of 2:1 could yield a higher recovery factor and greater CO₂utilization 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 (K_v/K_h) in CO₂-WAG can yield greater oil recovery and more CO₂storage; also, hysteresis has great impact on residual trapping. This study provides valuable insights into the potential of CO₂-WAG for CO₂sequestration and CO₂-EOR in mature mixed and interbedded reservoirs.