Polycrystalline Diamond Compact Cutters Research Articles

Prediction and Analysis of PDC Bit Wear in Conglomerate Layer with Machine Learning and Finite-Element Method

Polycrystalline diamond compact (PDC) bits experience a serious wear problem in drilling tight gravel layers. To achieve efficient drilling and prolong the bit service life, a simplified model of a PDC bit with double cutting teeth was established by using finite-element numerical simulation technology, and the rock-breaking process of PDC bit cutting teeth was simulated using the Archard wear principle. The numerical simulation results of the wear loss of the PDC bit cutting teeth, such as the caster angle, temperature, linear velocity, and bit pressure, as well as previous experimental research results, were combined into a training dataset. Then, machine learning methods for equal-probability gene expression programming (EP-GEP) were used. Based on the accuracy of the training set, the effectiveness of this method in predicting the wear of PDC bits was demonstrated by verifying the dataset. Finally, a prediction dataset was established by a Latin hypercube experiment and finite-element numerical simulation. Through comparison with the EP-GEP prediction results, it was verified that the prediction accuracy of this method meets actual engineering needs. The results of the sensitivity analysis method for the gray correlation degree show that the degree of influence of bit wear is in the order of temperature, back dip angle of the PDC cutter, linear speed, and bit pressure. These results demonstrate that when an actual PDC bit is drilling hard strata such as a conglomerate layer, after the local high temperature is generated in the formation cut by the bit, appropriate cooling measures should be taken to increase the bit pressure and reduce the rotating speed appropriately. Doing so can effectively reduce the wear of the bit and prolong its service life. This study provides guidance for predicting the wear of a PDC bit when drilling in conglomerate, adjusting drilling parameters reasonably, and prolonging the service life of the bit.

Torsion and vibration reduction mechanism of roller PDC hybrid bit

In directional drilling, torque fluctuation generates a big shock when using polycrystalline diamond compact (PDC) bits, causing problems such as instability in vibration tools. This study presents a new type of composite bit with a roller on the blade. The mechanism of torsion reduction and vibration of the roller PDC hybrid bit is studied through a rock breaking experiment on the roller unit and a comprehensive drilling experiment on the composite bit. Six roller contour shapes, different pressures into the depth of the roller, six roller radial positions and the relative height of roller teeth, and PDC cutters that affect the novel hybrid torsion on bit (TOB) and weight on bit (WOB) are investigated. The equivalent friction coefficient of the roller and axial force per unit depth is used to evaluate the torsional and vibration reduction ability of the Roller. The results show that the equivalent friction coefficient of different contour Rollers is lower than that of PDC cutters, and the torsion reduction ability of the Rollers is higher than that of PDC cutters. The vibration reduction in the roller teeth in different radial positions was 1.59–9.17 times higher than that of PDC cutters, The torque of the roller PDC hybrid bit is 71.72%–98.33% of the conventional PDC bit. The roller shares 1.59%–23.06% of the WOB of the whole bit. As the cutter teeth and the PDC cutter's relative height decreases, the compound drilling torque decreases, the roller bit's weight proportion gradually increases, the torsional capacity decreases, and the damping capacity increases.

