Nearly all of Husky's SAGD projects in the Lloydminster, Saskatchewan area are in reservoirs with bottom water. While operating several projects in this region, Husky observed that in some reservoir settings the outer SAGD pair had significantly decreased performance compared to inner SAGD pairs. When the outer pair was shut in to improve overall pilot performance, the performance of now exposed adjacent pair rapidly decreased.
It was hypothesized that operating the edge pairs protected inner pairs from the detrimental effects of the bottom water. Numerical simulation runs have confirmed this behavior. The conclusions reached are that in many bottom water settings poorer performing edge SAGD pairs protect and preserve the performance of inner SAGD pairs; and this should be taken into consideration during facilities design, operational strategy design, and production forecasting.
Use of SAGD is now being expanded to reservoirs with non-ideal conditions, including those in which active bottom water is present. This expanded application entails climbing a earning curve using both numerical simulation and field trials to improve performance.
In the Lloydminster, Saskatchewan area, an additional factor is the presence of mobile oil, which has less ability to contain the movement of both injected steam and bottom water. As a result, bottom water influx may be more difficult to control than it is where more viscous oil is present. A manifestation of the greater difficulty to control bottom water when mobile oil is present is poorer performance of the outer SAGD pairs that are exposed to both bottom water and edge water influx.
The field data used are from Husky's Pikes Peak and Pikes Peak South projects.
As shown in Figure 1, when the outer SAGD pair was shut in, the performance of the next SAGD pair immediately dropped. Also shown in Figure 1 is the rapid recovery of performance of the inner SAGD pair when the outer SAGD pair was returned to injection and production.
Numerical simulation for a simplified generic study of the behavior of outer and inner SAGD pairs in the presence of bottom water was initiated using CMG STARS software. As shown in Figure 2, the model contains an outer pair and an inner pair over bottom water with recharge. The numerical model was used to determine the optimum operating strategy for the outer pair.
Although field data are limited, the data that are available show the performance of outer SAGD pairs over bottom water in Lloydminster, Saskatchewan area channel sand reservoirs with mobile oil exhibit generally poorer performance than interior well pairs.
If the underperforming outer SAGD pair is shut in, the ‘new’ outer SAGD pair will under perform.
Runs using a simplistic numerical model of two SAGD pairs also illustrate the ability of an outer SAGD pair to protect an inner SAGD pair from the performance reducing effects of bottom water.