Drill String Research Articles

Reducing critical effects of torsional vibrations induced by PDC bits during rock destruction

Introduction.Torsional vibrations caused by drilling using a polycrystalline diamond compact (PDC) bit are analysed using wellbore data obtained under various drilling modes. The drilling caused by torsional vibrations are analysed. Two types of torsional vibrations, including those caused by the cutting action of a bit and by frictional forces.Aim.To investigate the induction of torsional vibrations as a result of rock destruction by PDC bits.Materials and methods.The research methodology was based on “Richard-Germay-Detournay” and “Tian-Detournay” bit designs and methods for reducing the critical effects of torsional vibrations induced by PDC bits during rock destruction.Results.Statistical analysis has shown that the drilling efficiency of the bit (DE) correlates well with the occurrence of torsion vibrations caused by the bit. The aggressiveness of the cutting structure of the PDC bit can be represented by an average DE rating. If the PDC bit is designed so that its DE exceeds a critical value, then the occurrence of torsion vibrations on the bit is unlikely during drilling.Conclusion.When drilling with a polycrystalline diamond PCD bit, two types of vibrations occur. If torsional vibration occurs due to the cutting action of the bit, then the fight against this process is focused on the design of the bit and operational parameters, the load on the bit and the speed of rotation of the bit. If the torsional vibration is caused by friction, vibration reduction methods should be focused on changing the design of the BHA and the drill string or the profile of the well. Therefore, it is very important to determine whether the torsional vibration is caused by the friction force of the elements, the cutting action of the bit, or both. In drilling, it is always necessary to search for the maximum penetration speed by changing the load on the bit under specified drilling conditions. To do this, it is necessary to evaluate the drilling efficiency of the DE bit, therefore, this article has evaluated the parameter depending on the size of the bit, which will determine the critical value of the drilling efficiency parameter.

Design and application of weight-on-bit control system for drilling process: A variable-gain integrated controller with multiple-model parameter estimation

Controlling weight-on-bit is essential in the drilling process, which determines the rock-breaking performance, drill bit wear, and wellbore quality. This paper aims to develop a weight-on-bit control system for field implementation, using a variable-gain integrated controller and a multiple-model parameter estimator. First, a weight-on-bit dynamic model for controller synthesis is derived according to the real drilling scenario. Then, the integrated controller is designed with two parts: a variable-gain proportional–integral controller and a dynamic output feedback controller, which separates the design of the system’s high-order from low-order parts. The controller gain is tuned through a multiple-model parameter estimator, which estimates uncertain parameters related to rock strength. The established control system is implemented in a typical electrical drilling rig. It meets the requirement of adapting to different formations, considering the flexible drill string and the variable feed motor speed threshold. Numerical and field application results confirm the effectiveness of the developed system.

Downhole heat management for drilling shallow and ultra-deep high enthalpy geothermal wells

Heat management while drilling high-enthalpy geothermal wells is crucial for successful drilling and commercial energy extraction from hot reservoirs (150+ °C). Accurate modeling of heat transfer between drilling fluids and rock is vital to predict the transient bottom hole circulating temperature (BHCT). This allows downhole tools and drill bits to operate within their temperature limits, thereby preventing premature failure while drilling from high-temperature exposure.In this study into the most effective ways to pro-actively manage downhole temperatures, a validated coupled thermal/hydraulic model is used to simulate the transient behavior of BHCT and the mud temperature profile inside the drill string and the annulus of geothermal wells. The developed model investigates the effectiveness of different heat management techniques for relatively shallow (< 3 km, 10,000 ft TVD) as well as deep (> 6.1 km, 20,000 ft TVD) high enthalpy geothermal wells. Simulation results show that higher drilling flow rates, use of mud coolers/chillers, use of certain fluid types and viscosities, as well as increased hole size can all be used as strategies to reduce the BHCT of shallow geothermal wells effectively. However, the situation changes for deep geothermal wells and/or wells with long geothermal reservoir exposure, with reduced effectiveness of e.g., mud cooling and other generally accepted cooling measures. For such wells, however, the use of an insulated drill pipe (IDP) in combination with drilling flow rate can be used effectively for heat management.As part of the results, correlation maps are presented to provide drilling engineers with a user-friendly tool to quickly gain an understanding of the impact of various downhole cooling strategies for drilling geothermal wells, which in turn can be used to technically and economically justify their selection in the well design and drilling programs.

