Abstract

Objectives/Scope: In 2021, Liberty Resources LLC (Liberty) deployed an enhanced oil recovery (EOR) pilot via a single huff ‘n’ puff (HnP) well in a 2560-acre Bakken spacing unit in Mountrail County, North Dakota. The primary goal was to demonstrate the economic viability of EOR using produced gas with water and surfactant. The pilot was designed, permitted, and conducted by Liberty in partnership with the Energy & Environmental Research Center (EERC) and EOR ETC. The objectives were to 1) repressure the reservoir above the minimum miscibility pressure (MMP), 2) prove the concept of using water coinjection to build hydrostatic pressure to inject gas at low surface pressures and to improve gas conformance, and 3) use a surfactant to enhance oil recovery through rock wettability alteration and interfacial tension reduction.

Methods/Procedures/Process: Numerical models were built and calibrated using data for the pilot site. Numerous injection scenarios were investigated to evaluate the effects of fluid injection and surfactant to inform the pilot design. A reservoir surveillance program was put in place to evaluate the pilot performance and reservoir response. The plan included the continuous monitoring and periodic sampling of the HnP well and the offset wells in the pilot area to collect data such as injection and reservoir pressure, injection rates, production data, surfactant recovery, produced gas composition, and fluid specific gravity. A coinjection technology developed by EOR ETC, the rapid-switched, stacked-slug (RSSS) injection system, provided the process to inject produced gas and surfactant-laden water in sequential and distinct volumes.

Results/Observations/Conclusions: The pilot was initiated on September 10, 2021, with no injectivity issues. The injection cycle was completed on October 11, 2021, with approximately 46 million standard cubic feet of produced gas, 40,000 barrels of freshwater, and 2400 gallons of surfactant injected. No major changes in the production of the offset wells were observed during the injection process. A minor gas breakthrough at one of the adjacent wells was detected but was apparently suppressed by increasing the water slug ratios. The coinjection technology allowed for a relatively low (<1300 psi) surface injection pressure while increasing formation pressure to ~4500 psi, well above the estimated MMP of 2450 psi. The initial production response was consistent with reservoir simulation predictions which indicate EOR occurred with a net gas utilization factor of ~6 thousand cubic feet per incremental oil barrel.

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