Diamond Compact Research Articles

The Effects of PDC Cutter Geometries to the Drilling Dynamics in Various Geothermal Rocks: A Comprehensive Study Using Advanced Drilling Dynamics Simulation

Polycrystalline Diamond Compact (PDC) bit has been used widely in drilling operations since a long time ago. However, the utilization of this bit in Indonesia’s geothermal drilling has just recently started in 2016, thanks to the rapid development of PDC cutter grades and shapes in the last decade. Prior to that, the geothermal drilling operation relied on Tungsten Carbide Insert (TCI) bit to drill the hard formations. The crushing action of the insert bit is very durable to remove the rocks. However, the seal and bearing limit the life of these bit types, causing multiple bit runs in a long drilling section.PDC bit offers longer drilling periods if the cutters are still in good condition. However, the nature of geothermal formation usually consists of hard rocks with fractures that can reduce the durability of the conventional flat PDC cutters. To overcome this weakness and optimize drilling operation,. Instead of having a flat surface, new PDC cutters have different 3D shapes. Each has different benefits: improving cutter strength against impact, increasing aggressiveness and depth of cut, or enhancing wear resistance towards abrasive formations.Advance drilling dynamics simulation helps analyze which cutter types will be more suitable for different geothermal rocks characteristics. The simulation is based on 4D Finite Element Analysis, which models the interaction between the cutting structures and the drilled formation. The result shows the dynamic drilling response of each cutter on different rocks, including the generated vibration, the cutter’s depth of cut distribution on the bit face, and the potential wear pattern on the bit. This paper then discusses which cutter and PDC bit type will be more beneficial for geothermal drilling based on the bit aggressiveness and durability to optimize the drilling operation.

A Review of Binderless Polycrystalline Diamonds: Focus on the High-Pressure-High-Temperature Sintering Process.

Nowadays, synthetic diamonds are easy to fabricate industrially, and a wide range of methods were developed during the last century. Among them, the high-pressure–high-temperature (HP–HT) process is the most used to prepare diamond compacts for cutting or drilling applications. However, these diamond compacts contain binder, limiting their mechanical and optical properties and their substantial uses. Binderless diamond compacts were synthesized more recently, and important developments were made to optimize the P–T conditions of sintering. Resulting sintered compacts had mechanical and optical properties at least equivalent to that of natural single crystal and higher than that of binder-containing sintered compacts, offering a huge potential market. However, pressure–temperature (P–T) conditions to sinter such bodies remain too high for an industrial transfer, making this the next challenge to be accomplished. This review gives an overview of natural diamond formation and the main experimental techniques that are used to synthesize and/or sinter diamond powders and compact objects. The focus of this review is the HP–HT process, especially for the synthesis and sintering of binderless diamonds. P–T conditions of the formation and exceptional properties of such objects are discussed and compared with classic binder-diamonds objects and with natural single-crystal diamonds. Finally, the question of an industrial transfer is asked and outlooks related to this are proposed.

An Alternative Formulation for Modeling Self-Excited Vibrations of Drillstring With Polycrystalline Diamond Compact Bits

Abstract This work describes an alternative formulation of a system of nonlinear state-dependent delay differential equations (SDDDEs), which governs the coupled axial-torsional vibrations of a 2 DOF drillstring model with a realistic representation of polycrystalline diamond compact (PDC) bits. The regenerative effect associated with the complex cutter layout for such bits can introduce up to 100 state-dependent delays in the equations of motion. This large number of state-dependent delays renders the computational efficiency of conventional solution strategies unacceptable. The regeneration of the bottom-hole surface can alternatively be described by the bit trajectory function, whose evolution is governed by a partial differential equation (PDE). Thus the original system of SDDDEs can be replaced by a nonlinear coupled system of a PDE and ordinary differential equations (ODEs). Via the application of the Galerkin Method, this system of PDE-ODEs is transformed into a system of coupled ODEs, which can readily be solved. The algorithm is further extended to perform a linear stability analysis of the bit motion. The resulting stability boundaries are verified with time-domain simulations. The reported algorithm could, in principle, be applied to a more realistic drillstring model, which may lead to an in-depth understanding of the mitigation of self-excited vibrations through PDC bit designs.