An experimentally-verified approach for enhancing fluid drag force simulation in vertical oilwell drill strings

ABSTRACT The oilwell drilling fluid flows cause viscous and hydrodynamic forces on drill strings. This effect is ignored or treated as a constant in most drill string models. The present study introduces mathematical models for lateral vibration damping and axial drag forces that are employable in lumped segment drill string models. First, the variables to which drilling fluid-generated forces are most sensitive were identified and the Response Surface Method was applied to design the experiment matrix. The lateral vibration-damping experiments, which were validated using a scaled-down physical model, and the axial drag experiments were done using Fluid-Structure Interaction simulations. The results were statistically analysed to acquire the models and were implemented in a 3D lumped segment bond graph developed using the Newton-Euler formulation and body-fixed coordinates. The results indicate a considerable effect of the extended treatment of damping and axial drag on bending moment fluctuation, wellbore interactions, and weight on bit.

Complex Mathematical Modeling of the Well Drilling Process

Recently, the technologies for the global modeling of the process of oil well drilling have become widespread. Mathematical modeling is used in well design, virtual testing of various drilling equipment, simulations of various emergency situations, and personnel training. Complex modeling of the well drilling process includes the simulation of such phenomena as the dynamics of the drill string and its contact interaction with walls, the flow of the drilling fluid and its interaction with the soil (considering influxes and leakages), soil crushing by the drill, the transfer of cuttings particles by the drilling fluid, heat exchange with the soil, and others. This paper provides a detailed review of the existing modeling approaches to solving such problems. Most of the studies included in the review focus on building a detailed mathematical model of one or several of the above processes. Moreover, all these processes mutually influence each other, which also needs to be considered in the analysis. It appears that further development of such a multiphysics approach will be the main direction of research in this area in the near future.

Nonlinear dynamic analysis of liquid-solid coupling and rubbing on drill string

Natural deposit of natural gas hydrate (NGH) exists in the colder regions on the earth, which is an unconventional energy and has great potential. About 97% of the NGH is located in the offshore and only 3% is located on the land. Due to its sensitivity to the external environment and easy decomposition, it is necessary to suppress the external environment changes caused by the complex vibration of the drill string during its exploitation. In this paper, the fluid-solid coupling nonlinear vibration of drill string is studied. The nonlinear dynamic model of drill string with drilling fluid is established. The relationship between variable parameters of drill string and complex dynamics of drill string is analyzed. Furthermore, the bifurcation diagram is given. The results have certain significance for discussing the non-linear dynamic mechanism of drill string under complex conditions and reasonably selecting the drilling process parameters of NGH.

Theoretical study on fatigue damage of sonic standing wave resonant drill-string

To achieve high-speed and undisturbed core drilling, the standing wave vibration of the drill string in a sonic drill is excited by a high-frequency inertial vibrator; the resulting high alternating stress cycle in the drill string can easily cause fatigue damage. In order to minimize the fatigue failure of drill-string at the stage of its design, it is necessary to assess the fatigue damage caused by alternating stress to guide engineering practice. In this paper, based on one-dimensional wave theory, we analyse the standing wave vibration in a drill-string excited by a sonic vibrator, and theoretically prove that the dynamic resonant stress of a drill-string is the key factor influencing the fatigue damage. By using the Palmgren–Miner fatigue damage rule, we establish a theoretical formula for the cumulative fatigue damage of a variable-length standing wave vibration drill string and reveal the fatigue damage mechanism of the variable-length resonant drill string. Furthermore, the effects of sonic drill systems and process parameters on the damage are quantified. It was found that by an appropriate choice of a drill-pipe length, the fatigue damage can be reduced whilst the axial stress concentration factor (aSCF) kσ on threaded connections can significantly increase it. At the fundamental frequency of the resonant sonic drilling, the maximum fatigue damage point, xf, is located approximately la/2 above the drill bit, not exceeding the theoretical sonic standing wave starting length, la, and unrelated to the hole depth. This study promotes the theoretical understanding and exploration of variable-length standing wave oscillators.

