The Bakken Formation in the Williston Basin is the most productive liquid-rich unconventional play. The middle Bakken member is the primary target for horizontal wellbore landing and hydraulic fracturing because of the better rock properties. Even with the new technology available, the primary recovery factor is believed to be around 10%. Previous studies have shown that gas injection could increase oil recovery factor for these types of reservoirs. In this study, the Elm Coulee Oil Field is selected as the area of interest, and CO2 is selected as the injection gas. The primary goal for this study is to build an EOS model for compositional simulation based on PVT tests from Bakken reservoir fluids and investigate different factors which might influence recovery for CO2 injection.
Liquid samples were collected as recombined fluid from the Elm Coulee Field; standard PVT tests were performed. By matching the reservoir fluid EOS model with PVT tests, an EOS-based compositional fluid model is created. The resulting EOS model is used to investigate production outcomes of CO2 injection for the Bakken Formation in the Elm Coulee Field.
It is believed that a logarithmically gridded reservoir model could represent hydraulically fractured reservoir better, and it is used in this study. The middle Bakken member could be divided into different lithofacies based on depositional environment; thus, different rock properties are assigned to different lithofacies. A homogenous reservoir model is used to compare the production outcomes with the heterogeneous cases.
Two producers and one injector are positioned in one section area. For base production run, producers produce without gas injection for 30 years; for gas injection cases, producers produce for 5 years without gas injection, and then production rate is simulated with CO2 injected for next 25 years. Without gas injection, oil recovery factor is around 12%, while gas injection could boost overall oil recovery factor by additional 8% to 14% of the OOIP. Lean gas and separator gas were also evaluated as injecting fluids, and it was discovered that these gases performed similar to CO2. Heterogeneous reservoir models result in slight higher oil recovery factors, but with earlier gas break-through and more gas production.