Polycrystalline Diamond Compact Bits Research Articles

Prediction of formation abrasiveness under the action of a PDC bit based on the fractal dimension of a rock surface

During rock drilling, a drill bit will wear as it breaks the rock. However, there is no uniform grading standard for rock abrasiveness. To solve this problem, the wear mechanisms of a polycrystalline diamond compact (PDC) bit and the formation it is drilling into are analyzed in depth, and an abrasiveness evaluation method based on the fractal dimension of the rock surface topography is established. Initially, a three-dimensional digital model is generated from a scanning electron microscope image of the rock after drilling; next, an evaluation of the irregularities on the rock surface is performed using an adapted Weierstrass–Mandelbrot (W-M) function to ascertain the fractal dimensionality. Then, the microcontact characteristics of the contact surface between the formation and the PDC bit are analyzed, and the distribution of the microconvex contact points of the two-body friction pair in a region is obtained. Because the sliding friction between the drill bit and the rock produces a large amount of heat, according to the contact area formula of the friction surface and heat conduction theory, the temperature rise and overall temperature distribution of the formation and PDC bit under the condition of sliding friction are revealed, and the real contact area between the formation and the drill bit within a certain temperature range is obtained. Finally, the evaluation index of rock abrasiveness under sliding conditions is established by adopting the wear weight loss of the rock cutting tool per unit volume as the index of rock abrasiveness, and the model is verified by a microdrilling experiment. The research in this paper is highly important for improving the rock-breaking efficiency and bit service life during drilling.

Stick-slip vibration analysis of a coupling drilling-rock system with two delays and one delay-dependent coefficient

Drilling depths exceeding 10,000 m have recently been achieved, but the increasing depth poses challenges due to the flexible drill-string, making it more susceptible to stick-slip vibration. This vibration can lead to accidents such as excessive bit wear and tool falling into the well. Previous studies on modeling polycrystalline diamond compact (PDC) bits have focused on symmetrically distributed complete blade bits, which are not commonly used. To address this gap, this study introduces PDC bits that closely resemble the actual ones, with a structural configuration of both complete blades and part-blades symmetrically distributed. We also consider the impact of mud and rock chips on the drill bit by introducing a delay-dependent coefficient of response damping change. This study develops a stick-slip vibration model for a coupling drilling-rock system with two state-dependent delays tn1 and tn2, and one delay-dependent coefficient. Using the crossing curve method and numerical studies, we obtain different types of crossing curves reflecting the stability switching characteristics of the system in the dimensionless delay τ2τ1 space. During stick-slip vibration, τ1 mutates from 1.69 s to 2.51 s, and τ2 mutates from 1.3 s to 2.16 s, both within the stable region of the crossing curve. The critical steady state and the stable state typically cross the stable and unstable regions of the crossing curve in their respective times. Increasing the torsional damping within the range of 0.04 to 0.08 can accelerate system stabilization or delay the arrival of the critical steady state. Moreover, increasing the drive rotation speed within the range of 4 to 6 eliminates stick-slip vibration and accelerates the system towards a steady state. In conclusion, this study predicts the drill-string dynamics of a symmetrically distributed PDC bit with complete blades and part-blades. The insights gained can help minimize stick-slip vibration and provide recommendations for drilling operation parameters.

Rock-Breaking Characteristics of Three-Ribbed Ridge Nonplanar Polycrystalline Diamond Compact Cutter and Its Application in Plastic Formations

Summary To improve the penetration performance of polycrystalline diamond compact (PDC) bits in hard-to-penetrate plastic formations, the study of three-ribbed ridge nonplanar PDC cutter technology was carried out. The rock-breaking characteristics of nonplanar cutters are analyzed by comparison with conventional planar cutters and axe-shaped cutters through simulation and indoor experiments. The simulation results show that the planar cutter mainly breaks the rock by shearing and extruding, the axe-shaped cutter mainly breaks the rock by shearing, and the nonplanar PDC cutter mainly relies on its convex ridge structure to penetrate and split the rock. Nonplanar cutter has better penetration performance and cutting stability than planar cutters and axe-shaped cutters. The field test shows that the rate of penetration (ROP) and footage of the developed PDC bit with three-ribbed ridge nonplanar PDC cutters are increased by 133.66% and 176.11% compared with the conventional PDC bit in the hard-to-penetrate plastic formations. The use of nonplanar PDC cutters improves the working stability, rock-breaking efficiency, and service life of the bit. The special three-ribbed ridge structure of the nonplanar cutter has changed the interaction mode between the cutter and the rock. Its successful application in the plastic formation provides a reference for faster drilling of PDC bits in hard-to-penetrate plastic formations.

