Role of Reservoir Lithofacies and Associated Well Placement on Optimizing SAGD Production
Enerplus Resources Fund, Calgary, AB, Canada
The geometry and reservoir lithofacies associated with the top and base of the SAGD pay interval has a significant impact on SAGD performance. Optimal well placement and understanding of the interaction of the steam chamber and associated production fluids within key reservoir lithofacies can lead to superior reservoir performance.
SAGD operators place the SAGD well pairs as close to the base of pay as possible; however, there is no common definition for SAGD pay. Some operators define pay as a continuous bitumen interval, averaging at least 65% oil saturation by volume, with no mudstones greater than 1 m thick. Other operators use a more conservative approach, limiting mudstones to less than 0.5 m in thickness. These data can be obtained from petrophysical log analysis without the need for core data in most instances. Other criteria used to define SAGD pay are based primarily on reservoir lithofacies, which requires core data and/or imaging tools. The top and base of the SAGD interval and the associated placement of SAGD well pairs will vary considerably, depending upon how pay is defined and interpreted.
Utilizing a reservoir lithofacies approach to define SAGD pay can significantly reduce risk and increase recovery factors. Placing both the producer and injector in continuous, high permeability cross-bedded sand lithofacies allows for more efficient startup resulting from unrestricted communication between the injector and producer wells, and ultimately, higher performance of the SAGD well pair. Cross-bedded sands, commonly exhibit a vertical permeability range from 2 to 6 Darcy (D) units. Thin, discontinuous mudstones and minor mudstone clast breccias, if present, do not appear to significantly impede vertical permeability.
In contrast, placing SAGD well pairs in sand-dominated inclined heterolithic stratification (IHS) can negatively affect operations. IHS lithofacies contain laterally continuous mudstone interbeds with 2 to 3 orders of magnitude reduction in vertical permeability (milliDarcy). Poor communication between producer and injector wells increases the time required during the circulation phase and potentially reduces peak production performance. Sand-dominated IHS meets the SAGD pay criteria of most operators. The role of reservoir lithofacies to optimally place well pairs for SAGD is critical for meeting production targets and maximizing recoverable reserves.
AAPG Search and Discovery Article #90075©2008 AAPG Hedberg Conference, Banff, Alberta, Canada