Polycrystalline Diamond Compact Cutters Research Articles

Multi-objective optimization of Stinger PDC cutter breaking rock parameters under intrusion-cutting action using Taguchi-based gray relational analysis.

To enhance the efficiency of the Stinger Polycrystalline Diamond Compact (PDC) cutter in breaking hard rocks, this study focuses on optimizing the cutter intrusion-cutting rock breaking parameters. A numerical calculation model for the rotational breaking of granite by a Stinger PDC cutter was established. A comprehensive statistical examination was performed to assess the influence of various factors on intrusion ability (IA), tangential force (TF), and mechanical specific energy (MSE). The Taguchi method was used to determine the optimal settings for each factor, while analysis of variance was employed to assess the significance and relative impact of these factors on the target outcomes. In addition, the multi-objective function was optimized using the gray relational analysis method. The primary process parameters obtained for the various performance characteristics are the cone top angle (α), the cone top radius (r), the cutter diameter (d), the cutter back inclination angle (β), and weight on bit (P). The impact ratios of these parameters are 6.20%, 7.66%, 3.93%, 17.20%, and 65.02%, respectively. The optimal geometrical parameters are α = 60°, r = 2mm, and d = 15mm, while the optimal working parameters are β = 30° and P = 800 N. In the optimal case, IA and MSE were reduced by 55.335% and 15.809%, respectively, compared to the initial case. Despite a 15.706% increase in TF, the overall GRG increased for all three evaluation criteria, with an overall increase in efficiency of 18.229%. The results of this paper can provide guidance for the design of Stinger cutter PDC drill bits.

The Rock-breaking Performance of the Special-shaped PDC cutter under Combined penetration and Cutting

The utilization of Special-shaped Polycrystalline Diamond Compact (SPDC) cutters improves the performance of PDC cutters and extends their lifespan. Researchers have extensively explored the rock-cutting mechanism of SPDC cutters. However, most existing studies view SPDC cutter cutting and penetration processes separately, resulting in the cutting depth not dynamically changing during the cutting process, which deviates from the actual drilling process. This study establishes a discrete element-based numerical model to simulate the fragmentation of heterogeneous granite using SPDC cutters. It comprehensively analyzes the rock-breaking characteristics of the SPDC cutters under combined penetration and cutting from multiple perspectives. The results show that for granite, the Stinger cutter (CS) has the best penetration effect, followed by the Axe-shaped cutter (AX), Cylindrical cutter (CY), and Three-blade cutter (TB) in descending order. Under combined penetration and cutting, CY demonstrates the highest comprehensive rock-breaking efficiency, followed by AX, TB, and CS. Under straight cutting, the rock primarily fails in tensile, and the cutting depth of each cutter linearly increases with the cutting distance. The expansion extent of cracks is a key factor affecting rock-breaking efficiency, with cracks tending to expand vertically, and the cutter shape has little impact on it. The rock debris predominantly consists of small-volume fragments, particularly noticeable with CS. When crushing granite, AX is suitable for working under a WOB of 1 kN, CY is recommended for 2 kN, TB is suitable for 3 kN, and CS achieves the best rock-breaking efficiency at 4 kN. The study provides new insights into the selection of PDC drill bits and the optimization of PDC cutter arrangements.

Investigation on rock-breaking characteristics of hard sandstone by non-planar PDC cutters under high confining pressure

