How Does a Stationary Sand Bed Affect the Flow Dynamics in an Eccentric Annulus?

In the drilling operations, it is common to have a stationary bed of the drilled cuttings in the high angle sections of the wellbore. The bed must be removed in the later stages before running the casing, or when it starts to cause high torque and drag on the drill string. The mere act of circulating drilling fluid, however, may not clean the well (i.e., critical flow rate and shear stress for bed erosion must be reached). In an effort to better understand the underlying mechanisms of bed removal process during hole cleaning, in this paper, we look at how the presence of a stationary sand bed affects the flow field in an eccentric annulus. Experiments simulating turbulent flow of water in an eccentric annulus with/without the presence of stationary sand bed have been conducted by using a 9m long horizontal flow loop (with an annular configuration of 95 mm ID outer pipe and 38 mm OD inner pipe). The flow loop was equipped with particle image velocimetry (PIV) system, which was used to collect velocity field data. The PIV data were then used to study the characteristics of the turbulent flow of water in the eccentric annulus. The velocity field and Reynolds stress profiles were analyzed in two planes, one perpendicular to the bed interface and off-center of the annulus, and the other along the center-line of the annulus. Experiments were carried out with the presence of two different height stationary sand beds and also without a sand bed as the control case. The extent to which the presence of the sand bed affects the flow appears to be a strong function of the bed height in the annulus. For a small bed height, deviation of the velocity field from the no bed case was slight. In this case, Reynolds normal and shear stress values were lower near the bed interface comparing to the annulus centerline. On the other hand, for a flow over a thicker bed, this behavior changed, and the flow became more uniform in the annulus (in terms of turbulence and mean flow properties). The results help in understanding the mechanism of bed erosion under constant pump flow rate. From the practical point of view, data presented here suggest that hole cleaning in an eccentric annulus progressively becomes more difficult as the bed becomes smaller. The results also explain why in long horizontal and extended reach wells often wiper trips are required for proper cleaning of the hole.