Over the last five years the use of oil based mud in the North Sea has increased. This is principally for drilling the Tertiary and Cretaceous mudstones in the Central and Northern sectors and the massive salts in the Southern sector. The technical reasons behind this increase have been examined as has the cost effectiveness of using an oil based mud system.

To compare the performance between oil based and water based mud, operations in six distinct zones on the UKCS were evaluated; these included areas where several exploration or production wells had been drilled using both mud types. Only drilling operations which could be affected by the mud system were selected for comparison. This evaluation was done for the 17 1/2" and 12 1/4" hole sections of each well: the operations selected for consideration were drilling, tripping, reaming, conditioning mud and hole, and any instances of stuck pipe. Performance was compared using a time per metre drilled standard.

Analysis of the data shows that well sections drilled with oil based mud require fewer wiper trips, well costs are lower, average drilling rates are higher, there is a reduced need for reaming and mud conditioning, and mud related problems are fewer. Additionally the risk of stuck pipe in 12 1/4" hole is significantly reduced with the use of oil based mud. Overall, the cost effectiveness of using oil based mud is shown to be significant. In some instances significant savings are being realised; such savings could make marginal field developments economically viable from reduced drilling costs alone.

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