Due to high hardness, strong heterogeneity, and low drillability in deep formations, conventional Polycrystalline Diamond Compact (PDC) bits with planar cutters usually get bit worn, short drilling footage, and low rate of penetration (ROP) when drilling into such formations. To improve the aggression and the impact resistance of PDC bit, a series of non-planar PDC cutters have been proposed and developed by scholars and oil & gas industries. However, the in-situ stress and hydrostatic pressure significantly weaken the rock-breaking performance of PDC cutters, which is a paucity in related studies. Consequently, this paper conducts rock-cutting experiments in hard sandstones by planar cutters and three types of ridged cutters under confining pressure and atmospheric conditions. The influences of cutting parameters on the rock-breaking performances of different cutters are investigated, focused on the changes in cutting force, mechanical specific energy, and rock cuttings distribution. Additionally, visual cutting experiments are conducted to compare the differences in the rock-breaking mechanisms of different cutters. The results indicate that in-situ stress and hydrostatic pressure exert a noticeable inhibitory effect on rock fragmentation, resulting in smaller cuttings and lower rock-breaking efficiency than those under atmospheric conditions. Furthermore, compared to the three types of ridge cutters, planar cutters exhibit a lower rock-breaking efficiency. Ridged cutters, which leverage the efficient stress concentration caused by their ridges, demonstrate a higher rock-breaking performance by effective and efficient rock failure near the ridges. Additionally, under different cutting parameter conditions, each cutter have a corresponding optimal rock-breaking efficiency range. Specifically, for the triple-ridged concave cutters, the highest rock-breaking efficiency is achieved when the cutting angle is approaching 20°. As the cutting angle increases, the axe-shaped cutter exhibits the highest rock-breaking efficiency. In addition, the triple-ridged concave cutter and the axe-shaped cutter possess unique rock-breaking advantages, with the former being more stable and the latter producing larger volumes of cuttings. The selection of appropriate cutter parameters can enhance the service life and the drilling speed of PDC bits. The experimental results and optimization charts have significant implications for selection on cutter shapes and the design of hybrid-cutter parameters in deep hard formation drilling.

Numerical investigation of rock-breaking mechanisms and influencing factors of different PDC cutters during rotary percussion drilling

While drilling deep or ultra-deep wells in hard rock formations, there are significant problems such as easy wear of drilling cutters and a low rate of penetration (ROP), and rotary percussion drilling technology is one of the essential techniques for improving drilling efficiency. However, there is currently limited research on the compatibility of rotary percussion drilling tools, polycrystalline diamond compact (PDC) cutters, and hard rock formations. Considering the motion mechanism of rotary percussion drilling tools, a three-dimensional rock-breaking numerical model was established for different cutters (planar, axe-shaped, and triple-ridged) under impact loads. Penetration depths, cutting widths, and rock-breaking volumes of different cutters under static and dynamic load coupling were analyzed. Changes in tensile, compressive, and shear stresses during the cutter rock-breaking process were compared, revealing the different cutter rock-breaking mechanisms under impact loads. The cutter rock-breaking laws under different impact amplitudes and frequencies were analyzed quantitatively, and the differences in rock-breaking effects of different cutters under various geological conditions were assessed. The results indicated that cutter penetration depth during rotary percussion drilling could be increased by 16.04% compared to that during conventional drilling. Under the same impact load, the rock-breaking effects of different cutters on hard and soft rocks exhibited similar variation patterns. The penetration depth followed the order axe-shaped > triple-ridged > planar cutters, with an average tangential order of triple-ridged < axe-shaped < planar cutters. When drilling through hard rock, the average penetration depths of planar, axe-shaped, and triple-ridged cutters were 5.57, 6.06, and 3.91 mm, respectively. The average tangential forces were 670.57, 541.76, and 347.89 N, respectively. During the transition from soft to hard rock during drilling, the tangential forces of the planar, axe-shaped, and triple-ridged cutters increased by 8.41%, 3.21%, and 2.64%, respectively. The planar cutter experienced the most significant instantaneous tangential-force change and was more prone to impact damage. Increasing the amplitude of the axial stress waves helps enhance the penetration depth of the drill bit, whereas increasing the frequency of the stress waves can reduce the intensity of fluctuations during drill-bit penetration and increase drill-bit stability. The research results provide insights into optimizing drilling cutters and engineering parameters during rotary percussion drilling.

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.

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.