Stick-slip vibration behaviors of BHA and its control method in highly-deviated wells

Stick-slip vibration is a common phenomenon met in oil gas drilling, especially in the complex trajectory wells such as highly-deviated wells, it will cause the earlier failure of BHA (bottom hole assembly) and lower drilling efficiency. In this present paper, a finite element model of highly-deviated well, which consists of a drill string system, a borehole, a PDC bit (Polycrystalline Diamond Compact bit) and a rock model, is carried out to investigate the stick–slip vibration behaviors of drill bit and BHA. Besides, the torsional impact drilling method is proposed in the context to control the stick–slip vibration of BHA. The research results show that, the phenomenon of stick–slip vibration of BHA gradually weaken and disappear with the distance from the drill bit increases, and the stick–slip vibration mainly occurs at the BHA which near drill bit. The high frequency torsional impact drilling method can reduce the resistance torque of the bit by 34.71%, and increase the drilling depth by about 58.28%. It can effectively improve the drilling efficiency of the bit, making the rotation of drill bit becomes relatively more stable, and reducing or even eliminating the stick–slip vibration. These results lead to an enhanced understanding of stick–slip vibration behaviors of drill bit and BHA in highly-deviated wells, and provide the basis for improving the drilling efficiency.

Wear and Degradation of Drilling Performance of Polycrystalline Diamond Compact Bit in Laboratory Test

Wear and Degradation of Drilling Performance of Polycrystalline Diamond Compact Bit in Laboratory Test

Rock cutting mechanism of special-shaped PDC cutter in heterogeneous granite formation

Special-shaped Polycrystalline Diamond Compact (PDC) cutters show the superiority for improving rate of penetration (ROP) in hard abrasive formation drilling. However, few attentions have been paid on its geometric design and rock cutting mechanism. To deepen the understanding of special-shaped PDC (SPDC) cutter's rock-breaking mechanisms and provide a certain reference for the selection and application of SPDC cutters in the specific target formation, this paper firstly proposes two new principles (loading performance (LP) and cutting performance (CP)) as well as their evaluation indexes for evaluating the cutting process of cutters from the relationship between geometry, aggressiveness, stress state, working life, and rock cutting efficiency of SPDC cutters. Then, using gray-white granite as the research object, the finite element heterogeneous granite model, whose relative percentage error of all calibration quantities with experiment are less than 7%, based on Voronoi tessellation and the granite cutting model of 10 types cutters under confining pressure are established. Finally, the cutting results are discussed, and the CP of all cutters are evaluated. It's found that the ductile-brittle failure mode in the removal area and the influence region of cutting process determine the rock failure mode of SPDC cutters. The confining pressure has an inhibitory effect on rock cutting efficiency, it's because the high confining pressure leads to ductile dominated failure mode in removal area. The confining pressure weakens the LP and CP of cutters, but has no obvious effect on their stress distribution state. With the increasing of confining pressure, “ductile-plastic stick-slip cutting” effect is produced in the removal area. The conical-shaped cutter (whose CP coefficient is 4.3 × 10 −3 - 5.4 × 10 −3 N −1 ) and saddle-shaped cutter (whose CP coefficient is 3.4 × 10 −3 - 4.8 × 10 −3 N −1 ) are the optimal selection for heterogeneous granite formation. On the contrary, the three-blade cutter and cusp-shaped cutter are unsuitable for heterogeneous granite formation. This research leads to an enhanced understanding of rock-breaking mechanisms of SPDC cutters, and provides the basis to select of SPDC cutters for the specific target formation. • A heterogeneous granite generation method based on Voronoi tessellation using finite element method. • The rock-cutting mechanisms and differences of 10 kinds of PDC cutters. • A new ideas and methods for parameter optimization and performance evaluation of special-shaped PDC cutter using FEM.

