CO₂ injection to improve oil recovery using zipper hydraulic fractures

Abstract

Conventional oil reservoirs are the main source of oil production because they are easier to produce. On the other hand, unconventional reservoirs have considerably lower oil recoveries in spite of their large initial oil in place (IOIP). Despite the innovative technologies of horizontal wells and hydraulic fracturing techniques, the recovery factor for the Bakken, the largest unconventional resource in the United States, is less than 10 percent. The low oil recovery factor from the Bakken resource reveals the need for enhanced oil recovery (EOR). The hydraulic fracture spacing, orientation, and fracture pattern affects the performance of EOR schemes. For instance, a zipper fracture arrangement is believed to improve permeability and create a larger stimulated reservoir volume (SRV) which, in turn, should improve both the primary and EOR recovery factors. In this thesis, the CO2 injection-and-soak method was the EOR technique. The CO2 EOR was modeled using the GEM® (from Computer Modeling Group) compositional reservoir simulator and Bakken reservoir parameters. While keeping reservoir parameters exactly the same, two different hydraulic fracture patterns, a regular pattern and a zipper pattern, were used in the evaluation of both primary production and CO2 EOR. The primary production was for one year and the CO2 injection-and-soak method for nine years. In this study, we did not observe much improvement in oil recovery using the zipper fracture pattern versus a regular fracture pattern. Furthermore, the molecular diffusion effect was insignificant even with values as large as10-2 cm2/s.