Steam assisted gravity drainage is the primary in-situ recovery method for bitumen from the large Athabasca deposit in Alberta, Canada. SAGD field operations encounter a significant decrease in production performance when low permeability shale barriers are present in the formation. These layers can reduce SAGD performance and impede the growth of the steam chamber. They also significantly limit the percentage of the deposit where bitumen can be economically recovered with SAGD. The concept of drilling vertical slimholes to create flow paths through barriers was conceived and investigated at Alberta Innovates - Technology Futures (formerly the Alberta Research Council). Use of slimholes has the potential to significantly increase the amount of recoverable bitumen (reserves) and the rate at which it is produced during SAGD. For shallow reservoirs, the slimholes could be drilled from the surface at relatively low cost.

2-D and 3-D field scale numerical simulations were performed using reservoir properties and operating conditions that were based on published information for the MacKay River SAGD operation in the Athabasca deposit. The reservoir depth was 135 m, the initial pressure 500 kPaa, the initial temperature 7.5 °C, and the initial oil saturation 0.8. The simulations explored the impact of vertical slimholes, which were 7 m laterally offset from the horizontal well pair (Figure 1), on reservoirs with and without shale layers or shale lenses. The effect on SAGD performance of slimhole cross-section (25 cm × 25 cm or 50 cm × 50 cm) and distance between slimholes (12 m or 24 m in the direction parallel to the well pair was investigated). The effect of reservoir and slimhole permeability and the impact of horizontal slimholes from the injector or producer were also explored.

The slimholes were represented by high permeability vertical channels using refined grids. Counter-current flow of gas and liquid through the slimholes was represented by adjusting the relative permeability in the slimholes.

For a reservoir, with a continuous shale layer, SAGD performance was improved by vertical slimholes due to the recovery of previously inaccessible oil from above the shale layer where a secondary steam chamber was formed.

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