Rock Breaking Mechanism Research Articles

Study on rock breaking characteristics of stress wave and blast-induced gas during CO2 gas blasting

To further distinguish the rock breaking mechanism of stress wave and blast-induced gas, and instruct the application of CO2 gas blasting technology in deep coal seam. In this work, the stress distribution on the surface of coal-rock is obtained by introducing the stress intensity factor into the orifice stress concentration equation; moreover, the internal relationship among the stress wave energy, the blast-induced gas energy and the surface stress of coal-rock is described by work-energy principle. The equation set of rock breaking characteristics under CO2 gas blasting is then established. Meanwhile, the stress and ultrasonic characteristics of samples while gas blasting are obtained by utilizing the self-developed CO2 gas blasting experimental platform. Ultimately, the consistency between the experimental results and the theoretical analysis is verified by comparative discussion. The results show that the closer the intensity factor of horizontal stress (Kx) and the intensity factor of vertical stress (Ky) are, the easier the cracking of coal-rock is. When the crack initiation angle is 45° and 225°, Kx is equivalent to Ky and the crack propagation is deepest. Under the synergistic effect of blast-induced gas that is positively correlated with stress, the attenuation coefficient and the stress will reach the maximum in the same period, and the rock breaking effect of stress wave is the strongest in this period. During blasting, the double-peak of ultrasonic is a non-accidental phenomenon, which reflected the accelerated accumulation of coal-rock internal energy under the synergy of blast-induced gas.

Experimental study on drilling efficiency with compound axial and torsional impact load

Compound axial and torsional impact (CATI) has been widely considered as a highly efficient assistant drilling technology. However, the rock-breaking mechanism of CATI is still unclear due to the lack of experiments on this new drilling method. Therefore, an impact-drilling experimental device was developed to investigate the influence of the impact angle (load direction), amplitude, and frequency on the drilling efficiency. The results show that impact angles that are too large or too small both have negative effects on the bit performance. There is an optimal impact angle for certain combinations of impact amplitude and frequency, and the range of the optimal impact angle is 40°–60° under the test conditions of this study. The optimal impact angle decreases with the increase of the impact amplitude. Increasing the impact amplitude or decreasing the impact frequency can increase the drilling efficiency of the Polycrystalline Diamond Compact (PDC) bit. This study reveals the rock-breaking mechanism of the compound axial and torsional impact drilling technique and provides a reference for designing such drilling tools.

Composite rock-breaking of high-pressure CO2 jet & polycrystalline-diamond-compact (PDC) cutter using a coupled SPH/FEM model

CO 2 drilling is a promising underbalance drilling technology with great advantages, such as lower cutting force, intense cooling and excellent lubrication. However, in the underbalance drilling, the mechanism of the coupling CO 2 jet and polycrystalline-diamond-compact (PDC) cutter are still unclear. Whereby, we established a coupled Smoothed Particle Hydrodynamics/Finite Element Method (SPH/FEM) model to simulate the composite rock-breaking of high-pressure CO 2 jet & PDC cutter. Combined with the experimental research results, the mechanism of composite rock-breaking is studied from the perspectives of rock stress field, cutting force and jet field. The results show that the composite rock-breaking can effectively relieve the influence of vibration and shock on PDC cutter. Meanwhile, the high-pressure CO 2 jet has a positive effect on carrying rock debris, which can effectively reduce the temperature rising and the thermal wear of the PDC cutter. In addition, the effects of CO 2 jet parameters on composite rock-breaking were studied, such as jet impact velocity, nozzle diameter, jet injection angle and impact distance. The studies show that when the impact velocity of the CO 2 jet is greater than 250 m/s, the CO 2 jet could quickly break the rock. It is found that the optimal range of nozzle diameter is 1.5–2.5 mm, the best injection angle of CO 2 jet is 60°, the optimal impact distance is 10 times the nozzle diameter. The above studies could provide theoretical supports and technical guidance for composite rock-breaking, which is useful for the CO 2 underbalance drilling and drill bit design.