A robust integrated control design for weight-on-bit fluctuation suppression in drilling process subject to fractured formations

Fractured formations lead to insufficient contact or loss of contact between the drill bit and rocks, which subsequently causes notable fluctuation in weight-on-bit due to flexible drill strings. This paper presents a robust integrated control design to suppress weight-on-bit fluctuation. First, a finite element drill-string longitudinal model is employed as the basis of control design, which has been verified using actual run data. The integrated controller contains a proportional–integral (PI) controller and a dynamic output feedback controller, dealing with the system’s first-order and high-order dynamics. The dynamic output feedback controller only needs to focus on the system’s high-order dynamics since the controller synthesis is based on the pre-designed PI controller. The controller decreases the flexible mode amplitude from disturbance to the output channel through H-infinity loop shaping, making the closed-loop system less flexible. The numerical results demonstrate that the developed approach can effectively suppress weight-on-bit fluctuation due to drill-string flexibility when drilling in fractured formations.

Control of drill string torsional vibrations using optimal static output feedback

Failures during oil extraction can have impacts on exploration costs and time, especially those caused by vibrations during the drilling phase. A proper way to reduce these detrimental failures is to design an active controller that efficiently mitigates vibrations with the constraint of using the limited amount of real time data available in field operations. In this context, a technique that might be suitable for the controller design is the optimal static output feedback (OSOF), that considers only the measured signals for feedback. Recently, the authors proposed some modifications to this technique, involving the addition of sensors locations and a new way of dealing with the dependence of the controller parameters on the system initial condition. The aim of this contribution is to apply this novel control technique to suppress drill string torsional vibrations and address all the particular aspects regarding the application of the proposed controller for the drill string problem, i.e., reformulation of the equations of motion as a stabilization problem, determination of the output matrix and linearization. Analyses and comparison of the proposed controller with proportional–integral (PI) and linear quadratic regulator (LQR) controllers are performed considering numerical simulations of a finite element model with a non-regularized dry friction function. Results indicate that the proposed controller has a performance similar to the LQR with a significantly reduced number of sensors, and also exhibits superior performance and robustness with an equivalent control effort over the PI controller.

Nonlinear ensemble gray and black-box system identification of friction induced vibrations in slender rotating structures

Due to the nonlinear bit-rock and drill–string–borehole interactions, the modeling and analysis of drill string dynamics are a challenging work. These slender structures are subjected to torsional, lateral, and axial vibration modes, which are generally prejudicial to the drilling process. For this reason, proper modeling of the system dynamics is necessary to optimize drilling performance. In the present paper, we focus on the torsional dynamics and model of an experimental system subjected to friction torque. An ensemble approach, combining gray and black-box modeling techniques, is used to calibrate some of the dynamical system parameters, particularly those related to friction. This combination is chosen with the aim of improving the prediction accuracy of a pure gray-box approach while retaining interpretability in the dynamic responses. The present paper compares four well-known friction models with increasing levels of complexity using experimental data. It is shown that the ensemble model proposed can improve the gray-box by up to 92.68% in terms of prediction errors, by adding a non-physical layer to it. The present analysis demonstrates that better results can be obtained by using an ensemble of gray and black-box identification techniques for friction modeling and simulation, aiming at improving data-driven model construction of complex torsional dynamics in slender structures.

Research on the Influence of a Cuttings Bed on Drill String Friction Torque in Horizontal Well Sections

