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Featured Application: It solves the bottleneck problem of the low permeability of deep coal seams and the difficulty of coalbed methane mining, greatly improves the efficiency of coalbed methane mining, and reduces the risk of coal mining, which is of great significance to the project. To solve the bottleneck problem of low deep coal seam permeability and difficult coalbed methane (CBM) mining. Combining hydraulic splitting technology and directional drilling technology, a directional hydraulic splitting enhancement method of deep CBM mining was proposed. The selection equation for the directional hydraulic splitting of deep coalbed was constructed. The numerical simulation reveals the variation in coal fractures around different split angles. The split angle under the maximum coal damage effect was obtained. It was found that the combined effect of the double crack damage disturbance region led to reciprocal stress fluctuations during crack development and, eventually, the formation of a zigzag fracture. The larger the splitting angle, the larger the fissure development length and the larger the coal-damaged area. A double crack takes 25% less time to complete propagation than a single crack. When the splitting angle is 90°, the disturbed area occupies 2/3 of the area around the borehole, and the overall fracturing effect is the best. In the application process, the new directional hydraulic splitting technology can increase CBM mining by 5.08%, greatly improve CBM mining efficiency, and reduce the coal mining risk, which is of great significance to the project. [ABSTRACT FROM AUTHOR]

The technology of coal breaking and punching by a high-pressure water jet can increase the permeability of coal seam and prevent gas explosion accidents. As one of the key components of this technology, the structural parameters of the nozzle have an important effect on the performance of the water jet. At present, the relationship between multiple optimization indexes and structural parameters of the nozzle is mostly studied separately. In fact, the influence of the nozzle structural parameters on different optimization indexes is different. When there are multiple optimization indexes, they should be considered collaboratively to achieve the best water jet performance of the nozzle. Therefore, a multi−objective collaborative optimization method is proposed which takes the maximum velocity in X-axis and effective extension distance in Y-axis as the performance evaluation indexes of the water jet. The numerical simulation of the nozzle jet is carried out by computational fluid dynamics(CFD) method, and an orthogonal test database is established. The weight of multi-objective is analyzed, and the key structural parameters of the nozzle are optimized by the combination of BP (back propagation) neural network and genetic algorithms. The results show that the primary and secondary sequence of each structural parameter on is γ > θ > l ∕ d , which could reflect the comprehensive influence on the maximum velocity in the X-axis and effective extension distance in the Y-axis. The optimal structural parameters of the nozzle are, θ = 42.512°, l/d = 2.5608, γ = 12.431°. The field erosion experiment shows that compared with the original nozzle, the water jet performance of the optimized nozzle has been improved, the punching depth has been increased by 72.71%, and the punching diameter has been increased by 106.72%. This study provides a certain reference for the design and optimization of coal breaking and punching nozzle. [ABSTRACT FROM AUTHOR]

Coal seam gas pressure is one of the fundamental parameters used to assess coal seam gas occurrence and is an important index in assessing the risk of gas disaster. However, the geological characteristics of coal seams become increasingly complex with increasing mining degree, thus decreasing the accuracy and success rate of direct methods for measuring gas pressure. To address such issues, we have developed a new method for direct measurement of gas pressure in water-bearing coal seams. In particular, we developed a pressure measurement device based on theoretical analysis and quantified the basic parameters of the device based on well testing. Then, we verified the applicability of our method based on comparative analysis of the results of field experiments and indirect measurements. Our results demonstrate that this new method can resolve the effects of water pressure, coal slime, and other factors on the estimation of gas pressure. The performance of this new method is considerably better than that of traditional methods. In particular, field test results demonstrate that our method can accurately and efficiently measure gas pressure in water-bearing coal seams. These results will be of great significance in the prevention and control of coal seam gas disaster. [ABSTRACT FROM AUTHOR]

