PDC Bit Research Articles

LSTM Recurrent Neural Network Based Method for Identification of PDC Drill Bit Operating Conditions and Its Application

Abstract PDC drill bit underground operating conditions identification is one of of the difficulties during the drilling operation. It is of great significant for the drilling improvement and accurate construction formation to accurately identify the complex operating conditions during drilling the operation such as PDC bit wearing and stick-slip vibration, etc. In this paper, data of relationship between torque, rate of penetration (ROP) signal and weight on bit (WOB), rev were obtained through lab simulation experiment, after which, a relationship database of torque, ROP signal and WOB, rev was established, a single-layer LSTM recurrent neural network (LSTM-RNN) was constructed, the number of neurons, batch size and optimizer type of LSTM layer were optimized. With BPTT algorithm, LSTM model was trained and tested on the training set containing 6000 groups of data, and achieved the goal of identifying the bit operation conditions against the input of WOB, torque, rev and ROP signal parameters. Based on this model, the PDC bit underground operating conditions identification software was compiled, and was connected to the logging unit to realize real-time identification of bit underground operating conditions, and field test application was successfully carried out in Mahu 1 Well Region. The research showed that the convergence speed was fast and the time cost was at the lowest when the number of neurons of LSTM model was 50, and that the optimized Adam optimizer model could meet the requirements of convergence speed and prediction accuracy with the lowest loss value. The classification accuracy of this model on the test set reached 94% and the accuracy of the prediction method was also verified by field test application. To sum up, the method of LSTM-RNN based PDC bit underground operating conditions real-time identification can better meet the requirements of accurate identification of PDC bit underground operating conditions in the practical drilling, and has guiding significance for the judgement of PDC bit operating conditions on site and formulation of scientific construction decisions.

Experimental study on rock drilling vibration of PDC bit in interbedded formations

Mastering the vibration characteristics of PDC bit, especially in interbedded formations, can help to explore reasonable bit failure control measures, achieve bit optimization and drilling optimization. This work used single rocks to construct different types of vertical and horizontal interbedded rocks with different interbedded properties, the experimental study obtained the bit vibration characteristics under single rock, sudden-changed rock, and alternating rock conditions. The vibration acceleration of a single rock is consistent with the rock strength. The acceleration increases with the increase of weight-on-bit and rotary speed, the influence of weight-on-bit is greater than that of rotary speed. The maximum amplitude of accelerations always occurs in the low frequency range of 0∼300 Hz, the difference between rocks is mainly reflected in the high frequency range greater than 400 Hz. When the drill bit suddenly crosses from soft rock to hard rock in the vertical interbedded formation, the acceleration increases rapidly, causing direct impact. When the drill bit suddenly crosses from hard rock to soft rock, the impact and acceleration attenuates slowly, which could cause fatigue damage to the drill bit and induce high-frequency vibration. The rapid and significant change in depth-of-cut is the fundamental reason for the impact generated when the bit crosses through the interlayer, and both the maximum and minimum depth-of-cut occur in the transition region. PDC bit alternately encountered soft and hard rocks in the horizontal interbedded formation, causing intensified vibration. The acceleration is consistent with rock difference. The influence of weight-on-bit on acceleration increases with the increase of rotary speed for horizontal interbedded formation with small rock differences. However, with the increase of weight-on-bit, the influence trend and degree of rotary speed on acceleration remain basically unchanged. The increase in weight-on-bit /rotary speed weakens the effect of rotary speed/ weight-on-bit on acceleration in horizontal interbedded formations with significant rock differences. The research results illustrate that the reason for the abnormal bit failure in interbedded formations is the frequent changes in depth of cut caused by rock mutation and alternation, and the resulting impact. This provides theoretical guidance for the implementation of effective measures such as reasonable cutter arrangement design, pertinent setting of auxiliary structures, and appropriate adjustment of drilling parameters.