Rock-Breaking Characteristics of Three-Ribbed Ridge Nonplanar Polycrystalline Diamond Compact Cutter and Its Application in Plastic Formations

Summary To improve the penetration performance of polycrystalline diamond compact (PDC) bits in hard-to-penetrate plastic formations, the study of three-ribbed ridge nonplanar PDC cutter technology was carried out. The rock-breaking characteristics of nonplanar cutters are analyzed by comparison with conventional planar cutters and axe-shaped cutters through simulation and indoor experiments. The simulation results show that the planar cutter mainly breaks the rock by shearing and extruding, the axe-shaped cutter mainly breaks the rock by shearing, and the nonplanar PDC cutter mainly relies on its convex ridge structure to penetrate and split the rock. Nonplanar cutter has better penetration performance and cutting stability than planar cutters and axe-shaped cutters. The field test shows that the rate of penetration (ROP) and footage of the developed PDC bit with three-ribbed ridge nonplanar PDC cutters are increased by 133.66% and 176.11% compared with the conventional PDC bit in the hard-to-penetrate plastic formations. The use of nonplanar PDC cutters improves the working stability, rock-breaking efficiency, and service life of the bit. The special three-ribbed ridge structure of the nonplanar cutter has changed the interaction mode between the cutter and the rock. Its successful application in the plastic formation provides a reference for faster drilling of PDC bits in hard-to-penetrate plastic formations.

Rock fragmentation mechanism of PDC cutter from the insight of cutting chips

To meet the global energy needs and demands, obtaining oil and gas resources from deep strata plays an increasingly important role in energy supply. However, it is difficult to drill since the special properties of rocks in deep formation. To further improve the breaking efficiency of polycrystalline diamond compact (PDC) bit in deep hard rock, this paper conducted several groups of experiments on cutting granite with PDC cutters. By changing the cutting depth, rake angle and linear velocity, the relationship between the mass proportion, fractal dimension and roughness index of cuttings and the mechanical specific energy (MSE) is explored, and the following conclusions are obtained: When cutting depth HC exceeds 0.8 mm, massive chips with particle size r > 2 mm begin to form and the brittle failure begins to occur, and the probability of brittle failure will increase with the further increase of cutting depth. With the increase of rake angle, more energy will be used to form new surfaces of cutting chips, resulting in a decrease in the roughness index and an increase in MSE. More massive chips will be formed with the increasing linear velocity, but MSE only decrease slightly. The variation of the fractal MSE (FMSE) that based on the fractal dimension of cutting chips is extremely consistent with the MSE. The relevant conclusions in this paper can provide some reference for the structural design of PDC bit, so as to improve the drilling efficiency in deep hard rock.

Study on Evaluation and Prediction for Shale Gas PDC Bit in Luzhou Block Sichuan Based on BP Neural Network and Bit Structure

Deep and ultra-deep shale gas resources have great potential, but well drilling faces many challenges. The Polycrystalline Diamond Compact (PDC) bit has become the primary rock-breaking instrument for oil and gas drilling. Reasonable bit structure designs can promote rock-breaking efficiency and extend service life. In this study, reverse modeling technology is used to analyze the structural characteristics of PDC bits collected in the field, and the influence of the structural characteristics of the bit at a specific interval on the rate of penetration (ROP) and drill footage is investigated using the Spearman rank correlation coefficient method. The number of blades, cutting angle of the cutters, crown rotation radius, internal cone angle, and diameter of the cutters are discovered to be the main structural characteristics that affect the ROP and footage of the bits, and the degree of influence varies depending on the formation conditions. The number of blades, crown rotation radius, inner cone angle, and cutting angle of the cutters have a significant impact on the ROP, whereas blade thickness, gauge length, gauge width, nozzle equivalent diameter have a significant impact on the bit footage. In addition, a back propagation (BP) neural network is utilized to build a prediction model of bit footage and ROP over a certain interval based on the structural characteristics of the bit. The goodness of fit of the model is greater than 85%, and its accuracy is high. Based on the usage of the bit, the evaluation and prediction of the bit can provide a reference for the structural design and optimization of the bit in a specific interval, guide the bit selection work, rationally plan the drilling operation, and reduce the drilling cost.

