Analysis About Drill String Vibration of Horizontal Well

Torsional vibrations experienced by drill strings can be detrimental to drilling operations. With a goal of understanding torsional vibrations experienced by drill strings and determining means to attenuate undesired vibrations, the authors have studied the effect of adding a sinusoidal modulation to a constant rotation speed of a drill string. A combination of modeling, analysis, and experiments is used to explore the influence of this rotation input modulation on the system response. The drill string is modeled as a modified Jeffcott rotor, which is described by a system with three degrees of freedom. Considering the case of forward whirling of a rotor in continuous contact with a stator, the equations of motion are reduced to a single degree-of-freedom nonlinear oscillator describing the torsional motions. In order to understand the fast time scale and slow time scale components of the motion, the method of direct partitions of motions is used to determine an approximate response to the nonlinear oscillator. The obtained results of the analysis illustrate that with the sinusoidal modulation of the rotor drive speed, the equivalent torsion stiffness can be enhanced and the character of the friction force at the contact can be made smooth. The analyses helps bring forth the stabilizing influence of the added sinusoidal input to the rotor drive speed. Over the considered parameter ranges, the numerical results obtained with the full three degree-of-freedom model and the reduced single degree-of-freedom model are found to be in agreement with each other. Furthermore, the results from these models are found to compare well with those obtained by using the method of direct partition of motions. Experiments with a laboratory scale drill-string arrangement are to be carried out to validate the analytical and numerical findings and further explore the effectiveness of the drive speed modulation on the rotor dynamics. Introduction Slender rotating structures are used in many engineering applications. Drill strings are long rotating slender structures, which are used in drilling operations. A schematic of a rotary drill rig is shown in Fig. 1 (e.g., Liao, Balachandran, Karkoub, and Abdel-Magid, 2011). Drill strings experience different types of vibrations (axial, torsional, and lateral vibrations) that may lead to detrimental failures of a drilling system. Drill-string vibrations have attracted the attention of many researchers, and many models have been developed to understand them. Since drill strings have a large length-to-diameter ratio, typically, the first torsional natural frequency and first lateral natural frequency are close to each other. This frequency proximity and the nature of the system allows for coupling and energy transfer between the associated vibration modes. Here, as a step towards developing further understanding, a drill string is modeled as an extended planar Jeffcott rotor with gravity acting normal to the rotor. Due to the planar motions, no gyroscopic effects are considered. The extended model, which was earlier considered in the work of Vlajic, Liu, Karki, and Balachandran (2014), is shown in Fig. 2. The model is described with three degrees of freedom (DOF), namely, x and y to account for lateral motions, and theta to account for torsional motion. A large number of research efforts have focused on controlling drill-string vibrations by using different feedback control algorithms, which need measurements along the drill string. In this work, the authors propose a different control approach to mitigate whirling motions during continuous rotor-stator contact. This can be compared to a situation of a drill string being in continuous contact with the borehole. The approach is open loop in implementation and this approach is based on adding a high frequency input to the drive speed of the drill string. Given the open-loop nature, the method does not depend on any measurements along the drill string or rotary table, which could be attractive for a practical stand point. Although the current focus is on motions of a rotor with continuous stator contact, it is planned to study stick-slip and non-contact cases in the future. System Modeling, Studies, and Results As previously mentioned, the drill string is modelled as a Jeffcott rotor with three DOF. Proceeding along the lines of the group's prior work (Vlajic, Liu, Karki, and Balachandran, 2014), after some approximations for the continuous rotor-stator contact case, the 3 DOF system is reduced to a single second-order nonlinear differential equation governing the torsional motion. In Fig. 3, for a representative case with a constant drive speed, the torsional state histories obtained for the full model and the reduced model are plotted. It is noted that the final state of the motion is captured quite well. To further analyze the response during forward whirling, the reduced-order model was nondimensionalized and an approximate solution was obtained by using the method of direct partitions. This method allows one to separate slow scale motions from fast scale motions. As discussed, in the group's prior work, this method can be useful to examine whirling motions. The results obtained by directly integrating the full model as well as the reduced-order model have been compared with that obtained by using the method of direct partitions of motions. It is seen that the perturbation analyses is able to provide an approximate solution that compares well with the numerical results obtained through direct integrations. Next, this analysis is used to examine the influence of the addition of secondary frequency component. It is seen that the addition of the secondary frequency, a high-frequency one, helps smooth out the friction coefficient variation with respect to the relative speed at contact. This is found to have a beneficial effect on the motion and helps suppress large-amplitude torsional motions. Stability analysis was also conducted to examine this effect. Concluding Remarks A study has been conducted to examine the influence of sinusoidal modulation of rotor drive speed in a system, wherein the rotor experiences continuous stator contact. A single degree of-freedom reduced model is developed to study the torsion response and it is found that the addition of a high frequency input can be beneficial in attenuating rotor motions. This is believed to be useful for developing open-loop control schemes for attenuating forward whirling motions of drill strings. Results obtained from a lab scale experimental arrangement will be used to examine the effect of this drive speed modulation further and they will be reported in the conference presentation. Future studies could build on the current effort to examine cases of backward whirling. Acknowledgment: The authors would like to gratefully acknowledge the support received from the Qatar National Research Fund for NPRP Project 7-083-2-041, to pursue this collaborative work between the University of Maryland, College Park, MD, USA and Qatar University, Doha, Qatar.

