Rock Breaking Efficiency Research Articles

Optimization of discharge circuit model based on electro pulse boring experiment

High-voltage electro pulse rock-breaking is a green breaking technology, which can achieve high efficiency of rock-breaking. It has a wide application prospect in deep and ultra-deep well drilling and tunnel driving in the future. Pursuant to the time delay characteristics of the gas discharge switch in the electro pulse boring (EPB) system, the equivalent model of the discharge circuit is established based on the time-varying resistance model. With this model, the real-time parameters such as voltage, current and discharge energy can be predicted in the EPB process of different rocks. In line with the arrangement of coaxial cylindrical electrodes, a 25 mm electrode drill bit is designed, and an EPB experiment system is developed based on the transformer step-up pulse power supply. The electro pulse breaking of red sandstone, yellow sandstone, concrete and granite, and the complete drilling of 60 mm diameter in red sandstone and concrete are realized, and EPB rules of different rocks are obtained. The real-time curves of electrical parameters in different rocks are collected by electrical parameter tester. The model parameters are identified based on the least square method of unconstrained nonlinear optimization. The current curve fitted by the identification parameters and the current curve obtained from the experiment are featured in high coincidence and good follow-up, which proves the accuracy of the optimized model and parameter identification. The optimization of the discharge circuit model and the identification of the model parameters based on EPB experiment are of great significance to guide the selection of the technological parameters of EPB, improve the efficiency of rock-breaking and reduce the energy loss in the drilling process.

Development of a real-time muck analysis system for assistant intelligence TBM tunnelling

Intelligent tunnelling has become an important direction for the development of TBM technology recently. As a result of the interaction between rock mass and TBM cutterhead, mucks are very important for predicting rock mass conditions and evaluating rock breaking efficiency. A real-time muck analysis system for assistant intelligence TBM tunnelling is proposed in this paper. Machine vision was applied to take the muck images continuously in the high-speed conveyor belt. The image segmentation and feature extraction of the mucks are conducted by using a deep learning algorithm. The proposed system also measured the mass and volume flow of the muck by installing a belt scale and a scanner to monitor the stability of the rock mass on the tunnel face. After the system was completed, it was installed on an indoor simulation experimental platform. A series of experiments were conducted to verify the design functions and measurement accuracy. Additionally, the system was applied to a TBM tunnelling project. The application results showed that the proposed system reached its design requirements and functions, and can provide muck data support for further assistant intelligent TBM tunnelling.

Using a high-pressure water jet-assisted tunnel boring machine to break rock

The concept of tunnel boring machine (TBM) disc cutter rock breaking coupled with high-pressure water jets has been proposed to overcome the difficulties that occur when TBMs encounter extremely hard rocks. Thus, to meet actual engineering requirements for the TBM construction of tunnels as part of the Wan’anxi water diversion project in Longyan City (Fujian Province, China), experiments were conducted on high-pressure water jet-assisted TBM disc cutter rock breaking. By varying kerf depth and width under different water jet parameters and performing disc cutter rock breaking tests on rock surfaces with no kerf, single kerf, and double kerfs, the effects of different kerf depths on the disc cutter rock breaking process, load, and efficiency were examined. The test results showed that high-pressure water jets can generate the regular kerfs required for the coupled disc cutter rock breaking of granite. Employing the coupled rock breaking method also resulted in a decrease in specific energy and an approximately 40% decrease in the normal force of the disc cutter, thereby significantly improving rock breaking efficiency. These results provide key technical parameters for the design and manufacture of high-pressure water jet-assisted rock-breaking TBMs and serves as a reference for similar processes.