A review of the frictional contact in rock cutting with a PDC bit

Drilling is one of the most expensive processes in the oil and gas exploration and exploitation processes which is commonly performed with polycrystalline diamond compact (PDC) fixed-cutter bits (or PDC bits as they are often referred to) due to their good stability and high flexibility in design. The PDC bit longevity strongly depends on the wear flat surface that changes during progression through rock materials. The wear flat size increases with time and can greatly influences the bit performance and efficiency. The increase in wear causes increasing weight-on-bit (WOB) on the PDC bit to keep the rate of penetration (ROP) constant. Over the past decades, contrary to the pure cutting process, not many investigations have been devoted to studying the friction mechanism at the wear flat–rock interface, resulting in poor understanding of the friction mechanism. The objective of this paper is to provide a detailed review of the state-of-the-art in rock cutting with particular focus on the frictional mechanism of the blunt PDC cutters and issues associated with the contact stress and the friction coefficient at the wear flat–rock interface. Moreover, the contact forces at the wear flat are governed by the wear flat inclination angle. Gaining a better understanding of the frictional contact process can eventually play a significant role in addressing practical issues such as drilling efficiency, optimization, and susceptibility to vibrations. Further, in spite of some obvious differences between the indentation and rock cutting processes, the main similarities between these two processes are also discussed which can provide new insights for the further studies. • A brief review of the cutting forces acting on the sharp PDC cutters. • An extensive review of the contact forces acting on the wear flat of the PDC bits. • Analysis of the factors affecting the contact stress and friction coefficient at the wear flat–rock interface. • A new comparison between the indention test and the rock cutting process. • Review of the factors affecting the development of the wear in the PDC bit/cutters.

Experimental and numerical investigations on rock-breaking mechanism of rotary percussion drilling with a single PDC cutter

To achieve high-efficiency drilling in hard formations, the rotary percussive drilling tool was designed to improve rock-breaking efficiency. Currently, few studies are conducted to analyze the relationship between the percussion parameters and rock breaking efficiency, considering the dynamic characteristics of rock during rock breaking. According to this situation, to quantify the influence of different percussion parameters on the rock-breaking efficiency, a series of dynamic experiments on hard rock and numerical simulations were carried out. Orthogonal experiments of split Hopkinson pressure bar (SHPB) device were conducted to analyze 13 parameters of the Riedel Hiermaier Thoma (RHT) material model. A polycrystalline diamond compact (PDC) cutter was selected as the research object, and a corresponding numerical model was established, considering the actual size of the drilling teeth and the load transmitted to the formation. Subsequently, the numerical simulation results of the SHPB test and cutting experiment were verified using the physical model. A sensitivity analysis was carried out to analyze the effect of different factors, including dynamic amplitude, frequency, and waveform, on the penetration depth and eroded volume fraction of formation. The results proved that rotary percussion drilling increased the penetration depth and eroded volume fraction of hard rock. The rock-breaking efficiency of the square wave was more obvious than that of the triangular and sine waves. Increasing the dynamic to static load ratio was beneficial for increasing the penetration depth and eroded volume fraction of formation. When the frequency increased, the cutting section became smoother, and the cuttings were finer, which was beneficial in cleaning the borehole. • Orthogonal experiments were used to obtain RHT material model parameters. • The physical model verified the rock failure of numerical simulation. • The rock-breaking efficiency of the square wave was most apparent.

DEM numerical investigation on the effect of back rake angle of PDC bit in drilling hard rock strata

Polycrystalline diamond compact (PDC) drill bits used in the Deep Borehole Placement (DBP) approach for nuclear waste disposal are subjected to highly competent, low-porosity, and low-permeability rock mass that cause severe wear and damage. To augment the penetration rate and minimize the wear-induced cost, a novel interaction rock cutting model with breakable granite and unbreakable cutter was established using two-dimensional discrete element method (2D-DEM). The particle element was used to simulate the granite sample, and the micro parameters of the contact elements between them were calibrated based on the uniaxial compression and Brazilian tests. While the rigid block element was used to reproduce the single PDC cutter including its elastic properties and geometric structure (i.e., back rake angle). The cutting process was simulated by applying normal force and tangential velocity on the single cutter to explore the effect of the back rake angle (5, 10, 15, 20 and 25°) on the stress condition of the cutter and penetration rate. The simulation results show that with the increase of the back rake angle, the maximum shear stress magnitude and stress concentration area decrease simultaneously, in according with the decrease of material removal rate, which do good help to decrease the wear and unexpected damage of bit cutter. In addition, the cutting process can be divided into the effective cutting stage when the elastic energy continuously stores and the fragmentation initiation stage when the stored energy suddenly releases and numerous cracks generate. In long term, the established DEM model can be used to optimize the structure of the PDC bit.