The longitudinal vibration of the bottom drill string is violent and the law is complex during the deep well drilling. The vibration of the drill string brings many adverse effects on the drilling pipe fracture and bit trampoling. Generally speaking, the effective way to control the vibration of drill string is to install damping device in bottom hole. The research group proposes a device that uses the longitudinal vibration energy of the deep well drill string to modulate the impact dynamic load, which converts the vibration energy of the downhole drill string that is not conducive to drilling into the mechanical impact energy that improves the rock breaking capacity of the bit. The impact load modulation device can use the drill string to apply the “mechanical WOB” and the differential pressure between the upper and lower piston to produce the “hydraulic WOB”, The simulation results show that the adjustable range of output load is 2 ~ 7T, and the change of each time is about 2T. The modulation law of impact load under the influence of longitudinal vibration of drill string and different parameters is analyzed. Through ground experiment and simulation, the damping performance and speed-up effect of the modulation device are compared and analyzed, and the impact load output characteristics of the device are analyzed, which provides a thinking for the design of damping and pressurization tools.

Vibrations are known as destructive loads and thus are leading to downhole fatigue failures, severe bottom hole assembly and bit wear and may cause wellbore instabilities. Downhole measurement of vibrations, however, can provide valuable information about downhole conditions and characteristics of the formations drilled. Uniaxial compressive strength is one of the representative characteristics of rock properties. It shows the capacity of a rock to withstand a certain amount of axial loads. Optimizing weight on bit and rotary speed of the drill string are the ways to continue drilling a hole - of course by exceeding uniaxial compressive strength of formations - and to keep rate of penetration in an optimal range during drilling operations. Therefore, it is expected that the axial vibrations change with variations of the uniaxial compressive strength of formations. Previous experiments with several layered concretes cubes proved that the drill string vibrations are related to the uniaxial compressive strength of concretes. To extend the study in this area, complex rock samples such as sandstone, dolomite and limestone were drilled with optimized ranges of weight on bit and rotary speed of drill string. Experimental drilling operations were conducted using a laboratory drilling rig (CDC miniRig) with a vibration sensor sub attached to the drill string. The uniaxial compressive strength of all samples was measured prior to the experiments. During all experiments, drilling parameters were recorded with a sampling rate of one second and the vibration data with a sampling frequency of 3200 Hz. Analysis of magnitude and higher order frequency moments of vibration data in combination with the mechanical specific energy demonstrated a change of the drill string's axial vibration measurements due to variations of uniaxial compressive strength. From that it is concluded that the formations can be recognized in real-time by incorporation of the vibration measurements. Introduction When talking about drill string vibrations, in most of the time downhole fatigue failure, severe bit wear and reduced rate of penetration come to the mind. Drill string vibrations are inevitable in the drilling operations and lots of researches have been investigated to manage the magnitude of vibrations by designing proper bottom hole assemblies (Macpherson et al., 1993; Chen et al., 2006). However, the vibrations can be utilized as a valuable tool to recognize formations drilled. In most of the cases, the vibrations are caused by bit and drill string interaction with the formations under certain drilling conditions. One of the dominant conditions is formation characteristics (mechanical and petrophysical properties). Uniaxial compressive strength (UCS) is one of the representative characteristic of the formation properties. It shows the capacity of a formation to withstand a certain amount of axial loads. Previous works show that the harder the formation, the bigger the magnitude of the vibrations and vice versa (Esmaeili et. al, 2012). Therefore monitoring vibrations in real-time can advance warning of formation changes. Myers et al. (2002) explained how the vibration data measured by an accelerometer are related to petrophysical properties of the formations in the Indian Ocean region. Al-Shuker et. al. (2011) presented changes in drilling dynamics driven by the bit's interaction with the formation can be a near instantaneous indicator of changes in formation characteristics. The key is real-time drilling dynamic measurement at the bit. Previous works (Esmaeili et al., 2012) presented a close relationship between the mechanical properties of layered concretes cubes and the measured vibrations. The study proved that the vibrations can be used as a tool to recognize the formation changes.

