SAGD Pad performance in a point bar deposit with a thick sandy base

Abstract

The McMurray Formation in Alberta, one of the largest oil accumulations in the world, hosts bitumen with viscosity typically greater than 1 million cP. This formation is heterogeneous containing sedimentological elements such as point bar deposits which in turn consist of inclined heterolithic strata of interlayered sand-shale/siltstone sequences and abandoned mud channels. Due to shale/siltstone interbeds, the reservoir presents severe lateral and vertical lithological heterogeneity which limit vertical flow of steam ascension and bitumen drainage. As part of an ongoing study on the performance of SAGD in point bar systems, we have evaluated the impact of the geology on pad performance in a clean sand unit (on average 30m thick) with an overlying highly heterogeneous point bar deposit (on average 20m thick). The results demonstrate that the performance of a SAGD pad depends on the orientation of the wellpairs within the reservoir even when it contains a thick clean sand interval where the wellpairs are placed near the base of the clean sand. Improved steam-to-oil ratio performance is achieved when the well pairs cut across the shale layers where steam can access multiple layers of oil-laden sand between the shale layers. Even when the steam chamber is relatively uniform in the basal sand zone, steam ‘fingers’ into the heterolithic point bar leading to chamber non-uniformities and less efficient utilization of steam along the well pair, as reflected by the steam-to-oil ratio.