Experimental investigations on mechanical performance of rocks under fatigue loads and biaxial confinements

In this research, a series of biaxial compression and biaxial fatigue tests were conducted to investigate the mechanical behaviors of marble and sandstone under biaxial confinements. Experimental results demonstrate that the biaxial compressive strength of rocks under biaxial compression increases firstly, and subsequently decreases with increase of the intermediate principal stress. The fatigue failure characteristics of the rocks in biaxial fatigue tests are functions of the peak value of fatigue loads, the intermediate principal stress and the rock lithology. With the increase of the peak values of fatigue loads, the fatigue lives of rocks decrease. The intermediate principal stress strengthens the resistance ability of rocks to fatigue loads except considering the strength increasing under biaxial confinements. The fatigue lives of rocks increase with the increase of the intermediate principal stress under the same ratio of the fatigue load and their biaxial compressive strength. The acoustic emission (AE) and fragments studies showed that the sandstone has higher ability to resist the fatigue loads compared to the marble, and the marble generated a greater number of smaller fragments after fatigue failure compared to the sandstone. So, it can be inferred that the rock breaking efficiency and rock burst is higher or severer induced by fatigue loading than that induced by monotonous quasi-static loading, especially for hard rocks.

Experimental Investigation and Numerical Analyses for Red Sandstone Rock Fragmentation

The effects of initial stress conditions and rock breaking parameters on rock fragmentation were investigated through indentation tests and numerical analysis with a discrete element method (DEM). Different initial stress conditions and the thrusting velocity of the cutter were the focus in the design of indentation tests. Some indicators were used to reflect the instability of the thrusting force, rock breaking efficiency, failure patterns, and the fluctuation of force-penetration response. The indentation results show that rock failure patterns have a closed correlation with initial stress conditions and loading rate. All tests showed several distinct chipping phases, and obvious chiseled pit and crushed rock could be observed on the top surface of each rock specimen. Moreover, rock brittleness was evaluated by the fluctuation of force-penetration curves; this study also revealed the changing law of peak and mean thrusting force. In addition, the numerical simulation of indentation tests reproduced the development process of rock fragmentation and crack propagation. Increasing confining stress restrained the propagation of vertical and lateral cracks, whereas the axial stress had the effect of restraining the propagation of radial cracks. However, there is a critical confining stress that causes different results of fractured depth. Therefore, this investigation is beneficial for optimizing the cutting parameters to promote rock breaking efficiency.

Theoretical and Experimental Study of the Effects of Impact Drilling Parameters on the Properties of Surrounding Rock Damage

Using a self-designed hydraulic impact drilling test-bed and rock core drill, six groups of cylindrical granite specimens (93 mm dia. × 200 mm) containing central axial holes formed either by impact or nonimpact drilling methods were tested in uniaxial compression to failure on an Instron 1346 universal testing machine to investigate their mechanics and damage properties. The longitudinal acoustic wave velocities were measured before testing. The rock specimens were grouped according to the method of drilling the central hole (impact load exerted by different impact power and different frequencies for an approximately identical impact power, or nonimpact drilling). In this study, a statistical constitutive damage model based on Weibull distribution was used to calculate the degree of rock damage after drilling center holes. The experimental curves were measured to analyze the damage evolution process and the radius of rock damage. These indicate that rock damage increased with the increase of impact power and decreased with increasing impact frequency at constant impact power. This was also verified by the measured longitudinal wave velocity in all rock specimens. These results have significance for guiding the design of composite rock drilling tools that are dedicated to improving rock-breaking efficiency.

Three-Dimensional Numerical Simulation of Energy Transfer Efficiency and Rock Damage in Percussive Drilling With Multiple-Button Bit

Abstract Percussive drilling is suitable for the fragmentation of rocks and other similar materials with hard and brittle properties. Research on energy transfer efficiency is of great significance to improving the rock-breaking efficiency of percussive drilling. This paper numerically studied energy transfer efficiency and rock damage in percussive drilling with a multiple-button bit. In this study, the finite element method was used, and the concrete was selected as the research object. Moreover, the damage-plasticity model for concrete and the explicit dynamics solver in abaqus were adopted. Based on the above model, we analyzed the effects of different parameters on the energy transfer efficiency and the concrete damage. Results show that the degree of concrete damage is positively correlated with the output energy, referring to the energy actually absorbed by the concrete, of the percussive system. Under the condition of the same input energy, the energy transfer efficiency decreases as the number of buttons increases. With the increase of impact velocity of the impact hammer, the energy transfer efficiency augments. Multiple repeated impacts will cause the repeated damaging of the concrete, which reduces the energy transfer efficiency.

