Polymer-Augmented Waterflood In The Rapdan Upper Shaunavon Unit
Authors: Campbell T.A. , Bachman R.C. 
Published: March 2010

A polymer-augmented water-flood pilot has been implemented to test the enhanced recovery process in the Rapdan Upper Shaunavon Unit. The Rapdan Unit has been under a peripheral water-flood 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, adsorption 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.

Introduction

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 (Fig. 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 It). 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 acres) well spacing and there are currently 31 active oilwells and 14 injectors in the unit.

The Rapdan Unit has an estimated 16 736 103m3 (105 MMBBLS) of original-oil-in-place (OOIP). Cumulative production is 2412 103m3 oil (15.2 MMBBLS) or 14.4% of the OOIP. Remaining reserves are estimated to be 608 103m3 oil (3.8 MMBBLS) for an ultimate recovery of 18% OOIP. Production in 1984 averaged 195 m3 oil/d (1226 BOPD) at a 70% watercut. The peripheral waterflood 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 waterflood would be a feasible enhanced recovery process. Polymer was chosen over other EOR schemes due to the high mobile oil saturation remaining in the reservoir, 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.

Reservoir Description

The Rapdan Sand (Upper Shaunavon Fonnation) 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 permeability sand occurs with a minimum amount of calcareous cementation. Thin section and X-ray diffraction studies show a low clay content, predominantly kaolinite, from 1% to 5% of rock mineralogy.