Investigation of the cutting force response to a PDC cutter in rock using the discrete element method

In the drilling industry, Polycrystalline Diamond Compact (PDC) bits with multiple fixed PDC cutters are widely applied to cut rock under high bottom hole pressure and high temperature. The bit-rock interaction is the main excitation source of drill string vibrations and needs to be understood and controlled better to improve the performance of the drilling process. Rock cutting mechanisms can be examined best for a single PDC cutter: Limitations of dynamic control and available sensors in experimental setups are eliminated in numerical simulations. Using the Discrete Element Method (DEM), deformation and strength of rock can be modeled through its full degradation in the PDC cutting process under typical environmental conditions. Stresses and strains in the rock material and on the cutter faces become available for examination.In this paper, numerical simulations of the cutting process are conducted using a modified bonded-particle model (specialized DEM), which is calibrated to a pressure series of high-strain triaxial laboratory tests. The simulation results are interpreted from the perspective of continuum mechanics providing stress and strain rate fields within the rock. These serve to identify two primary deformation modes predominant in two distinct zones moving with the PDC cutter: A crush zone below the chamfer and a shear zone ahead of the cutter face. The influences of two cutter design parameters, the back rake angle and the chamfer size, on these zones and the cutting force, are investigated. The shear angle is found to decrease with increasing back rake angle, resulting in a wider shear band. Moreover, the back rake angle has a considerable influence on the cutting forces both in the normal and tangential direction, whereas the chamfer size affects the normal cutting force predominantly. These results are in good accordance with experimental observations in the literature. Furthermore, the relationship between the cutting force and underlying rock failure mechanisms is clarified.This study enhances the understanding of the rock cutting process and gives first indications for dynamic cutting force control through cutter design.

The novel and facile electrolysis method for removing the cobalt binder phase from large diameter polycrystalline diamond compacts

The thermal stability, wear resistance and impact toughness of polycrystalline diamond compact (PDC) are the primary determining variables in determining the performance. The cobalt binder contributes significantly to the PDC's thermal stability and wear resistance. To increase the thermal stability of the PDC, this paper used electrolysis to remove the cobalt binder from the polycrystalline diamond (PCD) layer of the PDC with a diameter of 62 mm. The optimal process parameters for cobalt removal via electrolysis were determined by examining the electrolyte concentration, the electrolytic voltage, and the electrolytic time. The cobalt removal impact was evaluated using a scanning electron microscope (SEM), an energy dispersive spectrometer (EDS), an X-ray diffraction (XRD), and thermogravimetric analysis-differential thermal analysis (TG-DTA). In addition, the abrasion ratio of the PDC samples were tested. The experimental results indicated that the optimal electrolysis parameters for cobalt removal were obtained when the concentration of cobalt sulphate was 4 g/100 mL, the electrolysis voltage was 2.5 V, and the electrolysis time was 10 h; in this case, the cobalt removal rate from the PCD layer exceeded 80%, and the cobalt removal depth was 372 μm. TG-DTA analysis revealed that the thermal stability of PDC was significantly enhanced as a result of the initial graphitization temperature of the cobalt removal sample being increased from 1071 °C to 1113 °C, but the abrasion ratio rose by more than 20%. It was discovered that electrolysis may efficiently remove the cobalt binder phase from the PCD layer, therefore improving the thermal stability and wear resistance of PDC.

Numerical Study on Fragmentation Characteristics of Granite Under a Single Polycrystalline Diamond Compact Cutter in Rotary-Percussive Drilling

Abstract Fragmentation characteristics of granite in rotary-percussive drilling (RPD) are studied using the distinct element method. We developed a model to investigate the interaction between the rock and a polycrystalline diamond compact (PDC) cutter. The micro contact parameters in the model are calibrated by conducting a series of simulated mechanical tests of the rock. Sensitivity analyses are then conducted to according the drilling performances which are quantified as the penetration displacement, the fragmentation volume and the specific energy, as well as the lateral force and the particle size distribution. Results show that the model can well represent the typical fracture system under indentation of the cutter, the torque fluctuation phenomenon in drilling, and the formation of lateral chips, which verify the reliability of the model. The cutter with a back rake angle of 55 deg and impact frequency of 30 Hz has the best penetration performance in evaluated parameters. Increasing the frequency has a great effect on the rock breaking speed under the coupling effect of impact and cutting in the low frequency range. Considering crushing efficiency, 50 Hz is the recommended impact frequency. This paper provides a useful tool to represent the fragmentation performance of rotary-percussive drilling and sensitivity analyses shed light on the potential ways to improve the performance.

