Abstract
In 2023, Liberty Resources LLC (Liberty) deployed an enhanced oil recovery (EOR) pilot via a single huff ‘n’ puff (HnP) well in a 2560-acre spacing unit at the East Nesson site in Mountrail County, North Dakota. This was the second EOR pilot at this site and the third by this operator in the Bakken. The prior pilot at the East Nesson site was deemed effective, resulting in incremental oil production by using a novel technology for coinjecting pulses of produced gas and freshwater with surfactant. The objective of this second pilot was to improve economics and simplify injection operations by using readily available equipment and sources of water and gas in a water alternating-gas (WAG) scheme. Oil recovery mechanisms include raising reservoir pressure above the minimum miscibility pressure and improving gas conformance to displace incremental oil in a scalable process.
Numerical models were developed and calibrated using historical production data and learnings from the prior EOR pilots. Various injection scenarios were simulated including the use of surfactants and alternative WAG ratios. Studies included evaluation of surfactant mixtures in lab tests determined to be suitable for Bakken reservoir conditions, showing good stability, interfacial tension behavior, and contact angles with improved rock wettability. While surfactants were evaluated via simulations, ultimately the field pilot was simplified to use only produced gas and water with no additives. After completing the injection scheme, a reservoir surveillance program, including continuous monitoring of the HnP well, was implemented to evaluate oil production performance and reservoir response.
The WAG injection scheme was completed with no injectivity issues with approximately 70 million standard cubic feet of produced gas and 23 Mbbls of produced water injected. Production in the offset wells showed no indication of a major gas breakthrough compared to earlier pilots. The use of WAG injection appeared to improve gas conformance and containment, delaying and reducing gas breakthrough. Additionally, no gas or water breakthrough was detected outside the pilot area of the 2560-acre drilling spacing unit. Initial production results show increased oil production in the HnP well with predicted trends in gas and water rates during flowback. Results include analysis and assessment of oil recovery and efficiency compared to simulation and the prior Bakken EOR pilots.