Critical parameters investigation of rock breaking by high-pressure foam fracturing method

As a new and mild rock breaking method between blasting method with high stress loading rate and hydraulic fracturing method with low stress loading rate, high-pressure foam fracturing method has advantages of no spark, less dust, no harmful gas, controllable breaking shape and so on. In this paper, the rock breaking characteristics of 700 × 700 × 700 mm rock samples were studied by using high-pressure foam fracturing device. The influence of borehole depth, seal length and foam pressure on rock breaking were analyzed. The results show that the rock breaking weight increases with an increase of borehole depth and foam pressure, and decreases first and then increases with a decrease of seal length. When the borehole depth is 100–120 mm, the seal length is 30–10 mm and the foam pressure is 15–16.5 MPa, the failure mode of rock changed from blast crater to stripping large stone. These results have certain guiding value for revealing the rock breaking mechanism and engineering application by high-pressure foam fracturing method.

Orthogonal experiment on underwater rock breaking by gas–liquid two-phase jet

Cutting and breaking operations are common in underwater drilling and mining engineering. Because the resistance of the underwater environment is reduced, the gas-assisted water jet method improves the rock-breaking performance of ordinary water jets. However, the rock-breaking mechanism of a gas–liquid two-phase jet in an underwater environment is unclear. Furthermore, the influence of the jet parameters on its rock-breaking performance needs to be studied. Therefore, the fundamental structure of an underwater gas–liquid two-phase jet was analyzed further in this work. On this basis, a rock-breaking orthogonal experiment was conducted. The effects of the water jet system pressure, air flow pressure, transverse velocity, and erosion distance on the rock-breaking performance of underwater gas–liquid two-phase jet were investigated. The essential reasons for the variation in jet erosion performance were discussed. Furthermore, the order of importance and significance of the above four factors on the rock-breaking performance were analyzed, and reasonable jet working parameters were determined. The results show that rock breaking is the result of the joint action of the microjet formed by the water-phase jet impact and gas-phase cavitation collapse. In general, the system pressure has the highest impact on the rock-breaking performance. Therefore, a higher water-jet system pressure should be applied in underwater gas–liquid two-phase jet-breaking operations. The preferred values of the working parameters for the test conditions and selected parameters are as follows: air flow pressure = 0.4 MPa, traverse velocity = 3 m/min, and erosion distance = 10 mm. The research results can provide technical guidance and a reference for underwater cutting and breaking operations.

Flow field investigation of a straight–swirling integrated jet for radial jet drilling

Radial jet drilling (RJD) technology is applied to low permeability oil-gas reservoirs, coalbed methane, and shale gas. A self-propelled bit is an integral component. The external flow fields of a new-type of straight–swirling integrated jet (SSIJ) were tested under non-submerged conditions and the key parameters of the jet were studied in this paper. A rock-breaking experiment at a specific target distance was conducted to verify the PIV test results. It is indicated that, from the introduction of a central straight jet, the central low-axial-velocity zone can be eliminated, and the maximum radial and tangential velocities are not at the jet axis. Second, the hydraulic parameters do not affect the velocity distribution law, but significantly affected by nozzle structures. The straight-to-swirl ratio and the dip-angle of an impeller slot should be 0.8 and 45°, respectively, to obtain a flow field with both a higher central velocity and larger coverage area. Through the rock-breaking experiment, it was found that when the dip-angle is 45° (swirl number of 0.45), both the borehole depth and diameter are larger, and the bottom is the flattest, simultaneously, the correctness of the PIV test is verified. Finally, since the two jets (straight & swirling) have interacted and mixed after ejected from the nozzle, there is no constant velocity core, the SSIJ has no initial and basic sections compared with the conventional combined jet, and could be divided into a transition section and uniform-mixing section along the axial. By analyzing the swirl numbers at different positions, it was found that the SSIJ has a straight, weak-swirl, strong-swirl, and periphery jet zones along the radial. The results can guide rock-breaking mechanism revelation and the optimal design of an SSIJ bit for the RJD technology.

The mechanism of microwave rock breaking and its potential application to rock-breaking technology in drilling

The exploration and development of oil and gas resources have shifted from shallow to deep and ultradeep. The difficulty of rock breaking has also increased, introducing new challenges to traditional rock-breaking technology. Hence, there is an urgent need to develop new rock-breaking technologies to improve the development efficiency of deep oil and gas resources. Therefore, this study focused on the new microwave rock-breaking technology and conducted experimental and numerical simulation research on typical deep, hard rock granite. The research results showed that granite in the microwave field exhibited high-temperature melting and fracture, and the highest temperature could reach 550 °C. Under the irradiation of circulating microwaves, a minimum irradiation time threshold of 3 min was needed to cause irreversible damage to the rock. The numerical simulation results showed that the interaction of thermal stress and in situ stress would cause the inside of the rock stratum to separate into a disturbed deterioration area, disturbed unloading area and initial stress area. These results are expected to provide the necessary technical guidance and theoretical support for the research and development of high-efficiency rock-breaking drilling for deep hard rock.