Fluid–solid coupling simulation of a new hydraulic self-adaptive PDC cutter for improving well-drilling efficiency in complex formations

A new type of hydraulic self-adaptive polycrystalline-diamond-compact (PDC) cutter (SAPC) is designed for controlling cutting depth of the PDC bit flexibly and reducing the harm of stick–slip vibration to drilling operation. The fluid–solid coupling movement of SAPC under different well-drilling conditions are simulated and analyzed. The results show that the up-going time of SAPC can be increased by reducing the diameter of the thin connecting pipe, increasing the diameter of the SAPC liquid-cavity, increasing the height of the cavity, increasing the liquid viscosity and increasing the initial spring force/decreasing the elasticity coefficient. Among them, reducing the diameter of thin connecting pipeline and increasing cavity diameter are the most sensitive. The up-going time can also be increased by the way of changing liquid viscosity without changing the structure of SAPC. Increasing the initial spring force and decreasing the elasticity coefficient can also increase the up-going time of the SAPC, however, the response window to the linkage force is reduced. The double-pipeline embedded SAPC can effectively reduce the volume and installation difficulty. The reasonable thin pipeline diameter is 0.2–0.5 mm, and the thick pipeline diameter is more than 2 mm, and the cavity diameter is more than 18 mm. Based on the results of the numerical simulation study, the up-going time of the SAPC is designed over 1.25 s, which meets the performance requirements of the hydraulic SAPC using in complex reservoir drilling, such as carbonate-sandstone reservoir.

Modeling of Cutting Rock: From PDC Cutter to PDC Bit—Modeling of PDC Bit

Summary In this paper, we integrated our polycrystalline diamond compact (PDC) cutter model (Chen et al. 2021) into a PDC bit model that can predict the weight on bit (WOB), torque on bit (TOB), and imbalanced side force on a bit under given drilling conditions. We first proposed a method to determine the actual cutting plane and depth of cut of each cutter on a PDC bit. Once the two parameters for each cutter are determined, the cutter model can then be applied to calculate the cutting force of each cutter. The final bit force and moment (i.e., WOB, TOB, and imbalanced side force) are calculated as the resultant force and moment of cutting forces of all cutters. The PDC bit model in this paper considers all bit design parameters, including bit matrix geometry, blade profile, cutter layout, and the inclination of each cutter. Furthermore, the bit model also considers some bottomhole assembly (BHA) parameters (e.g., bit tilt angle, location of first fulcrum point, and tool face/steering plane angle), which allows the bit model to simulate a bit under different drilling modes. The bit model is also validated by published test data and field applications. Finally, case studies are conducted, and the influence of bottomhole stresses, BHA parameters, and drilling modes on bit force and moment are discussed. A field application of the bit model is also provided. The bit model can be directly used for PDC bit design and simulation. In fact, this paper presents a general way to integrate a cutter model into a PDC bit model. Readers are also encouraged to apply this method to integrate their own cutter model into a PDC bit model.

Identifying the causes of residual stress in polycrystalline diamond compact (PDC) cutters by X-Ray diffraction technique

An attempt was made to identify the sources of residual stress in polycrystalline diamond compact (PDC) cutters by X-ray diffraction (XRD) technique. PDC retains residual stress after sintering and this residual stress plays an important role for cutter performance. X-ray diffraction method of residual stress measurement is very common because it is nondestructive. Residual stress was measured in a finished 16 mm PDC cutter and changes in residual stress were observed as the substrate was removed by grinding. Residual stress was also measured after full removal of cobalt from the diamond table by leaching. Comparison was made between the as-pressed PDC cutter, after removing the substrate, and after removing the cobalt to understand the relative contribution of geometry and cobalt binder to overall residual stress. An additional measurement was made under High Pressure High Temperature (HPHT) compacted diamond powder in the absence of cobalt to evaluate the contribution of high temperature deformation of diamond particle on residual stress.

