The effect of well path, tortuosity and drillstring design on the transmission of axial and torsional vibrations from the bit and mitigation control strategies

As well designs become increasingly complicated, a complete understanding of drillstring vibrations is key to maximize drilling efficiency, to reduce drillstring dysfunction and to minimize drillstring, tool, and borehole damage. Torque and drag models exist that seek to quantify the effects of borehole inclination and tortuosity on static friction along the drillstring; however, the effects on dynamic friction remains poorly understood. This dissertation begins with a review of the past fifty years of work on drillstring dynamics models, an overview of the proposed control strategies and a summary deployed vibration mitigation applications within the drilling industry. Derivations from first principles of a series of computationally efficient axial and torsional drillstring models in both the frequency and time domains are then presented and verified with field data. The transfer matrix approach is used to predict the severity of axial vibrations along the drillstring and is verified using a series of case studies using field data. Harmonic axial vibrations within drillstrings are either induced intentionally, in the case of axial oscillation tools midway along the drillstring, or unintentional, in the case of bit bounce. Two case studies of bit bounce are first evaluated to ensure model validity for a harmonic excitation at a the bit and the model is found to accurately predict bit bounce based on surface rotation rates. Induced axial oscillations, generated by axial oscillation tools, are then investigated to quantify friction reduction and drilling efficiency improvements. Optimal placement is found to depend on wellbore geometry, but is usually restricted to periodic regions of the drillstring. These optimizations are then verified using field trials and suggest that improved placement can result in 20% or more reduction in friction along the drillstring. Two applications of torsional drillstring vibrations are then investigated -- stick slip mitigation and drillstring imaging. The time domain form of the torsional drillstring model is used first to evaluate the effectiveness of three types of top drive controllers -- stiff controllers, tuned PI controllers and impedance matching controllers -- in mitigating stick slip oscillations. Then, the transfer matrix method is applied to evaluate the effect of wellbore geometry on drillstring mobility to conclude that higher order modes of stick slip may become dominant in non-vertical wellbores. The feasibility of drillstring imaging using torsional signals from surface is then investigated to identify inputs and methods that show promise in three setups of varying complexity -- a hanging beam, a laboratory drillstring model and a drilling rig. Two techniques show promise -- white noise injection and model fitting of a step response -- in identifying larger features, including drillstring length and BHA location. However, low sampling frequencies and low bandwidth inputs reduce the ability to image small features such as friction points along the wellpath.Petroleum and Geosystems Engineerin

Dynamic analysis of the longitudinal vibration on bottom drilling tools

With extreme complexity, the drilling process is a dynamic process which is severely influenced by longitudinal vibration. Longitudinal vibration, as one of the most important reason, is directly generated by the fatigue failure of the bottom hole assembly. In this paper, the natural frequencies of longitudinal vibration along the drillstring are analyzed by the finite element method. The deformed plot, stress nephogram, and displacement contour map under 1 to 4 ordered the natural frequency of the longitudinal vibration are obtained. The analysis results show that the maximum deformation always appears in the central part of the string so that some technological process on these positions is required to reduce the collision between the string and wellbore wall. Additionally, a time series of longitudinal vibration of a bottom rotating drillstring is extracted from real-time field data, which is measured while drilling near the drill bit. Then the time-frequency and energy spectrum analysis of the longitudinal vibration is carried out. The results of the statistical analysis show that, when the drillstring uniformly rotates, the longitudinal vibration can be considered as a kind of random vibration. However, if the stick-slip phenomenon occurs during the drilling process, the energy concentration will appear in the time series spectrum of the longitudinal vibration, by which means the vibration could be regarded as random no longer

Detection of low-dimensional chaos in drill bit torsional vibration time series

The near-bit strap-down measurement-while-drilling (MWD) system has been developed in this paper. By means of triaxial magnetometers, calculation method for bit rotational velocity was developed to monitor the drill bit torsional vibration. A number of techniques were applied to perform a nonlinear analysis of the experimental data of torsional vibration. Estimate delay time with mutual information and calculated the embedding dimension through Cao’s method, after reconstruct the phase space, the chaotic characteristics of the system were analyzed by calculating the correlation dimension and the largest Lyapunov exponent. We show that the largest Lyapunov exponent is positive and the correlation dimension is more than two, which is a strong indicator for the chaotic behaviour of the system. We also found that chaotic characteristics of the drill bit torsional vibration even existed in the whole drilling process, and thus the techniques based on phase space dynamics can be used to analyze and to predict drill bit torsional vibration. The results of this paper are of interest to applied and theoretical mechanics and petroleum engineering

