A large-scale study of cuttings transport in directional wells is discussed in this paper. Previous investigators used unrealistically high fluid velocities and/or short test sections where steady-state conditions had not been established. This study used a 40-ft [12.2-m] test section. Pipe rotation and eccentricity, as well as several types of drilling muds and flow regimes, were studied. Annulus angles varied from 0 to 90°, and actual drilled cuttings were used.
The major factors affecting cuttings transport are drilling fluid velocity, hole inclination, and fluid rheological properties. Much higher annular velocities are required for effective hole cleaning in directional wells than in vertical wells. An increase in hole angle and/or drilling rate reduces the transport performance of drilling fluids. Hole angles of 40 to 50° are critical because of cuttings buildup and downward sliding of the bed of cuttings. High-viscosity muds were observed to provide better transport than low-viscosity muds.
Since the introduction of rotary drilling, the circulation of drilling fluid has become an integral part of the drilling operation. Two primary functions of a circulating drilling fluid are (1) to remove generated cuttings from the bottomhole and bit teeth and (2) to lift those cuttings to the surface through the annular space between the drillpipe and the hole wall. The ability of the fluid to lift such cuttings is generally referred to as the carrying capacity of the drilling fluid.
This study resulted from the need for accurate and realistic data to facilitate the optimum design of drilling-fluid systems for directional drilling. For vertical or near-vertical drilling, the problem appears to have been adequately contained. In directional well drilling, however, the inclined (usually eccentric) annulus poses several problems not encountered in vertical wells.
Previous investigators1–7 have listed the most relevant factors affecting the carrying capacity of drilling fluids:
fluid annular velocity;
drilling fluid properties;
annular velocity profile;
particle density; settling velocity, size, and geometry;
drillpipe rotary speed; and
pipe/hole diameter ratio. It is difficult and impractical to investigate the effects of all these parameters simultaneously. Consequently, our objective was to develop field-oriented cuttings-transport models that account for the most significant factors affecting particle and fluid dynamics in directionally drilled wells.
To achieve this objective, a two-pronged approach was adopted.
Design and construct an apparatus for experimental investigation of the behavior of actual rock cuttings at realistic fluid velocities, hole inclination angles, pipe/hole eccentricities, drillpipe rotary speeds, and other relevant variables.
Apply all theoretical considerations for the development of applicable mathematical relations based on detail analyses of relevant principles. These principles should include the dynamics of irregularly shaped particles in non-Newtonian fluids; the axial-velocity profile in inclined, eccentric annuli; and the tangential velocity produced by drillpipe rotation and pipe/hole eccentricity. This paper covers only the first phase of this study with brief comments on the second phase.
The cuttings transport in vertical wells has been covered extensively by many investigators.1–5,8–24 In contrast, very little has been contributed to the problem of directional well drilling. Fujii and Sato25 conducted laboratory experiments at 0, 45, and 60° angles from the vertical with water and carboxymethyl-cellulose-polymer solutions and a 1.33-in. [34-mm] pipe inside a 2.36-in. [60-mm] casing. In our opinion, their results are of little practical significance because of the high, unrealistic velocities used (up to 10 ft/sec [3 m/s]) and because the short test section (10 ft [3 m]) did not establish steady-state conditions.
Movsumov et al.26 attempted to solve the problem of drilled-cuttings transport in inclined, eccentric annuli purely from theoretical considerations. Their mathematical approach involved extensive trial and error and, therefore, is of little practical value, especially because their analysis was idealized to exclude the important phenomenon of bed formation that is discussed later. In the current work,6,7 a unique experimental facility was designed to provide flexibility for a comprehensive investigation of steady-state cuttings transport. Several angles of inclination, drillpipe rotations, pipe/hole eccentricities, and mud flow rates were investigated.