Drilling progress rates for individual sections of hole were obtained from the depth-time curves of a number of deviated wells in the North Sea. Within each field there was seen to be a good correlation between drilling progress rate and the average inclination for the section. This observation forms the basis of a mathematical model for estimating drilling times of deviated wells. The method requires that a standard design can be specified for all wells in a field. Casing points are defined with respect to the Geology and the drilling programme is set up as a sequence of unit operations, casing jobs and drilling intervals. The model can be calibrated from wells already drilled to provide a yardstick against which future performance can be judged. Alternatively it can be initialised with the performance of exploration wells and then be used to estimate the drilling times for a field development. The model has been used routinely for three years and has proved to be a cheap but powerful tool. Furthermore, it has provided insights into the potential for computer applications in drilling operations performance analysis and simulation.

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