Cyclic CO2-EOR in the Bakken Formation: Variable cycle sizes and coupled reservoir response effects

Abstract

Over the last decade, production from unconventional resources in North America, particularly shale gas and tight oil, has seen continuous growth and became an increasingly important consideration for the industry. In such reservoirs, sharp declines and limited drainage area has resulted in rapid abandonment conditions with significant amounts of oil in place left untouched. In order to maximize the recovery from unconventional resources, several different solutions including infill drilling, refracturing, and enhanced oil recovery (EOR) techniques have been researched. In this study, we have optimized the application of CO2-EOR methods for the Middle Bakken Formation. Numerical simulations have been paired with the experimental design method in order to identify the parameters that could control oil recovery, CO2 utilization, and CO2 retention factors of the cyclic CO2-EOR process. Unlike previous studies, a growing injection-production cycle size scheme was designed and optimized in this study. It was ultimately shown that for the same set of geological parameters, the variable cycle design resulted in an additional 1.4 per cent and 5.9 per cent in oil recovery and CO2 retention factors, respectively, with 10.6 MSCF/STB less CO2 utilized than typically seen in the constant cycle operation.Furthermore, this study analyzed the combined effect of operational and geological parameters on the three responses of oil recovery, CO2 utilization, and CO2 retention factors. In particular, fracture spacing, fracture half-length, start time of operation, oil gravity, as well as injection pressure were identified as the most influential variables. Interaction effects of the mentioned variables with geological parameters of porosity, pay-zone thickness, and permeability were also studied. The range of operational values which yields the optimum responses for specific geological parameters were identified. The statistical approach also helped provide screening strategies for an optimized operation of a CO2-EOR process in an unconventional reservoir.