Gas emission from the working face in a gas-bearing coal seam impairs safe production. Especially in the coal face, influenced by mining, pressure-relief gas in adjacent seams flows into the working face through the goaf. Moreover, caved overlying strata in different regions show differences in pore and seepage characteristics. Thus, the caving of overlying strata in the goaf of a working face was explored by carrying out physical similarity simulation. Then, based on the characteristics of the geometric shapes of caved overlying strata after mining, a trapezoidal three-dimensional model for gas extraction was established. According to the calculation result, the parameters of field high-level boreholes were optimized. Then, the controllability of gas concentration at the working face after gas extraction was assessed by applying statistical process control. The result showed that after the observed surface of the physical similarity model was lightened, it was more favorable for conducting the test. Moreover, the maximum gas concentration in the goaf was negatively correlated with the diameter of high-level boreholes and the negative pressure for gas extraction. A statistical process control chart revealed that the gas concentrations at the working face were safe after gas extraction based on high-level boreholes, which also validated the feasibility and effectiveness of the model.
相似文献With increasing demands for coal resources, coal has been gradually mined in deep coal seams. Due to high gas content, pressure and in situ stress, deep coal seams show great risks of coal and gas outburst. Protective coal seam mining, as a safe and effective method for gas control, has been widely used in major coal-producing countries in the world. However, at present, the relevant problems, such as gas seepage characteristics and optimization of gas drainage borehole layout in protective coal seam mining have been rarely studied. Firstly, by combining with formulas for measuring and testing permeability of coal and rock mass in different stress regimes and failure modes in the laboratory, this study investigated stress–seepage coupling laws by using built-in language Fish of numerical simulation software FLAC3D. In addition, this research analyzed distribution characteristics of permeability in a protected coal seam in the process of protective coal seam mining. Secondly, the protected coal seam was divided into a zone with initial permeability, a zone with decreasing permeability, and permeability increasing zones 1 and 2 according to the changes of permeability. In these zones, permeability rises the most in the permeability increasing zone 2. Moreover, by taking Shaqu Coal Mine, Shanxi Province, China as an example, layout of gas drainage boreholes in the protected coal seam was optimized based on the above permeability-based zoning. Finally, numerical simulation and field application showed that gas drainage volume and concentration rise significantly after optimizing borehole layout. Therefore, when gas is drained through boreholes crossing coal seams during the protective coal seam mining in other coal mines, optimization of borehole layout in Shaqu Coal Mine has certain reference values.
相似文献Methane content in coal seam is an essential parameter for the assessment of coalbed gas reserves and is a threat to underground coal mining activities. Compared with the adsorption-isotherm-based indirect method, the direct method by sampling methane-bearing coal seams is apparently more accurate for predicting coalbed methane content. However, the traditional sampling method by using an opened sample tube or collecting drill cuttings with air drilling operation would lead to serious loss of coalbed methane in the sampling process. The pressurized sampling method by employing mechanical-valve-based pressure corer is expected to reduce the loss of coalbed methane, whereas it usually results in failure due to the wear of the mechanical valve. Sampling of methane-bearing coal seams by freezing was proposed in this study, and the coalbed gas desorption characteristics under freezing temperature were studied to verify the feasibility of this method. Results show that low temperature does not only improve the adsorption velocity of the coalbed gas, but also extend the adsorption process and increase the total adsorbed gas. The total adsorbed methane gas increased linearly with decreasing temperature, which was considered to be attributed to the decreased Gibbs free energy and molecular average free path of the coalbed gas molecular caused by low temperature. In contrast, the desorption velocity and total desorbed gas are significantly deceased under lower temperatures. The process of desorption can be divided into three phases. Desorption velocity decreases linearly at the first phase, and then, it shows a slow decreases at the second phase. Finally, the velocity of desorption levels off to a constant value at the third phase. The desorbed coalbed gas shows a parabolic relation to temperature at each phase, and it increases with increasing temperature at the first phase, and then, it poses a declining trend with increasing temperature at the rest phases. The experimental results show that decreasing the system temperature can restrain desorption of coalbed methane effectively, and it is proven to be a feasible way of sampling methane-bearing coal seams.
