A Comparative Study of CO2-Flood Displacement Efficiency for Different CO2 Injection Strategies: Permian Basin vs. U.S. Gulf Coast

In CO2-EOR, there are two main field development strategies: water-alternating-gas (WAG) and continuous gas injection (CGI). The aim of our study is to compare these strategies in terms of their economic performance (from the basis of incremental oil recovery) and in terms of their environmental performance (from the basis of ultimate CO2 storage volumes). Within this framework and to demonstrate the efficiency of each strategy, we evaluate the distribution of Carbon Dioxide in oil, gas, and brine phases; the amount of total CO2 stored at the end of the project; the incremental oil recovery; and the CO2 utilization ratios. In this study, we model and compare two fields which represent two different reservoir settings: Cranfield (representative of the U.S. Gulf Coast sandstone reservoirs) and SACROC (representative of the Permian Basin carbonate reservoirs). CGI is the original operating strategy in Cranfield and WAG is the original operating strategy applied in the SACROC unit. High resolution geocellular models are used for both Cranfield and SACROC fields. The models are constructed based on wire-line logs, seismic surveys, core data, and stratigraphic interpretations. A comprehensive pressure-production history matching for primary, secondary, and tertiary recovery is conducted for both of the fields. Different operating strategies are designed for each field (e.g. CGI and WAG). After finishing the history-matching, CGI and WAG scenarios are simulated in both models to compare their performances. CO2 partitioning in oil, brine, and gas phase (mobile or residual) are compared for both scenarios (WAG and CGI). The partitioning of CO2 in oil results in CO2 miscibility in oil, the partitioning of CO2 in brine results in CO2 dissolution in brine, and CO2 partitioning in gas phase are divided into structural trapping and residual trapping of CO2. We plotted the contribution of different trapping mechanisms over a post injection period for WAG and CGI for both SACROC and Cranfield. Additionally, the total CO2 storage, the incremental oil recovery, and CO2 utilization ratios are compared in both scenarios for both fields. Although actual operating strategy in these two fields are different (CGI in Cranfield and WAG in SACROC), our numerical modelling results show that WAG could not only balance the CO2 storage, incremental oil recovery, and CO2 utilization ratio but also store the trapped CO2 with lower risk of leakage in both fields (by decreasing the amount of structurally trapped CO2). Because of multiple alternation of CO2 and water slugs in WAG, this approach reduces the viscous instability and therefore the efficiency of oil recovery. Our study shows that the distribution of CO2 in different phases is different for each field. Because of the lower minimum miscibility pressure (MMP) and lighter initial oil saturation in SACROC, the partitioning of CO2 in oil is much higher in SACROC than in Cranfield. The dissolution of CO2 in brine is much higher in Cranfield because of the presence of strong aquifer near injection wells. The present work provides valuable insights for optimizing oil production and CO2 storage in a CO2-EOR project. Additionally, this study clearly shows the impact of development strategies on the relative importance of different trapping mechanisms.