Numerical study on optimal impact angle of a single PDC cutter in impact rock cutting

In previous work (Liu et al., 2020), we studied the differences between conventional rock cutting and cutting with compound vertical and horizontal impact force. In this work, we investigated the relationship between the impact force direction and cutting efficiency, which is the key to the impactor design. We established an impact rock cutting model with a single sharp polycrystalline diamond compact (PDC) cutter, and the direction of the impact force was described by an impact angle (β), which ranged from 0 to 90°. Simulations with different β were conducted, and the fragmentation process, forces, and mechanical specific energy (MSE) were analyzed. We also investigated the effects of the amplitude and duration of the impact forces on the MSE. The results showed that there is an optimal β, for which the MSE is the lowest and cutting efficiency is the highest. The effects of the vertical and horizontal impact forces on the rock cutting process were different, and excessive vertical or horizontal impact forces were not conducive to improving the cutting efficiency. The optimal β decreased with an increase in the impact amplitude, and a similar relationship was found between the optimal β and the duration of the impact force. In practice, designers are suggested to set the β of the impactor between 30° and 60°, and a lower β may be necessary to adapt to a higher impact amplitude and lower frequency.

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.

Study on rock-breaking mechanism of axe-shaped PDC cutter

Polycrystalline diamond compact (PDC) cutter is the key component of a drill bit. Recently the shaped PDC cutters, especially the axe-shaped cutter (ASC), have been warmly welcomed by the drilling engineers. However, the rock-breaking mechanism of ASC is rarely studied. In order to investigate the cutting action of ASC, comparative experiments of ASC and conventional cylinder-shaped cutter were carried out based on a vertical turret lathe (VTL). VTL tests' procedure were modified to mimic the initial state, steady state of PDC cutter in the real down hole drilling. The cutting forces and temperature were measured using a dynamometer load cell (3-component force sensor) and a thermocouple, respectively. The results show that ASC has better wear resistance and cutting efficiency than the conventional cylinder-shaped cutter. ASC is easier to press into the rock with its pointed geometry, leading to lower normal force at the initial state of cutting. The cutter shape has large influence on the cutters’ resistance to impact. In addition, both the normal force and tangential force required by ASC during stable cutting are less than that of conventional cylinder-shaped cutter under the same test conditions. Although the cuttings produced by both ASC and conventional cutter have pretty similar particle size distributions, ASC consumes less energy to break the rock based on the fractal analysis of cuttings. • Two states of PDC cutters were simulated on vertical turret lathe. • Consider fractal analysis into cuttings analysis of PDC cutters. • The wear resistance of PDC cutters were compared. • A new experimental attempt to explore the impact resistance of PDC cutters.

Effects of the chamfer and materials on performance of PDC cutters

Polycrystalline diamond compact (PDC) cutters play a key role in the performance of PDC bits in the oil & gas drilling. The study of single cutter on the rock-breaking mechanism is very important for PDC bits to deal with challenging formations, especially when the wells become increasingly longer and complex. Both cutting efficiency and durability are always the focuses of attention for the researches on single PDC cutter. Of all the factors influencing the cutting efficiency, the chamfer is always ignored in theoretical and experimental studies. In this work, the rock-cutter interaction model was modified with considering the effect of the chamfer, which could be used to predict the cutting forces and cutting efficiency of PDC cutters while shearing the rocks. All cutters have been tested on a vertical turret lathe to verify the modified theoretical model. The accuracy of the theoretical prediction agreed well with the experimental results. It was found that the cutting efficiency could be greatly enhanced by reducing the chamfer size for the cylinder-shaped PDC cutters. Unfortunately, the small chamfer size made PDC cutters vulnerable to the premature failure. In order to improve the durability of small-chamfered PDC cutters while retaining their cutting efficiency, an optimized mix of diamond grains was developed and experimentally studied. The results illustrated that the cutter with optimized diamond mix exhibited much better wear resistance. On the other hand, the chamfer size has relatively weak effect on the wear resistance of PDC cutters. The increased chamfer size contributes to protect the cutter edge from chipping but will cause more cumulative friction heat and impair the cutter's durability. The findings in this work will provide a valuable guideline for the manufacturers of PDC bits. • A theoretical prediction model is modified by considering chamfer. • Obtain a relationship between MSE and chamfer size. • Explored the relationship between the diamond grains and wear resistance. • Both wear resistance and thermal durability of PDC cutters are more influenced by material composition than chamfer size.

