Influence of drillpipe rotation and eccentricity on pressure drop over borehole with Newtonian liquid during drilling

Field measurements have revealed that the pressure drop over a borehole dur ing drilling of a slim oil well or a well with a long reach can depend sign ificantly on the rotation speed of the drillpipe. In this paper we discuss analytical, numerical and experimental studies to explain this effect for t he case of a Newtonian liquid in the borehole. Special attention is paid to the inertial forces in the liquid (in the annulus), caused by the rotation and eccentricity of the drillpipe. It is concluded that for large values o f these inertial forces the axial pressure drop depends significantly on th e rotation speed of the drillpipe. A pressure drop simulator (also valid fo r the case of a non-Newtonian liquid in the borehole) has been developed, t hat includes this effect and is in good agreement with field test data.