Study on Composite Design Suitable for High Hardness and Strong Abrasive Formation

PDC (Polycrystalline Diamond Compact bit) composite is the most important cutting element of petroleum bit, which performance directly affects the service effect and service life of the bit. During the drilling process, the cutter will produce a large amount of friction heat when cutting the rock, resulting in a sharp increase in the internal temperature of the cutter. When the temperature reaches a certain value, thermal wear and tear are very easy to occur, which will not only cause diamond delamination but also reduce the wear resistance of the cutter. Under the action of impact load, impact failure is more likely to occur, which greatly reduces the service life of the cutter and the rock-breaking efficiency of the drill bit. Therefore, this paper studies the composite interface suitable for high-temperature drilling through the changes of cutting tooth temperature field and stress field with different interface shapes, which shows that the non-planar interface is more suitable for improving the cutting tooth life of composite under the action of comprehensive stress field.

The growth behavior and surface performance enhancement of diamond film deposited on polycrystalline diamond compact

Polycrystalline diamond compacts (PDC) are subject to thermal abrasive wear and impact damage when working in harsh downhole environments due to their internal diamond skeleton-cobalt microstructure. Here, the polycrystalline diamond film was deposited on PDC epitaxially by the hot filament chemical vapor deposition technique, which can firmly adhere to the PDC under severe working conditions and enable to enhance its wear resistance. A systematic study is presented into the effects of deposition parameters such as substrate temperature, methane concentration, gas pressure, and hydrogen flow rate on the film growth behavior, including the growth mode and the morphology evolution observed from the edge cross-section and surface by SEM. Under the deposition conditions of high substrate temperature, low methane concentration, low pressure, and large hydrogen flow, the crystal grains show square crystal planes with smooth surfaces and sharp edges, and the film is dominated by epitaxial growth and closely bonded with PDC without cracks and delamination. While under the opposite conditions, with the increase of secondary nucleation and the rapid shrinking of the square crystal plane, the as-grown film and the PDC substrate are loosely bonded with relatively poor adhesion, which is easy to cause edge chipping and peeling off when grinding and not suitable for the surface performance enhancement. After synthesizing the film adhesion, PDC integrity, the quality of crystallization and the film deposition rate, the optimal epitaxial conditions for CVD of PDC were summarized and tested. It was found that after 21 days of decobaltization pretreatment, the diamond films deposited on PDC for 16 h were able to firmly adhere to the substrate and provide the best surface performance enhancement.

A formation-based approach for modeling of rate of penetration for an offshore gas field using artificial neural networks

Due to the mounting concern in the oil and gas industry about creating well-designed drilling plans and efficiently drilling new wells, analysis of offset well data has received extensive attention over the past few decades. By doing this, bottom hole assemblies, drill bit for a given section, drilling fluid, and the operational parameters such as weight on bit (WOB) and drilling string rotational speed (RPM) can be accurately determined. This makes it possible to achieve the most favorable Rate of Penetration (ROP) without causing drilling problems or non-productive Time (NPT). The present paper proposes a unique formation-based Artificial Neural Networks (ANN) model of ROP, which suits deviated wells with the application of Polycrystalline Diamond Compact (PDC) bits. In this study, nine different drilling functions that account for various factors affecting ROP are considered. Unlike the conventional ANN-ROP modeling technique, which directly considers raw drilling variables as inputs into ANN, the drilling functions are considered as inputs to ANN in the proposed ANN-ROP model. The drilling functions for this study are available in literature. The proposed ANN-ROP model is trained and tested using data collected from the 12 ¼ inches drilling section, which is the most extended drilling section of these wells. Furthermore, the prediction performance of the proposed models is evaluated in various formations. The results indicate that the proposed model is able to predict drilling ROP with high accuracy with the average Absolute Percentage Error (AAPE) being 0.83% for the training wells and 4.5% for the testing wells and with the R-squared obtained being 0.97 for the training wells and 0.93 for the testing wells. The proposed formation-based ANN-ROP methodology is shown to surpass the conventional ANN-ROP in terms of both accuracy and efficiency and the principle-based methodology in terms of accuracy.