New understandings of the applications of PDC cutters in oil and gas drilling

Today, the polycrystalline diamond compact (PDC) cutters have already reached a pretty high level of quality and are still keeping advancing. The formations drillability is positively related to the quality of PDC cutters. With advances in PDC cutters, most formations have become easy-to-drill. Years of improvements in the PDC cutter quality and the drilling techniques have also upended some long-hold knowledge and understandings about the cutter applications. PDC designs abandoned long ago are once again applicable, such as unleached, thin diamond table, small chamfer, and non-pure diamond mix. From the perspective of saving money and environmental protection, these long-abandoned PDC designs are worth a shot, but are suggested to be used in the well-understood formations and lithologies. On the other hand, why do PDC cutters always perform poorly in challenging formations? The results revealed that this is mainly due to the irregular dynamic impacts and inefficient rock cutting, which are the primary causes of the premature failure of PDC cutters and bits. The challenging formations as well as the unknown formations in the exploratory wells still prefer the cutters with the best quality, such as pure diamond mix, thicker diamond table, deep leaching, excellent residual stress management, and the ones from the best manufacturer. In addition, how to select the right cutters depends highly on the bit failure forensics and the multi-index evaluation. The emphasis of cutter selection is on selecting the right design and the right materials for the right formations, rather than the most expensive ones.

Performance and Formula Optimization of Graphene-Modified Tungsten Carbide Coating to Improve Adaptability to High-Speed Fluid Flow in Wellbore

In order to improve the erosion resistance of steel PDC (Polycrystalline Diamond Compact) bit under high-speed fluid flow conditions underground, it is necessary to develop a high-performance erosion-resistant coating. In this paper, laser cladding was used to prepare the new coating by modifying tungsten carbide with graphene. And the effects of tungsten carbide content and graphene content on the coating performance have been thoroughly studied and analyzed to obtain the optimal covering layer. The research results indicate that, for new coatings, 60% tungsten carbide and 0.3% graphene are the optimal ratios. After adding tungsten carbide, the hardness has significantly improved. However, when the tungsten carbide content further increases more than 30%, the increase in hardness is limited. In addition, when the content of graphene is more than 0.3%, the branched structure becomes thicker. In detail, this is a phenomenon where the segregation of Cr, Si, and W becomes very obvious again, and the segregation of Fe occurs at the Ni enrichment site. The research results contribute to the development and optimization of high-quality erosion-resistant coatings under the high-speed flow conditions in wellbore. These are of great significance for improving the efficiency of oil and gas exploration and development.

Research on slim-hole drilling technology for shale gas geological survey in China

Over the last 10 years, the China Geological Survey has deployed 137 slim-hole shale gas geological exploration wells for coring entire wellbores. These wells are primarily located in new blocks and geological formations where neighboring well data are insufficient, beyond the scope of developed oil fields in China, or outside of oil and gas company mining-right areas. The drilling rig equipment, coring tools, and core drill bits of slim-hole shale gas drilling technology are different from those associated with traditional petroleum drilling. Many studies have been conducted on non-coring slim-hole drilling technology. This paper focuses on coring technology and drilling safety, summarizing a set of high-efficiency shale gas drilling equipment and technology systems based on geological drilling equipment and techniques (that can be used for solid mineral exploration). We report on: 1) an improved vertical shaft drilling rig adapted to shale gas well control safety; 2) high-efficiency core drilling techniques, focusing on coring tools, and techniques incorporating an inverted tower drilling tool combination, air circulation follow-through technology, and expanded casing technology; 3) research progress on high-efficiency core drill bits, including non-planar tooth polycrystalline diamond compact bits and impregnated diamond core bits, along with their application effects. This research provides substantial advances in drill-core technology and improvements in exploration efficiency. Moreover, it provides a reference frame for well structural design and selection of construction technology for shale gas exploration drilling projects.

