Abstract

Hole cleaning is a complex process as there are many variables affecting cuttings removal (e.g. drilling fluid type, density, flow rate and rheological properties, cuttings size, drill pipe rotation speed). With the increasing number of drilling small diameter wells (e.g. coiled tubing drilling applications, ultra-deep wells drilled for exploitations of unconventional oil and gas resources), the wall resistance of the micro annulus also emerges as one of the most critical factors affecting the cuttings accumulation in wellbore. The eccentricity of drill pipes commonly observed during the drilling process of ultra-deep well and coiled tubing well makes the wall resistance effect on the cuttings transport even more prominent. Understanding the wall resistance effect on the particle settling behavior in eccentric annuli is, therefore, crucial for hydraulic design of efficient cuttings transport operations in these wells. In this study, a total of 196 sets of particle settling experiments were carried out to investigate the particle settling behavior in eccentric annuli filled with power-law fluids. The test matrix included the eccentricity ranges of 0–0.80, the dimensionless diameter ranges of 0.13–0.75 and the particle Reynolds number ranges of 0.09–32.34. A high-speed camera was used to record the particle settling process and determine the influences of the eccentricity, the dimensionless diameter, the fluid rheological properties, and the solid particle characteristics on the wall factor and the particle settling velocity. The functional relationship among the dimensionless diameter, the particle Reynolds number, and the wall factor was determined by using the method of controlling variables. An eccentric annulus wall factor model with average relative error of 5.16% was established. Moreover, by introducing Archimedes number, an explicit model of particle settling velocity in the eccentric annulus with average relative error of 10.17% was established. A sample calculation of particle settling velocity was provided to show the application of the explicit model. Results of this study can be used as a guideline by field engineers to improve hydraulic design of cuttings transport operations in concentric and eccentric annuli.