Failure characteristics of rocks subjected to PDC cutter indentation

Indentation of rocks by the Polycrystalline Diamond Compact (PDC) cutter has great impacts on the cutting depth. However, a vast majority of previous studies only focused on the rock cutting process. Further insight into the failure characteristics of rocks subjected to PDC cutter indentation is required to optimize the design of PDC bits. In this paper, a set of experiments were conducted to investigate the effects of back rake angles and mechanical properties of rocks on the PDC cutter indentation. Penetration forces and Acoustic Emission (AE) signals were captured to determine rock failure modes (brittle failure or plastic failure), and the three-dimensional profilometer was used to identify the morphology features of penetration pits. Besides, the penetration efficiency and brittle failure index were defined to evaluate the crushing characteristics of rocks during penetration. The results show that plastic failure dominates the indentation process as the back rake angle is smaller than 20°, which significantly limits the growth of penetration depth. Based on the penetration force and the number of AE hit counts, chipping occurs in the indentation process when using the PDC cutter with greater back rake angles (>20°). Such phenomenon can be further confirmed by topography features of penetration pits. Moreover, the position of brittle failure moves from the front to the back of the cutter as the back rake angle exceeds 40°. Compared with granite and sandstone, chipping occurs more easily in the marble, indicating a higher brittle failure index. In conclusion, back rake angle has great effects on the rock cutting process and the indentation of rocks. Our current study is expected to provide theoretical guidance for the cutter layout in PDC bits.

Fragmentation Characteristics of Rocks Under Indentation by a Single Polycrystalline Diamond Compact Cutter

Abstract Polycrystalline diamond compact (PDC) bit accounts for the most drilling footage in the development of deep and geothermal resources. The goal of this paper is to investigate the PDC cutter-rock interaction and reveal the rock fragmentation mechanism. A series of loading and unloading tests are conducted to obtain the curves of contact force versus penetration displacement. A single practical PDC cutter is fixed on the designed clamping devices that are mounted on the servo experiment system TAW-1000 in the tests. The craters morphology and quantified data were obtained by scanning the fragmented rock specimen using a three-dimensional morphology scanner. Finally, a numerical model is established to get the stress and deformation fields of the rock under a single PDC cutter. The results show that there are two kinds of failure modes, i.e., brittle failure and plastic failure, in the loading process. Marble is more prone to brittle fracture and has the lowest specific energy, followed by shale and granite. The brittle failure in marble mainly occurs behind the cutter while that happens ahead of the cutter for shale. Curves of contact force versus penetration displacement illustrate that a cutter with a back rake angle of 40 deg has a better penetration result than that with a back rake angle of 30 deg. Enhancing loading speed has a positive effect on brittle fragmentation. The distribution of von Mises stress indicates the initiation point and direction, which has a good agreement with the experiment. The research is of great significance for optimizing the PDC bit design and increasing the rate of penetration.

The compacts of boron-doped synthetic diamond: Methods for the increasing of their electrochemical activity