Horizontal well drills are difficult to operate and require high borehole cleaning operations. The impact of cuttings accumulation on friction and torque cannot be ignored. In the process of horizontal well drilling, it is very easy to form a cuttings bed in the highly inclined eccentric annulus. The formation of the cuttings bed will not only increase the friction torque but also have a greater impact on the annulus flow. Based on the application of gas drilling in horizontal wells, this paper uses laboratory experiments to study the influence of cuttings with different particle sizes and lithology on the friction coefficient between the drill string and the borehole wall of the horizontal well section. In addition, in view of the difficulty in the migration of cuttings in gas drilling horizontal wells and the formation of cuttings deposits, this paper carries out an experimental study on the effect of the cuttings bed on drill pipe coating height and cuttings bed thickness on friction coefficient. Experimental results show that the friction coefficient of gas drilling horizontal wells is about 0.44–0.58. When the coating height of the cuttings on the drill pipe exceeds a certain value (26 mm), the friction coefficient tends to be stable, but its value is as high as about 0.55. When the thickness of the accumulation of cuttings exceeds a certain value (10 mm), the subsidence hindering effect between the particles in the cuttings bed is weakened, so the friction coefficient of the casing section and the open hole section tends to be stable, about 0.53.

Pattern recognition of stick-slip vibration in combined signals of DrillString vibration

Vibration signal measurement is one of the important methods to study the movement mode of the drill string. Through the interpretation of the vibration signal, the vibration pattern of the drill string can be accurately identified. MWD is a measurement method which can more directly reflect the vibration information of the drill string. In this paper, firstly, the drill string vibration signal is obtained by the sensors with a three-axis accelerometer and a gyroscope, and the sensor’s response has been solved and analyzed, which proves the effectiveness of this method for expressing drill string vibration information and the correlation between the measured signals. Then, the correlation coefficient, as the characteristic information basing on signal combination, is proposed; since the correlation coefficients of different patterns have different distribution characteristics, supervised machine learning is used to establish a classification model of vibration patterns. Finally, through the application of classification model in the actual drill data, the specific vibration types of various modes are clarified, which proves that the classification model can effectively distinguish different vibration modes and its accuracy can reach up to 95%.

The Review and Development of Devices with an Increasing Rate of Penetration (ROP) in Deep Formation Drilling Based on Drill String Vibration

The oil and gas resources stored in deep strata are an important replacement field of the petroleum industry. Accelerating the exploration and development of deep oil and gas is of great significance to the security of energy strategy. Drilling is the primary link and necessary means of deep oil and gas exploration and development. Slow drilling speed is one of the key problems restricting the exploration and development of deep oil and gas. The research and development of down-hole equipment with an increasing ROP provides a technical means for increasing the ROP. However, the energy of existing down-hole equipment with an increasing ROP comes from the drilling circulation medium, and the ROP increase effect of such equipment is relatively obvious in shallow and middle formations. However, with the increase in well depth, energy in the circulation medium increasingly struggles to reach deeper formations, and the ROP increase effect is not good at the later stage of drilling. In the drilling process, drill string vibration is a frequently encountered complex situation, but it also contains sufficient energy, and the energy of drill string vibration will increase with an increase in the well depth, which can meet the energy demand of increasing the ROP in deep oil and gas exploration. This paper analyzes the characteristics of drill string vibration, and introduces six kinds of devices that take drill string vibration as energy and realize drill string vibration reduction, bottom-hole pressurization, and high-pressure pulse jet, providing a new idea for the development of deep down-hole speed-increasing devices.

Research on deviation correction characteristics of automatic vertical drilling tool

The automatic vertical drilling tool can solve the problems of poor effect of conventional drilling. In order to effectively improve the ROP (rate of penetration) and quality of vertical wells, an automatic vertical drilling tool is developed in this article. This new innovative design has these performances, such as high temperature resistance, strong reliability. By analyzing the drilling trend with drill string system, the tool deflection correction ability was studied. Based on the longitudinal and horizontal bending continuous beam method, the BHA mechanical models of the automatic vertical drilling tool, including the calculation formulas of the lateral force and rotation angle, were established considering the variable cross-section treatments. The drilling trend prediction model was derived by formation interaction functions, and verified by numerical examples and experimental tests. On these bases, taking the trend angle of well inclination as the evaluation indicators, combined with the lateral force and rotation angle of drilling tool, the influencing factors of this tool deviation correction ability were analyzed. These calculation and experimental test results show that: the WOB, thrust force, bit anisotropy and formation anisotropy have a significant impact on the tool deflection correction ability. Furthermore, the research results can supply theoretical references for the vertical drilling and design of similar tools. Also, this research can provide basis for the optimization of drilling parameters, and the prediction and control of the wellbore trajectory.

