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As oil imports in the United States approach 60% of total daily consumption, more efforts are being expended to maximize recovery from known domestic oil fields. As part of this effort, CO2 flooding of reservoirs has been proven to be an effective means to increase the recovery of oil bypassed during primary production, albeit commonly at significant cost because of capture, compression, and transportation of adequate CO2. At the same time, global and national interest in the viable geological sequestration of anthropogenic CO2, a major greenhouse gas when emitted into the atmosphere, is also becoming more significant. In the Michigan Basin, the juxtaposition of the Devonian Antrim Shale natural gas trend, one that contains high levels of associated CO2, with the mature Niagaran (Silurian) reef oil play, characterized by reservoirs with high percentages of stranded oil, may provide an economically viable model to combine enhanced oil recovery (EOR) efforts with the geological sequestration of CO2.

Niagaran pinnacle reefs in the Michigan Basin have produced more than 450 MMBO since the late 1960s. Because of the complex heterogeneity of the reef reservoirs, however, primary production averages only around 30% with secondary waterflood programs typically capturing an additional 12%. The northern reef trend in the Michigan Basin comprises an immense hydrocarbon resource, located in hundreds of closely spaced but highly compartmentalized reef fields in northern lower Michigan. These geologically complex carbonate reef reservoirs present not only significant opportunity for EOR operations because of known traps, quantifiable remaining oil, existing infrastructure, and very few secondary recovery projects to date, but also great challenges to modeling for maximum sweep efficiencies and recovery factors during miscible CO2-EOR projects.

In the northern reef trend, a local source for subsequent CO2 flooding is readily available as a by-product of Antrim Shale production. The annual production of CO2 separated from Antrim gas is approximately 21 bcf, most of which is currently vented directly into the atmosphere. The close proximity of a source of high-quality CO2 from several gas-processing plants throughout the northern reef trend, a region with more than 800 Niagaran reef fields, provides an economically viable opportunity to combine CO2-flood EOR operations with geological sequestration of CO2 greenhouse gases. Initial results of a pilot project where CO2 from the Antrim Shale is being injected into several Niagaran reefs are discussed along with reservoir characterization issues associated with these heterogeneous reservoirs. Similar EOR projects throughout the northern reef trend could provide an economic foundation for CO2 sequestration programs. This is especially the case if they are designed alongside industrial activities that generate easily captured CO2 emissions streams, such as other gas-processing plants or future ethanol plants planned for the region.

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