Conducting diamond-compact electrode materials are obtained by graphite transformation to diamond at high pressure and high temperature (8–9 GPa, ~ 2500 К) in two- and three-component B4C–C (graphite), Pt–C, and Pt–C–B growth systems; their physical and electrochemical properties are studied. Results of voltametric studies are in good agreement with the electrochemical impedance spectroscopy data obtained in f-sweep and E-sweep modes. The compacts’ electroactivity can be improved in two ways: either extremely heavy doping with boron or adding with catalytically active metal, platinum. The boron content reached in the diamond compacts is as high as ~ 4 at %. The obtained compacts showed the highest electroactivity of all diamond, diamond-based, and diamond-like materials we have studied (by example of the reaction of anodic chlorine evolution from KCl solution). In compliance with the earlier found general trend for all studied diamond, diamond-based, and diamond-like materials, the increase in the material’s electroactivity occurs with the increasing of the compact doping level. The diamond electrode added with Pt in the Pt–C growth system behaves in the chloride-ion anodic oxidation reaction like a purely platinum one, with due allowance for the platinum actual amount at the electrode surface. The combined platinum and boron adding to the diamond grown in the Pt–C–B system produced electrodes electroactive with respect to the chloride-ion anodic oxidation reaction; the electroactivity increased with the increasing of the platinum content at the electrode surface.

Investigation of the Cross-Cutting Polycrystalline Diamond Compact Bit Drilling Efficiency

The parallel track scraping principle of conventional PDC bits largely limits the cutting efficiency and working life in deep formations. Cross-cutting polycrystalline diamond compact (PDC) bit may be an efficient drilling tool that increases the rock-breaking efficiency through both cross-cutting and alternate-cutting modes of the PDC cutter. The motion track equation of the cross-cutting PDC bit was derived by using the compound coordinate system, and the motion track was analyzed. Meanwhile, through the unit experiment and discrete element simulation, the cutting force, volume-specific load, and crack propagation were analyzed under different cutting modes. Through establishing a nonlinear dynamic model of the bit-rock system, the speed-up mechanism of the novel bit was analyzed based on rock damage, rock stress state, and motion characteristic of the bit during the rock-breaking process. Compared with unidirectional cutting, cross-cutting produces less cutting force, more brittle fracture, and a greater decrease of formation strength. The novel PDC bit can put more rock elements into a tensile stress condition than a conventional PDC bit, and the plastic energy dissipation ratio of the cross-cutting PDC bit is lower while the damage energy consumption ratio is higher than they are for conventional bits, which is beneficial to increasing the ratio of fracture failure and improving rock-breaking efficiency. Laboratory drilling tests show that the cross-cutting PDC bit can create mesh-like bottom-hole features. Drilling contrast experiments show that a mesh-like bottom-hole pattern can be obtained by using the cross-cutting PDC bit, of which the ROP is obviously higher than that of the conventional bit when drilling in sandstone or limestone formation. Meanwhile, the influence of deviation angle, weight on bit, and rock properties on cutting efficiency of the cross-cutting PDC bit are studied.

Impact of surface fluorination on phase stability of nanodiamond particles and inter-granular bonding in polycrystalline diamond under HPHT conditions

Polycrystalline diamond (PCD) compacts, made by conventional high pressure-high temperature (HPHT) technology, contain Co catalyst in inter-granular spaces causing degradation of PCD mechanical strength and thermal stability during rock cutting due to thermal expansion and graphitization. To solve this problem a catalyst-free synthesis of PCD based on HPHT sintering of fluorinated nanodiamond (FND) and submicron size aluminum mixtures was explored in the present work. The feasibility of using FND has been established by evaluation of phase stability of FNDs of 5, 10, and 90 nm sizes under industrially favorable P-T conditions (7.5–8 GPa, 1500–1700 °C) for PCD synthesis. It was found that surface fluorine slows down the graphitization of nanodiamond particles. Enlargement of diamonds from 5 and 10 nm up to a 10 μm has been observed for FND/Al systems. This phenomenon can be associated with the Wurtz-type reaction under the HPHT conditions leading to formation of inter-granular carbon‑carbon bonds between the nanoparticles and formation of the fluid Al-C-F phases in the samples volume, accelerating the mass transport and re-crystallization of nanodiamond. PCD samples sintered from mixtures of fluorinated micro-diamonds, nano-diamonds FND-90 and 3 wt% aluminum exhibited enhanced up to 1200 °C thermal stability and as high wear resistance as leached commercial PCD.

