Summary 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. Introduction 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;hole inclination;drilling fluid properties;penetration rate;pipe/hole eccentricity;hole geometry;annular velocity profile;particle density; settling velocity, size, and geometry;drillpipe rotary speed; andpipe/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.

Summary The effects of field-measured mud rheological properties on cuttings transport in directional well drilling were studied experimentally. Water and bentonite/polymer muds were used, and angles of annulus inclination ranging from 0 to 900 from vertical. Experimental data were processed to express the cuttings transport quantitatively through annular cuttings concentration (vol%) at steady state. Three separate regions of hole inclination can be identified regarding cuttings transport: 0 to 45deg., 45 to 55deg. and 55 to 90deg. The effect of laminar flow dominates cuttings transport in low-angle wells (0 to 45deg.). In high-angle wells (55 to 90deg.), the effect of turbulent flow predominate. In the range of intermediate inclination (45 to 55deg.), turbulent and laminar flow generally have similar effects. In laminar flow, higher mud yield values and yield-point/ plastic-viscosity (YP/PV) ratio provide better cuttings transport. plastic-viscosity (YP/PV) ratio provide better cuttings transport. The effect of mud yield value is significant in the range of 0 to 45deg. hole inclination and becomes small or even negligible in the range of 55 to 90deg.. The effects of mud yield value and YP/PV ratio are more significant for lower annular fluid velocities. In turbulent flow, the cuttings transport was generally not affected by the mud rheological properties. properties. Introduction and General Discussion The problem of cuttings transport was studied by many investigators. An extensive literature review is given by Tomren. Recently, increasing attention regarding cuttings transport has been given to directional drilling. Tomren, Iyoho, and Becker, among others, have conducted studies in this area. On the basis of detailed analyses of previous and current work, several factors affect the cuttings transport in an inclined annulus. Axial and Radial Components of Particle Slip Velocity. According to gravity laws, only the axial component of the slip velocity exists in the case of a vertical annulus: ..........................................(1) This situation changes while the annulus is inclined gradually. The component of the slip velocity appears as ..........................................(2) and ..........................................(3) This situation is shown in Fig. Obviously, when the angle of inclination is increased, the axial component of the slip velocity decreases, reaching zero value at the horizontal position of the annulus. At the same time, the radial component reaches a maximum in the position mentioned. By taking these conditions into account, one can say that all factors that may lead to improved cuttings transport by a reduction of the particle slip velocity will have a diminishing effect while particle slip velocity will have a diminishing effect while the angle of inclination is increased. Annular Mud Velocity. The annular mud velocity in vertical drilling has to be sufficient to avoid cuttings settling and to transport these cuttings to the surface in reasonable time. As discussed earlier, in the case of an inclined annulus, the axial component of particle slip velocity plays a less important role, and one could conclude that to have a satisfactory transport, the annular mud velocity in this case may be lower than in the vertical annulus. The, however, would be a misleading conclusion. The increasing radial component of particle slip velocity pushes the particle toward the lower wall of the annulus, causing a particle toward the lower wall of the annulus, causing a cuttings (particle) bed to form. Consequently, the annular mud velocity has to be sufficient to avoid (or at least to limit) the bed formation. Studies show that to limit cuttings bed formation, the annular mud velocity in directional drilling has to be generally higher than in vertical drilling. Flow Regime and Regime of Particle Slippage. When the cuttings-transport phenomenon is considered, the regime of flowing mud and vertical slippage should be considered simultaneously. A mud in turbulent flow always induces turbulent regime of particle slippage, independent of the cuttings shape and dimensions. Therefore, in this case, the only factor that determines the particle slip velocity is the momentum forces of the mud; there is no influence of mud viscosity. P. 297

This paper presents a new design model that will enable the drilling engineer to select the proper hydraulics for problem-free drilling in high-angle holes (from 55 to 90° from vertical). Empirical correlations have been developed after carrying out an extensive experimental study of cuttings transport in a 5-in. full-scale flowloop. The model predicts the required critical transport fluid velocity (CTFV), the average cuttings travel velocity (CTV), and the annular cuttings concentration under most given sets of drilling operating conditions.

