Friction Coefficient Measurements for Casing While Drilling with Steel and Composite Tubulars

For the calculation of drilling loads knowledge of the Coulomb friction coefficients for friction between drilling tubulars and casing or drilling tubulars and open hole is essential. To reduce drilling costs for geothermal wells the casing while drilling, CwD technique is considered. CwD reduces drilling time by eliminating round trips, because the well is drilled and cased simultaneously, thus improving efficiency. Drilling loads are the result of friction in the borehole, the weight of the string, and the borehole and drillstring geometries. Drilling costs will be reduced even more if the drilling loads can be reduced. In the Coulomb friction model the ratio between the friction force and the normal force is constant. Thus, if the normal force is reduced, the friction reduces. This can be achieved by replacing the regular heavy steel casing by lighter composite tubulars. Dynamic drilling loads like torsional vibrations can be triggered by the difference in static and dynamic Coulomb friction coefficients. A large difference increases the chance on the occurrence of such friction-induced vibrations. The objective of this study was to compare the Coulomb friction coefficients for steel and composite tubulars, under both static and dynamic conditions, through experiments with the different samples in sand and steel 'boreholes'. If the friction factors for composite casing are known, the dynamic drilling loads for CwD with composite casing can be predicted more accurately. Unfortunately, with the set-up used to measure the dynamic friction coefficients no conclusive results have been obtained. The Coulomb friction coefficients for steel on steel and for steel on ‘rock’ were constant for all applied loads, but the coefficients for composite on steel and for composite on ‘rock’ behaved unexpectedly. In particular, decreasing the normal force seemed to increase the friction coefficients to unrealistically high values. Also the static friction coefficient measurements of the composite casing showed some inconsistencies, which can be contributed to the irregular surface caused by the production process.Petroleum EngineeringSection Petroleum EngineeringCivil Engineering and Geoscience