The development of heavy oil assets in Canada, and worldwide, often relies on Steam Assisted Gravity Drainage (SAGD). Directional drilling and surveying these horizontal wellbores presents a host of challenges to the service provider and operator alike. One such challenge relates to conventional surveying techniques and the cumulative errors associated with this approach. Once integrated over the length of the horizontal wellbore, downhole positional uncertainty can typically be 30 m or more in lateral uncertainty, and 5 m or more in vertical uncertainty. These errors present a number of complications in the development of the asset.

To address the issue of cumulative uncertainty in horizontal wellbores, a method of absolute referencing to a surface generated magnetic signal has been developed and successfully deployed in the field. The surface installation is engineered to provide a precise location based on the measurable magnetic field for any point in a desired wellbore placement region. The surface installation consists of conductors arranged on surface above the intended well path, and the exact location of the installation is determined by way of a differential Global Positioning System (GPS) unit. While drilling, the installation is energized, and the Measurement While Drilling (MWD) tool samples the superimposed magnetic field vector. These results are transmitted to surface via standard real time telemetry, and a North, East and True Vertical Depth (TVD) position of the sensor is calculated via surface software in real time.

The absolute positioning technique has been deployed on eight SAGD drilling wells at subsurface depths of 100 to 250 m, for two different operators in Northern Alberta, with encouraging results. On six of the eight wells, the calculated position of the sensor agreed with the integrated position as determined through standard MWD surveying. However, on two of the eight test wells, the absolute ranging determined position disagreed from the MWD integrated position by 10 m or more. The discrepancy observed could be accounted for by systematic errors in MWD surveying. Here we report in detail on a three well redrill program from this set. For shallow horizontal wellbores, the accuracy of the method described can be far superior to surveying techniques such as MWD or gyro at the end of the horizontal section, with estimated uncertainties that can be an order of magnitude smaller than the accumulated uncertainty associated with conventional methods.

With non-cumulative uncertainty, the absolute positioning method can be used to complement SAGD drilling operations in a number of ways. Some potential uses of the technique include: no-access active wellbore twinning, vertical observation well avoidance, infill drilling, and TVD confirmation. Through these applications, it is predicted that operational costs at the rig site will be drastically reduced (via the elimination of wireline, tractor units and survey management techniques) and reservoir drainage will be better optimized by the deployment of the system.

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