An experimental study followed by comprehensive flow modeling is presented. The experiments were conducted on a horizontal well setup with drillstring under compression, considering the influence of rotation on frictional pressure losses of Yield Power Law (YPL) fluids. Flow through various buckling configurations with and without drillstring rotation was investigated. A new correlation is presented for the transition from laminar to turbulent regions in concentric and eccentric annuli. A broad model of flow of YPL fluids is proposed for concentric, eccentric and buckled configurations. The model includes the effects of rotation in laminar, transitional and turbulent flow. A 91 ft. inner pipe was rotated while applying axial compression during flow. At the no-compression case, eccentricity of the inner pipe is varied as the drillstring rotated. The aim for such a design was to simulate actual drilling operations. The test matrix involves flow through sinusoidal, transitional and helically-buckled drillstring. The effect of pitch length is investigated. Helical modes with two different pitch lengths were tested. Eight distinct YPL fluids were used to examine the dependence of pressure losses on fluid parameters. In the theoretical part, a stability criterion is modified to determine the onset of transitional flow of YPL fluids and a correlation is proposed for practical purposes. In addition, pressure loss prediction models are presented for the flow of YPL fluids through concentric, eccentric, free and buckled configurations of the drillstring, with and without rotation. The proposed models are compared with data from the literature and the experiments. It has been observed that increasing eccentricity and rotation causes an earlier transition from laminar to turbulent flow. Increasing eccentricity decreases pressure losses. In addition, the buckled configurations show a further decrease in frictional pressure losses as the compression increases. In the helical mode, decreasing the pitch length results in a decrease in pressure losses. Rotation tests with free drillstring show an increase in pressure losses as the rotary speed of the drillstring increases. Also, rotating the drillstring while it is compressed suggests an elevated increase in pressure losses due to amplified vigorous motion of the drillstring. Distinct differences in the effects of buckling and rotation are observed in laminar, transitional and turbulent flow. The greatest differences are found in the transition region. Flow of YPL fluids is one of the greatest challenges in the modern drilling industry. Studies that correspond to actual drilling conditions are substantially important in reducing these challenges. The information obtained from this study can be used to improve the control of bottomhole pressure during extended reach (ERD), horizontal, managed pressure (MPD), offshore and slim hole drilling applications. Consequently, this theoretical and experimental research has the potential to lead to safer, deeper and more precisely controlled oil/gas well drilling operations.

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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.

Summary An experimental study followed by comprehensive flow modeling is presented. The experiments were conducted on a horizontal annulus with drillstring under compression, considering the influence of rotation on frictional pressure losses of yield-power-law (YPL) fluids. Flow through various buckling configurations with and without drillstring rotation was investigated. Correlations of critical Reynolds numbers are presented that predict the onset and offset of transition from laminar- to turbulent-flow regions in concentric and eccentric annuli. A broad model of flow of YPL fluids is proposed for concentric, eccentric, and buckled configurations. The model includes the effects of rotation in laminar, transitional, and turbulent flow. A 91-ft inner pipe was rotated while applying axial compression during flow. At the no-compression case, eccentricity of the inner pipe is varied as the drillstring rotated. The aim for such a design was to simulate actual drilling operations. The test matrix involves flow through sinusoidal, transitional, and helically buckled drillstring. The effect of pitch length is investigated. Helical modes with two different pitch lengths were tested. Eight distinct YPL fluids were used to examine the dependence of pressure losses on fluid parameters. In the theoretical part, a stability criterion is modified to determine the onset of transitional flow of YPL fluids and a correlation is proposed for practical purposes. In addition, pressure-loss-prediction models are presented for the flow of YPL fluids through concentric, eccentric, free, and buckled configurations of the drillstring, with and without rotation. The proposed models are compared with data from the literature and the experiments. It has been observed that increasing eccentricity and rotary speed causes an earlier transition from laminar to turbulent flow. Results suggest reduced pressure losses with an eccentric pipe. In addition, buckled configurations showed a further decrease of frictional pressure losses as the compression increases. In the helical mode, two pitch lengths are compared, and decreasing the pitch length resulted in a decrease in pressure losses. Rotation tests are conducted with free and buckled configurations. Rotation in the free-drillstring mode showed an increase in pressure losses as the rotary speed of the drillstring increases. Amplified vigorous motion of the drillstring is visually observed as the drillstring is buckled while rotating. Rotating the drillstring while buckled showed a further increase in pressure losses compared to rotating in free mode. This additional increase in pressure losses is attributed to the more-dynamic motion of the drillstring. Distinct differences of pressure losses in the effects of buckling and rotation are observed in laminar, transitional, and turbulent flow. Significant differences are measured in the transition region.

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 applications. To overcome this challenge, accurate estimation of frictional pressure loss in the annulus is essential, especially for multilateral, extended reach and slim hole drilling applications usually encountered in shale gas and/or oil drilling. A better estimation of frictional pressure losses will provide improved well control, optimized bit hydraulics, a better drilling fluid program and pump selection. Field and experimental measurements showed that pressure loss in the annulus is strongly affected by the pipe rotation and eccentricity. Eccentricity will not be constant throughout a wellbore, especially in highly inclined and horizontal sections. In an actual wellbore, because of rotation speed and the applied weight, some portion of the drillstring will undergo compression. As a result, variable eccentricity will be encountered. At high compression, the drillstring will buckle, resulting in sinusoidal or helical buckling configurations. Most of the drilling fluids used today show highly non-Newtonian flow behavior, which can be characterized using the Yield Power Law (YPL). Nevertheless, in the literature, there is limited information and research on YPL fluids flowing through annular geometries with the inner pipe buckled, rotating, and eccentric. Furthermore, there are discrepancies reported between the estimated and measured frictional pressure losses with or without drillstring rotation of YPL fluids, even when the inner pipe is straight. 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 YPL fluids. The test matrix includes flow through the annulus for various buckling modes with and without rotation of the inner pipe. Sinusoidal, helical and transition from sinusoidal to helical configurations with and without the rotation of the drillstring are investigated. Results show a substantial difference of frictional pressure losses between the non-compressed and compressed drillstring. 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 safer and enhanced optimization of drilling operations.