Numerical solution to binary quasi-linear partial differential equations for drilling surge pressures based on the characteristic-lines method

Accurately predicting surge pressures caused by tripping drill string or running casing string is fundamental to implementing Managed Pressure Drilling in areas with narrow safety mud density margin. In this work, the region of interest is divided into 3 coupled regions, i.e., inside moving pipe, annulus, and that below the moving pipe. Mass conservation equations and momentum conservation equations depicting the 3 regions are developed considering mud compressibility, pipe elasticity and wellbore wall elasticity. Applying the characteristic-line theory for partial differential equations, a series of discretized equations have been developed for the 3 regions, including internal nodes within uniform size pipe conduit, coupling nodes between two adjacent different size conduits, surface and bottom-hole nodes, and the conjunction node of the 3 regions. Based on the discretized equations, a computational flowchart is programmed to get numerical transient solutions. Both the transient surge pressure variation with time at the given interested point and the surge pressure variation with depth along the whole well axis can be depicted using this method. Two case studies show that computational values are in good consistent with the measurements with a relative error less than 10%.

Measurement-While-Milling (MWM): An innovative approach for increasing the casing milling efficiency in deep drilling operations

Deep boreholes are secured by steel tubes (casings) which are run in the hole and cemented in place. In most cases, these casings are considered a permanent installation. However, sometimes they have to be removed in order to repair or abandon the well. As the casing is cemented in place, it cannot be pulled, but needs to be milled to small chips which are flushed out of the borehole by the drilling mud. One of the main challenges in casing milling operations is continuous and complete chip removal. If the metal chips are too long, chip nests will grow around the milling string. As a result, this will restrict the annulus flow area and affect the chip removal in boreholes. The obvious solution in such condition is to do round tripping and clean the chip nest which is associated with the risk of injuries, as well as, increasing the none-productive time. In the worst case, the poor cleaning and circulation of chips can even end up with the milling string stucking problem in boreholes, consequently long-time fishing job. According to the available literatures, hardly any study for identifying the chip shapes and accordingly adapting the operation parameters to the casing milling process environment downhole to keep milling within desired generated chip shapes and sizes could be found. This paper presents an encouraging idea to monitor the milling process in real time by utilizing the acoustic emission signals (vibration modes) accompanied with the milling process to identify the desired chip shape and size range. Initial laboratory tests have been carried out to investigate and study the acoustic emission signals accompanying the casing milling process to identify the chip shapes and sizes. The preliminary test results show very good correlation and agreement between the chip length formed during those specific tests and the observed burst events in the measured signals. The study results have demonstrated the functionality of the new concept, and thus confirmed that it is a very promising idea towards developing a practical real time downhole monitoring system for milling operations. Adapting the milling operation parameters downhole in real time to keep the milling process within the desired generated chip shapes and sizes will offer better cleaning and removal of the chips and will prevent the development of chip nest around the drill string and its consequences such as round tripping, risk of drilling crew injury, none-productive time and even milling string stucking problems.

Assessment of frictional torque and lubricity of waste-tire bio-oil as lubricant additive for stuck pipe

The optimal approach to reducing surface torque when drilling uses oil-based or synthetic-based mud systems instead of aqueous mud systems. However, lower surface torque and drag can be achieved by lowering the water-based mud friction factors using lubricant additives. Torque can be reduced up to 25% with the suitable mud lubricant additives. Lubricant additives are viable methods for decreasing frictional resistance in drill strings during oil and gas drilling operations. In this study, bio-oil was produced from waste tire distilled pyrolytic oil and added to formulated standard aqueous drilling fluid to investigate its ability to reduce frictional torque during drilling. The lubricating properties of the proposed lubricant additive was compared with commercially used lubricant additive as reference samples (control). The simulation results were compared with measurements from the OFITE Lubricity Tester. The result shows that adding the distilled waste tire pyrolytic oil and produced bio-oil to the drilling mud decreased the coefficient of friction and torque to generate oily films on the contact-surface. The distilled pyrolytic oil and produced bio-oil torque values were in the range of 23.50–32.98 and 21.30–30.82 respectively. The commercial lubricant additive (control) gave 42.85% maximum torque reduction, while the produced bio-oil gave a similar maximum torque reduction of 41.88%. The result shows that, for a water-based drilling fluid, the friction coefficient between the drill string and rock formation, can be reduced with the application of the produced bio-oil. Thus, the produced bio-oil can be used to prevent differential sticking or free the stuck pipe during drilling.