Study on super impact resistant polycrystalline diamond compacts with homogeneous PCD/WC-Co interlayer

With ultra-deep wells exceeding 10,000 m, the downhole drilling environment has become increasingly harsh and challenging and poses higher and more urgent requirements for the improvement of mechanical properties of polycrystalline diamond compact (PDC) cutters. In this work, four different types of polycrystalline diamond (PCD) structures were utilized with the purpose of exploiting both the advantages of homogeneous PCD/WC-Co composites in the impact toughness and the advantages of pure PCD composites in the wear resistance. Each PCD layer was characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and Raman spectroscopy. The indentation hardness, wear resistance and impact resistance of PDC cutters were evaluated using nanoindentation instrument, vertical turret lathe and progressive drop tester, respectively. The field trials of the developed PDC cutters in the ultra-well above 8000 m were also carried out. The experimental results demonstrated that PDC cutters with homogeneous PCD/WC-Co interlayer have excellent impact resistance while the pure PCD top layer ensures the cutter's hardness and wear resistance. Through observing the cracks in the diamond layer after mechanical performance testing, it was discovered that the homogeneous PCD/WC-Co interlayer has the capability of promoting the deflection of cracks on a macroscopic level. In addition, the inclusion of WC-Co alters the fracture mode of the PCD layer from transgranular fracture to mixed-mode fracture, which well explains the toughening mechanism of PCD/WC-Co cutter. This work provides valuable insights for the development of high-quality PDC cutters, and has important significance for the further design of efficient rock-cutting materials and comprehensive improvement of the drilling performance of PDC bits.

Numerical Investigations for Rock-Breaking Process and Cutter Layout Optimization of a PDC Drill Bit with Dual-Cutter

PDC (polycrystalline diamond compact) drill bits are widely employed for rock-breaking in many industries like underground engineering and building constructions. The cutter layout would directly affect the overall performance of the drill bits. Field applications show that the staggered cutter layout strategy of dual-cutter can increase the drilling efficiency of the PDC bit. In order to explore the rock breaking mechanism of this type of drill bit, a numerical model of a dual-cutter and rock breaking with damage evolution based on a hybrid finite and cohesive element method (FCEM) has been established in this work. The model is verified through Brazilian disk tests. The rock breaking processes of this type of bit have been analyzed, including crack initiation, propagation, and the formation of rock debris. Moreover, the effects of horizontal and vertical offset of the back cutter on the MSE (mechanical special energy) have been investigated. Results demonstrate that the dual-cutter can prominently reduce the MSE compared to a single-cutter. The vertical offset of the back cutter has a minor effect on the MSE, while the horizontal offset is of great significance on the MSE. On this basis, the relationships between the MSE and both the vertical and horizontal offset coefficients have been built based on the response surface methodology (RSM). Finally, an optimized layout solution, with optimal vertical and horizontal offset coefficients of 0.641 and 0.497, is determined via the Gray Wolf algorithm.

Numerical and analytical models of the mechanism of torque and axial load transmission in a shock absorber for drilling oil, gas and geothermal wells

The paper proposes an improved design of a shock absorber used in drilling deep oil and gas and geothermal wells with polycrystalline diamond compact (PDC) bits. The proposed innovations successfully combat the dangerous phenomenon of self-excited vibrations, which can lead to malfunctions such as stick-slip and whirling of the drilling tool. Most conventional drill shock absorbers are designed to only absorb longitudinal vibrations, which was sufficient when using roller cutter bits for the drilling process. However, the design features of PDC bits and the phenomenon of interaction of their cutters with interlayered rocks during deep drilling impose new requirements on the properties of the drill shock absorber. To protect the downhole tool from abnormal torque values and torque oscillations, it is proposed to equip the shock absorber with a special torque transmission unit in the form of a fourteen-thread self-releasing screw pair. This unit is capable of transforming increases in external torque into increases in the force that loads the elastic element of the shock absorber. The numerical and analytical models of the mechanism of transferring external axial load and torque to the elastic element of the drill shock absorber are constructed. The distribution of contact pressures on the interacting surfaces of the screw pair and the distribution of equivalent stresses in the screw pair parts are analysed. The strength of the proposed drill shock absorber assembly was evaluated using the Huber-von Mises energy criterion. The dependence of the load transmitted to the elastic element of the shock absorber on changes in the external torque and external axial force is investigated. In general, it is determined that the external load is distributed evenly between all turns of the screw pair, and the limit state of the parts of the proposed assembly is not reached even under high-torque operating load. The obtained analytical dependencies will allow to effectively determine the required strength and stiffness of the elastic element of the drill shock absorber at the design stage. The obtained analytical results were verified using a finite element model.

A novel experimental setup for axial-torsional coupled vibration impact-assisted PDC drill bit drilling.