Study and Application of Rock Breaking Mechanism of Concentrated Water Hydraulic Smooth Blasting in Broken Sand-Stone Geological Conditions

The use of polyenergy water pressure controlled blasting technology in tunnel construction is gradually being promoted, and the technology is often used in hard rock, and the mechanism of rock breaking in fractured sandstone strata is still lacking systematic research. The above mechanism was investigated using a combination of field experiments and ANSYS/LS-DYNA numerical simulation, and the results showed the following: (1) In the case of joint-hole blasting, the concentrating jet formed by the concentrating tube can effectively achieve controlled directional blasting of fractured sandstone. (2) The use of gun clay to seal the hole can effectively improve the stability of the blasting effect, reduce the waste of explosive gas, and effectively extend the duration of action. (3) The water bag enhances the blasting effect through the water wedge effect in the broken surrounding rock and also has the function of energy storage, which can effectively improve the blasting effect when combined with the gun clay. (4) The rock-breaking mechanism of polyenergy hydropressure smooth blasting in fractured sandstone geological conditions is the dual rock-breaking action of “polyenergy jet + water wedge action.” (5) In this article, based on the analysis of blasting mechanism, the deployment method of polyenergy hydropressure smooth blasting is designed and has been well applied in engineering practice.

Cutting performance and contact behavior of partial-wear TBM disc cutters: A laboratory scale investigation

The rock cutting performance and cutter-rock contact behavior of partial-worn cutters are investigated via scaled rock cutting experiments. The results reveal that the partial-wear of the cutter reduces the normal force, while the tangential force increases. Changes are also observed in the rock breaking mechanism. Although the rock breaking capacity of the partial-worn cutters is not significantly reduced, the increase of volume ratio of the rock powder results in a rise in specific energy, indicating that the energy utilization of the partial-worn cutters is relatively low. In addition, plastic deformation and abrasive wear are observed on the partial-worn cutter surface. The changes of motion modes and surface geometry play a key role in the above-mentioned phenomena.

Numerical investigations on rock breaking mechanism and parameter influence of torsional percussive drilling with a single PDC cutter

Torsional percussive drilling tool was designed to reduce the risk of stick-slip and improve drilling efficiency, but there are few studies on the influence of torsional impact parameters, including amplitude, frequency, waveform, on the rate of penetration (ROP) while drilling the hard rock. The design of the torsional percussive drilling tool was proposed, and the mathematical model of torsion hammer movement was developed, the Riedel-Hiermaier-Thoma (RHT) rock dynamic material model was applied to analyze the rock-breaking process of the tool. A numerical model of hard rock cutting with a single polycrystalline diamond composite (PDC) bit cutter was established, the influences of amplitude, frequency, time parameters, and waveforms on the penetration depth of the PDC bit cutter were analyzed in the process of torsional percussive drilling. The results showed that the numerical simulation results were in good agreement with the indoor test results, which verified the correctness of the numerical simulation. Compared with conventional drilling, the penetration depth of torsional percussive drilling was increased by 20.0%, and the cutting debris particles were smaller, which was conducive to improving ROP and reducing stick-slip vibration of the bit. With the increase of the peak value of torsional percussive amplitude, the penetration depth of the cutter first increased and then decreased. When the ratio of torsional impact load to axial static load was 1.0, the penetration depth of the drilling cutter reached a maximum. When the torsional impact frequency was 40 Hz–100 Hz, the penetration depth decreased with the increase of the frequency, but the diameter of cuttings became smaller and smaller, which was conducive to cleaning the wellbore with drilling fluid. The latter has the highest rock-breaking efficiency compared with triangular, sine, and rectangular waves. Finally, suggestions are given on how to adjust these parameters by adjusting engineering parameters and tool structure parameters to optimize the efficiency of the torsional percussive drilling tool.

