Diamond Compact Bits Research Articles

Mechanism and Data Fusion Method for Predicting Wear and Life of Polycrystalline Diamond Compact Bits Based on the Gaussian Process Regression

Mechanism and Data Fusion Method for Predicting Wear and Life of Polycrystalline Diamond Compact Bits Based on the Gaussian Process Regression

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.

Uniaxial compressive strength prediction based on measurement while drilling data: A laboratory study

The uniaxial compressive strength is one of the most important basic parameters of rock, which is essential for surrounding rock stability analysis and support scheme design of underground engineering. At present, it is time consuming and costly to take a large amount of core samples for laboratory testing, and the mechanical properties of the cores may be affected by mining disturbance, which can easily lead to inaccurate result. The measurement while drilling (MWD) technology provides a new approach to solve the above challenge. The key to implementing this technology is to establish a correlation model between drilling parameters and rock mechanics parameters. Based on the characteristics of polycrystalline diamond compact (PDC) bits in drilling and rock breakage, this paper analyzes the mechanical state of the bit in breaking rock. A theoretical correlation model between the torque, feed force of the bit and the uniaxial compressive strength of the rock has been developed. To verify the accuracy of the theoretical model, the uniaxial compressive strength of five different types of rocks (red sandstone, green sandstone, limestone, marble and shale) was obtained through laboratory mechanical tests. The torque Mb, feed force Fb and other parameters in the drilling process of these five rocks were tested through the newly developed MWD test system. The correlation between the drilling parameters and the uniaxial compressive strength of rock was established. The results showed that the feed force Fb and torque Mb measured at five different types of rocks indicate a linear increasing trend with an increase in depth of cut h. Meanwhile, a strong linear relationship between the feed force Fb and torque Mb is evident. This paper proposes an MWD-based method to rapidly conduct the in-situ measurement of the uniaxial compressive strength of various rocks.

Drilling performance analysis of a polycrystalline diamond compact bit via finite element and experimental investigations

The significance of improving the drilling productivity and reducing the cost and non-productive time of drilling process, substantially relies on the efficiency of drilling performance. This paper provides a comprehensive understanding of drilling process, aiming to predict drilling performance and investigate drilling parameters using a validated finite element (FE) model. Experimental validation of the FE model was achieved through testing on a laboratory drilling rig, ensuring the accuracy and reliability of the numerical simulations. To accurately capture the nonlinear characteristics of bit-rock interaction, the Riedel–Hiermaier–Thoma model was adopted as a material model, and its parameters were identified through a series of carefully conducted experimental tests. The numerical results obtained from the FE rock failure model during the compressive and tensile tests demonstrated a robust correlation with the experimental data. The verified material model was then employed into another FE drilling model to simulate rock breaking in an actual drilling scenario. This analysis sheds light on the impact of drill-bit interaction with the rock formation, providing valuable insights into its behaviour during drilling operations. The FE drilling model was further utilised in a parametric study to predict the effects of critical drilling parameters, like loading rate and rotary speed, on the weight on the bit, torque on the bit, and rate of penetration. Both the FE drilling and experimental results provided a significant consistency when the drilling parameters were compared, and nonlinear dynamic phenomena, such as stick–slip and bit-bouncing, were observed. By investigating these effects, this study contributes to optimising drilling operations, enabling better control of premature vibrations and enhancing drilling efficiency.

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.

Research on the Temperature Field Distribution Characteristics of Bottomhole PDC Bits during the Efficient Development of Unconventional Oil and Gas in Long Horizontal Wells

Unconventional tight oil and gas resources, including shale oil and gas, have become the main focus for increasing reserves and production. The safe and efficient development of unconventional oil and gas is a crucial demand for the energy development strategy. Deep tight oil and gas resource development generally adopts horizontal well drilling methods. During drilling, especially in long horizontal sections, the high temperature frequently causes failures of downhole drilling tools and rotary steering tools. The temperature rises sharply during rock breaking with the drill bit. Existing wellbore heat transfer models do not fully consider the impact of heat generated by the drill bit on the wellbore temperature field. This paper aims to experimentally study the temperature rise law of the cutting tooth of the bottom polycrystalline diamond compact (PDC) bit during rock breaking. A set of evaluation devices was developed to study the temperature field distribution characteristics at the bottom of the PDC bit during rock breaking under different experimental conditions. The results indicate that the flow rate of drilling fluid, bit rotation speed, and weight on bit (WOB) significantly affect the distribution of the temperature field at the well bottom. This experimental research on the temperature field distribution characteristics at the bottom of the PDC bit during rock breaking helps reveal the heat transfer characteristics of the long horizontal section wellbore, guide the optimization of drilling parameters, and develop temperature control methods. It is of great significance for the advancement of efficient development technologies for unconventional resources in long horizontal wells.