Investigation on dynamics of drillstring systems from random viewpoint

Drillstrings are one of the critical components used for exploring and exploiting oil and gas reservoirs in the petroleum industry. As being very long and slender, the drillstring experiences various vibrations during the drilling operation, and these vibrations are random in essence. The first part of the thesis focuses on stochastic stick-slip dynamics of the drill bit by a finite element model and a single degree of freedom drillstring model in Chapters 3 and 4, respectively. In the single degree of freedom model, the path integration (PI) method is firstly used to obtain the probability density evolution of the dynamic response. Then Monte Carlo (MC) simulation is used for validating PI results and conducting the parametric study. The second step of my research is to study the stochastic dynamics of a vertical, multiple degrees of freedom drillstring system. The work of this part is presented in Chapter 5. The novelty of this work relies on the fact that it is the first time that the statistic linearization method is applied to a drillstring system in the bit-rock interaction to find an equivalent linear dynamic system which is then solved with the stochastic Newmark algorithm. After that, the stick-slip and bit-bounce phenomena are analyzed from random viewpoint. The third step of my research move on to directional drilling. A static study of directional drillstring from random viewpoint is presented in Chapter 6. The finite element method (FEM) based on the soft string model is employed and built. Then two strategies are taken to model the random component for hoisting drag calculation. The purpose of this work is to analyze the effects of the random component on hoisting drag calculation by the MC simulation method

Modeling and simulation of vibration in deviated wells

During the engineering of deviated well, drillstring is in the complicated moving state, strong vibration is the main reason that induces drillstring failure. Drillstring vibrations usually have axial vibration, lateral vibration, torsional vibration and the drillstring near the bottom of well usually coupled vibrates strongly. A dynamic model to predict the effect of drillstring parameters on the type and severity of vibration is desired by the oil industry, to understand and prevent conditions that lead to costly downhole tool failures and expensive tripping or removal of the string from the wellbore. High-fidelity prediction of lateral vibrations is required due to its coupling with potentially destructive axial and torsional vibration. This research work analyses the dynamics of a horizontal oilwell drillstring. In this dynamics, the friction forces between the drillstring and the borehole are relevant and uncertain. Drillstring contact with its borehole, which can occur continuously over a line of contact for horizontal shafts such as drillstrings, generates normal forces using a user-definable stiff spring constitutive law. Tangential contact forces due to friction between the drillstring and borehole must be generated in order for whirl to occur. The potential for backward whirl and stick-slip requires the transition between static and dynamic Coulomb friction. The proposed model computes the relative velocity between sliding surfaces when contact occurs, and enforces a rolling-without-slip constraint as the velocity approaches zero. When the surfaces become ‘stuck’, a force larger than the maximum possible static friction force is required to break the surfaces loose, allowing sliding to resume. The drillstring bottom-hole-assembly has been modeled using a three-dimensional multibody dynamics approach implemented in vector bond graphs. Rigid lumped segments with 6 degrees of freedom are connected by axial, torsional, shear, and bending springs to approximate continuous system response. Parasitic springs and dampers are used to enforce boundary conditions. A complete deviated drillstring has been simulated by combining the bottom-hole-assembly model with a model of drill pipe and collars. The pipe and collars are modeled using a lumped-segment approach that predict axial and torsional motions. The proposed dynamic model has been incorporated the lumped segment approach which has been validated with finite element representation of shafts. Finally, the proposed contact and friction model have been validated using finite element LS-DYNA® commercial software. The model can predict how axial and torsional bit-rock reactions are propagated to the surface, and the role that lateral vibrations near the bit plays in exciting those vibrations and stressing components in the bottom-hole-assembly. The proposed model includes the mutual dependence of these vibrations, which arises due to bit-rock interaction and friction dynamics between drillstring and wellbore wall. The model can simulate the downhole axial vibration tool (or Agitator®). Simulation results show a better weight transfer to the bit, with a low frequency and high amplitude force excitation giving best performance but can increase the severity of lateral shock. The uniqueness of this proposed work lies in developing an efficient yet predictive dynamic model for a deviated drillstring

