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In this paper, research was carried out on borehole protection technology using screen pipes, aiming to address the problems of gas drainage borehole collapse and low gas drainage efficiency in soft coal seams, which have low gas permeability and high gas content. An experiment was conducted to study the compressive resistance and borehole protection effects of polyvinylchloride (PVC), polypropylene (PP), and acrylonitrile–butadiene–styrene (ABS) screen pipes with different sieve opening spacings. The optimal screen pipe material and sieve opening spacing were determined. The borehole protection effect and gas drainage efficiency using three borehole protection methods, namely, bare boreholes, traditional PVC screen pipes, and new ABS screen pipes, were compared and studied on site, and the influence of pipe diameter on borehole protection efficiency was revealed. The results show that the compressive resistance and borehole protection effects of the three types of screen pipes are in the following order: PVC < PP < ABS. ABS screen pipes can more effectively inhibit boreholes from collapse, and the best borehole protection is achieved with a sieve opening spacing of 50 mm. The field tests showed that the gas drainage concentration of the bare borehole in the creep deceleration stage was 113% higher than that of the borehole protected by the ABS screen pipe and 255% higher than that of the borehole with the PVC screen pipe. The screen pipe began to play a role in borehole protection at the beginning of the stable working stage, as the borehole wall began to deform and collapse. On the 30th day of gas drainage, the gas drainage concentration of the borehole installed with the ABS screen pipe was about 9 times that of the bare borehole due to the collapse and blockage of the bare borehole. The new ABS screen pipe with an outer diameter of 40 mm and a sieve opening spacing of 50 mm had a better borehole protection effect and gas drainage efficiency than the traditional PVC pipe. Proper sizing of the screen pipe diameter can improve borehole service life and gas drainage efficiency. The research results are of great significance for solving the problem of gas drainage borehole instability and for improving gas drainage efficiency in soft coal seams. [ABSTRACT FROM AUTHOR]

The paper is devoted to the problem of increasing energy efficiency of coalmine methane utilization to provide sustainable development of geotechnologies in the context of transition to a clean resource-saving energy production. Its relevance results from the fact that the anthropogenic effect of coalmine methane emissions on the global climate change processes is 21 times higher than the impact of carbon dioxide. Suites of gassy coal seams and surrounding rocks should be classified as technogenic coal-gas deposits, while gas extracted from them should be treated as an alternative energy source. Existing practices and methods of controlling coalmine methane need to be improved, as the current “mine – longwall” concept does not fully take into account spatial and temporal specifics of production face advancement. Therefore, related issues are relevant for many areas of expertise, and especially so for green coal mining. The goal of this paper is to identify patterns that describe non-linear nature of methane release dynamics in the underground boreholes to provide sustainable development of geotechnologies due to quality improvement of the withdrawn methane-air mixture. For the first time in spatial-temporal studies (in the plane of CH4-S) of methane concentration dynamics, according to the designed approach, the parameter of distance from the longwall (L) is introduced, which allows to create function space for the analyzed process (CH4 of S-L). Results of coalmine measurements are interpreted using the method of local polynomial regression (LOESS). The study is based on using non-linear variations of methane concentration in the underground boreholes and specific features of their implementation to perform vacuum pumping in the most productive areas of the undermined rock mass in order to maintain safe aerogas conditions of the extraction block during intensive mining of deep-lying gassy seams. Identification of patterns in the influence of situational geomechanical conditions of coal mining on the initiation of metastable gas-coal solution transformation and genesis of wave processes in the coal-rock mass allows to improve reliability of predicting methane release dynamics, as well as workflow manageability of mining operations. Presented results demonstrate that development of high-methane Donbass seams is associated with insufficient reliability of gas drainage system operation at distances over 40 m behind the longwall face. Obtained results confirm a working hypothesis about the presence of spatial migration of methane concentration waves in the underground gas drainage boreholes. It is necessary to continue research in the area of estimating deviation angles of “advance fracturing” zone boundaries from the face line direction. Practical significance of research results lies in the possibility to use them in the development of scientific foundation for 3D gas drainage of a man-made coal-methane reservoir, taking into account spatial and temporal advancement of the production face.

Abstract Nowadays, the water jet technique has been more frequently used for coal bed methane stimulation. As the ultimate component of the water jet system, the nozzle has important influence on the rock breaking ability of water jet. To optimize the nozzle, a simplified model for impinging water jet was established. The effects of geometric structure (cylindrical, conical, and conical‐straight) and parameters (outlet diameter d, conical angle α, and length L) on the flow field of the impinging water jet are simulated through computational fluid dynamics software Fluent14.0. Besides, the effects of inlet pressure (Pin) and target distances (S) on the jet dynamics are also discussed based on the simulation results. The results show that the jet dynamic parameters including dynamic pressure, velocity, and impinging pressure vary with different nozzles. The jet dynamic parameters linearly increase with the inlet pressure, while they first increase then decrease with the increase in target distance. Additionally, the optimal nozzle designed based on the numerical simulation was used in the field test, and the results indicate that this technique can effectively improve the gas drainage efficiency.

Gas drainage in boreholes is characterized as an essential means to control abnormal gas emissions during mining. Unfortunately, its drainage efficiency is terribly restricted by high in-situ stress in deep underground coal seams. Gas drainage involves gas emission under stress constraint conditions (SCCs), as well as multi-scale gas migration (gas desorption, diffusion, and seepage). Nevertheless, how SCCs influence gas emission behaviors remains unclear. Considering this status quo, this paper focuses on researching He and CH 4 emissions under different SCCs, and analyzes the influence of SCCs on gas emission by integrating the initial gas diffusion coefficient and the diffusion attenuation coefficient. In addition, a novel method for calculating equivalent diffusion and seepage pore radii by the T 2 spectrum of nuclear magnetic resonance was proposed. Finally, according to the multi-mechanism gas emission model that tracks the dynamic evolutions of the equivalent diffusion and seepage pore radii, the influences of equivalent pore radius and pressure on the conductance of different diffusion mechanisms were further quantitatively analyzed. The research in this paper is expected to provide a theoretical reference for efficient gas drainage in deep coal seams. • The influence of stress on the apparent gas diffusion coefficients was analyzed. • The equivalent diffusion and seepage pore radii were proposed and calculated by NMR. • A multi-mechanism gas emission model considering dynamic pore radius was developed. • A testing method of adsorbed CH 4 desorption amount in fractured coal was proposed. [ABSTRACT FROM AUTHOR]

Nowadays, the water jet technique has been more frequently used for coal bed methane stimulation. As the ultimate component of the water jet system, the nozzle has important influence on the rock breaking ability of water jet. To optimize the nozzle, a simplified model for impinging water jet was established. The effects of geometric structure (cylindrical, conical, and conical‐straight) and parameters (outlet diameter d, conical angle α, and length L) on the flow field of the impinging water jet are simulated through computational fluid dynamics software Fluent14.0. Besides, the effects of inlet pressure (Pin) and target distances (S) on the jet dynamics are also discussed based on the simulation results. The results show that the jet dynamic parameters including dynamic pressure, velocity, and impinging pressure vary with different nozzles. The jet dynamic parameters linearly increase with the inlet pressure, while they first increase then decrease with the increase in target distance. Additionally, the optimal nozzle designed based on the numerical simulation was used in the field test, and the results indicate that this technique can effectively improve the gas drainage efficiency. [ABSTRACT FROM AUTHOR]