Stuck Pipe Detection in Geothermal Operation with Support Vector Machine

One of the biggest problems during drilling operation is a stuck pipe in which the drill string would stick or freeze in the well. This challenge leads to a significant amount of remedial costs and time. Many researchers have investigated different factors regarding the stuck pipe. These factors include poor hole cleaning, improper mud design, key seating, balling up of bit, accumulation of cutting and caving, poor bottom hole assembly configuration, differential pressure, etc. (Chamkalani, Pordel Shahri, and Poordad 2013). Since geothermal drilling targets lost circulation zones at reservoir depth, the chance of getting stuck pipe events becomes higher. Many publications reported that lost circulation events that lead to stuck pipe events have become the top non-productive time (NPT) contributor to costs in many geothermal drilling projects. The consequences of a stuck pipe are very costly, that include lost time when releasing the pipe, time and cost of fishing out the parted Bottom Hole Assembly (BHA), and efforts to abandon the tool(s) in the hole. Despite many observations that have been done to develop a system in avoiding stuck pipe incidents in oil and gas drilling operations using artificial intelligence (AI), few works have been developed for geothermal drilling operations. In this research, we propose a method to build an early warning system model for stuck pipe conditions based on a Support Vector Machine. Based on the experiment result Support Vector Machine Algorithm showed good performance with 89% accuracy and 81% recall for limited training dataset.

Drill string dynamics and experimental study of constant torque and stick-slip reduction drilling tool

Aiming at the problems of large torque fluctuation of drill bit and low weight-on-bit(WOB) transmission efficiency during the drilling process, a novel drilling tool generating constant torque and reducing stick-slip is proposed. For investigating the further dynamics behavior of drill string system, the mathematic models are established based on the new tool output characteristics. Also, compared with field test and application, the drilling tool effects on increasing ROP (rate of penetration) and reducing drill bit wear are verified. The research results show that the peak value of drill bit angular velocity increases gradually with the bigger value of the pre-compression of disk spring system, so as the rotary table inputting speed, during drill bit downward movement. About the drill string influence, the larger the length of drill string, the longer response time is the rotating angular velocity of drill bit. Another one, the shorter the steady time of drill bit angular velocity, the bigger ratio is the stick-slip within one period. This new drilling tool can effectively improve the drill bit force conditions, and reduce the stick-slip stage, which can also profitably increase drilling efficiency and the service life of downhole tools, especially in ultra-deep or long distance horizontal wells.

Response characteristics of drill-string guided wave in downhole acoustic telemetry

Modeling of a drill-string acoustic channel has been an important topic in downhole telemetry for a long time. The propagation of drill-string guided waves in the borehole contains excitation, attenuation, and mode conversion issues that have not been considered by existing modeling methods. In this article, we formulate a hybrid modeling method to investigate the response characteristics of a fundamental-mode drill-string wave in various borehole environments. This hybrid method provides channel functions, including transmitting and receiving deployments, periodicity of the structure, and formation property changes. The essential physics of the drill-string wave propagation is captured with a one-dimensional model. The analytical solutions of the wavefield in multilayered cylindrical structures are introduced into a propagation matrix to express drill-string-wave interactions with the borehole environments. The effectiveness of the proposed method is confirmed through comparison with the finite-difference method. In addition, by designing numerical models, we investigate the conversion effect of the drill-string wave at the tool joint. We demonstrate that the conversion intensity of the drill-string wave is positively correlated not only with the cross-sectional area of the tool joint but also with the wave impedance of the outer formation. Hard formation outside the borehole reduces the energy leakage while intensifying the conversion of drill-string waves to Stoneley waves, and the opposite is true for the drill string in an infinite fluid. The converted Stoneley waves interfere with the drill-string waves, resulting in variations of bandgap distribution, which challenges the reliability of the data transmission.