Study on Composite Rock-Breaking Mechanism of Ultrahigh-Pressure Water Jet–PDC Cutter

Summary The drilling industry is paying increasing attention to deep and ultradeep wells because of the gradual decline and depletion of recoverable resources on the shallow surface. However, the difficulty of conventional mechanical rock-breaking grows significantly with increasing drilling depth. It has been found that the effect of a high-pressure water jet combined with a polycrystalline diamond compact (PDC) cutter is significant and can greatly increase the efficacy of rock breaking. A composite rock-breaking experimental device with a high-pressure jet was designed to carry out composite rock-breaking experiments. Meanwhile, a composite rock-breaking numerical model of high-pressure water jet-PDC cutter was created by smoothed particle hydrodynamics/finite element method (SPH/FEM). After verifying the reliability of the numerical model through experiments, the key factors, including rock stress field, cutting force, and jet field, were extracted to analyze the composite rock-breaking mechanism. The results show that the enhancing effect of jet impact on rock breaking is mainly reflected in three aspects: (1) The high-pressure water jet can create a groove and crater on the rock surface, effectively unloading the rock stress at the bottom of the well and increasing the area of rock damage; (2) PDC cutter vibration can be efficiently reduced with high-pressure jet; and (3) the rock debris in front of the cutter is cleaned in time, avoiding the waste of energy caused by the secondary cutting and reducing the temperature rise of the PDC cutter. Besides, it has been investigated how parameters like jet pressure, nozzle diameter, impact distance, and cutting depth influence the effect of jet rock breaking. The findings indicate that the best rock-breaking efficiency and economy occur at jet pressures of 30–40 MPa. Correspondingly, in terms of nozzle angle, nozzle diameter, and impact distance, the ideal ranges are 60°, 1.0–1.5 mm, and 10 mm, respectively, wherein the ideal impact distance is approximately 10 times the nozzle diameter. This research is critical for the advancement of high-pressure jet drilling technology and the design of supporting drill bits.

Dynamic response and damage behavior of impact wear for polycrystalline diamond compact under low kinetic energy impact

Polycrystalline diamond compact (PDC) cutters are the main load-bearing components in shale gas drilling, and their performance significantly affects the efficiency of shale gas extraction. One of the primary reasons for cutter failure is the dynamic impact between the rock and the cutter due to the complex geological conditions of shale gas extraction. Investigating the impact wear behavior and damage mechanisms is vital to designing superior-performing PDC structures and improving their effectiveness in engineering applications. This work investigates the kinetic response and damage behavior of PDCs under low kinetic energy impacts to explain the characteristics and mechanisms of PDC impact wear. The results show that the impact force fluctuates with impact owing to changes in the interface state. Owing to the elastic-plastic properties of PDC, 87 % of the kinetic energy is used for plastic deformation and fracture. Secondly, the contact time and kinetic energy absorption rate only relate to the impact mass. The contact time positively correlates with mass, while the kinetic energy absorption rate changes in the opposite direction. In addition, diamond fracture occurs with a low kinetic energy accumulation of only 0.059 J. Cobalt catalysis and stress effects were found to induce graphitization and carbon rehybridization jointly during impact wear. It is concluded that the PDC impact wear mechanism under low kinetic energy impact is a brittle fracture with a combination of transcrystalline fracture and intergranular fracture. This work provides referable basic research data for designing impact-wear resistant PDC structures and improving the efficiency of shale gas extraction.

Investigation on the rock-breaking performance of specially-shaped PDC cutter under confining pressure conditions

