Abstract

Application of steam-assisted gravity drainage at the Alberta Oil Sands Technology and Research Authority Underground Test Facility (UTF) Phase B has demonstrated the commercial viability of in-situ recovery in Athabasca oil sands deposits. This process has obtained high bitumen recovery (>50%), low steam/oil ratios (<2.5), and relatively high production rates (100m³/day) from each of three well pairs.

Characterization of the UTF reservoir and estimates of the lateral extent of heterogeneities are illustrated by interlocking detailed cross-sections and outcrop studies. In addition, representative petrophysical core samples provide quantitative data for characterizing key lithofacies in the reservoir. Interpretation of the reservoir is based on a largescale, estuarine point bar geological model. From sedimentological and reservoir perspectives, three flow units are recognised: trough cross-bedded sands, a transition zone and an inclined heterolithic stratification (IHS) unit associated with the lower, middle and upper portions of the reservoir, respectively.

Emphasis is placed on mudstone-dominated beds within the IHS unit due to their strong influence on delaying steam rise demonstrated during the first year of production. Mudstones of the IHS unit display two to three orders of magnitude reduction in absolute permeability, compared to interbedded sands of the IHS and sands within the trough cross-bedded unit.

These mudstones are laterally continuous over an area of approximately 150 metres by 200 metres and dip 5° to 12° perpendicular to paleoflow, but are nearly horizontal parallel to paleoflow. Optimal placement of the horizontal well pairs in the east-west direction likely enhances the production success obtained. This well orientation is perpendicular to the dipping IHS beds, creating numerous breakthrough points for steam injection and allowing production fluids to flow along bedding planes to the production well.