Rock cutting mechanism of special-shaped PDC cutter in heterogeneous granite formation

Special-shaped Polycrystalline Diamond Compact (PDC) cutters show the superiority for improving rate of penetration (ROP) in hard abrasive formation drilling. However, few attentions have been paid on its geometric design and rock cutting mechanism. To deepen the understanding of special-shaped PDC (SPDC) cutter's rock-breaking mechanisms and provide a certain reference for the selection and application of SPDC cutters in the specific target formation, this paper firstly proposes two new principles (loading performance (LP) and cutting performance (CP)) as well as their evaluation indexes for evaluating the cutting process of cutters from the relationship between geometry, aggressiveness, stress state, working life, and rock cutting efficiency of SPDC cutters. Then, using gray-white granite as the research object, the finite element heterogeneous granite model, whose relative percentage error of all calibration quantities with experiment are less than 7%, based on Voronoi tessellation and the granite cutting model of 10 types cutters under confining pressure are established. Finally, the cutting results are discussed, and the CP of all cutters are evaluated. It's found that the ductile-brittle failure mode in the removal area and the influence region of cutting process determine the rock failure mode of SPDC cutters. The confining pressure has an inhibitory effect on rock cutting efficiency, it's because the high confining pressure leads to ductile dominated failure mode in removal area. The confining pressure weakens the LP and CP of cutters, but has no obvious effect on their stress distribution state. With the increasing of confining pressure, “ductile-plastic stick-slip cutting” effect is produced in the removal area. The conical-shaped cutter (whose CP coefficient is 4.3 × 10 −3 - 5.4 × 10 −3 N −1 ) and saddle-shaped cutter (whose CP coefficient is 3.4 × 10 −3 - 4.8 × 10 −3 N −1 ) are the optimal selection for heterogeneous granite formation. On the contrary, the three-blade cutter and cusp-shaped cutter are unsuitable for heterogeneous granite formation. This research leads to an enhanced understanding of rock-breaking mechanisms of SPDC cutters, and provides the basis to select of SPDC cutters for the specific target formation. • A heterogeneous granite generation method based on Voronoi tessellation using finite element method. • The rock-cutting mechanisms and differences of 10 kinds of PDC cutters. • A new ideas and methods for parameter optimization and performance evaluation of special-shaped PDC cutter using FEM.

Rock breaking characteristics of cyclic electrohydraulic shockwaves

In recent years, to further increase the rate of penetration (ROP), cyclic electrohydraulic shockwaves drilling (EHSD) has been proposed. It is of great significance to study the rock breaking characteristics of this technology. Firstly, a series of experiments were carried out using three different types of sandstone rock samples. The results showed that all the rock samples were fractured after 15 to 40 impacts. Before fracture, the damage of rock samples can be divided into internal damage and external damage, including surface pits, orthogonal central tensile cracks developed from the bottom face, and other tensile cracks. Secondly, based on stress wave theory, a theoretical model for calculating the P-wave peak stress in the rock sample was established, and the calculation was carried out by MATLAB. The simulation results for S2-1 showed that the maximum transmitted compressive stress near the top face reaches 163.5 MPa. The P-wave reflected from the lateral faces formed a superposition area of tensile stress, and the maximum tensile stress reaches 5.44 MPa. With the increase of impact times, the maximum tensile stress gradually decreases and approaches the top face, which explains the formation of the orthogonal central cracks. In addition, it was found that rock samples are subject to the alternating changes of compressive and tensile stress under a single impact. Finally, based on the analysis and discussion of the above results, the rock breaking mechanism of EHSD was analyzed. The results showed that the EHSD mainly damages the rock through the water wedge effect, the tensile stress near the crack boundary, and alternating stress. The research can improve our understanding of how EHSD breaks rocks, which can promote the application of this technology in field drilling.