Hydration reactions of expansive materials are typically very safe, easy to induce, and low in cost, while the crushing of such materials is typically free from noise, dust, vibration, and toxic gases. In the present study, to realize the application of expansive materials in the prevention of coal seam gas disasters, the microstructure, heat release rate, and expansive pressure of expansive materials were investigated for different degrees of hydration based on temperature and pressure measurements and using a scanning electron microscope (SEM); fracture characteristics were determined based on fracture tests of coal-like materials. The results show that the expansive material with +30% water has the lowest hydration temperature (100°C). The expansive pressure of the steel tube was found to reach 57 MPa, which is deemed suitable for application in coal seams. The strain and displacement of coal-like materials were found to increase with time, with four main cracks appearing. Based on these results, it is feasible that hydration reactions of expansive materials could increase both gas drainage and permeability in coal seams, thus reducing the risk of rock burst around boreholes in coal seams. [ABSTRACT FROM AUTHOR]

Based on airfoil curves that can effectively balance the rectification and drag reduction effects in flight hydrodynamics, we designed an internal streamline structure of jet nozzle for coalbed methane (CBM) mining. The three types of nozzles originating from three typical airfoil curves are compared with the conical nozzle. Results showed that the thin-type streamlined nozzle had the largest effective shock range and least radial divergence and was thus selected as the best nozzle. Moreover, the pressure distribution at the outlet of the nozzle was found to be related to the range and number of small-pressure fluctuations near the wall. A larger number of small-pressure fluctuations and a larger range caused faster pressure of the jet water column to decay along the axial direction. Rectification with a concentrated effect also slowed down the attenuation velocity of the jet-water-column pressure between the concentration point and the nozzle. The variation rule of shock pressure with range was further determined experimentally. We found that the shock pressure of jet water column initially increased within a short distance and then decreased rapidly. The effective shock range of the thin-type streamlined nozzle in air was 1.417 times that of the conical nozzle, and the effective reaming area was 1.104 times greater. Thus, the effect of reaming was effectively improved. The length of the water column at high pressure was larger than that of the conical nozzle, and the shock efficiency was relatively high. [ABSTRACT FROM AUTHOR]

• Studied the migration mechanism of gas bubble in the meso coal fissure. • Explained the mechanism of gas bubbles aggregation in the water injection fissure. • Revealed the correlation between fissure structure and bubble influence. Coal seam water injection is a key technology for achieving the co-mining of coal and coalbed methane. In response to the unclear mechanism of bubble migration in meso fissures that currently dominate the displacement of coalbed methane, this paper proposes a numerical simulation method based on CT images for exploring the migration mechanism of gas bubbles in the meso fissures of coal water injection. The meso fissure geometric modeling of coal water injection was reconstructed using CT scanning image analysis technology. Based on level-set theory, a bubble migration model under a two-phase fluid environment was constructed, and the influence characteristics of different fissure structures on bubble migration were explored. Furthermore, the mechanism of gas bubble migration within the meso fissures of coal water injection was revealed. Results show that the shape of the bubble with a radius of 0.03 mm oscillates obviously in the fissure structure with a length–width ratio of 1.3. The smaller the fissure structure diameter is, the larger the structure length–width ratio is, and the easier the bubble is to form gas resistance. Meanwhile, the larger the curvature of the fissure structure, the greater the contact angle between the bubble and the wall, and the easier it is for the bubble to detach from the wall. During the migration process, the size of the bubble constantly changes. With the increasing length–width ratio and width of the fissure structure, the equivalent radius of the bubble initially increases and then decreases. A negative correlation function is observed between bubble size and flow velocity. At low flow velocity, the volume of the bubble can increase by 18.17 %, and as the flow velocity increases, the volume of bubble can decrease by 54.53 %. In the bubbles aggregation process, an exponential function relationship can be observed between the bubble leading edge position and the bubbles spacing with a negative correlation. The influence of fissure structure on the bubbles is less than the interaction among bubbles. These results provide a theoretical basis for the efficient and increased extraction of coalbed methane. [ABSTRACT FROM AUTHOR]