Experimental Research on Rotation-Percussion Drilling of Diamond Bit Based on Stress Wave

Rotary-percussive drilling is a common form of drilling in geotechnical engineering. in order to study the effect of rotational speed and impact speed on rock-breaking efficiency, a drilling simulation platform developed by ourselves and a drill pipe equipped with diamond bit were used to conduct simulated drilling experiments. By monitoring the stress wave during drilling, the strain, stress and energy changes at different rotational speeds and impact velocities can be obtained, and the optimal drilling parameters can be determined. The results show that both speed and impact velocity have direct influence on the drilling process. When the rotational speed is kept constant, the drilling efficiency increases first and then decreases with the increase of the impact velocity. When the impact velocity remains constant, the drilling efficiency decreases with the increase of rotating speed. With the increase of rotational speed, the optimal impact velocity also increases. It is helpful to improve the drilling efficiency by reasonable choice of speed and impact speed.

An Axial and Torsional Percussion Hammer with Considerable Potential to Increase the Drilling Speed of High-Temperature Geothermal Wells

Geothermal energy is a kind of green and sustainable energy and an essential alternative energy in the future. Improving the drilling speed of geothermal wells is helpful for the efficient exploitation of geothermal resources. Axial & Torsional Percussion Hammer (ATPH) was proposed in this paper for drilling enhancement. ATPH can convert the kinetic energy of the drilling fluid to percussion load, and improve the rock-breaking efficiency of the bit. Because of its mechanical seal design, it can operate efficiently at temperatures over 200 °C, which proves that it can be used in geothermal wells. The large eddy simulation (LES) method is chosen to calculate the percussion load characteristics of the ATPH oscillator, including its main frequency and amplitude. In the field drilling experiments of two high-temperature wells, ATPH showed better ROP enhancement ability and longer working life than PDM. In the well interval drilled by ATPH, the maximum temperature at the well bottom reached 202.54 °C.

Structure design of weight-on-bit self-adjusting PDC bit based on stress field analysis and experiment evaluation

The complex formations such as soft-hard interbedded and strong abrasive strata in ultra-deep wells make the PDC bit inefficient in rock-breaking, which greatly increases the development cost of oil and gas reservoirs. There are two basic ways to solve the above problems: one way is to relieve the force condition of the bit by changing the working mode and the other is to release the stress of the rock at bottomhole with special tools. A method is proposed in this paper to make full use of the stress-releasing effect to reduce the value of compressive stress of the rock at bottom hole, even turn it into partially tensile stress, and take advantage of the cutting method of ‘Hybrid Bits’ to improve rock-breaking efficiency (RBE). A supporting Weight-on-bit (WOB) Self-adjusting Dual-diameter PDC Bit (WSADB) is invented to realize the above idea. Based on Biot's theory of poroelasticity and considering the fluid-solid coupling effect, numerical calculation of the stress field at well bottom is performed. The results show that the WSADB can improve the RBE by enlarging the volume of the stress unloading area at well bottom; The rock damage function based on the 3D Mohr-Coulomb Criterion is used to analyze the effect of structural parameters of the bit on RBE and the optimal structural parameters are obtained. When the diameter ratio of the pilot bit to the reaming bit is 0.5, the rock to be drilled at the well bottom will be damaged to the greatest extent; When the length of the pilot bit is longer than 0.5 times of the borehole diameter, the longer the length is, the greater the degree of damage to the rock at well bottom will be. To verify the accuracy of the above calculations, a comparative test between the WSADB and conventional PDC bit is performed. The results show that the WSADB has great superiorities in improving the rate of penetration (ROP), lowering the torque on bit (TOB), reducing the mechanical specific energy (MSE) and enhancing the RBE. Making full use of the advantages of higher ROP, smaller TOB and higher mechanical energy utilization of the WSADB is expected to provide a solution to the problem of low RBE of conventional PDC bit in ultra-deep wells.