Drillability Evaluation and Sensitivity Analysis of the Fencheng Formation Strata in Maye Exploration Area

Abstract Predicting drillability ratings and selecting suitable drill bits are crucial for improving drilling efficiency. This investigation employed Principal Component Analysis (PCA) to normalize adjacent well logging parameters such as natural gamma, well diameter, natural potential, and others as influencing factors and established a mathematical model for the drillability ratings of hard rock formations. A sensitivity analysis of drilling parameters for the Fencheng Formation rocks revealed that sonic time difference, formation density, natural gamma, and resistivity were closely associated with drillability ratings. Empirical results showed that a multiple logarithmic linear regression approach accurately predicted drillability ratings for hard rock formations with over 95% precision compared to experimental values. The Fencheng Formation strata’s high hardness and abrasion resistance result in poor drillability (with ratings above 7.5), with rotational speed sensitivity found to be higher than drilling pressure sensitivity for expediting drilling in hard formations. For the “turbine + PDC bit” drill tool combination, it is recommended to use a drilling pressure of 40 - 80KN and a rotary table speed of 50 - 70r/min. High rotational speeds and low drilling pressures effectively reduce bit sticking effects, while using PDC bits with turbines reduces bit wear, improving the economic efficiency of drilling operations without sacrificing mechanical drilling speed.

Experimental study on the rock breaking effect of different tooth shapes of inserted teeth in a hobbing-cone hybrid PDC bit under complex stress environment

The hobbing-cone hybrid PDC bit has emerged with the increasing exploitation of deep oil and gas resources. As an important component of new bits, studying the mechanism of toothed fracturing of deep rocks is crucial for improving rock-breaking efficiency. In this paper, the spherical teeth and conical teeth were used to intrude sandstone under different confining pressures, and the brittleness index (BIm), specific energy (SE), and average fragment size (dm) were used to analyze the rock-breaking effect. The 3D contour scanner quantitatively analyzed the crushing pit area. Finally, the in-situ CT scanning and discrete element numerical simulation were used to summarize the crack propagation law. The results show that with the increase of confining pressure, the BIm of inserted teeth increases continuously, and the growth rate of conical teeth increases, while that of spherical teeth decreases. The SE and dm exhibit a symmetrical distribution with a confining pressure of 10MPa as the axis of symmetry. When the difference in principal stress is 5MPa, the SE is the lowest point, the rock-breaking efficiency is the highest, and the corresponding dm is at the highest point, but the curve trend is the opposite. When the stress difference is 5MPa, it is conducive to propagating surface cracks, thus forming a larger crushing area and the highest rock-breaking efficiency. Based on CT results, the dense particle core is formed near the inserted teeth, and that of the spherical teeth is located below. In contrast, that of the conical teeth is located on the side, which is also the reason for the difference in crack propagation between different tooth shapes.

Improving breaking efficiency of hard rock: Research on the mechanism of impact-shear rock breaking technology

To expedite drilling operations in hard rock of coal mines, a new type of impact-shear drill bit was developed, and its mechanism of speed-up and efficiency increase was studied. The RHT constitutive model was used to describe the structural behavior of rock, and the rock-breaking simulation model of full-size bit was established. Compared with PDC bit and hammer bit, the rock-breaking force, bit torque, and rock stress characteristics of impact-shear bit were analyzed. The results show that, in comparison to PDC bit and hammer bit, the axial force of impact-shear bit was reduced by 68.25% and 71.40%, respectively, and the average torque was reduced by 91.79% and 83.36%, respectively. Notably, for the impact-shear bit, the fluctuation of drilling force was effectively mitigated, the stick–slip vibration of bit was weakened, the rock-breaking energy consumption was drastically reduced, and the rock-breaking efficiency and bit’s life were finally improved. In terms of rock stress characteristics, the pre-impact effect of the central hammer bit of the impact-shear bit can release the internal stress of the rock well, and the stress of the rock element on the hole wall was relatively reduced, thus making it easier for the external PDC bit to break the rock. Field test results show that, under the condition of the small drilling rig, the impact-shear bit can give full play to the pre-crushing function of the impact mechanism, thereby effectively protecting the PDC cutter of external PDC bit, and realizing the fast hole-forming in hard rock of coal mine.

