There is tremendous potential for shale oil reservoirs such as the Bakken formation, Eagle Ford and Niobrara to have a lasting impact on the U.S energy situation due to the multi-billion barrel resource base that these formations contain. Horizontal drilling and multi-stage hydraulic fracturing technologies have allowed significant oil to be produced; however, the primary recovery factors are still less than 10%, which means enhanced oil recovery methods needs to become the next big push in shale oil research. Miscible gas injection may become the most effective method in such lower permeability fields, because conventional water flooding may result in extremely lower injectivity. This work expands on previous research from Shoaib and Hoffman (2009) which focused on the Elm Coulee field in Eastern Montana and showed miscible gas injection may be a possible solution for shale oil reservoirs. The wells in their study had longitudinal hydraulic fractures, whereas today most wells have transverse fractures. The significance of this research is to evaluate the reservoir performance of the CO2 flooding with different hydraulic fracture orientations and recommends the best hydraulic fracture orientation which can maximize the oil production.
In this paper, separate simulation models with multiple transverse hydraulic fractures wells and longitudinal hydraulic fractures wells have been built based on a sector of the Elm Coulee field. Two different grid models (uniform grid models and local grid refinement (LGR) models) have been applied for the two types of hydraulic fracture for primary recovery and secondary recovery. Breakthrough time, total oil production, ultimate recovery factor and injection effectiveness for different cases have been determined and compared to find the best hydraulic fracture orientation in Elm Coulee field. Hydraulic fracture permeability sensitivity and bottom hole pressure (BHP) sensitivity analysis have also been made based on the LGR CO2 injection models.
Results show that both transverse and longitudinal fractures produce similar amounts of oil, but with transverse fractures, the CO2 breakthrough time is much earlier, and the CO2 production rate and cumulative production are much higher. Thus, the injection quantities are much greater for the transverse cases, and its overall injection efficiency is less.
This work forms the foundation to begin understanding how to best perform CO2 injection enhanced oil recovery (EOR) in Bakken shale oil reservoirs.