An experimental study of the relationship between cutting efficiency and cuttings size in rock cutting using a PDC cutter

Rock cutting efficiency is the most common parameter used to evaluate the performance of polycrystalline diamond compact (PDC) bits. Many studies have shown that cuttings size might be used to evaluate rock breaking efficiency; however, there are few quantitative studies of the relationship between rock cutting efficiency and cuttings size in rock cutting when a PDC bit is used. Several linear cutting tests were conducted to explore the relationship between the mechanical specific energy (MSE) and cuttings size to solve this problem. The cuttings size distributions were also discussed. Then, the coarseness index (CI) was selected as the most appropriate indicator of cuttings size and its relationship with the MSE was investigated. In addition, rock cutting tests based on the Taguchi method were conducted to analyze cutting parameters such as the depth of cut (DOC), the back rake angle (θ) and the cutting speed (v) effects on the CI and MSE. The contribution of each parameter to the response was obtained. The results show that the distribution of cuttings size conforms to the Rosin–Rammler equation, and there is a significant relationship between the MSE and cuttings size. The MSE decreases with increased cuttings size, furthermore, the relationship between the MSE and the CI could be described by an exponent function. The Taguchi experiment results show that DOC and θ have significant impacts: the CI value increases with increased DOC, and the CI increases slightly with increased θ. The most influential factor on the CI is the DOC, followed by θ. However, within the range of experiments, the cutting speed has an insignificant effect on the CI value. This work is highly pertinent to better understanding of the relationship between rock cutting efficiency and cuttings size in rock cutting using a PDC cutter.

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.

3D numerical simulation study of rock breaking of the wavy PDC cutter and field verification

The structure of polycrystalline diamond compact (PDC) bits is a key factor in improving their rock-breaking efficiency. To further study the rock-breaking mechanism of the wavy PDC cutter, a 3D finite element model of the dynamic rock breaking of the PDC cutter was established, and a comparative study on the rock-breaking process of the wavy PDC cutter and the conventional PDC cutter was conducted. The results show that the wavy PDC mainly breaks the rock by ploughing and shearing, which results in a higher rock-breaking efficiency. The cutting force of the wavy PDC cutter is 6.64% lower than that of the conventional PDC cutter, and its fluctuation is obviously smaller. With an increase in back-rake angle, the cutting force, its fluctuation, and the mechanical specific energy (MSE) increase, and the optimal back-rake angle is 10°–15°.With an increase in cutting depth, the cutting force and its fluctuation in the wavy PDC cutter increase, the MSE decreases, and the optimal cutting depth is 2 mm. As the number of waves increases, the cutting force and MSE of the PDC cutter increase first and then decrease, and the greater the number of waves, the larger the fluctuation of the cutting force. The cutting temperature of the wavy PDC cutter is lower than that of the conventional cutter, it decreases with an increase in the back-rake angle and number of waves, and gradually increases with an increase in cutting depth. The main results of this work will help to reveal the process of rock breaking and the mechanism of the wavy PDC cutter, thus contributing to the achievement of a high rate of penetration and efficiency in the drilling process of hard formations with PDC bits.