Cutter shape has a significant influence on the rock-breaking performance of drill bit. Compared with conventional Polycrystalline Diamond Compact (PDC) cutter, the emergence of specially-shaped PDC cutters (SSPCs) with different geometric shapes is of great benefit to solve the problems of low rate of penetration (ROP) and rock-breaking efficiency in deep hard and hot dry rock formations. However, the high confining pressure and high in-situ stress (HCHS) have a significant influence on the rock-breaking performance of PDC cutter which cannot be ignored. Given that the above facts, this paper selects five kinds of PDC cutters with different cutter shapes, and implements a series of cutting experiments of granite under confining pressure conditions (CPCs) and atmospheric pressure condition (APC) using a true triaxial rock cutting device. The influence of cutter profile on the rock-breaking performance of PDC cutter was studied from the perspective of cutting force, mechanical specific energy (MSE), cuttings, coarseness index (CI) and fractal dimension. The results show that the rock breaking under CPC is more difficult than that under APC, resulting in the increased cutting force and MSE. Moreover, the cuttings obtained under CPC are smaller in size and the cutting grooves are smoother. Cutter shape has a great influence on the rock-breaking efficiency of PDC cutter, the rock-breaking efficiency of axe-shaped cutter is higher, while that of stinger cutter is lower. Moreover, PDC cutters with different shapes have different sensitivities to confining pressure, and stinger cutter has the lowest sensitivity to confining pressure, in other words, confining pressure has little influence on the rock breaking process of stinger cutter. What is more, the distribution of cuttings is characterized by the fractal dimension and particle size distribution parameter from the perspective of cuttings morphology and size respectively, and they present completely opposite changing trends. The obtained results are of great significance to the optimization of cutter profile, the design of bit cutter arrangement and the drilling of deep formation.

Numerical simulation study of PDC cutter rock breaking based on CT scanning to construct a rock model and apply a cohesive element to construct a discretization element

To more accurately simulate the interaction between a polycrystalline diamond compact (PDC) cutter and rock, the structural characteristics of basalt were analysed using computed tomography (CT), and a hard particle–matrix–pore three-phase rock model was established due to its structural characteristics. Three-dimensional models of the interaction between PDC cutters and rock were established by using the finite–discrete element method (FDEM). Using Voronoi diagrams to randomize the grid structure and optimize the propagation of cracks. This study revealed that a broken zone near the cutters will form during the rock breaking process, resulting in the affected area being larger than the area in contact with the formation rock. The rock breaking ability of circular cutters is stronger than that of conical cutters. When a conical cutter breaks hard particles, they are mainly striped or forced away from the rock, but circular cutters can divide hard particles into small pieces. The numerical model used in the research institute can effectively calculation results of the process of interaction between PDC cutter and rock, this proves that the application of the cohesive method to simulate rock breaking by PDC cutters is reliable.

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.

Optimizing rock breaking performance: The influence of chamfer on polycrystalline diamond compact (PDC) cutters

Research on the rock-breaking performance of the Polycrystalline Diamond Compact (PDC) cutter has primarily focused on sharp cutters, often overlooking the influence of chamfer. Notably, the design of chamfer parameters has been largely unreported. In this study, we established a theoretical model of cutting force that takes chamfer into account. We analysed the primary and secondary relationships of four factors – back rake angle, depth of cut, chamfer angle, and chamfer length – on the force of the PDC cutter. This was done through a pseudo-level orthogonal level test. A numerical simulation, based on the Smooth Particle Hydrodynamic (SPH) method, was conducted to analyse the rock-breaking force and stress distribution characteristics of PDC cutters with different chamfer angles. Combined with a drop hammer impact test, we provided an optimized design of chamfer parameters. Our findings revealed that while the chamfer had a relatively minor influence on the force of the PDC cutter, it contributed to the optimal distribution of stress on the PDC cutter. This effectively protected the cutting edge and prevented early cracks and spalls of the cutter. When the chamfer angle was less than or equal to the back rake angle, the resultant force of the PDC cutter increased with the increase of the chamfer angle. However, when the chamfer angle was greater than the back rake angle, the resultant force of the PDC cutter first increased and then slightly decreased with the increase of the chamfer angle. Additionally, the resultant force of the PDC cutter increased approximately linearly with the increase of chamfer length. When the chamfer angle of the PDC cutter was between 30° and 45°, the fluctuation of the cutting force was relatively smooth, the rock-breaking process was stable, and the cutter’s impact resistance energy was relatively higher. These findings will provide valuable guidelines for the design of chamfered PDC cutters.

