Abstract

One of the major concerns during drilling horizontal and directional wells is proper hole cleaning. If there exists a stationary bed, generally, an increase in the fluid velocity will erode the bed significantly. However, depending on the drilling conditions, very high fluid velocities are needed for bed removal, which may not be applied due to hydraulic and physical limitations. In such cases, pipe rotation may enhance the cuttings transport mechanically, and effective hole cleaning can be achieved even at fluid velocities lower than the critical annular fluid velocities required to prevent stationary bed development. In this study, the effect of pipe rotation on hole cleaning is investigated for horizontal and directional wells. Cuttings transport experiments have been conducted at METU Cuttings Transport Flow Loop using pure water as well as water-based muds consist of different concentrations of xanthan biopolimer, starch, KCl and soda ash, weighted with barite. Effect of pipe rotation have been observed for hole inclinations from horizontal to 50 degrees, for rate of penetrations from 15 to 45 ft/hr, for flow velocities from 2.1 ft/s to 7.2 ft/s, and for pipe rotations from 0 to 120 rpm. Pressure drop within the test section, and stationary and/or moving bed thickness are recorded besides the other test conditions. It has been observed that, pipe rotation has a significant improvement on cuttings transport, especially if the pipe is making an orbital motion. Also, pipe rotation drastically decreases the critical fluid velocity required to remove the stationary cuttings bed totally. However, after a certain rotation speed, no additional contribution of pipe rotation on hole cleaning is observed. When there are no cuttings present, the frictional pressure losses are increasing as the pipe rotation speed is increased. However, as the cuttings are introduced, due to the reduction in the stationary cuttings bed area, frictional pressure losses may decrease. Additionally, the effect of pipe rotation on hole cleaning as a function of fluid viscosity is also examined. It has been observed that, as the fluid viscosity is increased, contribution of pipe rotation on hole cleaning is increasing when compared with no rotation case.

Introduction

Hole cleaning has been one of the major challenges that the drilling industry has to face since early days. For vertical wells, in order to improve the cuttings transport performance, increasing the fluid viscosity or flow rates usually work. However, as the hole inclinations start deviating from vertical to horizontal, additional challenges arise, such as cuttings bed development. Especially as the inclinations are around 40–60 degrees, the hole cleaning is the most difficult due to the back sliding of the cuttings inside the wellbore. For such cases, higher flow rates are required. Also, for inclined wellbores, as mentioned in many studies available in the literature, fluids with high viscosities do not perform successful hole cleaning performances as observed in vertical cases. Moreover, during drilling operations, drillstring is usually rotating, which literally increases the complexity of the behavior of the drilling fluid and cuttings transport inside the wellbore. It is very difficult to explain the true behavior of fluid and cuttings inside the wellbore during the drillstring rotating.

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