Abstract High‐voltage electric pulse rock breaking has excellent potential for exploiting deep geothermal resources. Numerous researchers have conducted experimental studies on this topic, particularly in rock mechanics, where the breakdown occurs. However, there has been limited scholarly research on drilling fluid. Therefore, the study focuses on the drilling fluid suitable for electric pulse drilling, considering the characteristics of electric pulse rock breaking, which differ from traditional rock breaking. The study focused on the impact of various drilling fluid parameters on the effectiveness of electric impulse rock breaking using red sandstone as the experimental material. This was investigated using the finite element method, and indoor electric rock‐breaking tests were conducted in a drilling fluid environment. The results indicate that the plasma channel mainly grows in the permeable layer of the drilling fluid, resulting in shallow rock breaking depth in the drilling fluid environment. The pore permeated by drilling fluid guides the growth of the plasma channel. The higher the conductivity of the drilling fluid, the closer the ion channel of rock breaking by electric pulse is to the rock surface. This results in a smaller crushing volume and shallower damage depth, which is more detrimental to rock breaking by an electric pulse. The viscosity of drilling fluid can impede the breakdown to some extent. In this paper, the influence of drilling fluid parameters on electro‐pulse rock‐breaking technology is preliminarily studied, which has significant reference value for the selection of actual drilling fluid.
Summary High-voltage electric pulse (HVEP) drilling technology offers advantages such as high rock-breaking efficiency and low energy consumption. However, its effectiveness is influenced by parameters including pulse voltage magnitude and the shape and structure of the high-voltage electrode drill bit (HVED). Currently, there is limited research on the mechanisms by which high pulse voltages (>100 kV) affect rock dielectric breakdown and the patterns of pulse voltage generation. To better reflect the impact of various parameters on rock-breaking performance during HVEP drilling, this study conducts laboratory experiments on HVEP rock breaking. The effects of pulse voltage magnitude, different rock samples, and various HVED shapes on HVEP rock-breaking efficiency were investigated. The experimental results indicate that the cracks generated after the electric pulse breakdown of rock are predominantly tensile, with a hackly pattern, and propagate to significant depths. When shear cracks form inside the rock, they predominantly propagate as small crack growths, resulting in a larger area of the rock being affected by the cracks. The pentagonal prism-shaped HVED exhibited the highest average standard deviation in rock-breaking performance, indicating the greatest variability. Cylindrical and conical HVEDs showed better rock-breaking performance with deeper fractures, though concave surfaces were observed in the center of the fractures. Triangular and quadrangular prism-shaped HVEDs demonstrated the most consistent rock-breaking quality. In addition, increasing the electrode bit diameter reduced the maximum electric field strength within the rock, increased the average electric field strength, and expanded the breaking range. This study provides valuable insights for the development of electric pulse rock-breaking tools and the advancement of HVEP drilling technology.
Hydraulic-electric rock fragmentation (HERF) plays a significant role in improving the efficiency of high voltage pulse rock breaking. However, the underlying mechanism of HERF remains unclear. In this study, considering the heterogeneity of the rock, microscopic thermodynamic properties, and shockwave time domain waveforms, based on the shockwave model, digital imaging technology and the discrete element method, the cyclic loading numerical simulations of HERF is achieved by coupling electrical, thermal, and solid mechanics under different formation temperatures, confining pressure, initial peak voltage, electrode bit diameter, and loading times. Meanwhile, the HERF discharge system is conducive to the laboratory experiments with various electrical parameters and the resulting broken pits are numerically reconstructed to obtain the geometric parameters. The results show that, the completely broken area consists of powdery rock debris. In the pre-broken zone, the mineral cementation of the rock determines the transition of type CⅠ cracks to type CⅡ and type CⅢ cracks. Furthermore, the peak pressure of the shockwave increased with initial peak voltage but decreased with electrode bit diameter, while the wave front time reduced. Moreover, increasing well depth, formation temperature and confining pressure augment and inhibit HERF, but once confining pressure surpassed the threshold of 60 MPa for 152.40, 215.90, and 228.60 mm electrode bits, and 40 MPa for 309.88 mm electrode bits, HERF is promoted. Additionally, for the same kind of rock, the volume and width of the broken pit increase with higher initial peak voltage and rock fissures will promote HERF. Eventually, the electrode drill bit with a 215.90 mm diameter is more suitable for drilling pink granite. This research contributes to a better microscopic understanding of HERF and provides valuable insights for electrode bit selection, as well as the optimization of circuit parameters for HERF technology.
With the progress of engineering technology, the natural frequency of structures can be easily obtained by dynamic testing. If the relationship between the fundamental frequency and the constraint stiffness is analysed, the constraint stiffness of the foundation can be identified. To this end, the vertical vibration of piers is first carried out, and a dynamic identification method for constraint stiffness is proposed. Relying on a project example of a bare pier, the vertical fundamental frequency of the test pier is measured by the pulsation method. Then finite-element software is used to establish a test pier model. The stiffness identification is simulated in the completion stage by adding elastic support and concentrated mass on the top of the model pier. The results show that the difference between the identification results of the bare pier and the calculated value according to the empirical formula is only 2.97%. The error of the identification results obtained by simulating the completion stage is less than 2.34%, and decreases with the increase of the constraint stiffness of the pier top. This method is highly accurate and therefore suitable for identifying the vertical restraint stiffness of the foundation of constant section piers of continuous beam bridges.
Abstract The high‐voltage electric pulse fracturing (HVEPF) technology represents a novel and highly promising approach in rock fracturing. The investigation of thermal damage inflicted upon rocks by high‐voltage electrical pulses under multi‐physical field coupling is of great significance in the development of deep geothermal energy. This study establishes a damage model for rocks under electric fragmentation conditions by integrating electric field, heat transfer field, and solid mechanics field. Based on the developed damage model, the insulating properties, temperature variations, and forms of damage of rocks during electric fracturing are explored. Subsequently, the influence of voltage on rock damage status is investigated. The findings reveal that damage to the rock does not occur immediately after electrical breakdown; rather, it increases with the growth of current and temperature within the breakdown channel. Initial damage occurs at the ends of the breakdown channel, followed closely by damage in the central region of the channel. The predominant form of damage in rocks is tensile failure, with shear failure playing a secondary role, and the volume of damage increases with voltage. These results elucidate the characteristics of rock damage during electric fracturing, providing valuable insights for the engineering application of electric fracturing techniques.
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