Cuttings transport in aerated mud drilling for inclined annuli with inclination angles between 30° to horizontal was investigated. Experiments were carried out for over 300 flow conditions using a 9-m long un-pressurized annular test section with 5-in. CSG and 2-in. DP, concentric or eccentric annulus geometry. Two types of base liquids, water and PHPA solution, were used for aerated muds. In the experiments, cuttings were fed into the test section at a constant rate corresponding to a supposed ROP, and the volumetric concentration of cuttings in the annulus under steady state was measured. From the analysis on the variation of cuttings concentration with mud flow rates ranging from sub-critical to critical regions, the critical flow rate (CFR) criterion avoiding a formation of cuttings bed was found to correspond to the cuttings concentration of about 4 vol%. The annular velocity (AV) equivalent to the CFR was as high as 2 m/s in highly inclined eccentric annulus using non-Newtonian conventional PHPA fluid. Air injection significantly reduced the CFR and improved the cuttings transport efficiency in highly inclined annulus except for the horizontal annulus in which much higher AV of about 1.5 m/s was still required to transport cuttings effectively. The result indicates that the flow pattern between gas and liquid considerably affects the cuttings transport performance.

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