Use of Roller Reamers Improves Drilling Performance in Wells Limited by Bit and Bottomhole Assembly Vibrations

Abstract

Abstract Stick-slip and whirl are vibrational problems that commonly limit drilling performance in hard formations and extended reach wells. The introduction of roller reamers has resulted in significant performance improvement in a number of the operator's drill teams with chronic stick-slip or whirl-induced borehole features. Whirl is a dysfunction characterized by lateral vibration that may occur both at the bit and in the bottomhole assembly (BHA). When whirl becomes severe, lateral vibrations cause significant side forces in stabilizers. Frictional drag resulting from these side forces causes high levels of torque to be generated at stabilizers and can result in stick-slip (a condition characterized by fluctuations in the rotational speed of the BHA). The operator refers to this as "coupled stick-slip" because the condition is caused by whirl in stabilizers. When these conditions exist, the replacement of stabilizers with roller reamers significantly reduces the potential for torque-generation at the contact points. Consequently, more torque becomes available to the bit and the driller may raise weight on bit (WOB). This results in reduced levels of bit whirl and improved rate of penetration (ROP). The reduction of bit whirl and elimination of stick-slip prevent damage to bit and BHA components. Roller reamers are also seen to serve the historical purpose of conditioning the borehole. The operator's field experience indicates that the need for conditioning is often due to whirl-induced features in the borehole. These features include spiraling and ledges. The use of roller reamers greatly reduces the prevalence and impact of whirl features. This paper details the conditions where roller reamers provide benefits allowing improved performance. A conceptual model for coupled stick-slip is provided as well as a summary of whirl-induced borehole features which may be mitigated with roller reamers. Representative examples are provided that show drilling performance improvements using roller reamers to mitigate these conditions. Introduction When lateral and torsional vibrations in the BHA become coupled, a torque or torsional vibration limit of the drilling system often prevents the driller from running sufficient WOB to prevent bit whirl. Bit whirl can be detrimental to drilling operations in many ways. These detrimental effects include the development of ledges (Brett et al. 1990), borehole oscillations (Pastusek and Brackin 2003, Boualleg et al. 2006), hole spiraling (Gaynor et al. 2001), and premature failure of downhole tools (Bailey et al. 2008, Mitchell and Allen 1987). Dupriest et al. (2005) demonstrated that the presence of whirl in the bit itself may be observed by evaluating the impact of changes in drilling parameters in real time on mechanical specific energy (MSE). MSE measures the amount of energy consumed in drilling a unit volume of rock. Real-time surveillance of MSE across global operations suggests that over 40% of footage is affected by a detectable level of whirl. When the driller is not constrained by a limit of the rig or drillstring, parameters may be adjusted to reduce the level of bit whirl. Typically, weight on bit (WOB) is increased resulting in a reduction in MSE. This indicates that the amount of energy lost to side-cutting and heat generation from whirl vibrations was reduced. For example, Boualleg et al. (2006) documented a reduction in the amplitude of whirl-induced borehole oscillations that resulted from drilling the interface between two samples of rock with a contrast in compressive strength as WOB was increased. Field evidence confirms the presence of these whirl-induced oscillations in field conditions at the interface of laminated rocks. The operator has noted a couple of recent instances where the BHA became mechanically stuck as a result of whirl-induced borehole patterns after drilling a transition between rocks of varying strength.