Structure Design of Bionic PDC Cutter and the Characteristics of Rock Breaking Processes

The rational structural design of polycrystalline diamond compact (PDC) cutters effectively enhances the performance of drill bits in rock fragmentation and extends their service life. Inspired by bionics, a bionic PDC cutter was designed, taking the mole claw toe, shark tooth, and microscopic biomaterial structures as the bionic prototypes. To verify its rock-breaking effectiveness, the finite element method was employed to compare the rock-breaking processes of the bionic cutter, triangular prism cutter, and axe cutter. The study also investigated the influence of different back rake angles, cutting depths, arc radii, and hydrostatic pressures on rock breaking using the bionic cutter. Prior to this, the accuracy of the finite element model was validated through laboratory tests. Subsequently, a drill bit incorporating all three types of cutters was constructed, and simulations of rock breaking were conducted on a full-sized drill bit. The results demonstrate that the bionic cutter exhibits superior load concentration on the rock compared to the triangular prism cutter and the axe cutter. Additionally, its arc structure facilitates the “shoveling” of the rock, making it more susceptible to breakage under tensile stress. As a result, the efficiency of the bionic cutter surpasses that of the triangular prism and axe cutters. Similarly, it exhibits minimal fluctuations and values in cutting force. As the back rake angle and cutting depth increase, the MSE and cutting force of all three cutters also increase. However, the bionic cutter consistently maintains the lowest MSE and cutting force, confirming the superiority of its bionic structural design. The MSE and cutting force of the bionic cutter fluctuate with the increase of the arc radius, and the optimal arc radius falls within the simulation range, between 21 mm and 23 mm. Compared to the other two types of cutters, bionic cutters possess a unique structure that allows for better release of internal stress within the rock, thereby ensuring higher efficiency in rock-breaking, particularly in deep geological formations. The rock breaking simulation results of full-sized drill bits show that the use of a bionic cutter can improve the drill bit’s ability to penetrate the formation, reduce the possibility of drill bit bounce during the rock breaking process, prevent the occurrence of stick-slip, improve the drilling stability, effectively improve the efficiency and service life of the drill bit during the rock breaking process, and reduce the drilling cost. It is concluded that the research results of bionic PDC cutters are helpful to the development of high-performance drill bits and the reduction of drilling costs.

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.

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.

Evaluation of the Wear Resistance and Electrochemical Behavior of Drill Bits for Drilling Oil and Gas Wells

This work aims to study the relation between the hardness and behavioral wear of tungsten carbide inserts (TCI) and polycrystalline diamond compact (PDC) cutters, used as drill bits for drilling oil and gas wells, through wear testing conducted using the pin-on-disc test and the measure of microhardness Knoop test. The morphology of the worn surface was characterized using scanning electron microscopy (SEM), and the chemical composition was taken before and after the wear test inside the tracks using energy-dispersive X-ray spectrometry (EDX). Additionally, the behavior of the corrosion resistance was evaluated with the electrodynamic polarization technique and was compared with a coating of diamond-like carbon (DLC) in order to determine whether this coating applied to tool steel could serve as a substitute for insert-based coatings in future applications, as its cost is lower than that of the inserts. A paragraph discusses what characteristics must be taken when selecting or manufacturing materials for drill bit applications. In conclusion, it was found that the TCI insert presented cracks in the test tracks, while the PDC insert showed soft wear; this was due to the uncertain hardness, which was greater than the indenter used. Concerning the Tafel test, the curves showed that the PDC insert presents better behavior under corrosion; however, the DLC can be a good substitute.

Experimental investigation of the rock cutting process and derivation of the 3D spatial distribution of the formation strength using in-cutter sensing

Recent advances in downhole measurements allow to accurately measure drilling forces at the drill bit, and estimate the evolution of the rock strength along the well profile. This paper presents an experimental methodology that allows to measure drilling forces, at the cutter scale, with a sensor embedded behind a polycrystalline diamond compact (PDC) cutter, and to infer the 3D spatial distribution of the rock strength. Two experimental campaigns have been performed on a laboratory-scaled drilling rig and complemented with standard mechanical tests to validate rock strength estimations. In the first campaign, homogeneous synthetic rock samples have been prepared. The average rock strength of each sample derived from cutter force measurements and a cutter–rock interaction model, compares well with the one derived from mechanical tests. In the second campaign, heterogeneous synthetic rock samples have been prepared. They are made of two layers of gypsum mixtures of different strengths, separated by a slanted bedding plane. Based on the instrumented cutter measurements, the 3D spatial distribution of the rock strength has been reconstructed along its path. Rock strength estimations are consistent with results obtained from mechanical tests, and the reconstructed geometry of the bedding plane matches well its actual geometry. The experimental methodology and technology presented in this paper lay the foundations for estimating rock properties in 3D, at the drilling stage. It has the potential to provide geoscientists information about complex lithological structures at an early stage, reducing the need for expensive and time-consuming coring and logging operations.