相似文献Pseudo-reservoir stimulation in horizontal well is an effective technique for indirectly extracting coalbed methane (CBM) in soft coal from the surrounding rocks (pseudo-reservoir). However, systematic studies of the theory and on-site application of this technique are still lacking, which severely hinders its application. In this paper, the technical principles of pseudo-reservoir stimulation are analyzed firstly, and then, the technical advantages are demonstrated by experimental tests and theoretical analysis. The results show that the pseudo-reservoir generally possesses considerable gas adsorption capacity, with the gas content of 1.56–4.22 cm3/g (avg. 2.51 cm3/g) in Well XC-01, which can be extracted as supplementary resources. The fracability of the pseudo-reservoirs is 0.73–0.92, which is much higher than that of the coal seam, i.e., 0.03–0.43. Meanwhile, the compressive and tensile strength and cohesion of the pseudo-reservoir are higher than those of the coal seam, indicating pseudo-reservoir stimulation is more conducive to forming fracture network, and maintaining wellbore stability and fracture conductivity. The technical feasibility of pseudo-reservoir stimulation is determined by the regional geological conditions, showing simple tectonic conditions and well-developed surrounding rocks with high fracability and mechanical strength but low permeability, water sensitivity and water content are beneficial for the technique application. Note that the fracture conductivity in pseudo-reservoir is more stable and higher than that in coal seam, pseudo-reservoir stimulation is beneficial for the CBM extraction from both hard and soft coal seams. By minimizing the gas diffusion distance, this technique overcomes the technical obstacles to the CBM commercialized production in soft coal.
相似文献The permeability of a coal seam is an important index for coal mine gas control and coalbed methane development, and its magnitude determines the degree of difficulty of gas drainage. To obtain the permeability value, a dimensionless mathematical model for dual-porosity borehole gas-coupled flow in a coal seam was established and adopted using a simulator developed by our group. A new method of inversion was developed to determine the fracture permeability coefficient λf and the matrix micro-channel diffusion coefficient Km by fitting the simulated results with onsite measured data. A range of simulations quantified the effects of different dimensionless parameters on gas migration. The results verified the feasibility of the inversion method based on the high matching degree of the fitted results, and the dimensionless mathematical model was accurate. The desorption and release of adsorbed gas from the center to the surface in coal matrices were heterogeneous, and unsteady states and gas migration times in coal matrices cannot be neglected. The new method can be introduced to analyze the problem of gas migration in different coal reservoirs, simplify the corresponding calculation and computational processes, and provide guidance in determining the permeability of coal seams.
相似文献In situ stress is not only a vital indicator for selecting explorative regions of coalbed methane (CBM), but also a pivotal factor affecting CBM production. The present study explored whether in situ stress affected the development potential of CBM in western Guizhou, China. To this end, we collected injection/falloff well test data and gas content data from 70 coal seams in 28 wells. The study found that from top to bottom, strike slip fault stress fields (<?500 m), normal fault stress fields (500–1000 m) and strike slip fault stress fields (>?1000 m) were successively developed in western Guizhou. The distribution features of vertical permeability in western Guizhou are consistent with the stress fields' transformation location. The coal permeability in the western part in Guizhou presents a tendency of increase followed by decrease as a result of increased burial depth. The vertical development characteristics of coal seam gas content are controlled mainly by reservoir pressure, and the relationship between reservoir pressure and buried depth shows a linear increase. The CBM in western Guizhou is divided vertically into three development potential regions dependent on the characteristics of burial depth, permeability and gas content of coal seams. The most favorable vertical development potential region in western Guizhou is 500–1000 m. This region exhibits high gas content, high permeability and moderate burial depth, which are favorable for the production of CBM. These research results can provide basis for geological selection and engineering implementation of CBM in western Guizhou.
相似文献Since most coalfields in China are commonly characterized by high gas content and low permeability, there is an urgent need to improve coal seam permeability and further enhance coal bed methane (CBM) extraction efficiency. As an emerging fracturing technology, the CO2 gas fracturing (CGF) technology has been widely used because of its advantages of low cost, environmental protection and high fragmentation efficiency. In order to improve the fracturing ability of CGF technique and optimize the release orifices of discharge head, computational fluid dynamics model was used in this paper to simulate the flow fields of dynamic pressure of gas jet released from the orifices with different structures and other geometrical parameters. The results show that the orifice structure has a great influence on the flow field of gas jet, but little influence on the magnitude of the dynamic pressure. Besides, the maximum dynamic pressure of gas jet linearly decreases with the increase in the number of release orifices. Based on a series of simulation results, the discharge head which has single group of orifices with structure c, diameter of 24 mm can be identified as the best choice for fieldwork. Then, two field experiments were conducted in Pingdingshan and Changping coal mines to evaluate the enhanced CBM extraction efficiency by CGF. The results indicate that the CGF can effectively create a large number of cracks in a large range around the fracturing borehole in the coal seam and further significantly improve the permeability. And the CBM extraction efficiency can be improved to a higher level from a lower level and maintained for a long time. Besides, the effective influence radii caused by CGF in Pingdingshan and Changping coal mines are 15.19 m and 12.5 m, respectively. Compared with other fracturing techniques, the CGF technique has a promising application prospect.
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