Shale Barrier Effects on the SAGD Performance

Abstract

Abstract The SAGD process has already been implemented for commercial production in Alberta Oil Sands areas in Western Canada since early 2000. SAGD performance is very sensitive to reservoir heterogeneities such as shale barriers, bottom and/or top water zones, and a gas cap. In the SAGD process, low permeability zones such as shale layers may act as a flow barrier depending on their size, vertical and horizontal locations, and continuity throughout the reservoir thus making it very important to understand and characterize the effect of shale layers. In this study, the impact of various sizes and locations of shale barriers have been investigated through two-dimensional hypothetical simulation models. The various simulation models have been designed to investigate the shale size and vertical location in both BIP (shale between the injector and producer) and AP (shale above the producer) cases. Two different types of models were designed to look at the effect of flow path existence between the injector and producer: type-A is designated as having a no flow path directly above the producer and type-B has a flow path directly above the producer. The simulation results show that type-A has a greater impact than type-B on SAGD performance especially for the BIP case. Small shale sizes of 3 and 5 m have a slight impact on performance; however, cases with 10 m shale have a greater impact due to the disruption of gravity drainage to the producer. Type-A BIP may require a longer pre-heating period for successful SAGD operation. Generally, shale barriers of 5 to 25 m are not critical for an AP case regardless of vertical location of shale barriers; however shale barriers greater than 50 m may act as a barrier and reduce the effective pay thickness of the reservoir depending upon the its vertical location. Introduction Steam Assisted Gravity Drainage (SAGD) is the most promising in-situ recovery method to recover bitumen from the Athabasca oil sands deposits located throughout Western Canada. The reservoir heterogeneities such as shale barriers, bottom and/or top water zones, and a gas cap have been identified as key issues that affect the success of a SAGD development project. Low permeability zones may act as flow barriers depending upon size, vertical and horizontal location, and continuity throughout the reservoir therefore making it important to understand and characterize the effects of shale layers. Even though the SAGD recovery process has been implemented in Alberta's Oil Sands since the 1980's, there are still many issues that need to be understood and solved in order to have a successful SAGD project and is such, reservoir heterogeneity remains to be a significant factor throughout the optimization process. There have been several publications outlining the effects of impermeable layers on SAGD performance. Yang and Butler (1992) studied the effects of reservoir heterogeneities through laboratory experiments; they found that short horizontal barriers did not affect the general performance whereas longer horizontal barriersdecreased the oil production rate but, in some configurations, not nearly as much as one may have expected with top steam injection. When long horizontal barriers were placed in the middle of the reservoir, above the well-pairs, the bitumen above the barrier could not be produced.