The current interest in directional survey calculations is related to the increased number of highly deviated holes drilled from offshore platforms. Positional accuracy is important because, when two wellbores closely approach one another, it is essential to avoid intersection. Also, when a relief well is drilled, it is important to achieve intersection with the wild cat well. Unfortunately, there is no set of calculations that can be used to analytically determine exact bottomhole position relative to the top hole. Therefore, uncertainty with respect to estimating the wellbore position should be determined using directional surveys and the different models being used.

Errors in estimating the target primarily derive from two sources — model errors and measurement errors. Model errors occur in the calculations used to estimate borehole position because of a lack of complete information, which can only be tackled by having an infinite number of survey stations throughout the well path. On the contrary, measurement errors occur in the readings of inclination angle (I), azimuthal angle (A), and the distance between two stations (L). This paper focuses on estimation of measurement error, which is an inherent property of measuring instruments being used for the survey and can be evaluated numerically after making some assumptions necessary for such an analysis. The said analysis would ultimately provide ellipses of uncertainty for the target point based on different survey models.

The study is based on a paper by Walstrom et al. [1], which uses the angle averaging (AA) method for survey calculations. The study is extended for calculating measurement errors in the following models: the radius of curvature (ROC), minimum curvature (MC), and natural curve (NC) methods.

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