The drilling industry has long had problems with debris left in the hole after completion of drilling and prior to a trip or to the running of a casing string or a production liner. Debris left in the hole can also lead to poor cement bonding with the formation. This problem can be especially bad in high angle or horizontal wellbores, where often large quantities of the debris can accumulate on the low side of the hole, thereby creating a lengthy interval potential cleaning problems.

Historically the industry has used centralizers and other tools to achieve a certain level of stand-off, whereby the axial flow of drilling fluid or completion fluids can flow past the debris and slowly remove the unwanted solids accumulation. The rotation of drill pipe to aid in wellbore cleanout has been used for many years, but the question of ‘how fast?” remains. In addition, many other ‘creative’ techniques have been tried in order to solve the problem, sometimes with some success and other times with little success. However, no model specific for wellbore cleanout in cases such as this exists.

In the last few years, the modeling of drill pipe rotation in drilling situations has advanced to the point where the helical flow characteristics of non-Newtonian drilling fluids can now be mapped in an annulus subject to axial and rotational flow. The axial and tangential velocities can be coupled to produce the annular point velocities for concentric and eccentric geometry. In this work, the velocity modeling is applied to predict wellbore cleanout efficiency for various particle sizes as functions of fluid shear stress in the annular flow stream and drillpipe rotation speed. Results will show rotational speeds required for the cleanout of small particles such as sand and for much larger-sized drilled cuttings.

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