The geological conditions of hot dry rock (HDR) reservoirs are complex. The geothermal mining of HDR faces major challenges in the drilling and construction of wells, fracturing to create storage, and flowing to extract heat. Vibration impacts help improve the rock-breaking efficiency, where the axial-torsional coupled vibration impact technology can increase the bit penetration depth and reduce the stick-slip effect. To study the feasibility and efficiency of the axial-torsional-coupled vibration impact-assisted Polycrystalline Diamond Compact (PDC) bit to break high-temperature and high-pressure rocks, a new experimental setup was designed. The system includes a drilling fluid circulation system, an axial-torsional coupled impact drilling system, a formation simulation system, and a data acquisition and control system. This setup can produce a rock-breaking torque of 2000 N·m, a drilling speed of 200rpm, a weight on bit of 100kN, an axial vibration frequency of 100Hz, and a torsional vibration frequency of 50Hz. It can simulate the formation pressure of 70 MPa and the rock temperature of 400 °C. A series of rock-breaking drilling experiments were successfully conducted using this setup. The results show that the axial-torsional coupled vibration-impact assisted PDC bit has a good performance in breaking high-temperature and hard rocks, which can accelerate the application of this new technology in deep formation drilling.

Failure Forensics of Shaped PDC Cutters Using Image Analysis and Deep Learning

Summary One of the major advances in polycrystalline diamond compact (PDC) bits in the last 10 years is the global adoption of 3D-shaped PDC cutters. By manipulating the cutter shape based on the understandings of cutter–rock interaction mechanisms, the cutting efficiency and mechanical properties of PDC cutters have been greatly improved. Ongoing innovations in 3D-shaped PDC cutter technology are critical to overcoming the more and more challenging formations in ultradeep wells, such as the 10 000-m-deep wells being drilled in China. Such an important role for 3D-shaped PDC cutters in oil and gas drilling applications necessitates a complete and effective failure analysis method. However, the current International Association of Drilling Contractors (IADC) dull grading cannot fulfill this objective. It is out of date in judging the damages to PDC bits and exhibits more limitations in addressing the unique challenges presented by complicated cutter shapes. To address this issue, an intelligent recognition model for PDC bit damage identification was developed based on the image analysis technology and the YOLOv7 algorithm. More than 10,000 dull bit images were used to train and validate this intelligent recognition model, which were collected from 363 PDC bits that suffered different degrees of damage after being used to drill 185 wells in the Sinopec Shengli Oilfield. Compared to the existing models, the developed intelligent recognition model has several notable contributions. First, the developed model is capable of recognizing the damages of various shaped PDC cutters commonly used by the global bit manufacturers, enabling a more accurate assessment of the failure behaviors of shaped cutters and their bits. The detection accuracy of the developed model exceeds 80% based on the confusion matrix. The recognition results by the developed artificial intelligence (AI) model are consistent with the actual failure modes judged by experienced drilling engineers. Second, the developed AI model provides direct qualitative identification of the failure modes and failure reasons for both cutters and PDC bits rather than the quantitative evaluation of the missing diamond layer used by IADC dull grading. Furthermore, the developed model eliminates the effect of reclaimed cutters on the AI detection results based on the implicit use of spatial cues in the YOLOv7 algorithm. The intelligent recognition model developed in this work can provide reliable and valuable guidance for the post-run evaluation, the bit selection for the next run, and the iterative optimization of bit design.

Dynamic characteristic response of PDC bit vibration coupled with drill string dynamics

A thorough understanding of the rock breaking process is key to increasing the rate of penetration and reducing bit wear. Currently, many models of drill bit–rock interaction have been developed. However, these models inaccurately reflect the complex movement of polycrystalline diamond compact (PDC) bits in the well. In this study, the finite element method was used to discretize and simulate the rock removal process, and drill string nodes were taken as the boundary conditions of drill bit movement. A model combining drill string dynamics and drill bit–rock interaction was established to describe the complex dynamic behavior of a PDC bit in a well. The accuracy of the model was verified by a PDC bit drilling experiment. Then, the influences of rock type, weight on bit (WOB), rotational speed, and PDC bit blade number on PDC bit vibration characteristics were studied. Results show that drill bit vibration is stronger when drilling in rocks with high hardness and roughness, and the bit runs more smoothly with more blades. Therefore, drill bits with more blades should be used when drilling hard rocks. Bit vibration is enhanced by increasing rotational speed and WOB, but the effect of rotational speed on bit vibration is much smaller than that of WOB.