Research on kinematics analysis of spherical single-cone PDC compound bit and rock breaking simulation verification

The single-cone bit has become the first choice for slim hole sidetracking and deep well drilling with its unique rock breaking method and high ROP (Rate Of Penetration), with its main failure mode being of early excessive wear of the cutting teeth. In order to improve the adaptability of single-cone bits to hard and highly abrasive formations, a spherical single-cone Polycrystalline Diamond Compact (PDC) compound bit is designed. According to the characteristics of the tooth profile, the way of tooth arrangement and the way of contact between the cutting teeth and the rock, the acceleration equation to the cutting teeth of the spherical single-cone PDC compound bit is established. The acceleration of the single-cone bit is verified by numerical simulation experiment of rock-breaking. The shaft inclination angle of the cone, the position and height of the PDC teeth, the radius of the PDC teeth, the lateral rotation angle and the front inclination angle on the acceleration are studied. The results show that as the shaft inclination angle increases, the bit transmission ratio gradually increases, and the harder the rock formation, the larger the transmission ratio of the single-cone bit; the shaft inclination angle and the position of the PDC tooth have a greater influence on the acceleration of the PDC tooth, and the radius, lateral rotation angle and front inclination angle of the PDC tooth have a small influence on the acceleration of the PDC tooth; rock properties have an impact on the acceleration of the cutting teeth, with the acceleration of the cutting teeth in hard rock formations being higher than that in soft rock formations; near the top of the cone, the absolute acceleration of the cutting teeth will fluctuate sharply and cause severe wear of the cutting teeth, so the tooth distribution in this area should be strengthened; on the premise that the bearing life of the single-cone bit is allowed, the value of the shaft inclination angle β can be approached to 70°. The relative error between the theoretical analysis results of the acceleration of the PDC cutter and the rock-breaking simulation experiment results is between −0.95% and −2.24%. This research lays a theoretical foundation for the dynamic research of spherical single-cone PDC compound bit.

Non-linear Axial Vibrations of Composite Drill Strings Considering Interaction of Roller Cone Bit and Polycrystalline Diamond Compact Bit with Rock

Due to the world increasing energy demands, optimizing the drilling system parameters such as the weight on bit (WOB), and the structure of drill string and bit, also vibrations and dynamic behavior of drill strings are of significant interest to researchers and energy industries. Specially, to overcome limitations of drilling operations in oil and gas industry, composite drill strings as high-tech devices are under development. In this research, the fully coupled non-linear axial vibrations of composite drill strings due to the interaction of two common bits namely; Roller Cone (RC) and Polycrystalline Diamond Compact (PDC) with rock, considering the major non-linear terms, the drill string-wellbore contact, the different weight on bit (WOB) and the different composite configurations using the finite element method (FEM) and the Lagrangian approach were studied. This study proved that the different configurations of composite drill strings showed specific dynamic behavior at different conditions. Therefore, composite drill string can be designed for particular purposes. Also, the results imply the remarkable effects of weight on bit (WOB) and type of bits on the axial vibrations of composite drill strings.

Analysis of Rock Dynamic Stresses During the Drilling by Polycrystalline Diamond Compact Bits

In view of shortening the development period of polycrystalline diamond compact (PDC) bits, the finite element method was adopted to simulate the dynamic stress of rocks. By employing drilling related theories, the three dynamic principal stresses of rock were analysed and the dynamic rock-breaking criterion was established. Second, the drilling model of PDC core bit was constructed, and the stress was simulated and calculated. Finally, laboratory tests were carried out to verify the simulation results. The analytical results demonstrate that the two obvious stages in the rock-breaking process are the initial rock-breaking stage and the normal one. The dynamic rock-breaking stress in the normal drilling stage varies from 66.3 to 99.6 MPa, which is lower than 278.4 MPa in the initial rock-breaking stage. During spud drilling, the axial force and the tangential force are 1.85 and 1.60 kN, respectively. During normal drilling, the axial force ranges from 0.2 to 0.9 kN, and the tangential force from 0.15 to 0.6 kN. The load of normal drilling is lower than the spudding load, and the bit is more likely to be damaged during spudding. The bit is normally worn during normal drilling.