Research on the mechanism and performance of free-surface rock breakage by a disc cutter

To reveal the mechanism of free-surface rock cutting by a disc cutter and study the effect of cutting parameters on rock cutting performance, a numerical model of rock breakage by a disc cutter is established with the particle flow code (PFC). The numerical model of rock breakage by a disc cutter has been verified by rock breaking tests and can satisfactorily reproduce the rock cutting process. Hence, a numerical model was used to further study the rock breaking mechanism and effects of cutting parameters. The obtained results show that the penetration force first linearly increases and then instantaneously declines. The crushing zone gradually formed with increasing penetration force, and after that, the main crack within the rock rapidly extended to the free surface, which resulted in a decline in the penetration force. The formation of the crushing zone and main crack propagation were influenced by compression stress and tension stress, respectively, and the dissipation of energy was mainly in the stage of crushing zone formation. The peak penetration force has a linear relationship with breaking depth. The specific energy first increases and then decreases with increasing breaking depth, and the minimum value is in the range of 20–25 mm. With increasing tip width, the peak penetration force linearly increases, and the specific energy first increases and then decreases. The optimal rock breaking performance was obtained when the tip width was approximately 1 mm. Compared with the traditional rock breaking method, the preslotting method can significantly reduce the cutter load. When the breaking depth is 20 mm, the peak penetration force under the condition with a preslotting depth of 5 mm decreases by over 87% compared to that without preslotting. The preslotting method provides a potential approach for efficiently breaking hard and extremely hard rock.

Study on Rock-Breaking Mechanism of Axe Cutter Under High-ROP Condition and its ROP Increase Practice

Study on Rock-Breaking Mechanism of Axe Cutter Under High-ROP Condition and its ROP Increase Practice

Full-scale linear cutting tests to study the influence of pre-groove depth on rock-cutting performance by TBM disc cutter

To improve the tunnelling performance of tunnel boring machines in hard rock and extremely hard rock environments, abrasive water jet assisted rock breaking is a possible solution that has currently attracted considerable attention. In this approach, the depth of the pre-cut groove is a key factor affecting the performance of assisted rock breaking. To study the influence of the pre-groove depth on assisted rock breaking performance, a full-scale linear cutting test of pre-grooved rock was conducted with a disc cutter. The influence of the pre-groove depth on the rock breaking effect under the same-trajectory mode (SM) and different-trajectory mode (DM) of assisted cutting was explored, and the mechanism of assisted rock breaking with different groove depths was revealed. Compared with the complete cutting mode (CM), the results showed that the normal force (FN), rolling force (FR), side force (Fs), and specific energy (SE) under SM decrease as the groove depth increase. Under DM, the FN, FR, and SE decrease as the average groove depth on both sides increase, and the Fs decreases as the minimum groove depth increases. In SM and DM, the cutting SE first decreased rapidly and then tended to decrease slowly as the groove depth increased, indicating the existence of an optimal groove depth. The rock fracture patterns under SM and DM are more prone to tensile failure, which significantly reduces the rock cutting force and SE of the disc cutter. These results can provide inspiration for the design and construction of new types of tunnel boring machines that are assisted by abrasive water jet pre-grooving.

Research on the Optimization of a Drilling Rock Breaking Method Based on Fuzzy Cluster Analysis

Improving drilling efficiency is the best way to reduce drilling costs and the choice of the drilling mode is instrumental in doing so. At present, however, a standard approach for the optimization of these processes does not exists yet. Through a comparative statistical analysis of the rock-breaking mechanisms and the characteristics of different drilling methods, this research proposes a set of cues to achieve this objective. Available statistical data are classified by means of a fuzzy cluster analysis according to the anti-drilling characteristic parameters of formation. The results show that different drilling methods rely on their own rock breaking mechanisms and have distinct characteristics. The rotary table drilling method is the most commonly used drilling mode, however, it displays some limitations with regard to deep wells, ultra-deep wells and difficult formations. The combined drilling method has the advantages of both the rotary table drilling and the down-hole power drilling modes. Polycrystalline diamond compact (PDC) drill bits can lead to good results for medium hardness and weakly abrasive formations. Underbalanced drilling for formations with high hardness and strong abrasiveness displays some limitations.