Individualized Design and Field Application of Annular-Groove Polycrystalline Diamond Composite Bit

Summary Aiming at the problems of slow drilling speed, low service life, and poor stability of conventional polycrystalline diamond compact (PDC) bits in deep formations that are difficult to drill, a new annular-grooved PDC bit is proposed. The bit adopts discontinuous tooth arrangement to set cutters, and a circumferential annular groove around the center of the bit is set on the bit body to form a fragile convex annular ridge at the bottom of the well, improving the bit’s ability to invade the formation and its rock-breaking efficiency. Combined with the formation characteristics of Shapai 11 well and Shixi 102 well in Xinjiang Oil Field, the PDC bit crown shape, annular-groove design, cutter selection, and other aspects are designed individually, and the dynamic rock-breaking and hydraulic characteristics of the annular-groove PDC bit are simulated and analyzed. Finally, two PDC bits with different diameters are developed—a Φ215.9-mm annular-groove PDC bit and the field application bit. The field application bit results show that compared with the bit in the same layer of adjacent wells, the mechanical penetration rate of the annular-groove PDC bit is increased by 29.8–176.7%, and the footage is increased by 142.5–273.1%. It is concluded that the annular-groove PDC bit can significantly reduce the rock-breaking energy consumption of the bit and improve the mechanical rate of penetration (ROP) of the bit. At the same time, the raised annular ridge can reduce the lateral vibration of the bit and extend the service life of the bit, which will accelerate the exploration and development of deep difficult-to-drill formations. It is of positive significance to reduce drilling costs.

Mechanism Analysis and Mathematical Modeling of Brittle Failure in Rock Cutting with a Single Sharp Cylinder-Shaped PDC Cutter

Summary The drilling efficiency of a polycrystalline diamond compact (PDC) bit plays a vital role in oil and gas exploration, which is greatly affected by the rock-cutting performance of a single PDC cutter. Although many research efforts have been put in, the rock-cutting mechanism of a single PDC cutter is still indistinct. In this work, the rock-cutting process of a single sharp cylinder-shaped PDC cutter was captured using a high-speed camera. The brittle failure mode mechanism in the rock cutting of the PDC cutter was thus revealed by this real-time observation combined with the findings in previous publications. The brittle rock-cutting failure zones in front of the cutter were separated into three different zones: crushing zone, plastic flow zone, and rock chipping zone. The crushing zone grew while the cutter cut forward and generated a plastic flow zone. When the crushing zone was large enough, a tensile crack would tear apart the rock, forming the rock chip. Based on this rock-cutting mechanism, a new mathematical model of brittle failure in rock cutting of PDC cutter was developed, considering the rock properties and cutting parameters. The boundary geometry of the crushing zone was calculated using elastoplastic theory and the Mohr-Coulomb criterion. All forces on the boundaries of these three failure zones were calculated and combined into the tangential and normal forces in the 3D mathematical model. Furthermore, a new parameter, named as crescent area, was proposed in the mathematical model. When compared to previous publications, the newly developed mathematical model had no variables that needed to be calibrated with experimental data fitting. Moreover, a series of single PDC cutter cutting tests were carried out at various depths of cut (DOCs) and backrake angles to validate the mathematical model. The results showed that the model-predicted forces basically matched the experimental data. The modeling and experimental results shared the same trend for both tangential and normal cutting forces. The experimental phenomena could be well explained by the developed mathematical model. For example, the cutting forces increase with increasing DOC and backrake angle, which is caused by the changing of the crescent area of the rock-cutter interaction. All resultant forces have almost the same inclination angle to the horizontal plane because of the almost constant boundary shape of the crushing zone. The differences between modeling and experimental results could be attributed to several reasons, one of which was the oversimplified plastic flow zone. This work presents a mathematical model that can guide the PDC bit design at different formation properties.

