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Introduction

Steam-assisted gravity drainage (SAGD) method is a heavy-oil in situ recovery technique used for bitumen production of the Athabasca Oil Sands, where bitumen reserves from oil sands are estimated at 173 billion barrels (Alberta Energy and Utilities Board, 2008). The typical configuration of the SAGD includes two horizontal wells of 750 m in length and vertically separated by 5 m, in which the upper well is used for steam injection to increase the mobility of the bitumen and the lower well is for bitumen production. Feasible bitumen recovery from oil sands by SAGD is limited within the lateral perpendicular distance of approximately 50 m from the horizontal well pairs. Therefore, profitably viable bitumen production performance requires that the SAGD well pair location be at thick reservoir sands. The complexity of facies distribution in the target formation requires effort in understanding detailed distribution of the reservoir sands. A deterministically constructed geologic model was visualized to better understand 3D distribution of the reservoir sands in the study area (Takahashi et al., 2006). Because lateral continuity of the lithologic facies in the area of interest is shorter than typical interwell distance, the deterministically predicted facies distribution leaves inherent uncertainty in terms of the bitumen production forecasting.

The reservoir sand facies can contain impermeable thin mudstone layers and impermeable mudstone clasts. Previous works including Schmitt (2004) and Takahashi et al. (2006) often refer to the thin mudstone layers adversely affecting the growth of the steam chamber during the SAGD process and consequent bitumen production performance. Although the mudstone clasts have not been often discussed in regard to the impact on the bitumen production performance, our field operation has experienced unexpectedly lowered bitumen production from the reservoir containing the mudstone clasts.

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