The evaluation criteria for rock brittleness based on double-body analysis under uniaxial compression

The brittleness of rocks plays an important role in the fracture network fracturing process of unconventional oil and gas reservoirs, the drilling efficiency of rock breaking and wall stability, and mining engineering. Rock brittleness has become one of the key parameters in the study of many rock mechanics and related engineering problems. However, there is still no widely accepted definition of brittleness. Although many criteria have been proposed to characterize rock brittleness, their applicability and reliability have yet to be verified. Therefore, brittleness evaluations require further study. In this paper, we divided the rocks under an external load into the unruptured area (body I system) and ruptured area (body II system). We considered that the body I system provided energy for the fracture of the II body. The brittleness of rock is understood as the ability of energy release when a post-peak fracture occurs in the II system. Therefore, the quasi-static energy-balanced equation of a double-body system was established and a new brittleness index was defined as the elastic energy release between an unsteady quasi-static state to the stable state of body II. The new index for brittleness evaluation proposed in this paper can not only quantify the brittleness of rock through the relation between the extremum of a softening modulus at the inflection point in the post-peak deformation curve of body II and the elastic modulus of body I but also ensure the position of the failure strain in the post-peak stage as well as reviewing the instant characteristics of the rock’s brittle crack. It is a new method to evaluate rock brittleness. Taking sandstone with different contents of brittle minerals as an example, the variation of the brittle index with the content of brittle minerals was studied.

Dynamic response and strength failure analysis of bottomhole under balanced drilling condition

Study of the distribution of the stress field at the bottom-hole in ultra-deep wells is one of the keys to understand the rock-breaking mechanism and to optimize the design of drilling tools and bits, thus improving rock-breaking efficiency. To date, investigations of the stress field at bottom-hole considered only the static solution after stress redistribution, and neglected the dynamic evolution of the stress field and only considered uniaxial stress or biaxial stresses. Based on Biot's theory of poroelastic mechanics, the complex loading state of the rock at bottom-hole is considered and a fully coupled model of seepage field and stress field is proposed. Time effects are taken into consideration, and the model is combined with the three-dimensional Mohr-Coulomb criteria to predict strength failure of the rock at the bottom-hole. The dynamic response of the rock at the well bottom is obtained by numerical solution and the fail function is proposed to analyze the influence of stress at the well bottom on rock strength failure. The results show that the time effect of the poroelastic medium can be reflected by the dynamic response of the borehole or damping fluctuation of the pore pressure at the central line of the well bottom, and the influence range of this stress or the changes in pore pressure is within 5 well radii (R) of the borehole. For the scenario considered, under balanced drilling conditions and within 0–0.6R at the well bottom, the effective radial and hoop stresses remain in the tensile stress state after the borehole being drilled for several seconds, and the maximal values of effective radial and hoop stresses (compressive stress) appear near the borehole wall. The areas where tensile failure may occur are limited between the well bottom and 25 mm below the well bottom. The failure criteria calculated with the fail function shows that areas with smaller fail function values are concentrated in the convex part in the center of the wellbore and the junction part between the wellbore and the wellbore wall, where the rock strength failure is most likely to occur. Larger fail function values are found in a spherical area below the center of the wellbore, where the rock strength is greater due to the stress redistribution and resembles a tightly compacted core.

Fracture characteristics of coal jointly impacted by multiple jets

Drilling technology using multiple jets can achieve an ultra-small hole radius or well, and achieves good engineering results in soft rock such as coal. In this study, multiple jet impingement experiments were conducted to reveal the rock-breaking mechanism for coal. Computed tomography scanning was applied to observe the propagation law of internal rock fractures formed by the impact of multiple jets. By analyzing the relationship between the angle and length of a conical fracture and the arrangement mode of multiple jets, two types of interference fracture propagation laws were obtained. Combined with scanning electron microscope images of the impact debris, the fracture characteristics of coal impact by multiple jets is revealed, and a novel rock-breaking mode for multiple jets containing a center jet based on the combined impingement fracture propagation is proposed. The pilot hole formed by the center jet provides a free boundary for the side jet impact to avoid interference between adjacent jets. A conical fracture on the side near the center jet can also expand on the other side. When the cone length reaches the distance between the side jet and center jet, the erosion pits respectively generated by the adjacent jets will be connected, thus forming a joint erosion pit. Compared with multiple jets without a center jet, the rock-breaking efficiency of multiple jets containing a center jet can be improved by 47.3%. The present research results are a supplement to the rock-breaking theory of water jet impingement, providing theoretical support for the design of a multi-orifice nozzle.