Enhancement on PDC bit based on Archimedean spiral control method

The Longmaxi Formation shale in the Sichuan Basin has a large burial depth, narrow horizontal windows, and stringent requirements for controlling well deviations. When PDC bits are used for drilling, the cutting structure experiences significant challenges in terms of stability and rock-breaking efficiency. The secondary design cycle of PDC bits is lengthy, and the cost of engineering trials is high. Thus, bit development relies on strong empirical knowledge. Additionally, the spiral degree of the blades has not been uniformly defined, and there was a lack of quantitative studies about its effect on the rock-breaking performance of PDC bits. In this study, thus Archimedean spiral was used as the quantitative control equation for the spiral degree of the blades. The cutting depth control distance ΔL2 (or designed depth of cutting, designed DOC) and the level of different track (LODT) were used to adjust the radial cutting section of the interaction process between the bit and shale. An orthogonal coupling study was conducted on the rock-breaking efficiency and stability of 8 1/2ʺ PDC bits with six spiral span angles θspan, six cutting depth control distances ΔL2, and six LODTs as design parameters. The results indicated that using the triaxial compressive strength under the liquid column pressure as the mechanical specific energy (MSE) benchmark, the shear failure mode reduced the mechanical specific energy below this benchmark when cutting depth control distance ΔL2 of the front and rear elements exceeded 1.0 mm. However, increasing the cutting depth control distance ΔL2 led to an increase in torque and circumferential vibration amplitude. In addition, compared to straight blade designs, spiral blade designs significantly reduced the axial/circumferential vibrations, with reductions in circumferential vibration amplitude ranging from 37.87% to 48.33%. The spiral blade designs had a suppressive effect on axial vibrations only when the spiral span angle θspan was between 12 and 36° or 60°, reducing the axial vibration amplitude by approximately 20.09–25.69%. Different ΔL2 must be considered in conjunction with cutter material properties when selecting the spiral span angle based on the simulation results. When the ΔL2 exceeded 1.0 mm, LODT of 3/6 could maximize the reduction in the mechanical specific energy. Using MSE as the objective function, this study provided several PDC bit enhancement design schemes suitable for the Longmaxi Formation. The research results contributed to a quantitative understanding of the coupling effects between spiral span angle θspan and cutting sections on the rock-breaking efficiency and stability of PDC bits.

Study on Mechanism of Stick–Slip Vibration Based on Torque Characteristics of PDC Bit

Stick–slip vibration (SV) of drill string systems is the main cause of fatigue failure of PDC bits under complex drilling conditions. Exploring its mechanism is helpful for identifying the causes of bit failure and developing preventive measures to prolong bit service life. In this study, the influence of various factors on torque characteristics is tested by drilling rock breaking with various PDC bits and the variations in torsion variables and torsion speed of drill string systems under different torque loading conditions of drill bits are ascertained. Through a finite element simulation of the drill string–bit system, the influence of the PDC bit on the torsional deformation with variable torque is determined, and the influence mechanisms of bit size, tooth structure, invasion depth, rock strength, and other factors on the SV induced by a PDC bit are established. The results show that the change in the reaction resistance moment of the formation rock leads to variation in the driving speed of the drill string system, which is one of the main reasons for the SV. Even if the torque change in the bit is minor, SV will occur if the drill string is too long.

Study of rock-breaking performance of V-axe-shaped PDC cutter in tight sandstone formation

In the field of oil and gas drilling engineering, tight sandstone strata are one of the strata that are difficult to drill. The V-axe-shaped cutter has a good effect in crushing tight sandstone strata. However, there were few studies on its specific structural parameters and rock-breaking performance in tight sandstone formations. To further explore its structural parameters and rock-breaking performance, this paper studies the rock-breaking performance of a V-axe-shaped PDC cutter in tight sandstone formation by combining experiments with finite element simulation. Mechanical tests show that the compressive strength is high, with values of 130.3 MPa, 223.3 MPa, and 271.6 MPa under confining pressures of 0/15/30 (MPa), respectively, with the condition of 0.02 mm/min loading rate. Rock lithology tests that the sandstone in this formation has strong abrasiveness. The penetration ability should be considered when evaluating the rock-breaking performance of cutting cutters. The results show that the V-axe-shaped PDC cutter mainly uses shear, extrusion, and point load to break rock together. It has a stronger ability and stability to attack the rock. With a 100° V-shaped angle (α) and 140° Axe-shaped angle(β) has the best rock-breaking property. The back-rake angle of the cutter can reduce cutter vibration and significantly improve PDC bit rock-breaking efficiency. The optimal rock-breaking back-rake angle of the V-axe-shaped cutter is 20° in dense sandstone formations. With the increase of wear height(0–3 mm), the synthetical mechanical specific energy (MSE) is reduced by 11.53%, 6.30%, 10.30%, and 33.28%, respectively, compared with the conventional cutter. The tests indicate that the rock-breaking efficiency of the V-axe-shaped PDC cutter is 12.9% higher than that of the conventional PDC cutter. The application of V-axe-shaped cutters on the bit is expected to improve the rock-breaking efficiency and footage of PDC bit in tight sandstone formations and realize the development of low-cost and high-benefit oil and gas resources.

