Effect of Drillstring Deflection and Rotary Speed on Annular Frictional Pressure Losses

Abstract

Keeping the drilling fluid equivalent circulating density in the operating window between the pore and fracture pressure is a challenge, particularly when the gap between these two is narrow, such as in offshore, extended reach, and slim hole drilling applications usually encountered in shale gas and/or oil drilling. To overcome this challenge, accurate estimation of frictional pressure loss in the annulus is essential. A better estimation of frictional pressure losses will enable improved well control, optimized bit hydraulics, a better drilling fluid program, and pump selection. Field and experimental measurements show that pressure loss in annuli is strongly affected by the pipe rotation and eccentricity. The major focus of this project is on a horizontal well setup with drillstring under compression, considering the influence of rotation on frictional pressure losses of yield power law fluids. The test matrix includes flow through the annulus for various buckling modes with and without the rotation of the inner pipe. Sinusoidal, helical, and transition from sinusoidal to helical configurations with and without the drillstring rotation were investigated. Helical configurations with two different pitch lengths are compared. Eight yield power law fluids are tested and consistent results are observed. The drillstring rotation patterns and buckling can be observed due to experimental facility's relatively longer and transparent test section. At the initial position, inner pipe is lying at the bottom due to its extensive length, suggesting a fully eccentric annular geometry. When the drillstring is rotated, whirling, snaking, irregular motions are observed. This state is considered as a free drillstring configuration since there is no prefixed eccentricity imposed on the drillstring. The reason for such design is to simulate the actual drilling operations, especially the highly inclined and horizontal drilling operations. Results show that rotating the drillstring can either increase or decrease the frictional pressure losses. The most pronounced effect of rotation is observed in the transition region from laminar to turbulent flow. The experiments with the buckled drillstring showed significantly reduced frictional pressure losses compared to the free drillstring configuration. Decreasing the length of the pitch caused a further reduction in pressure losses. Using the experimental database, turbulent friction factors for buckled and rotating drillstrings are presented. The drilling industry has recently been involved in incidents that show the need for critical improvements for evaluating and avoiding risks in oil/gas drilling. The information obtained from this study can be used to improve the control of bottomhole pressures during extended reach, horizontal, managed pressure, offshore, and slim hole drilling applications. This will lead to improved safety and enhanced optimization of drilling operations.