Evaluating PDC bit-rock interaction models to investigate torsional vibrations in geothermal drilling

Polycrystalline diamond compact (PDC) bits are superior for drilling geothermal wells because of their superior drilling performance compared to conventional roller cone bits. However, the shear action of PDC bits generates detrimental vibrations during drilling. The main objective of this study was to establish a methodology to analyze and predict the stick-slip severity in hard rocks for geothermal wells. Two non-linear coupled axial-torsional bit-rock interaction (BRI) models are presented: one is based on a velocity-decaying friction model (VDF), and the other is based on a state-dependent delay friction model (SDDF). The capabilities of the two models were evaluated to assess the axial and torsional dynamic stabilities of drill stems in deep geothermal wells. The comparative analysis, along with the results from both models, were validated using geothermal well downhole data. Five distinct zones were selected for analysis, and the stick-slip severity value (SSV) was calculated using these two models (VDF and SDDF). The results from these two models for the five different zones were compared with the field data. The results indicated that VDF demonstrated superior quality when compared with field values, as the results of VDF were within the interquartile range of the observed SSV in each zone. A sensitivity analysis employing spider plots was performed for both models, considering parameters related to rock, bit, operational, and frictional aspects. In terms of the operational parameters, the weight-on-bit (WOB) and revolutions per minute (RPM) exerted the most significant influence on the SSV for both models. For the VDF model, the sensitivity analysis indicated that the frictional parameter, uniaxial compressive strength (UCS), and number of cutters (NOC) had the most pronounced impact on the SSV. In the case of the SDDF, the Intrinsic specific energy (ISE), bit diameter, and number of blades (NOB) are the key factors that predominantly affect the SSV.

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.

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.

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.

Real-time automatic identification methods for downhole whirling based on mechanical specific energy model of drill bit

In the drilling exploitation of hot dry rock for geothermal energy, whirling is one of the main low-efficiency conditions that affect the efficiency of Polycrystalline Diamond Compact (PDC) bit in horizontal well drilling. Realizing automatic and real-time identification with whirling is of great significance to save non-productive time and ensure drilling safety and benefit. In this paper, we first constructed a real-time drilling mechanical specific energy (MSE) model combined with while-drilling testing to reflect the real-time drilling conditions. The MSE model is used to normalize multi-source parameters. Secondly, we constructed a Back Propagation (BP) artificial neural network (ANN), and then the normalization effect verification, optimization of network parameters, and identification effect verification of whirling were carried out. The final results show that the established MSE model has a favourable effect on data normalization, which could also reduce the complexity of the required network model, and shorten the training time by 20–30 s/step. The optimal algorithm is Trainscg, whose optimal number of hidden layer nodes is 5, and the optimal maximum number of iteration steps is 1000. The established ANN model can accurately identify whirling based on MSE, the accuracy is about 0.94, and the average relative error is 1.3%. The method established in this paper provides a reference for the automatic identification of various low-efficiency conditions based on MSE.

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.

Effect of Polishing on Cutting Efficiency and Mechanical Properties of PDC Cutters

Summary Polycrystalline diamond compact (PDC) bits equipped with polished cutters have shown improvements in drilling performance compared to the bits using nonpolished cutters. Despite the positive feedback from numerous global field runs, the merits of polished cutters are still not fully studied and not taken seriously, for example, by the bit manufacturers and drilling engineers in China. In this work, the effect of polishing on the rock-cutting efficiency and mechanical properties of PDC cutters was comprehensively analyzed through laboratory tests and field trials. The underlying mechanism was also investigated through theoretical modeling and experimental results. A rock-cutting force model of a single PDC cutter was developed to elucidate the effect of polishing on the rock-cutter interaction considering the friction between the cutter surface and rock cuttings. The results revealed that the polished cutter has better rock-cutting efficiency because the polishing reduces the friction on the cutter surface. This reduction in friction facilitates the evacuation of rock cuttings from the crushing zone and plastic flow zone, leading to lower mechanical specific energy (MSE) compared to the nonpolished diamond surface. Moreover, the polished cutters exhibit improved thermal stability and better impact fatigue resistance while maintaining comparable wear and impact resistance to nonpolished cutters. To further validate the findings, two field trials were conducted in Sinopec Shengli Oilfield. The first field trial using four PDC bits with polished cutters and one bit with nonpolished cutters found that the bits with polished cutters obtained a higher rate of penetration (ROP) in drilling hard and plastic mudstone, which agreed well with the theoretical and experimental results. In the second field trial, it was noted that the polished cutters presented comparable mechanical properties to nonpolished cutters, which was also consistent with experimental results. However, the advantages of polished cutters in thermal stability and impact fatigue resistance were not distinguished in the field trials. This work elucidated the beneficial effect of polishing in enhancing the drilling efficiency of PDC cutters and, more meaningfully, without sacrificing the mechanical properties of PDC cutters, which provided solid evidence to convince bit manufacturers and drilling engineers for the broader adoption of polished cutters.