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Correcting errors inherent in directional surveying with MWD tools

Traditionally, there is a cone of error associated with directional surveys taken while drilling with any given survey tool to account for the inherent resolution within and accuracy of the tool. However, there are many other, more significant errors that are recognized to exist by certain sectors of the industry, but not generally accepted and accounted for in practice. This article does not propose any one solution, or endorse any commercial product. However, when collision prevention and accurate wellbore placement are required, the errors discussed herein should be borne in mind. REAL HOLE CURVATURE BETWEEN SURVEYS Suppose a man was deprived of sensory input, apart from knowing the inclination of his foot. While walking, this man would be unable to tell if he was walking along a flat surface, walking up stairs or walking down stairs, as long as the surface on which his foot rested was always horizontal, Fig. 1. Fig. 1. Graphical illustration of error inherent in the “minimum curvature” method in MWD.