Drill string models are of two major types- Conventional and Periodic. Conventional drill string models are widely used due to their simplicity in manufacturing and dynamics analysis. Drill strings are affected mainly by torsional, lateral and axial vibrations. The dispersive or propagative nature of these vibrations can only be analyzed by using periodic drill string models. Drill pipe makes up the majority of the drill string. In reality, at each end of the drill pipe, tubular, larger-diameter portions called the tool joints are located. Since the dimensions of the tool joint are negligible when compared to the drill pipe, conventional drill string models assume drill strings to be uniform structures. The tool joints in drill strings introduce a geometrical periodicity in the structure.A periodic structural component is comprised of a repeating array of cells, which are themselves an assembly of elements. The elements may have differing material properties as well as geometric variations. The periodic structures act as mechanical filters for the wave propagation. A material or geometrical change in the structure causes a disruption to the continuous propagation of vibration waves.There are three types of waves propagating in rotating structures: torsional, flexural (bending) and longitudinal. Of these waves, torsional and longitudinal waves are dispersive while flexural waves are non- dispersive. The proportions of the waves which are dispersed vary according to the Wavenumber. The wavenumber is a property of the wave and depends on the structural, material and geometrical properties of the wave guide. Waveguide is the medium through which a wave travels. The wave number, and hence the dispersion and propagation characteristics of waves can be tuned by varying the material and geometrical periodicity in the drill strings.This paper presents the vibration propagation and attenuation characteristics in conventional and periodic drill string models. The periodic drill string model is developed such that it can be easily scaled to a real drilling rig model. The research also features a laboratory setting which comprises a mini drilling rig and actual drilling samples. These features ensure robustness of the dynamics analyzed for the periodic drill string model.The research findings are analyzed under zero and non- zero borehole drill string interactions. Vibration information is collected from upper and lower parts of the drill string models to compare the propagation and attenuation characteristics. Proximity sensors are used for the data acquisition which is then analyzed using frequency and power spectrum graphs. The time and frequency domain vibration propagation characteristics are investigated for both the conventional and periodic drill string models. Laboratory testing analyzes (1) introduction of stop and pass band regions in frequency spectra by periodic drill string models, and (2) vibration attenuation in torsional and lateral vibration modes. The tests confirm the introduction of stop bands in frequency spectra for the periodic drill strings, while they were absent and had an all pass-band for conventional drill string models.The research opens up a new method of passive control of drilling vibrations- by introducing tool joint designs which could be tuned to efficiently dampen out the harmful vibrations affecting the drilling rigs.

Drilling a wellbore can result in several types of vibration that lead to inefficient drilling and premature failure of drill string components. These vibrations are subdivided based on their operating direction into lateral, torsional, and axial vibrations. Especially in hard and dense formations, high-frequency torsional oscillations are found in the bottom-hole assembly (BHA). These critical vibrations are induced by a self-excitation mechanism caused by the bit–rock interaction. Self-excitation mechanisms are regenerative effects, mode coupling, or a velocity-dependent torque characteristic at the drill bit. To increase drilling performance and reduce tool failure due to high-frequency torsional oscillations, the critical vibration amplitudes localized at the bottom-hole assembly need to be minimized. Increasing the damping of self-excited systems to affect the energy output during vibration is a common approach to mitigate self-excited vibrations. In drilling systems, the achievable damping is naturally limited by the small installation space due to the drilled borehole diameter. Therefore, alternative methods to influence vibrations are necessary. Applying parametric excitation in self-excited systems can result in a parametric anti-resonance and therefore in an energy transfer within different modes of the structure. This allows, among other benefits, improved utilization of the structural damping. In this article, the influence of additional stiffness–based parametric excitation on self-excited torsional vibration in downhole drilling systems is investigated. For this purpose, a finite element model of a drill string is reduced using the component mode synthesis and analyzed with the goal to mitigate torsional vibrations. The multiple degree of freedom drill string model is investigated regarding the additional energy transfer due to the parametric excitation. Robustness of various parameters, especially with regard to the positioning within the bottom-hole assembly, is analyzed and discussed. Additionally, the problem of multiple unstable self-excited modes due to the nonlinear velocity-dependent torque characteristic in drilling systems is addressed.