Research on the PDC rock cuttings image instance segmentation method based on improved U-Net++ network and multi-feature fusion

Currently, PDC bits dominate the petroleum bit market. It is a small cut of polycrystalline diamond, which is embedded in the body of the drill bit. Due to the small number of cuttings generated through the PDC bit, the individual cuttings are small, and the cuttings edge is blurred, which results in poor effect and low precision of traditional image segmentation. Therefore, this paper proposes an image segmentation method regarding deep learning. Firstly, a multi-task learning method is introduced based on the U-Net segmentation model. A multi-task-learning-U-Net++ instance segmentation model is proposed. The results of semantic segmentation and edge segmentation are obtained by this segmentation model. Then, the cuttings are segmented by superpixel to obtain the sub-blocks of superpixel. Finally, a multi-feature fusion method is proposed, which integrates semantic information, edge information and superpixel sub-blocks to obtain the segmentation result of cuttings image. Experimental results show that the proposed method can effectively segment each cutting in the image and this method has certain robustness. Compared with the segmentation algorithms such as U-Net and U-Net++, this algorithm performs better in multiple image segmentation performance indexes.

SUBSTANTIATION OF SOLUTIONS TO INCREASE THE DURABILITY OF THE INNER ROWS OF STEEL TEETH OF ROLLER CONE BITS DURING DRILLING OF BOULDER-PEBBLE ROCK FORMATIONS

The article deals with issues concerning the increase in the productivity of roller bits when drilling soft and soft-medium rocks with inclusions of boulder-pebble formations by justifying solutions to increase the relative safety margin of steel teeth expressed in an increase of the impact strength of the material for making cones in combination with an increase in the geometry of steel teeth. The efficiency of the rock-crushing tool and, as a result, the destruction of rock during the entire drilling cycle, directly depends on the resource and durability of the elements of the roller bit. When drilling the upper intervals of deposits in the East Siberian region, composed of soft and soft-medium rocks with inclusions of boulder-pebble deposits, there is a problem of premature wear of the drill bit. Both PDC bits are subject to catastrophic wear due to chips and fractures of PDC cutters, as well as roller bits. The use of first-class roller bits with steel teeth leads to advanced wear, expressed by the scrapping of the teeth of the inner rows. This leads to a «hanging» of the bit and a loss of mechanical penetration speed. Based on this, the work provides recommendations for increasing the durability of steel teeth for first-class roller cone bits.

On the issue of improving the technical and economic indicators of well wiring in difficult mining and geological conditions

In this work, the vulnerability of standard PDC bits was established when conducting wells in difficult mining and geological conditions, in particular, with frequent layering of rocks with different hardness. The use of adaptive TerrAdapt chisels, which automatically change the depth of cutting and the aggressiveness of the chisel, is proposed in Russian deposits with frequently alternating rocks. These bits can be combined with overhead dampers if the dynamic coefficient is in the critical range. This solution will improve the technical and economic performance when drilling wells.

Intelligent optimization for a full-sized PDC bit with composite percussive rock breaking drilling

Intelligent optimization for a full-sized PDC bit with composite percussive rock breaking drilling

A high-fidelity model for nonlinear self-excited oscillations in rotary drilling systems

This paper presents an integrated model to analyze self-excited axial and torsional vibrations in rotary drilling systems using realistic PDC bits. The model combines a multi-degree-of-freedom (MDOF) drillstring representation with a detailed PDC bit–rock interaction model, accounting for cutter layouts and rock properties. The bit–rock interaction is described by rate-independent laws for reactive force and torque, encompassing cutting and frictional contact at the cutter face and wear flat. The regenerative rock cutting introduces state-dependent delays (up to 100) due to complex cutter layout of the bit, while the unilateral nature of frictional contact is formulated as a nonlinear boundary condition. The state-dependent delays are transformed into constant angular offsets prescribed by the bit design by introducing a bit trajectory function whose evolution is governed by a partial differential equation (PDE), which is coupled with the drillstring dynamics described by ordinary differential equations (ODEs). The Galerkin method with Chebyshev polynomials transforms the set of coupled PDE–ODEs into a system of ODEs. This approach bypasses the need to search for multiple delays at each time step, allowing implementation of this integrated rock-bit-drillstring model. Simulation results replicate field observed torque-on-bit (TOB) effects including velocity-weakening and hysteresis due to torsional stick–slips, attributing them to cutting depth variation rather than downhole friction. Preliminary findings regarding the influence of bit design on torsional stick–slips align with field drilling practices. The results of parametric study correspond to field test observations, enabling analysis of bit design, drillstring configuration, and drilling parameters on self-excited torsional stick–slips.