Rock-Breaking Mechanism and Parameter Optimization of Ridged Pdc Cutters

Rock-Breaking Mechanism and Parameter Optimization of Ridged Pdc Cutters

Research on cutting track and working load of directional drilling PDC bit

Polycrystalline diamond compact bit directional drilling technology has proven to be an important means to effectively improve drilling efficiency and save drilling costs. Under the directional drilling condition of PDC bit, due to the existence of screw angle or offset mechanism, the center axis of the bit deviates from the center of the borehole, and the cutting teeth of the bit do complex alternating trajectory cutting motion at the bottomhole and the trajectory lines of the cutting teeth themselves intersect in space. (1) The kinematics analysis of directional drilling PDC bit under different working conditions was carried out, including motion analysis at the stage of sliding steering deflection and motion analysis at the stage of composite steering drilling. The kinematics equation of PDC bit under sliding steering deflection and motion space equation of PDC bit cutters at the stage of composite steering drilling were deduced respectively. (2) By establishing the calculation model of cross Angle of cutters under directional drilling conditions, the influences of transmission ratio, tooth placement, offset and deflection Angle on the cross angle are analyzed. The movement law of PDC cutters and the bottom hole morphology formed by bit are studied under directional drilling conditions. (3) Based on the digital simulation system of PDC bit rock breaking, the cutting load under the state of alternating trajectory was studied. On this basis, the cutting mechanics model of PDC bit cutters in directional drilling was established. At the same time, the accuracy of simulation analysis was verified by laboratory unit experiments. It is considered that PDC cutters can form alternate cutting path in directional drilling, which can easily invade rock and improve mechanical efficiency. The larger the intersecting angle of the cutting teeth, the more likely the rock is to form a through crack and then be broken. When the intersecting angle is 90°, the average energy consumption of rock breaking in cross cutting is reduced by about 32.9% compared with parallel cutting. The research results have improved the understanding of the rock breaking mechanism of PDC bit directional drilling, provided a new technical means for bit performance analysis and have also provided a reference for the design of new personalized bits under the discussed working conditions.

Experimental and numerical investigations on rock-breaking mechanism of rotary percussion drilling with a single PDC cutter

To achieve high-efficiency drilling in hard formations, the rotary percussive drilling tool was designed to improve rock-breaking efficiency. Currently, few studies are conducted to analyze the relationship between the percussion parameters and rock breaking efficiency, considering the dynamic characteristics of rock during rock breaking. According to this situation, to quantify the influence of different percussion parameters on the rock-breaking efficiency, a series of dynamic experiments on hard rock and numerical simulations were carried out. Orthogonal experiments of split Hopkinson pressure bar (SHPB) device were conducted to analyze 13 parameters of the Riedel Hiermaier Thoma (RHT) material model. A polycrystalline diamond compact (PDC) cutter was selected as the research object, and a corresponding numerical model was established, considering the actual size of the drilling teeth and the load transmitted to the formation. Subsequently, the numerical simulation results of the SHPB test and cutting experiment were verified using the physical model. A sensitivity analysis was carried out to analyze the effect of different factors, including dynamic amplitude, frequency, and waveform, on the penetration depth and eroded volume fraction of formation. The results proved that rotary percussion drilling increased the penetration depth and eroded volume fraction of hard rock. The rock-breaking efficiency of the square wave was more obvious than that of the triangular and sine waves. Increasing the dynamic to static load ratio was beneficial for increasing the penetration depth and eroded volume fraction of formation. When the frequency increased, the cutting section became smoother, and the cuttings were finer, which was beneficial in cleaning the borehole. • Orthogonal experiments were used to obtain RHT material model parameters. • The physical model verified the rock failure of numerical simulation. • The rock-breaking efficiency of the square wave was most apparent.

Experimental study on rock-breaking mechanism of micro-coring PDC bit

In this paper, in order to study the rock-breaking mechanism of the micro-coring PDC bit, a series of unit breaking experiments containing 2 breaking forms (static-pressure breaking and fracture breaking) are conducted on core columns of sandstone, limestone, and granite. Besides, a full-sized micro-coring PDC bit with a diameter of 152.4 mm is designed and manufactured and is used to conduct an indoor experiment on multiple sized sandstone core columns. The unit breaking experiment results show that ROP (rate of penetration) of the fracture breaking is higher than the static-pressure breaking. The indoor experiment results of the full-sized bit show that ROP of the micro-coring PDC bit is 49–112% higher than the conventional bit and that the diameter of core column shows greater influence on ROP, while the height of the column shows smaller. Moreover, the micro-coring PDC bit realizes volumetric fracture on all of the three types of rock samples. Since volumetric fracture produces large rock debris, the rock-breaking efficiency and ROP of the micro-coring PDC bit will be improved significantly.