In the oil drilling process, measurement-while-drilling system is usually used to provide real-time monitoring of the position and orientation of the bottom hole. Usually the bottom drill string attitude is measured on the condition that the drill string is static. Particularly, in rotary steerable drilling application, it is a challenging problem to accurately measure the spatial attitude (inclination, azimuth, and tool face) of the bottom drilling tool with the drill string rotating. A set of “strap-down” measurement system was developed in this paper, the triaxial accelerometer and triaxial fluxgate were installed near the bit, and real-time well deviation and azimuth can been measured. Laboratory experiments show that the dynamic solver methods will meet the project requirements. However, according to the drilling field tests, playback analysis on storage data of the underground measurement indicates that drill string vibration is the main reason for measurement errors. So, by setting a time window, the real-time measurement of the accelerometer and the flux gate signal are processed and analyzed, the movement patterns of the rotary drilling tool at the bottom were judged within the time window. Stick-slip phenomenon is utilized to improve the measurement accuracy and to reduce the impact of lateral vibration of the drilling string on the measurement system. We could take advantage of stick-slip phenomenon because of it cannot be avoided completely, although it should be avoided in order to improve the drilling string operating life. Simulation results showed that recommended dynamic survey approach could effectively enhance measurement precision, and deduce effect of vibration on resolve results.

On the basis of the study on drill string vibration rules in vertical well drilling and the idea of transformation of drill string vibration energy to drilling fluid hydraulic energy, a novel tool, down-hole drill string absorption & hydraulic supercharging device (DSAHSD), is designed to further improve the rate of penetration (ROP). The DSAHSD combines the advantages of reducing the axial vibration of the drill string and generating more than 120 MPa ultra-high pressure jet to effectively assist rock breaking. When the drilling fluid flows through this tool, part of the drilling fluid will be modulated to the ultra-high pressure jet by periodic compression of piston drove by drill string vibration. During this process, the ultra-high pressure jet ejected through ultra-high pressure nozzle that fixed on the bit edge in bottom hole. Because of hydraulic auxiliary rock breaking of the ultra-high pressure jet, the ROP will be improved. This tool has been applied to more than 40 wells in four oil fields throughout China. The results show that the maximum density of the test drilling fluid is 1.9g/cm3, the maximum footage is 1480m, and the operation time of the tool is more than 200 hours. Furthermore, the average ROP was increased by approximately 11% to 831%. In conclusion, the DSAHSD has the characteristics of a simple structure, has a long lifetime, is able to adapt well with existing drilling equipment, and can provide a safe and efficient drilling technique for deep wells. In addition, a novel design idea of down-hole hydraulic booster was proposed, so this is a major breakthrough of down-hole supercharging technique.

The drilling system usually encounters detrimental stick-slip vibration to the drilling process. One of the approaches for controlling the stick-slip vibration is using high-frequency torsional impact (HFTI). However, how the HTFI will affect the vibration of the drill string is still unknown. This study proposed a mechanical model of the drill string by using continuous system to investigate the effect of HFTI on the vibration of a drill string in slip phase, wherein the HFTI is considered in the model. The mechanical model is investigated through using mode superposition method and conducting case studies. Results show that the HFTI is more sensitive to the drill string close to the drill bit and that the HFTI has little effect on the vibration of drill string. During the drilling process, the HFTI aggravates the rock damage and rock failure, which actually improves the drilling efficiency and mitigates the stick-slip vibration.

The effect of gas and fluid flows on nonlinear lateral vibrations of rotating drill strings

Part of the process of exploration and development of an oil field consists of the drilling operations for oil and gas wells. Particularly for deep water and ultra deep water wells, the operation requires the control of a very flexible structure which is subjected to complex boundary conditions such as the nonlinear interactions between drill bit and rock formation and between the drill string and borehole wall. Concerning this complexity, the stick-slip phenomenon is a major component related to the torsional vibration and it can excite both axial and lateral vibrations. With these intentions, this paper has the main goal of confronting the torsional vibration problem over a test rig numerical model using a real-time conventional controller. The system contains two discs in which dry friction torques are applied. Therefore, the dynamical behaviour were analysed with and without controlling strategies.

