The Rapdan Unit is located in the southwest corner of the province of Saskatchewan, Canada approximately 30 km from the Montana border.The 11,200-acre Rapdan Unit was discovered in 1953 and unitized in 1957.It produces from the Upper Shaunavon (Jurassic) formation at a depth of 4,590 ft.The Upper Shaunavon formation is a shallow marine sand bar deposit which has been breached and reworked.The lithology ranges from dolomitic quartz sandstone to sandy dolomite and includes zones of sandy oolitic limestone, dolomitic limestone and fossiliferous and oolitic dolomite.Thin, discontinuous and wavy dolomite laminations are common.A brown, coarse grained, dolomite-cemented quartz sandstone has the best porosity and permeability.The highest permeability sand occurs with a minimum amount of calcareous cementation.Thin section and X-ray diffraction studies show a low clay content, predominately kaolinite, from 1-5% of rock mineralogy. The orientation of the field is in a north-northeasterly direction.A net pay isopach is depicted in the figure to the left.The polymer flood shown in the isopach, is located in the southeast flank, containing the thickest sand deposit of approximately 30 ft and the best reservoir quality.The remainder of the field is thinner sand averaging about 16 ft.
The Rapdan Unit has 105 MM bbls of original oil in place.The 23o API gravity oil is highly under-saturated at an initial pressure of 1,958 psi with a solution gas-oil ratio (GOR) of 114 scf/stb.Bubble point is 813 psi with the oil formation volume factor at initial pressure of 1.08.The oil viscosity is 10.6 cp at a reservoir temperature of 132 o F.No significant aquifer is present, although some formation water was produced at the southeast edge of the pool during primary production.Average connate water saturation is 30%.
In 1986, a 760-acre pilot polymer flood was implemented in the Rapdan Unit.In the polymer flood area, seven production wells and four injection wells were drilled on 40 acre spacing during 1985.The total pilot consists of five injection wells and thirteen oil wells on inverted five-spot patterns as shown in the isopach above.Pilot pore volume was 28,688 Mbbls with an original oil in place of 18,590 Mbbls.Water injection commenced in January 1986.Beginning in February 1986, 1,500-mg/L polyacrylamide polymer made in blended injection water was injected.Based on the numerical simulation, the polymer concentration was decreased to 1,100 mg/L and mixed in 100% source water beginning in November 1986.Cumulative injection to August 1993 was 4.12 MMbbls or 14.4% of the total pilot volume (27% of the total flooded volume).
The figure to the right shows that the total polymer flood pilot increased in production from 409 bbls/day in the 1986-87 time period to a peak production of 1,132 bbls/day in June 1990 with a corresponding oil cut increase from 18% to a peak of 36% in June 1990.Through June 1, 2001 a total 7,828,961 Mbbls of oil or 42.1% OOIP have been produced by primary, waterflood, and polymer flood.Numerical simulation waterflood prediction was 5,821,000 bbls of oil or 31.3% OOIP.Incremental oil produced was 2,007,961 bbls or 10.8% OOIP.The ultimate performance results in an incremental recovery of 2,010 Mbbls or 20.0% OOIP due to the alkali-polymer injection.Chemical, plant and operations cost per incremental barrel are US$0.99.
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