The Winter pool in Saskatchewan has been developed using horizontal wells 800 to 1400 meters long at 75 meter nominal spacing. The horizontal sections are located near the top of the reservoir to maximize the area swept by the coning behavior of the bottom water. Section 31-42-25 W3M is now being infill drilled to 37.5 meter spacing in an attempt to increase reservoir drainage. The heels of the infill wells are being placed between the toes of the original wells to avoid expected greater oil depletion and water coning at the heels of the original wells.

Several initial infill wells encountered lost circulation and/or magnetic interference from offsetting original wells, indicating the locations of the original wells obtained from MWD data were not accurate. For subsequent infill wells gyros were run in the original offsetting wells to verify trajectories. The gyro-based well locations disagreed with the MWD-based well locations. This prompted an examination of existing drilling, Gyro, MWD, and magnetic North positioning data and technology.

This paper will discuss the theoretical and practical aspects of (1) establishing the locations of existing horizontal wells, and (2) placing infill horizontal wells in optimum locations between the existing wells in the Winter pool.

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