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 146561, ’Mitigation of Torsional Stick/Slip Vibrations in Oilwell Drilling Through PDC-Bit Design: Putting Theories to the Test,’ by Jayesh R. Jain, SPE, L.W. Ledgerwood III, SPE, Olivier J. Hoffmann, SPE, Thorsten Schwefe, SPE, and Danielle M. Fuselier, SPE, Baker Hughes, prepared for the 2011 SPE Annual Technical Conference and Exhibition, Denver, 30 October-2 November. The paper has not been peer reviewed. Stick/slip vibration of drillstrings is gaining renewed interest as operating parameters for polycrystalline-diamond-compact (PDC) bits move into the stick/slip regime of higher bit weight and lower rotary speed to enhance drilling performance. The main objective of this investigation was to determine the influence of bit design on stick/slip behavior of the drilling system. Pairs of PDC bits were designed and manufactured. Each pair had a bit with a standard design and one that embodied a stick/slip theory. Full-scale tests were performed under controlled conditions with a research drilling rig. Introduction Drilling vibrations can result in failure of bits and bottomhole-assembly (BHA) components. Axial and torsional vibrations are prominent with roller-cone bits. With PDC bits suffering catastrophic failure caused by backward whirl, the focus has been on lateral vibrations. Much attention has been given to combating backward whirl by use of antiwhirl bit designs. Meanwhile, cutter technology progressed dramatically with thermally stable PDC cutters that are more impact and abrasion resistant. Consequently, operating parameters for PDC bits have shifted to higher weight on bit (WOB) and lower rotary speed to enhance drilling performance. This parameter combination also avoids the low-WOB and high-rotary-speed regime (Fig. 1) in which backward whirl can destroy bits after drilling only a few feet. Unfortunately, higher WOB and lower rotary speed instigate torsional instability that matures into stick/slip vibrations. It has been shown in the field that for a surface rotary speed of 60 rev/min, the measured downhole rotary speed reached 400 rev/min during the slip phase. Lateral vibrations during the slip phase correlated well with observed damage. More-severe cases of measured stick/slip have been reported in the literature when the bit experienced reverse rotation at the end of the slip phase before coming to a standstill. A desirable solution for mitigating stick/slip is an inherently stable sys-tem that expands the stable-operating zone through improved bit and BHA/drillstring design to allow drilling at more-efficient operating parameters of high WOB and low rotary speed. The BHA design and the geometry (size and number) of drillpipe and drill collars can have a substantial effect on the stick/slip behavior of the system. Although larger drillpipe can mitigate stick/slip and enable more-efficient drilling, its availability and cost may limit its effectiveness as a solution. Prevailing industry perceptions were reviewed, and five leading theories regarding the root cause of stick/slip were identified.

The vibration of a drill string is the topic under study in drilling and mining engineering. Drill string vibrations cause early failure of bits due to fatigue, drill pipe fracture and bearings damage. A modal and harmonic vibrational analysis of a drill string up to 60-65m long was done in a vertical water well by carrying out parametric studies. This paper investigates vibration of a drill string under longitudinal and mainly lateral which is the most dominant under the action of parameters such as drill pipe lengths and weight on bit. Finite element modelling of a drill string was made in Ansys workbench 2020, mechanical. Modal and harmonic frequencies were determined to obtain optimal drill pipe length and possible weight on bits. The drilling environment was assumed by using different weights on bit.

Drilling engineering is a high input and risk underground project, which needs the real-time down-hole diagno- sis and forecast technique. Drilling string vibration signals contain full and complex vibration waves, which contains the vibration of the drilling string by itself, the vibration stimulated by the interaction of bit and the formation, the vibration induced by colliding between drill string and bore hole. By adopting drilling string vibration testing system, down-hole vibration can be measured, and use it to the learn and analyze the state of down-hole. Drilling string vibration testing Sys- tem adopts the ADuC841 MCU as the core chip, and uses the high reliability technology to complete the software and hardware design of the testing system. The Storable down-hole drilling string vibration testing system has advantages of small in size, low power consumption, vibration resistance and high temperature resistance. High temperature test, vibra- tion test, laboratory test and spot test show that the down-hole drilling string vibration testing system has the ability to ac- curately record the actual down-hole drill string dynamic vibration status, and system can meet the needs of the drilling site testing.

Vibration control of the drilling process in the gas and oil industries is of great importance.Applying an effective control system diminishes both costs and process time and increases work efficiency.Typically, the drill string vibrates in a combination of three distinct modes of transverse, torsional, and axial vibrations.Due to the importance of torsional vibration and the associated stick-slip phenomenon, many works have been devoted to this area.However, transverse vibration is another major source of vibration arisen from the long length of the drill string structure.In this paper, a tunable vibration absorber as a semiactive controller is designed to suppress the transverse vibration.After modeling the drill string as an Euler-Bernoulli beam and formulating the problem, the optimum specifications of the absorber, such as spring stiffness, absorber mass, and its position, are determined using an algorithm based on the mode summation method.The effect of bit rotational speed under the non-resonance and resonance condition is studied.It is shown that the best position of the absorber depends on the spring stiffness and bit rotational speed.

Stress Analysis on Drilling String Vibration in Gas Drilling