Summary Inducing vibrations while drilling is a relatively new concept that shows promise in directional applications. Instigating motion of the drillstring, particularly in the sections lying on the low-side of the wellbore, can lead to substantial improvements in managing the two limiting factors of the long horizontal wells that are typically associated with current unconventional shale plays: removal of formation cuttings from the borehole and frictional drag between nonrotating drillstrings and the wellbore wall. Minimizing these effects by introducing controlled vibrations increases the overall drilling efficiency and reduces the cost associated with the well. The difficulty in implementing such actions, however, is consistency. Because the behavior of drilling assemblies is inherently nonlinear, it has been difficult in the past to reliably predict their response to dynamic events. This, in turn, creates a challenge when trying to optimize the performance of arbitrary vibration-inducing devices (VIDs). This study presents a detailed analysis of the fully coupled, 3D, nonlinear behavior of drillstrings under the action of induced vibrations. Specific focus is given to the dynamic characteristics of a drillstring, during horizontal-drilling operations, in long unconventional wells and how this behavior affects the success of the well. The response of the drillstring, due to induced axial and lateral oscillatory motions, is examined through linearized dynamic analysis and nonlinear time-domain simulations. Solutions obtained from the study provide a clearer understanding of the dynamics experienced downhole and lead to suggestions for improved practices when drilling with lateral VIDs, particularly with regard to hole cleaning. Further insights into the sliding behavior of the drillstring, during the use of these types of tools, are drawn from animated modeling results. Finally, the future applications of this technology are discussed.

Since the pioneering work of Bailey and Finnie in 1960, drillstring dynamics continues to be an active area of research within the industry. While much effort is put into developing accurate descriptions of the complexities associated with the downhole environment, proper modeling of the various damping mechanisms acting on the drillstring remains underdeveloped. A typical approach to modeling the damping downhole consists of utilizing a generalized, or proportional, damping model based on measurements of the actual system response. This technique can be fairly useful when done properly but does not actually quantify the effects of various environmental or operational parameters, such as fluid characteristics or string rotation, on the overall behavior of the drilling assembly. This study presents a nonlinear, semi-analytical, fluid-force model specifically developed to account for the various downhole characteristics that contribute to energy dissipation such as pipe eccentricity, lateral velocity, rotation speed, fluid rheology, and flow rate. This new fluid-force model is combined into an already proven drillstring model which was developed to embody the fully coupled flexibility of the drillstring, arbitrary wellbore curvature, frictional contact, and complex tool geometry. Using the improved model, the paper analyzes the nonlinear behavior of drillstrings with a focus on lateral vibrations in modern unconventional wellbores. Specific attention is given to studying the damping effects on the dynamic response of the drillstring and BHA during rotation with a Rotary Steerable System (RSS). The results shown through this investigation help to quantify the dynamics associated with modern drilling operations. Effects of fluid properties, flow rate, and rotation speed on the nonlinear behavior of the drillstring are examined through numerical studies of rotating an RSS assembly in an unconventional horizontal wellbore. Through the results, it is shown that proper modeling of the fluid forces acting on the drillstring helps to explain how BHAs, under certain conditions, can be safely operated within a range of resonant frequencies. Advanced visualizations of these time-domain simulations also reveal a unique observation that could have a significant influence on expanding the drilling envelope in automated operations.

Inducing vibrations while drilling is a relatively new concept that shows promise in directional applications. Instigating motion of the drill string, particularly in the sections lying on the low-side of the wellbore, can lead to substantial improvements in managing the two limiting factors of the long horizontal wells that are typically associated with today's unconventional shale plays: removal of formation cuttings from the borehole, and frictional drag between non-rotating drill strings and the wellbore wall. Minimizing these effects, by introducing controlled vibrations, increases the overall drilling efficiency and reduces the cost associated with the well. The difficulty in implementing such actions, however, is consistency. Because the behavior of drilling assemblies is inherently nonlinear, it has been difficult in the past to reliably predict their response to dynamic events. This, in turn, creates a challenge when trying to optimize the performance of arbitrary Vibration Inducing Devices (VIDs). This paper presents a detailed analysis of the fully coupled, three-dimensional, nonlinear behavior of drill strings under the action of induced vibrations. Specific focus is given to the dynamic characteristics of a drill string, during horizontal drilling operations, in long unconventional wells and how this behavior affects the success of the well. The response of the drill string, due to induced axial and lateral oscillatory motions, is examined through linearized dynamic analysis and nonlinear timedomain simulations. Solutions obtained from the study provide a clearer understanding of the dynamics experienced down-hole and lead to suggestions for improved practices when drilling with lateral VIDs, particularly with regards to hole-cleaning. Further insights into the sliding and rotational behavior of the drill string, while using these types of tools, are drawn from animated modeling results. Finally, the future applications of this technology are discussed.

Axial Excitation Tools, or AETs, are known to help improve ROP in modern horizontal wells. These devices instigate an axial vibration in the drillstring, which then propagates along the length of the horizontal assembly. The improvement in ROP is thought to originate from a more effective WOB transfer due to this induced axial movement from the AET. However, the exact mechanics of how these tools affect the drillstring is not known due to a lack of adequate modeling. Additionally, little work has been done to examine the potential consequences of inducing an axial resonance within the drillstring while utilizing these types of tools. This study explores these questions through advanced dynamic modeling. Using a newly-developed, validated, comprehensive drillstring model, the dynamic response of the drillstring, due to the axial excitation provided by an AET, is examined via a forced-frequency approach. The resulting sensitivity analysis illustrates the dependence of the system on the various operational and structural characteristics of the drillstring. Parameters examined include applied WOB, wellbore trajectory, design characteristics of the AET, and various boundary conditions. Nonlinear time- domain simulations are also conducted in order to quantify how AETs affect the axial force transfer to the bit. It is shown that various operational parameters will have a noticeable impact on the dynamic response of the drillstring when inducing axial vibrations while drilling. This observation suggests the necessity to properly plan for the use of AETs in order to avoid unnecessary failures of downhole components and subsequent non-productive time (NPT). While the modeling itself is advanced, the outputs are designed to help visualize the response of the system in a "user-friendly" manner, reducing the learning curve and bridging the gap between the "tech-savvy" and non-technical personnel. This study presents an efficient way of determining the potential of harmful vibrations when drilling with an Axial Excitation Tool. The modeling approach shown is comprehensive and provides new insights into the fundamental operation of these types of tools and how axial excitations help to improve drilling efficiencies. Results presented show that induced vibrations do not simply "reduce friction," as is so often claimed; instead, these tools generate a dynamic axial force, oscillating about the average value, which may help work past "tight-spots" along the well. In turn, this would give the impression of reduced friction along the wellbore based on hook load measurements.