Performance Of South Swan Hills Tertiary Miscible Flood
Authors: Derochie L.J. 
Published: March 2010

Abstract
A review of the performance of the tertiary hydrocarbon miscible flood which has been operating in the West Waterflood area of the South Swan Hills Unit since September 1982 is presented. The production trend clearly indicates that the tertiary flood has reversed the steady decline experienced under waterflood to the extent that the oil production has increased from 450 m3/day to as high as 1200 m3/day in 1986.
The ultimate recovery factors for the project area are estimated at 38% original oil-in-place (OOIP) by waterflood and 59% OOIP by miscible flood. The projected 20% incremental recovery is based on a black oil model simulation and is in line with the incremental recovery predicted for the secondary miscible flood in the South Swan Hills Unit(1). The early performance of the tertiary flood is consistent with model simulation work and indicates that the projection in incremental recovery is achievable.

Introduction
The South Swan Hills Unit is located 190 km northwest of Edmonton, Alberta (Fig. I). Production is from the Swan Hills member of the Beaverhill Lake Formation which is a reef of Upper Devonian age. The unit is subdivided into three project areas (Fig. 2). The tertiary hydrocarbon miscible flood was installed in what was originally the West Waterflood Project area in September 1982. The project area encompasses approximately 4450 ha and contains 83 wells drilled on 64-ha spacing.
Current plans are to inject a total hydrocarbon slug of 47.2% HCPV. This slug will comprise 15% HCPV solvent slug followed by a 32.2% HCPV dry gas slug. Both slugs are to be injected at an injection ratio with water conducive to optimum flood performance. Following hydrocarbon injection, the flood will revert to straight water injection.

Geological Description
The South Swan Hills reservoir is a Beaverhill Lake carbonate reef which comprises four oil-bearing geologic zones (Fig. 3). In the Tertiary Project area, only Zones 1 and 2 are of concern because, excluding the northern portion of the project area, Zone 3 (table reef zone) is predominately water-bearing. Zone 1 is a porous calcarenite which is characteristically of good porosity and high permeability. Zone 2 varies from a continuous reef front facies in the western part of the project area to a lagoonal facies in the eastern portion. Except for intermittent porous stringers, the lagoonal facies is characteristically poorer in quality. The reservoir rock averages 25 m in thickness and 8% in porosity.

Waterflood Performance
Waterflood operations began in 1963 at which time a semi-peripheral line drive was installed along the southwestern edge of the unit (Fig. 4). Additional voidage replacement capability was added as unit allowable were increased. Prior to start-up of the secondary miscible flood in the central portion of the unit in 1973, an estimated 5.0 × 106 m3 of oil migrated from the West Waterflood Project area into the Secondary Miscible Flood Project area due to a high pressure gradient which existed easterly across the unit(1). Waterflood operations continued in the West Waterflood area until September 1982. Figure 5 shows the production performance of the West Waterflood area since 1963.