Influence of blades’ shape and cutters’ arrangement of PDC drill bit on nonlinear vibration of deep drilling system

In this paper, the surface morphology method is improved and the bit-rock interaction model between the rock and the PDC bit is studied, considering the influence of blades’ shape and the cutters’ arrangement. Based on this model, the dynamic model of the deep drilling system equipped with arbitrary shape PDC bit is established, and the stability prediction method is proposed. Based on the improved surface morphology method, the dynamic characteristics of the drilling system equipped with four different types of bit (an actual bit, a Tian bit, a Tian bit with blades, and a traditional bit) are studied, comparing with the Tian model based on polygon clipping. The results indicate that the influence of blades’ shape and cutters’ arrangement of PDC bit on the nonlinear vibration of the drilling system cannot be ignored. By applying the proposed method, the dynamic model of the drilling system equipped with an actual PDC bit presents excellent computational efficiency, accuracy, and avoids unreasonable behavior, which provides a reliable theory for the dynamic analysis of the drilling system. In addition, the proposed uncut volume error and repeated cutting volume error can be used to evaluate the shape design of the drill bit and the accuracy of the bit-rock interaction model.

Development and application of hobbing-cone hybrid PDC bit

In order to solve the problems of low ROP and short service life of the bit drilling in deep and hard formation, a new technology of hobbing-cone hybrid PDC bit which has combined the advantages of conventional PDC bit and the hobbing-cone is proposed in this paper. The bit is developed and applied on basis of the structure design and the contrast experiment between the hobbing-cone bit and the conventional roller-cone hybrid PDC bit. The indoor drilling experiments show that the hobbing-cone hybrid PDC bit can form groove-shaped crushing pits in the bottom-hole. The teeth on the roller have high circumferential coverage on bottom-hole rock and an obvious pre-fracture effect on the rock. The experiment result shows that ROP of the hobbing-cone hybrid PDC bit is 20–30% lower in soft rock but 15–20% higher in hard rock, indicating that the bit is more suitable for hard rock drilling. Moreover, in the field application, average ROP of the hobbing-cone hybrid PDC bit is 126.1% higher and the footage is 89.6% longer than the comparisons, while wear status of the bit after drilling is quite good. Field application results show that the hobbing-cone hybrid PDC bit has good drilling effect in directional and hard formations, which can be well adapted to the drilling of applied wells (blocks) and directional sections with good steering performance, making it an optimization to improve the working performance in complex and hard-to-drill formations.

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.

Optimization Method for PDC Drill Bits in Hechuan

The Hechuan region is located in the eastern part of the Sichuan Basin, with extremely complex crustal structures, including geological structures such as fault zones and fold zones. In the Hechuan area, the geological conditions are diverse and complex, including volcanic rocks, sedimentary rocks, metamorphic rocks, etc. The physical properties and strength of these formations vary, posing great challenges for drilling operations. In order to improve drilling efficiency, it is necessary to optimize the type of PDC bit. Firstly, the rock breaking principle of PDC drill bits was elaborated. Secondly, we optimize PDC drill bits based on their drillability, rock drillability, and compressive strength. Through the comprehensive analysis of the above three methods, the suitable types of PDC drill bits for the Hechuan area of the Sichuan Basin can be determined. According to the complexity of geological conditions and the different properties of rocks, PDC bits with strong adaptability, high wear resistance and impact resistance, strong cutting ability and guidance can be selected to improve drilling efficiency and reduce drilling costs.

