A Polymer Augmented Waterflood Pilot is being implemented to test the tertiary recovery process in the Rapdan Upper Shaunavon Unit. Injection is to commence by late 1985. The Rapdan Unit has been under a peripheral waterflood since 1962. The performance of the water flood has been characterized by early water breakthrough due in large part to an oil/water viscosity ratio of 20.
This paper describes the studies undertaken to determine the feasibility and to design a polymer flood to improve oil recovery in the Unit. Laboratory work was done to select the most suitable polymer and to determine polymer adsorption, effective viscosity and incremental oil recovery. Field testing was conducted to determine polymer injectivity, absorption and degradation. A reservoir simulator was used to optimize polymer concentration, slug size, well spacing and pattern type and to predict production and injection rates.
The 4530 ha (11,200 acre.) Rapdan Unit is located in the southwest corner of the province of Saskatchewan approximately 30 km from the Montana border (Figure 1). The field was discovered in 1953 and unitized in 1957. The 23 °API oil is produced from the Upper Shaunavon (Jurassic) formation at a depth of 1400 m (4590 '). A peripheral waterflood was implemented in 1962 with several internal injectors added since. Extension of the field to the west occurred in the early seventies. Development is primarily on 64 ha (160 acre) well spacing and there are presently 31 active oilwells and 14 injectors in the unit.
The Rapdan Unit has an estimated 16,736 103m3
(105 MM BBLS) of original oil in place (OOIP). Cumulative production is 2,412 103m3 oil (15.2 MMBLS) or 14.4% of the OOIP. Remaining recoverable reserves are estimated to be 608 10 3m3 (3.8 MMBBLS) for an ultimate recovery of 181 of COIP. Production in 1984 averaged 195 m oil/d (1.226 BOPD) at a 70% watercut. The peripheral waterf100d has experienced early water breakthrough at adjacent oilwells due to the high oil-water viscosity ratio. Also areas of low pressure have resulted from poor communication to peripheral injection wells.
Screening of the reservoir concluded that polymer augmented water flood would be a feasible enhanced recovery process. Polymer was chosen over other EOR schemes due. co the high mobile oil saturation, the relative simplicity of the process and lower front end costs to implement. This paper presents the results of the polymer feasibility study which includes laboratory corefloods, a polymer injectivity test and reservoir simulation.
The Rapdan Sand (Upper Shaunavon Formation) is a shallow marine sand bar deposit which has been breached and reworked. A log type section is shown in figure 2. 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 permeabllity sand occurs with a minimum amount of calcareous cementation.