High Pressure, Dry Gas Miscible Flood Brazeau River Nisku Oil Pools

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Summary This paper describes the reservoir study and implementation of three high-pressure natural gas miscible floods in the Brazeau River field in Alberta. Results of miscibility conditions by various methods are presented. A simple approach to predict pool performance from coning and cross-sectional model results also is introduced. Introduction The Brazeau River Nisku field is approximately 94 miles (150 knQ southwest of Edmondton, Alta., as shown in Fig. 1. A map of the field is shown in Fig. 2. Development began in Jan. 1978 when the first well, 11-31-48-12 W5M, was drilled into the Nisku Pool A. Nisku Pool D was discovered in June 1978 by drilling Well 7-33-48-12 W5M. Well 2-26-48-13 W5M opened the Nisku Pool E in July 1978. The initial-oil-in-place (IOIP) values determined by material-balance calculations are 33 million, 16 million, and 13 million bbl (5300 × 10(3), 2600 × 10(3), and 2100 × 10(3) M3) for Pools A, D, and E, respectively. These three reefs contain light to volatile oil having 45 degrees API gravity (800 kg/m density). The GOR ranges between 760 and 2,300 scf/bbl (135 and 410 std m3/m3). Average porosity for the three pools ranges from 7 to 10%, while net pay varies between 130 and 256 ft (40 and 78 m). Permeability varies between 50 and 400 md. Pools A and E were overpressured, with initial pressures above 6.670 psia (46 000 kPa). Table 1 summarizes the reservoir parameters for the Brazeau River Nisku oil pods. Reservoir Description The oil-bearing reservoirs consist of isolated pinnacle reefs in the Nisku formation of the Upper Devonian Winterburn group. The Ireton structure, having a regional dip of approximately 73 ft/mile (14 m/km) to the southwest, forms the base of the Nisku reef's. The Ireton platform contains variably dolomitic but dominantly calcareous beds. No substantial porosity was noted within the platform, with the exception of a few stringers of minor moldic pores in dolomites at the top of the unit in a couple of wells. The reefs are extremely variable in terms of primary facies and diagenetic modifications; therefore reservoir properties are difficult to correlate between wells. The reefs are virtually entirely dolomitized. Some calcite exists in the form of relict fossils that have escaped both dolomitization and dissolution, and in the form of sparry calcite cement, which completely or partially plugs both intercrystalline and moldic vuggy pores. This phenomenon is more pronounced in Pool D, which results in porosity and permeability reduction. The edges of the pinnacle reef are assumed to have a slope of 30 degrees from the horizontal. Pools A and D show no evidence of either a gas/oil or an oil/water contact. However, a nonactive aquifer is associated with Pool E. Rock and Fluid Properties Core data from five wells were used to obtain porosity and permeability data for reservoir simulation studies. Porosity/permeability correlations were developed by plotting core porosity vs. maximum air permeability. Compensated neutron and formation density log values were correlated to core porosities. For the zones where no core data were available, porosities were obtained from the log correlations and permeabilities from the permeability/porosity correlations. Separate correlations were developed for Pools A and D. Pool D correlation was used for Pool E since no core data were available to develop a separate correlation. Oil properties were obtained from laboratory tests by using recombined fluid samples from each pool. JPT P. 2503^