This paper describes a new model for obtaining analytical solutions to the problem of non-Newtonian fluid flow through eccentric annuli. A discussion on non-Newtonian rheology is presented, followed by the development and solution of applicable differential equations using the Ostwald de Waele power-law model and a nonrectangular slot.Results indicate that velocity values are reduced greatly in the reduced region of the eccentric annulus. This is important in directional drilling where the drill pipe tends to lie against the hole. Design of mud flow for cuttings transport on the basis of the nominal average velocity could lead to serious problems associated with cuttings buildup in the lowvelocity region of the annulus. Other practical applications of this work include the determination of velocity distribution in chemical processes involving fluid flow through eccentric annuli -e.g., heat exchangers and extruders -and more accurate velocity profiles inside journal bearings, particularly for small diameter ratios.The main advantage in the new approach is that iterative finite difference methods used by previous investigators are avoided. Previous work along present lines used a linearized model and resulted in velocity profiles of unacceptable accuracy. This study improves both the accuracy and the solution technique.

The effect of drillpipe rotation on hole cleaning during directional well drilling is investigated. An 8" diameter wellbore simulator, 100 ft long, with a 4 1/2" drillpipe was used for the study. The variables considered in this experimental work are: rotary speed, hole inclination, mud rheology, cuttings size, and mud flow rate. Over 600 tests were conducted. The rotary speed was varied from 0 to 175 rpm. High viscosity and low viscosity bentonite muds and polymer muds were used with 1/4" crushed limestone and 1/10" river gravel cuttings. Four hole inclinations were considered: 40, 65, 80, and 90 degrees from vertical. The results show that drillpipe rotation has a significant effect on hole cleaning during directional well drilling, contrary to what has been published by previous researchers who forced the drillpipe to rotate about its own axis. The level of enhancement due to pipe rotation is a function of the simultaneous combination of mud rheology, cuttings size, and mud flow rate. Also it was observed that the dynamic behavior of the drillpipe (steady state vibration, unsteady sate vibration, whirling rotation, true axial rotation parallel to hole axis, etc.) plays a major role on the significance in the improvement of hole cleaning. Generally, smaller cuttings are more difficult to transport. However, at high rotary speed and with high viscosity muds, the smaller cuttings seem to become easier to transport. Generally, in inclined wells low viscosity muds clean better than high viscosity muds, depending on cuttings size, viscosity, and rotary speed level. Introduction Numerous studies on cuttings transport have been conducted for the past two decades. Although several investigators have made observations on the effect of drillpipe rotation, most have focused their studies on mud rheology and annular velocities. This is the first time an extensive experimental study is conducted with the sole purpose of investigating the effect of drillpipe rotation on hole cleaning. In the past, the effect of drillpipe rotation was thought to be minimal. This belief was based on the results of experiments which were conducted in flow loops that used centralizers to constrain the pipe to rotate on its own axis, avoiding any orbital motion. Although the motion of the pipe will change at different positions along the well, it is now believed that in most cases the drillstring will have both rotary and orbital motion, even when in tension. In this case, it is the orbital motion and not the rotation that improves hole cleaning. When the pipe is rotating only along its axis it will cause a shift and a slight increase in the velocity profile in the annular area, causing the velocities on one side of the hole to be higher than on the other. Generally, a no slip condition at the boundaries in the annulus is assumed. These include the boundary between the hole or casing and the fluid, the boundary between the fluid and the drillpipe, and the boundary between the fluid and the cuttings bed. If the pipe is not rotating, the velocity of the fluid particles at these boundaries is zero. When the pipe rotates this boundary condition means that the velocity of the fluid particles adjacent to drillpipe is equal to the rotational speed of the pipe, perpendicular to the hole axis, resulting in a pseudo-helical flow. The minor effects observed in tests conducted under this configuration (using centralizers) indicate that the shift and the increase of the annular velocities are minor and do not affect cuttings transport significantly. On the other hand, the orbital motion of the pipe improves the transport of cuttings significantly in two ways: first, the mechanical agitation of the cuttings in an inclined hole sweeps the cuttings resting on the lower side of the hole into the upper side, where the annular velocity is higher. Second, the orbital motion exposes the cuttings under the drillstring cyclically to the moving fluid particles. P. 459^