(754b) Laboratory Investigations of CO2-Enhanced Oil Recovery and Associated Storage in Reservoir and Source Rocks from the Bakken Petroleum System

Laboratory studies are being performed in support of field-based projects to both increase oil recovery and store CO₂ in unconventional reservoirs, with a focus on the Bakken Petroleum System in North Dakota (USA). As is common in unconventional reservoirs, oil recoveries in the Bakken are low, typically < 10% of known oil in place,^1,2 which has led to increasing interest for injecting CO₂ both for increasing oil recoveries as well as achieving associated CO₂ storage.^3-5 Although CO₂-based enhanced oil recovery (EOR) in conventional reservoirs has been practiced for decades, the tight-fractured nature of unconventional systems like the Bakken are likely to require different approaches for successful field operations, and will require deeper mechanistic understandings of the parameters controlling oil recoveries based on laboratory studies designed to mimic processes in tight fractured systems.⁶

Methods have been developed at EERC to investigate CO₂ EOR and CO₂ storage in the Bakken system at reservoir temperature (230 °F) and include: (1) Measuring minimum miscibility pressures (MMPs) using a capillary-rise vanishing interfacial tension (VIT) technique for crude oils produced from the target Middle Bakken and Three Forks formations,⁷ (2) determining the ability of CO₂ to dissolve light and heavy hydrocarbons via vaporization gas drive from crude oil at different pressures, (3) determining the ability of CO₂ at different pressures to recover crude oil hydrocarbons from both Middle Bakken and Three Forks reservoir rocks and Upper and Lower source shales,⁶ and (4) measure sorption isotherms of CO₂ with Bakken source and reservoir rocks using a magnetic suspension balance.

Results from these four experimental approaches can be summarized as: (1) Measurements of MMP for crude oils produced from the target Middle Bakken and Three Forks formations show that MMP can be reached at pressures about ½ or lower than the original reservoir pressure of about 5000 to 7000 psi. (2) Hydrocarbon compositions dissolved via vaporization gas drive from crude oil into the CO₂-dominated “miscible” phase show dramatic increases with higher pressures in both the total oil hydrocarbons dissolved and in the ability of CO₂ to dissolve heavier hydrocarbons. (3) Oil recoveries from rock core samples exposed to a “bath” of CO₂ (rather than a typical core-flooding experiment) under conditions designed to mimic fracture flow of injected CO₂ also show that higher pressures yield better total oil recoveries as well as better recoveries of heavier hydrocarbons. Mechanistic studies demonstrate that rock surface area, CO₂ pressure, and CO₂/rock contact time are the major factors controlling oil recovery from Middle Bakken and Three Forks rocks, as well as from the Upper and Lower shales. (4) Finally, measured isotherms show low to moderate storage capacities of the Middle Bakken and Three Forks rocks (ca. 0.5 to 2 mg CO₂ per gram rock), but very high capacities in the Upper and Lower Bakken shales (ca. 12 to 16 mg CO₂ per gram rock), indicating that the Bakken shales represent an opportunity for substantial storage of injected CO₂, as well as acting as effective seal caps to retain CO₂ in the Middle Bakken and Three Forks formationsMcNally, M.S.;

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3. Shoaib, S.; Hoffman, B.T. In CO₂ Flooding the Elm Coulee Field. SPE Rocky Mountain Petroleum Technology Conference; Denver, CO, 2009. DOI: 10.2118/123176-MS.
4. Yu, W.; Lashgari, H.R.; Wu, K.; Sepehrnoori, K. CO₂ Injection for Enhanced Oil Recovery in Bakken Tight Oil Reservoirs. Fuel 2015, 159, 354-363.
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6. Hawthorne, S.B.; Grabanski, C.B.; Miller, D.J.; Kurz, B.A. and Sorensen, J.A. Hydrocarbon Recovery from Williston Basin Shale and Mudrock Cores with Supercritical CO₂: Part 2. Mechanisms That Control Oil Recovery Rates and CO₂Energy and Fuels 2019, 33, 6867-6877.
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