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.

Experimental study on drilling of soft rocks

Drilling is commonly occurring material deformation process in vast range of industries including manufacturing, mining, petroleum, infrastructure, biomedical etc. Due to its three-dimentional nature and multi-axial deformation of material, drilling is a very complex process involving severe plastic deformation of material, high strain and strain rates, volume change, tempretarure gradient etc. In addition to this, drilling in rocks becomes further more complicated due to additional complexity added by brittle fracture. Therefore, it becomes important to have a complete understanding of the process and the effect of drilling paramerets on the material response in order to make it more energy efficient and improve the productivity in industrial applications.&#x0D; An attempt to understand the drilling in metals has been made by using a simplistic approach of decoupling the process into simpler process where, the penetration of the drill into the material and the rotatory motion has been correlated to indentation and cutting, respectively [1, 3, 8]. Similar framework has been adopted for rocks by many researchers where drill bits consist of many polycrystalline diamond compact cutters mounted on a tingesten carbide matrix [2, 5, 7]. In order to understand the interaction of these bits with rock, simplified models have been proposed for the cutters mounted on these bits [6]. This research work intends to understand the drilling mechanism experimentally by comparing drilling response (thrust force and torque) with the findings of intentation and cutting studies. Also, the force response (thrust and torque) were compared with the results obtained from the model.&#x0D; Drilling experiments were performed on samples prepared using gypsum, which is an ideal model material for soft rocks [4, 9]. Sample porosity was controlled by controlling the water content during casting of the samples (46 mm x 46 mm x 20 mm). Twist drills with helix angle of 300 and 600 were used. The diameter of the drill (6 mm) was kept significantly smaller (~ 8 times) than the width of the sample. The tip angle was kept constant (1200) for both the drills. The penetration rate was varied from 1 – 100 mm/min at constant rotation speed of 100 rpm. The penetration rate in combination with the rotation speed can be seen as depth of cut per revolution of the drill. The thrust force and torque were normalized with respect to the drill radius to compare the signatures with indentation [10, 11] and cutting [12] experiment results, respectively.&#x0D; On comparing normalized thrust and torque signatures with indentation and cutting results, a remarkable similarity was observed. Figure 1 and 2 show normalized thrust and normalized torque, respectively, as a function of depth of cut per revolution (penetration rate/ rotation speed). Similar to indentation results, Fig.1a shows that the thrust force has a strong dependence on material porosity but remained independent of drill geometry (Fig. 1b). This indicated that the drill geometry (which is responsible for cutting the material) does not influence the indentation part (vertical penetration). In contrast to this, the normalized torque showed dependence on both, drill geometry and porosity (Fig. 2) which was evident from studies performed gypsum cutting [12]. Also, the torque signatures were remarkably similar to cutting forces and cyclic peaks were observed which indicated cyclic removal of the chips during cutting. The drilling results were compared with the theoretical model as well [6]. However, results corresponding to the 300 helix angle drill did not align with the model predintions. Suitable modifications in the model was suggested. Based on some preliminary experiments performed by imaging the drilling process, it was believed that there exists an optimum helix angle for which the model presdintions hold good.&#x0D; This study shows experimentally that drilling, which is a complex 3 dimentional process, can be studied by decoupling it into simpler 2 dimentional processes: cutting and indentation. The similarity between the experimental findings of cutting, indentation and drilling suggest that the findings of cutting and indentation studies can be clubbed together and can be used to understand drilling better.