Impact of intraformational water zones on SAGD performance

Approximately 80% of the 173 billion barrels of recoverable crude bitumen in the Athabasca oil sands require in-situ recovery due to the depth of burial. Steam-Assisted Gravity Drainage (SAGD) has become the preferred in situ recovery process for producing Athabasca oil sands reservoirs. It is well known that oil sands reservoirs are heterogeneous with respect to porosity, permeability, phase saturations, and fluid composition. A large-scale heterogeneity that has been encountered in SAGD operations is top and bottom water thief zones. However, evidence of intraformational water zones has also been found: seismic, log, and core analysis suggest that these water zones can exist as both isolated pockets and extensive connected regions within the oil column. The effect of these high water saturation zones on the SAGD process is largely unexplored. In this research, detailed thermal reservoir simulation is used to study the movement of water during SAGD operations in reservoirs with several intraformational water zone configurations. The effect of water zone extent, connectivity, saturation and operating conditions on water movement and SAGD performance is examined. Fluid migration out of the reservoir is found to be relatively small, with ‘oil plugs’ effectively sealing water zones in front of the growing steam chamber. However, water zones are found to have a significant effect on production. The inclusion of water channels led to an average drop in production of 73% while water zones in the form of interconnected nodes led to a 12% reduction. This indicates that the spatial distribution of water zones largely determines the effect on SAGD performance. The variation in production is the result of heterogeneous steam chamber growth that leaves large areas of the reservoir unswept. This appears to be largely due to the influence of water zones on near wellbore pressure distributions, particularly at early times. By acting as either ‘pressure sinks’ or ‘pressure transmitters’ water zones are able to both ‘seed’ and suppress steam chamber growth.