Advances in horizontal well drilling and multistage hydraulic fracturing make it a reality to unlock the tremendous oil resources in the Bakken Formation of the Williston Basin. Currently, the monthly production rate from the formation is a record 35 million barrels. Although the production rate is encouraging, the recovery factor (averaging 7% per well) is small compared to the huge amount of oil in place. How to improve oil recovery becomes a critical step toward future development of the basin.
Unlike conventional reservoirs, the ultratight matrix and high-conductivity microfracture network reservoir conditions of the Bakken make many traditional improved oil recovery (IOR) technologies extremely challenging to implement. In this study, we first reviewed the application conditions of available IOR technologies and their feasibility in the Bakken. Then we carried out a systematic procedure, including rock characterization and gas extraction experiments under reservoir conditions (e.g., 5000–6000 psi and 230°–240°F), to investigate the oil recovery effect of various gases. Dozens of core plugs were collected from three wells which penetrate through the major oil pay, including two shale intervals: Upper and Lower Bakken and one somewhat more permeable interval that is the target for drilling: Middle Bakken. Detailed evaluation of rock/reservoir properties was conducted using photomicrography, scanning electron microscopy (SEM), x-ray diffraction (XRD) mineralogical analysis, and x-ray fluorescence (XRF) analysis. More than ten rock samples from Bakken units were selected to test different gases for use in hydrocarbon extraction, including CO2, N2, CH4, and C2H6.
24-hour gas extractions of 11-mm-diameter rods cut from rock samples clearly demonstrated the improvement of oil recovery in all Bakken rocks: as much as 95% of the oil was extracted from Middle Bakken and 8% to 35% from Lower and Upper Bakken samples, respectively. Results also showed that CO2 and C2H6 yielded better recovery efficiency than CH4 and N2, while N2 recovered the least oil during the same experimental time. The dramatic improvement of oil recovery could be partially attributed to the reservoir conditions, which induce high gas diffusivity triggering for miscible gas extraction in the experiments. This observation is favorable for the Bakken Formation, where the burial depth is usually over 9000 ft with initial pressure greater than 6500 psi. Thus high-pressure gas injection may help operators to recover additional percentages of oil from the formation beyond hydraulic fracturing.