Typical EOR screening criteria suggest that naturally fractured reservoirs are poor candidates for miscible CO2 flooding. Despite this rule of thumb, the unusual geology and considerable EOR target of the fractured Midale field persuaded its owners to study implementation of CO2 flood technology.

The 31,000 acre Midale field (Figure 1) is part of a trend of Mississippian carbonate reservoirs in southeastern Saskatchewan. Since 1962, the Midale Unit waterflood has recovered 20% of the 500 million barrels of original oil in place (OOP); ultimate waterflood recovery is expected to be 24%. The reservoir section consists of a heavily-fractured vuggy limestone (the "Vuggy") overlain by a less-fractured chalky dolomite (the "Marly"). Although naturally fractured, well productivities are modest, with average rates of 75-100 STB/D.

In 1984 the Unit owners began a 4.4 acre tertiary miscible CO2 flood pilot. Important pilot data sources included residual oil measurements; single- and multi-well pressure transient tests; fluid analyses; time-lapse logging programs at injectors, producers, and observation wells; chemical, radioactive, halogen, and alcohol tracer programs; phase behavior studies focussing on wax-asphaltene deposition; geotomographic efforts to track flood fronts and displacement insitu; and core/CATSCAN studies of CO2 diffusion in Midale core. The $18.6 million pilot project was completed in 1989.

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