We present a novel method for analysing the position uncertainty of directional surveys. By this method, the uncertainty can be accurately calculated by conceptually simple analytical formulae. The theory covers any correlation in measurement error and any correlation in toolface, for both stationary and continuous surveys.

The results demonstrate the importance of handling the toolface dependence properly. A direct benefit is that too optimistic or overly pessimistic accuracy estimates are avoided. Furthermore, the difference between rotating and sliding operation is significant for many error terms. For example, a misalignment error typically causes 10 times higher position uncertainty in sliding mode than in rotating mode, for a 3000 m long survey.

These findings should have implications for the future specification of survey instrument operation procedures, and thus the design of directional survey programs.

Keywords:
azimuth, expression, inclination, expectation value, wellbore position accuracy torgeir torkildsen, error term, coefficient, upstream oil & gas, covariance, drilling operation
Subjects:
Well Planning, Drilling Operations, Trajectory design, Directional drilling
Copyright 2000, Society of Petroleum Engineers
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