Investigation on the rock temperature in fiber laser perforating

The characteristics of rock temperature in laser perforating influence the rock breaking efficiency and energy consumption. We report a series of fiber laser perforating experiments on granite, sandstone and basalt to systematically examine temperature variation and correlate these with laser irradiation parameters and rock property. It is found that the rise of rock temperature in laser perforating area could be divided into steep rise, gentle rise and balanced stages. Rock temperature increases linearly in steep rise stage. The rate of temperature rise increases with the laser power, and the temperature rises faster when the rock has higher thermal conductivity in steep rise stage. The temperature limit of the steep rise stage (T0) is the critical point of rock melting. The rock temperature turns into the gentle rise stage when the rock starts melting. The duration of gentle rise stage would be prolonged if the rock has more refractory phases (quartz), and the temperature rate of gentle rise stage falls more sharply under higher laser power. The rock temperature in the perforating area will remain stable when the rock melts completely. The balanced temperature (Tb) increases with the laser power, and it will be higher when the rock has more refractory phases (quartz). Tb is the sign of complete melting of the perforating rock. Importantly, the phase transition of the rock is the main reason for the inhomogeneous-step rise of temperature in laser perforating, the irradiation power and rock composition are the key influencing factors of the rock temperature profiles.

Three-dimensional simulation of rock breaking efficiency under various impact drilling loads

Percussion drilling is believed to be an efficient way to enhance the drilling performance, compared with the conventional drilling in deep wells, which could be achieved by applying impact loads above the drill bit. The finite simulation is an efficient method to predict the drilling performance when the drill bit interacts with rocks. In contrast, as results of finite element technology limitation, the conventional way is to apply the displacement or velocity boundary conditions on the bit; however, this is completely different from the real field drilling process. Therefore, in this paper, a 3-D rock-bit physical model is presented to simulate the practical percussion drilling under various types of loads; at the same time, experiment was conducted to validate the effectiveness of the rock-bit model. The results demonstrated that the drilling performance with the rectangular and sinusoidal loads was promoted than the other types of load, and rate of penetration (ROP) increases with increasing of the amplitude of dynamic load, which could provide the theoretical guidance to the down-the-hole drilling tool design, thereby proving a quite efficient and lower cost drilling in the exploration of hard formations.

Numerical simulation analysis of hybrid impact cutting and its comparison with torsional impact cutting

Hybrid impact drilling is a new drilling method proposed in recent years, the PDC (polycrystalline diamond compact) bit impacts the rock in torsional and axial directions during its rotation. From the perspective of field application, hybrid impact drilling can increase the rate of penetration (ROP), especially in hard heterogeneous formations. However, its rock breaking mechanism and difference from torsional impact drilling are not clear, this leads to aimless in choosing these two drilling methods. In this paper, the quasi-3D numerical simulation model of hybrid impact cutting is carried out to investigate the rock breaking mechanism, including the chip formation, mechanical specific energy (MSE) etc. moreover, its comparison with torsional impact cutting is also conducted for evaluating the applicability of these two methods for the same formation. The results show that, the rock breaking efficiency of hybrid impact cutting is higher than torsional impact cutting for shallow depth of cut (DOC), on the contrary, the rock breaking efficiency of hybrid impact cutting is more lower for the medium DOC; but for the deep DOC, both of these two cutting methods cannot improve the rock breaking efficiency. The axial impact amplitude and frequency have large influence on rock breaking efficiency, the optimal axial impact amplitude and frequency exist for specific formation. Both of these drilling methods are not applicable to soft formations. This study leads to an enhanced understanding of rock breaking mechanisms in hybrid impact drilling, and contributes to the improvement in the design of impact tools and determination of the related parameters.

Properties of Drillstring Vibration Absorber for Rotary Drilling Rig

During drilling into hard rocks, the vibration of telescopic drillstring for the rotary drilling rig becomes more severe, which not only leads to the fracture of drillstring, but also reduces the rock-breaking efficiency, the service life, and the reliability of the rotary drilling rig. A drillstring vibration absorber for the rotary drill rig was proposed based on nonlinear targeted energy transfer technology, and the theoretical model of the absorber was adopted. By applying the instantaneous nonlinear energy absorption rate of the absorber, the vibration responses of the system were predicted on different structure parameters and impact amplitudes. To evaluate the feasibility of the absorber, experiments were carried out. Moreover, the amplitude decrease rate was generated for various working conditions. Furthermore, the analytical results were verified by the experiment. The results indicate that the absorber has a distinct effect on vibration reduction.

