Measurement While Drilling (MWD) is a common survey tool used in wellbore positioning. The industry often uses the Industry Steering Committee on Wellbore Survey Accuracy (ISCWSA) error models for estimating the Wellbore Position Uncertainty (WPU). However, the model's sensitivity to direction and nature of trajectory has not been discussed in detail. In this paper, a software model has been developed to better understand the influence of the individual error sources on measurement and position uncertainty in various drilling directions and hole sections.

Most operators have classified accurate wellbore positioning and directional design as one of the pillars of safety. It is commonly known that measurement accuracy of frequently used MWD instruments decreases with increasing hole inclination. Survey accuracy is also influenced by North to East direction. This work provides a detailed understanding of the behavior and contribution of each survey error source and screens the error terms contributing most towards WPU. Visualizing the contribution of each error source as a function of well path direction and inclination will support the understanding of position uncertainty of individual wells.

The results in this paper are based on analysis of the ISCWSA Non-mag error model. It has been observed that some errors are most dominant in the North/South drilling direction while others are most dominant in the East/West direction. Similarly, some error sources are most effective in the vertical hole section and least effective in the build-up or horizontal sections. This process continues for all different error sources in various hole sections and drilling directions. Therefore, this paper has summarized the most important error sources for the vertical, build-up and horizontal sections of the three wells, i.e North/South (NS), North/East (NE) & East/West (EW). The visualization of error sources will provide a more focused approach towards ultimately reducing the WPU.

The work in this paper is a part of a new digital well planning and well construction software tool. The working title of this software is Life Cycle Well Integrity Model – LCWIM.

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