Rock-breaking performances of innovative triangular-shaped polycrystalline diamond compact cutter

Due to high hardness and high abrasion, conventional planar polycrystalline diamond compact (PDC) cutters can easily get broken and dull when drilling in (ultra-)deep formations. To enhance the drilling performance, an innovative kind of non-planar PDC cutter, namely, a triangular-shaped PDC cutter, has been developed by altering the 2D planar cutting face into a 3D cutting structure of a triangular trustum of a pyramid. According to the numerical simulation results, the triangular-shaped PDC cutter can easily break hard rocks by a smaller cutting force than the conventional planar PDC cutter. Furthermore, it requires less mechanical specific energy for breaking the same volume of rock than the planar PDC cutter. The triangular-shaped PDC cutter shows great potential in improving the drilling performances of the PDC bit in hard and abrasive formations.

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.

Modeling of Cutting Rock: From PDC Cutter to PDC Bit—Modeling of PDC Cutter

Summary The main purpose of this paper is to present our polycrystalline diamond compact (PDC) cutter model and its verification. The PDC cutter model we developed is focused on a PDC cutter cutting a rock in 3D space. The model studies the forces between a cutter and a rock and applies the theory of poroelasticity to calculate the stress state of the rock during the cutting process. Once the stress state of the rock is obtained, the model can then predict rock failure by the modified Lade criterion (Ewy 1999). This work also developed a trial-and-error procedure to predict cutting forces, and the stress state of a rock before cutting process is also considered. A complete verification of the cutter model is conducted. The model results (i.e., predicted cutting forces) are compared with measured cutting forces from cutter tests in multiple published articles. The major influencing factors on cutting forces—backrake angle, side-rake angle, depths of cut, worn depth (or wear flat area), and hydrostatic pressure—are all studied and verified. A good agreement between the model results and cutter test data is found, and the overall mean relative error is approximately 15%. The influence of inhomogeneous precut stress state of a rock is also studied. Overall, the cutter model in this paper is complete and accurate. It is ready to be integrated into a PDC bit model.

A novel experimental apparatus for single polycrystalline diamond compact cutter tests.

Polycrystalline diamond compact (PDC) bits are increasingly favored in the drilling field due to their high efficiency in rock breaking together with their longevity. To investigate the rock failure mechanism and further improve the performance of PDC bits, innovative experimental equipment is proposed in this paper. With its assistance, we can study the characteristics of rock-breaking using a PDC cutter under different conditions, e.g., high pressure and high temperature (HPHT), confining pressure, and jet impingement. The setup can be grouped into three parts: a rock cutting system, high-pressure jet generation system, and controlling system. A series of experiments was conducted to demonstrate the reliability of the setup. The results demonstrate the improvement in performance of PDC bits in the exploration of HPHT formations.

Rock Strength Determination Based on Rock Drillability Index and Drilling Specific Energy: Numerical Simulation Using Discrete Element Method

In the present research, two real-time rock strength determination models based on the drilling parameters were studied. Firstly, a discrete element software named Particle Flow Code in 2 Dimensions (PFC2D) was used to analyze the applicability of rock drillability index and drilling specific energy, in which graded particle assemblies were created to simulate the rock behaviour. In the numerical models, Weibull distribution was used to randomize the bonds between particles and make the assembled rock model with different heterogeneity. The drilling process of a PDC (polycrystalline diamond compact) cutter was modelled in two steps: horizontal linear rock cutting and vertical pressing-in progress. From the horizontal linear rock cutting process, the peak cutting forces were obtained and vertical pressing-in process outputs the relationship between normal force and cutting depth. Subsequently, the methods for calculating rock drillability index and drilling specific energy were proposed in the discrete element model. Then, the relationship between the calculated indexes and rock strength was investigated (supported with the regression analysis). Moreover, the effect of rock heterogeneity, along with error comparison between the above two indexes, were discussed. The results showed that the rock drillability index is more accurate than drilling specific energy in rock strength assessment (The error is about 10% smaller).