A new type of high hardness coating for improving drill bit stability in unconventional oil and gas development

In deep unconventional oil and gas development, the problem faced is that PDC bits are eroded by solid-liquid high-speed fluids, resulting in damage. It has led to serious damage to the stability of the drill bit, a decrease in the service life of the drill bit, and an increase in the difficulty in efficient drilling. The essence is that the surface hardness and erosion resistance of the drill bit are not strong enough. Therefore, improving the stability of drill bits is a crucial and urgent problem to be solved. In this paper, Ni60A + 20% WC + 0.3% graphene composite coatings were prepared on a Q235 steel substrate, which is a new type of high hardness coating. Moreover, the effects of microstructure and microhardness of the composite coatings at different laser powers (800 W, 1200 W, 1600 W, and 2000 W) were investigated. The results show that the laser power can significantly affect the microstructure of the coating. The phase composition of the composite coatings is essentially the same at different laser powers. However, there are significant differences in the content of each phase. When the laser power is higher than 1200W, the content of M23C6, Cr3C2 and Fe3C in the composite coating increases and the microhardness of the coating decreases. When the laser power is below 1200 W, the dilution rate of the substrate is low and a metallurgical bond cannot be formed between the composite coating and the substrate.

Numerical Simulation Study on Vibration Reduction Effect of Flexible Cutting-Tooth Unit

Under the conditions of drilling in gravel-bearing and heterogeneous stratas, the movement and force of the PDC bit during drilling are highly unstable. Irregular impact loads often cause fatigue failures such as tooth fracture, tooth breakage and delamination of the composite sheet. Dynamic impact load is the main cause of fatigue failure of cutting-tooth, which seriously affects the rock-breaking performance of PDC bits. This paper proposes a flexible cutting tooth unit consisting of a central tooth, an elastic element, and a base. The technical concept of flexible-cutting rock breaking is to reduce the impact load amplitude suffered during the cutting process to a certain threshold range by setting elastic elements or reducing the support stiffness of the cutting tooth, so as to inhibit the expansion of micro defects caused by the impact dynamic load of cutting teeth and prolong the service life of drill bits. The finite-element models of flexible cutting teeth for rock cutting were established. The influence of cutting depth, front rake angle and stiffness of elastic elements on the cushioning and vibration-absorption effect of flexible cutting was analysed. The results show that flexible cutting can reduce the peak and average value of von Mises stress at the cutting edge. Under different cutting-depth conditions, the decline rates were 21.45–49.02% and 9.42–17.8%, respectively. Then, under different front-rake-angle conditions, the decline rates were 20.51–24.02% and 9.41–17.8%, respectively. There is a suitable stiffness of the elastic element, which makes the peak and average value of von Mises stress at the cutting edge of the flexible cutting-tooth unit perform better and the effect of improving the uneven stress distribution at the cutting edge better. Flexible cutting technology can effectively improve the adaptability of the PDC bit in heterogeneous formations and reduce the occurrence of abnormal failure of cutting teeth. The research results of this paper can provide theoretical support for the drilling speed of PDC bits in hard formations.

Theoretical study on rock-breaking performances of PDC cutter by fractal characteristics of cutting size

Given the technical problems in the process of deep oil exploration and exploitation, different shapes of special-shaped PDC cutters have emerged. The conical PDC cutter has been widely used in the design of hybrid-cutters PDC bit, leading to remarkable improvements both in the drilling speed and in the bit footage. However, the fundamental knowledge of the rock-breaking mechanisms of conical PDC cutters is still short of research in the deep hard formations. We hope to explore the rock-breaking characteristics of conical PDC cutters from different perspectives with the fractal theory widely applied in rock mechanics. Based on the fractal characteristics including the fractal dimension and the maximum cutting size of rock cuttings generated by single cutter cutting experiments, a theoretical model of mechanical specific energy is established for the PDC cutter in this study, and the validity can be verified by available experimental data. Meanwhile, comparing the rock-breaking performances of the conical PDC cutter with the conventional PDC cutter, the effects of the cutting parameters on the fractal dimension of cutting size, maximum cutting size and fractal characteristics on the transverse cutting force, friction work, mechanical specific energy are analyzed. Analysis results show that the predicted values of the model are highly fitting to the experimental values, and the prediction accuracy of the model is relatively high. The fractal dimension and the maximum cutting size of conical PDC cutters are larger than those of conventional PDC cutters. During the process of rock-breaking with conical PDC cutters, the lower the degree of rock-breaking, the larger the maximum cutting size, and the lower the mechanical specific energy of rock-breaking with conical PDC cutters. These also verify the stronger brittle failure ability of conical PDC cutters and the significant effect of conical PDC cutters’ plowing effect. Those findings can provide a theoretical understanding of the rock-breaking characteristics of conical PDC cutters and on for the cutter-layout method of the hybrid-cutters PDC bit in deep hard rocks.