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Abstract. Based on the gas-geology rules of Chinese coalmines, the outburst accidents are characterized and the main reasons are found out on serious coal and gas outbursts in China. The development history of coal and gas outburst prevention and control are briefly introduced, and various related technologies and management system for outburst prevention were discussed in detail. The problems and development trends of outburst prevention and control in China were put forward in this paper. The results show that the geological conditions and mining conditions are highly complicated compared to other countries; coal seams in China have characteristics of high ground stress, high gas content and gas pressure, low strength and permeability. Current outburst prevention technology is mainly a combination of both regional and local measures. Regional comprehensive outburst-prevention measures include four integrated parts, prediction of the regional outburst danger, regional outburst prevention measures, validation of the measures and onsite verification check. Local comprehensive outburst-prevention measures include prediction of working face outburst danger, working face outburst prevention measures, validation check, and safety protection. With the goal to totally eliminate all outburst accidents effectively and efficiently, supportive policies, both financial and technologic, are necessary to lead and guide coal mining enterprises to manage outburst mine actively, plan production activity reasonably and utilize new technologies and equipments eagerly.

To study the shrinkage rule of borehole diameter and its effect on gas extraction, a viscoelastoplastic model for boreholes considering strain softening and the dilatancy characteristic was established to obtain the expressions of the coal stress, variation in diameter, and pressure relief range. The stress distribution and pressure relief effect of the boreholes in soft and hard coal seams were comparatively analyzed. The shrinkage rule of the borehole diameter was studied. The reasons for the rapid reduction in the extraction concentration of the borehole in soft coal seams were described. A technology of improving the gas extraction effect in soft coal seams was developed. The research results showed that the radius of the plastic softening zone is 0.405 m for a borehole in a soft coal seam and 0.224 m for that in a hard coal seam. This indicates that the borehole in a soft coal seam has a better pressure relief effect. The boreholes in both hard and soft coal seams will incur a shrinkage phenomenon; however, the soft coal seam has low strength and a weak ability to resist damage, and thus the surrounding coal will have a more intense creep deformation, leading to an instability failure during a short period of time and thus, a blocking of the extraction channel, thereby causing a rapid reduction in the gas extraction concentration. The borehole in a hard coal seam also shows a shrinkage phenomenon, but remains in a stable state without a blockage; thus, high-concentration gas can be extracted from this borehole for a long period of time. The geo-stress and coal strength are the two main factors controlling the amplitude of borehole shrinkage. From an increase in stress, the borehole in a hard coal seam shows a more intense creep deformation in a deep mine, which may lead to blockage. The key to improving the gas extraction effect in soft coal seams is to maintain a smooth extraction channel. The full screen pipe is installed through the drill pipe to retain an extraction channel, leading to an average gas extraction increase from 0.043 m 3 /min to 0.12 m 3 / min, an increase of 2.77 times. These research results are consistent with actual production, and can provide theoretical guidance for determining the gas extraction parameters.

The premixed abrasive jet possesses a strong strike ability and is widely used in oil and gas exploitation, machining, rust removal, and other fields. The superstrong, forceful impact of the premixed abrasive jet is mainly provided by high-speed abrasive groups. Hence, the abrasive velocity is the basis of this research, by applying the distribution law of abrasive impact force. In this paper, the particle velocity of the premixed abrasive jet is analyzed theoretically, and the corresponding particle velocity model is established. The real-time contrast interpolation method is employed to solve the problem of the variable drag coefficient. Factors such as the nozzle structure, average abrasive diameter, abrasive density, and jet flow are utilized to determine the abrasive velocity of the nozzle outlet. The numerical solution for the abrasive velocity is obtained by dividing the high-pressure pipe and nozzle into several sections, along the axis. Finally, the calculated particle velocity is compared with the particle image velocity measurement (PIV), to verify the correctness of the established model. These results demonstrate that the model calculation is in effective agreement with the experimental results. The deviation between the theoretical value and the experimental mean is 0.18 m/s. The standard deviation of the experimental results is 3.81-4.22 m/s, while the average error is less than 4%.