Resevoir modelling of lithofacies scale heterogeneities within the middle McMurray formation, Alberta, Canada

Abstract

In 2007 Statoil acquired 1108 km2 of the Athabasca oil sands. The plan is to produce the oil using Steam Assisted Gravity Drainage (SAGD), with start-up in the end of 2010. The reservoir lies within the Lower Cretaceous tidal dominated Middle McMurray Formation. The main reservoir interval consists of mainly clean sands with excellent permeability and porosity (C&C Reservoirs, 1999). Above the good reservoir sands there are inclined heterolithic stratification (IHS). These are inclined beds deposited in pointbars, containing thin layers of sandstone and mudstones.

This project studies the effects from small-scale heterogeneities in the Middle McMurray Formation using the SBED software for lithofacies modelling. The models are then upscaled, giving results that may be used in the next scale of modelling.

The results show that thin mud-laminas found within clean sands have a large effect on the vertical permeability, even in small quantities. This will also have a high influence on the SAGD, which is especially sensitive to barriers affecting the vertical flow. In the IHS beds the results show that if the bioturbated mudstones contain more than 70% burrows, and is filled with sands having more than 1D permeability, steam may be able to flow trough the thin mudstone layers. This may create connectivity between the sands in the IHS, suggesting that it might be possible to produce from these intervals using SAGD.

The Middle McMurray Formation also contains local occurrences of shale-clast breccia related to channel bank collapse. The breccia contains clasts with random size and concentrations. A set of SBED models has been created to study the effects different shale-clasts will have on the permeability. The results show that the permeability will gradually decrease with the amount of shale-clasts present, and at ~70% the permeability will be to low for SAGD.