The Effect of Drillpipe Rotation on Hole Cleaning During Directional Well Drilling

Abstract

Abstract The effect of drillpipe rotation on hole cleaning during directional well drilling is investigated. An 8" diameter wellbore simulator, 100 ft long, with a 4 1/2" drillpipe was used for the study. The variables considered in this experimental work are: rotary speed, hole inclination, mud rheology, cuttings size, and mud flow rate. Over 600 tests were conducted. The rotary speed was varied from 0 to 175 rpm. High viscosity and low viscosity bentonite muds and polymer muds were used with 1/4" crushed limestone and 1/10" river gravel cuttings. Four hole inclinations were considered: 40, 65, 80, and 90 degrees from vertical. The results show that drillpipe rotation has a significant effect on hole cleaning during directional well drilling, contrary to what has been published by previous researchers who forced the drillpipe to rotate about its own axis. The level of enhancement due to pipe rotation is a function of the simultaneous combination of mud rheology, cuttings size, and mud flow rate. Also it was observed that the dynamic behavior of the drillpipe (steady state vibration, unsteady sate vibration, whirling rotation, true axial rotation parallel to hole axis, etc.) plays a major role on the significance in the improvement of hole cleaning. Generally, smaller cuttings are more difficult to transport. However, at high rotary speed and with high viscosity muds, the smaller cuttings seem to become easier to transport. Generally, in inclined wells low viscosity muds clean better than high viscosity muds, depending on cuttings size, viscosity, and rotary speed level. Introduction Numerous studies on cuttings transport have been conducted for the past two decades. Although several investigators have made observations on the effect of drillpipe rotation, most have focused their studies on mud rheology and annular velocities. This is the first time an extensive experimental study is conducted with the sole purpose of investigating the effect of drillpipe rotation on hole cleaning. In the past, the effect of drillpipe rotation was thought to be minimal. This belief was based on the results of experiments which were conducted in flow loops that used centralizers to constrain the pipe to rotate on its own axis, avoiding any orbital motion. Although the motion of the pipe will change at different positions along the well, it is now believed that in most cases the drillstring will have both rotary and orbital motion, even when in tension. In this case, it is the orbital motion and not the rotation that improves hole cleaning. When the pipe is rotating only along its axis it will cause a shift and a slight increase in the velocity profile in the annular area, causing the velocities on one side of the hole to be higher than on the other. Generally, a no slip condition at the boundaries in the annulus is assumed. These include the boundary between the hole or casing and the fluid, the boundary between the fluid and the drillpipe, and the boundary between the fluid and the cuttings bed. If the pipe is not rotating, the velocity of the fluid particles at these boundaries is zero. When the pipe rotates this boundary condition means that the velocity of the fluid particles adjacent to drillpipe is equal to the rotational speed of the pipe, perpendicular to the hole axis, resulting in a pseudo-helical flow. The minor effects observed in tests conducted under this configuration (using centralizers) indicate that the shift and the increase of the annular velocities are minor and do not affect cuttings transport significantly. On the other hand, the orbital motion of the pipe improves the transport of cuttings significantly in two ways: first, the mechanical agitation of the cuttings in an inclined hole sweeps the cuttings resting on the lower side of the hole into the upper side, where the annular velocity is higher. Second, the orbital motion exposes the cuttings under the drillstring cyclically to the moving fluid particles. P. 459^