A novel perspective to study rock failure characteristics during single PDC cutter tests: Based on the combined cutting and penetration

Polycrystalline diamond compact (PDC) cutter tests have been extensively conducted to shed light on the rock failure mechanism, thus further enhancing the performance of PDC bits. However, most of these studies primarily focus on the cutting process. Specifically, the cutting depth is pre-set rather than calculated based on rock penetration process. In the present paper, a physical model is developed to examine the combined effects of cutting and penetration on rock breaking. Experimental and theoretical analyses are also performed to study the individual impact of cutting and penetration. Based on rock failure characteristics under varying back rake angles and penetration forces, cutting parameters are recommended. The findings indicate that the back rake angle should not exceed 40° to prevent cutter breakage. Increasing the back rake angle simultaneously improves penetration efficiency and limit cutting efficiency. For rocks with high strength and it is challenge to load the weight on bit (WOB), a back rake angle of 20–30° is advised for efficient cutter penetration. However, the smaller the back rake angles, the more obvious the enhancement in penetration depth and rock-breaking volume with the further increase of penetration forces. At an equivalent cutting depth, larger back rake angles yield higher rates of penetration (ROP), while smaller angles enhance drilling efficiency. The determination of drilling parameters should further depend on field requirements. The present study provides an in-depth understanding of the rock failure process and the arrangement of PDC bits.

Optimization of functionally graded polycrystalline diamond compact based on residual stress: Numerical simulation and experimental verification

Polycrystalline diamond compact (PDC) is composed of a polycrystalline diamond layer with a tungsten carbide substrate, which is commonly used on drill bits. PDC bits are widely used in geological and petroleum drilling. There are frequently issues with failure, such as chipping of teeth during drilling, attributed to the residual stress within the PDC. Residual stresses arise from the difference in material properties between the polycrystalline diamond layer and the tungsten carbide substrate after sintering and cooling. To address the residual stresses inside the PDC, this research work optimized the design of a PDC with a functionally graded structure is reported in this paper. The effect of gradient structural parameters on the residual stresses of the PDC is analyzed using the finite element method. The results show that as the thickness of the gradient structural layer increases, the maximum residual tensile stress gradually decreases and stabilizes. Since the total thickness of the gradient structural layer is fixed, increasing the layer number (decreasing the thickness of the single layer) cannot reduce the maximum tensile stress, but it can minimize the sudden change of axial stress across the interface, thus resulting in less interface cracking. The optimum gradient structure design parameters are obtained and the optimal graded PDC is prepared via fused deposition modeling and sintering technology, resulting in a 62% reduction in maximum shear stress, a 70% decrease in maximum axial stress, and a 40% drop in maximum radial stress compared to conventional PDC. The accuracy of the finite element calculations is demonstrated by the Raman spectroscopy results. This work will provide a reliable guide for the design of functional graded structure PDCs and lay the foundation for their preparation and application.

Selection and Optimization Design of PDC Bits Based on FEM Analysis for Drilling Long Horizontal Sections of Shale Formations

Well structures with ultra-long sections have become one of the most applied technologies in the field of shale gas development. While there have been many technical challenges, enhancing the breaking efficiency and stability of polycrystalline diamond compact (PDC) bits has become an essential issue of focus. Since 2013, the well structure in the Duvernay area has been optimized multiple times, and the rate of penetration (ROP) of the entire wellbore has nearly doubled. However, there are significant differences in terms of the performances of different PDC bits, and there is still room for improvement to optimize these drill bits. For this reason, a confined compressive strength test was conducted to obtain the rock mechanical parameters from shale cores extracted from the long horizontal section. Using these data, a finite element model (FEM) was developed with a corresponding scale. A calibration of the elastic-plastic damage constitutive models was then performed using the FEM. The breaking mechanism of three different PDC bits was examined using a “PDC bit-bottom hole” interaction FEM model, facilitating guidance for bit selection and design optimization: (1) The type B PDC bit, which has four blades and 20 cutters, exhibited the highest mechanical specific energy (MSE) and the lowest vibration across three directional mechanical characteristics. This design is recommended for engineering applications. (2) Lower axial vibrations were produced when the CDE was used as the rear element when compared to those when using the BHE. However, an increase within an acceptable range was observed in the TOB and circumferential vibrations. Thus, for redesigning work on the type B bit, the assembly of the CDE is suggested. (3) A decrease in the MSE and vibration in three directional mechanical characteristics was observed when the depth of cut (DOC) was varied between 1.5 and 2.0 mm. A broadening in the range of lateral forces was noted when a DOC of 2.0 mm was used. Therefore, for the redesign of the type B bit, the assembly of CDEs as rear elements at a DOC of 1.5 mm is recommended. In conclusion, a new practical method for the selection and optimization of PDC bit design, based on rock mechanics and the FEM theory, is proposed.