For a deepwater operator facing the challenges of directional drilling and wellbore stability, surveying with high spread costs in excess of USD 1 million per day, any approach which promises to reduce or eliminate cost and risk has great potential benefit. This paper showcases how careful planning, a fit-for-purpose survey program, and, most importantly, an effective, real-time geomagnetic referencing service (GRS) can significantly improve the operator's ability to hit both geological and financial targets. The authors describe recent breakthrough improvements in the accuracy of GRS techniques and present a case study to illustrate the benefits of this approach for the industry, especially in deepwater operations.

Each magnetic survey tool has an associated degree of uncertainty in the accuracy of data acquired. This error is cumulative, increasing with depth as a well is being drilled. In three dimensions, this area of uncertainty takes an elliptical form around the projected well path, a zone of possible well positions known as the ellipse of uncertainty (EOU). In high-angle wells, the EOU may grow so large that it exceeds the dimensions of the geological target, decreasing the driller's level of confidence and increasing the risks and potential costs of missing the target. Therefore, the development of more accurate yet cost-effective surveying techniques has become increasingly critical.

The authors will describe a survey approach that meets this challenge by incorporating breakthroughs in real-time GRS and which offers the producer the following benefits:

• Achieving planned geological targets
• Saving the cost of running gyroscopic surveys
• Avoiding costly post-gyro well path corrections by staying on-path in real time
• Preventing stuck pipe, lost time, and other risks often associated with gyro surveys

The example case to be discussed illustrates how to realize all of the above benefits.