On the rock-breaking mechanism of plasma channel drilling technology

Plasma channel drilling (PCD), which is also known as high-voltage electric pulse drilling, is a highly efficient rock-breaking drilling technology with great development potential. This paper presents a method to study the rock-breaking mechanism of PCD technology. Then based on the predecessor's foundation, the factors affecting the development of plasma channels are explored using the dielectric breakdown model (DBM). Finally, a numerical rock-breaking model considering electric-thermal-mechanical three fields is established. This paper reveals the basic rock-breaking process of PCD and how the formation pressure affects the rock-breaking process of PCD technology. The main conclusions are: the formation law of plasma channel is related to the electrode structure and the inputting electrical parameters, the electrical properties of the rock and the type of rock; the plasma channel has an internal dynamic effect on the rock breaking process, and this internal dynamic effect of the plasma channel is sharply weakened after the channel is formed; shear cracks play a lead role in PCD, and the number of shear cracks is larger than that of conventional mechanical-rotary drilling; the formation pressure has little effect on the rock failure, the total volume of the chips, and the total number of cracks in the PCD, and the rock-breaking efficiency is not limited by the drilling depth. Finally, in order to improve the rock breaking-efficiency, it is recommended that the PCD technology be used in conjunction with slim hole drilling technology or jet drilling technology.

Experimental study on the adaptability of cutters with different blade widths under hard rock and extremely hard rock conditions

To investigate the matching rules of cutters with different blade widths in hard rock and extremely hard rock environments, this study carries out a full-scale cutting test of 432-mm disc cutters with blade widths of 12 mm, 19 mm and 30 mm. A multifunctional cutter performance experimental system was used to test the cutting loads and rock-breaking quantity of rust stone granite (hereinafter referred to as hard rock) and granite (hereinafter referred to as extremely hard rock) as well as to analyse the specific energy consumption for rock breaking under different cutting parameters. The experimental results show that (1) the blade width has a greater influence on the normal force than on the rolling force. The rock-breaking resistance of the 12-mm-blade-width cutters under extremely hard rock conditions is 36.7% lower than that of conventional cutters. (2) Under the same conditions, the smaller the blade width is, the smaller the specific rock-breaking energy consumption is and the smaller the optimal cutter spacing is. Under extremely hard rock condition, the specific energy consumption of the 12-mm-blade-width cutters is 37.9% lower than that of conventional cutters. (3) With a certain gross thrust of a single cutter, the rock-breaking quantity of a single 12-mm-blade-width cutter is much larger than that of the 19-mm-blade-width cutter, regardless of the types of rock. Under extremely hard rock conditions, the S/P of the 12-mm-blade-width cutter with the highest rock-breaking efficiency is approximately 10–12.

Study on rock breaking mechanism of PDC bit with rotating module

In view of the problems of low ROP, short bit life and high energy consumption of bits in deep and difficult formations. Basing on the 360°rotating teeth and disc PDC bit, a rotating modular PDC bit is proposed. The rock breaking efficiency of the bit is improved by “cross scraping” of the rotating module element (RME) and fixed cutting teeth. In addition, the rotating module element works alternately, which cools the cutting teeth in time, slowing the wear of the cutting teeth and prolongs bit life. This paper introduces the structural characteristics and working principle of the novel bit, and carries out variable parameter experiments on the rotating module element (side angle, journal angle, and tooth arrangement density, etc.) The experimental results show that the rotating speed of the rotating module element increases with the increase of the side angle, and decreases with the increase of the axial angle. The variation rules of cutting load and MSE of rotating module element under different structural parameters are studied. The indoor test shows that the ROP of rotary modular PDC bit is increased by 15% and the MSE is reduced by 14%, It is implies that the “cross scraping” of the rotary module element and the fixed cutting teeth can reduce the rock breaking specific work, and provide theoretical basis and support for the design of the subsequent rotary modular PDC bit.