Dynamic Analysis of Stick-Slip and Bit Bounce in Oilwell Drillstring

Drilling process consisting of drag bits which are used over drilling boreholes concerning generation along with research about oil and gas regularly undergo against caustic vibrations. As mentioned, vibrations will be able to cause the drag bit along with drillstring various breakdown concerning equipments. This project, a non-linear design concerning rotational and axial motions of drillstring plus bit act suggested. Furthermore, dynamics concerning two drive complexes considering translational along with rotational motions regarding drillstring is being advised. Regarding mentioned model, translational along with rotating motions concerning drag bit are reached in the process of result regarding total dynamic action. The consequences concerning numerous viable criteria covering dynamic action are considered including objective in producing an undisturbed drilling. Using appropriate selection concerning operational criteria can help in minimizing the consequences regarding bit-bounce also stick-slip. It is anticipated to aid lower time lost in drilling operation along with costs sustained because of caustic vibration

State Dependent Delayed Drill-string Vibration : Theory, Experiments and New Model

L.J. Pei would like to acknowledge NNSF of China (No. 11372282) and China Scholarship Council.Peer reviewedPublisher PD

A digital twin development framework for fatigue failure prognosis of a vertical oil well drill string

This thesis presents a novel methodology for fatigue life prognosis of vertical oil well drill strings through the development of a digital twin frame work. A technique is proposed to classify vibration types with their severities and estimate the remaining useful life time of the drill string based on various indirect measurements made at the surface level. The classification was done using a machine learning algorithm developed based on a Hidden Markov Model HMM). Training data for the algorithm were generated using a bond graph simulation of a vertical drill string. A three-dimensional lumped segment bond graph element and an interface element available in the literature were used to develop the simulation. The bond graph elements are developed based on a Newton-Eular formulation and body-fixed coordinates. The simulation was upgraded by introducing a fluid drag model and refining it with accurate element compliance values. Non linear fluid drag force statistical models were developed through the design of experiments(DoE) approach considering the non-linear geometry of the drill pipes,the drilling fluid rheology, and fluid velocity. A series of fluid-structure interaction(FSI) simulations were employed to develop the statistical models for the lateral vibration damping and the axial drag force dueto the drilling fluid flow through the pipe and the annular space. An apparatus was designed and fabricated to verify the FSI simulation. Further, a method was introduced to accurately determine the axial, shear, bending, and torsional compliances of geometrically-complex drill string segments represented by the bond graph elements. The trained HMM-based classifier using bond graph-generated training data selects the appropriate parameter set for the same bond graph to generate stress history for fatigue life prognosis. A generalized fatigue life estimation method was developed using SalomeMecaᵀᴹ, an open-source finite element analysis code. A detailed workflow for multi-axial, non-proportional, and variable amplitude (MNV) fatigue analysisis also provided. Three case studies are presented to demonstrate the significance of the nonlinear fluid drag models, the fatigue prognosis framework, and the digital twin development framework. In the first case study, the bond graph with the developed drag models showed higher stress fluctuations at the drill pipe threaded connection than the one with a static model. The second case study demonstrated the function of the proposed fatigue life prognosis framework as an optimization tool. In the case study, the optimum placement of the stabilizers reduced the drill collar damage by 66% compared to the worst-case scenario. The third case study used a laboratory-scale vertical drill string vibration simulator apparatus designed and fabricated to implement the framework as a proof of concept. It demonstrated the potential to use surface measurements to classify the vibration type and its severity for fatigue life prognosis

CFD simulation of downhole thruster performance evaluation

The present study investigates the rule that the downhole Thruster could contribute towards enhancing drilling performance of oil and gas wells. The core study was numerical simulation implemented ANSYS Software using different fluids of various viscosities. As a main part of the numerical study, an evaluation of the variation of fluid velocities and their resultant pressures at several planes within the Thruster geometry was included. The evaluation methodology included a comparative study of “With-Thruster” versus “Without-Thruster”, which represent two drilling modes involving drilling with axially induced oscillations and drilling without axially induced oscillations; respectively. This was performed to simulate different drilling modes of unconventional (i.e. with controlled and desirable axial oscillations) and conventional (i.e. rigid drilling system); respectively. By implementing the downhole Thruster, the conventional drilling can be shifted to the unconventional drilling mode that produces controlled axial vibrations empowered hydraulically through generating pressure pulses. At first, the drilling performance was simulated by implementing Maurer model, which showed significant increase in the rate of penetration (ROP) when using the Thruster for all fluid velocities. The improvement in ROP was noticed to increase from quarter unit to a one unit induced by a generated force of at as low as 6000 (N) to as high as 10000 (N); respectively. The clear increase in ROP in the simulation work was then carried out for further simulation of the Thruster for comprehensive evaluation under various conditions, including applying back pressures as well as using various fluid viscosities, which also showed improvement in ROP with Thruster implementation