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

SPE J 9535Summa..,. An experimental study comparing the effects of mud rheological parameters on annular drill-cuttings buildup was conducted in a large-scale flow section. Rheological parameters examined were mud yield point (YP); plastic viscosity (PV); YP/PV ratio; power-law exponent; consistency index; Fann™ V-G Meter dial readings at 600,300,200, 100,6, and 3 rev/min; effective viscosity (calculated by the power-law derivation); and initial and 100minute gel strengths. Fifteen bentonite/polymer water-based muds were used, ranging from water to a mud with yP and PV values of 20 Ibf/l00 ft2 and 40 cp [19.6 Pa and 0.04 Pa's], respectively. For each mud, flow tests were run at annular velocities from 120 to 240 ftlmin [37 to 73 m/min] and hole angles from 30 to 90° from vertical. The effective drilling rate for all tests was 50 ftIhr [15 mIh]. Altogether, 180 tests are analyzed.When data were correlated with annular cuttings volume in steady-state flow, the best fit was obtained with low-shear-rate parameters. These parameters include the 6-and 3-rev/min dial readings and initial gel strength. Correlation coefficients for these were often from 0.9 to 0.95. For hole angles ::s45° (from vertical), increasing values of these parameters improved cuttings removal (Le., reduced cuttings volume). The same was observed at angles near horizontal, but only in the laminar flow regime. Cuttings volumes were always smaller in the turbulent regime during near-horizontal flow. IntroductionAdequate cuttings removal from a well during rotary drilling is critical for cost-effective drilling. High annular cuttings buildup is often responsible for stuck drillpipe and other impediments to standard drilling and completion procedures. Increasing hole inclination from vertical aggravates the tendency for annular cuttings accumulation, intensifying the need for mud hydraulics aimed at acceptable hole cleaning.Many factors affect cuttings transport, 1 but the options available for effective transport are few. Such influential parameters as mud density and hole angle are predetermined for reasons ranging from rock and fluid in-situ characteristics to overall well-planning strategy. Other variables, such as drill-cutting physical properties, are uncontrollable. The windows of opportunity for ensuring adequate hole cleaning during drilling lie, to a considerable extent, with mud flow rate and mud rheology.While mud rheology does not (during high-angle drilling) affect annular cuttings flow as strongly as such factors as mud flow rate, mud density, and hole angle,l-6 its effect is significant. Furthermore, its effect depends appreciably on interactions with mud annular velocity and hole angle. 2The complex viscous nature of drilling muds has led to many proposed means of varying sophistication to characterize mud rheology. In tum, questions are (or should be) raised concerning the appropriate rheological parameters best suited for particular hydrodynamic applications. This study entertains the applicability of various parameters to annular s...