To reveal the evolution law of coal skeleton deformation during the process of CO2 flooding and displacing CH4 in coal seam, a fluid-solid coupling mathematical model of CO2 injection enhanced CH4 drainage was established based on Fick’s law, Darcy’s law, ideal gas state equation, and Langmuir equation. Meanwhile, numerical simulations were carried out by implementing the mathematical model in the COMSOL Multiphysics. Results show that the CH4 content of both regular gas drainage and CO2 enhanced gas drainage gradually decreases with time, and the decreasing rate is high between 10 and 60 days. Compared with regular gas drainage, the efficiency of CO2 enhanced gas drainage is more obvious with greater amount of CH4 extracted out. When coal seam gas is extracted for 10, 60, 120, and 180 days, CH4 content in coal seam is reduced by 5.2, 17.2, 23.6, and 26.7%, respectively. For regular gas drainage, the deformation of coal skeleton is dominated by the shrink of coal matrix induced by gas desorption, and the strain curve shows a continuous downward trend. For CO2 enhanced gas drainage, the strain curve of coal skeleton showed a decrease—rapid increase—slow increase trend. The evolution of permeability is opposite to the evolution of coal skeleton strain. Higher gas injection pressure will lead to a greater coal skeleton strain. The pumping pressure affects the deformation of coal skeleton slightly compared with that of initial water saturation and initial temperature. Greater initial water saturation leads to larger deformation of coal skeleton in the early stage. The strain value of coal skeleton gradually tends to be consistent as gas injection prolongs. Higher initial temperature leads to greater reduction in coal skeleton strain when the gas injection continues. Research achievements provide a basis for the field application of CO2 injection enhanced CH4 drainage in underground coal mines.
Owing to the exhaustion of shallow coal resources, deep mining has been occupied in coal mines. Deep buried coal seams are featured by the great ground stress, high gas pressure, and low permeability, which boost the risk of gas disasters and thus dramatically threaten the security about coal mines. Coal seam gas pressure and gas content can be decreased by gas extraction, which is the primary measure to prevent and control mine gas disasters. The coal mass is simplified into a continuous medium with dual structure of pores and fractures and single permeability. In consideration of the combined effects of gas slippage and two-phase flow, a hydraulic-mechanical coupling model for gas migration in coals is proposed. This model involves the equations of gas sorption and diffusion, gas and water seepage, coal deformation, and evolution of porosity and permeability. Based on these, the procedure of gas extraction through the floor roadway combined with hydraulic punching and ordinary drainage holes was simulated, and the gas extraction results were used to evaluate the outburst danger of roadway excavation and to verify the engineering practice. Results show that gas extraction can reduce coal seam gas pressure and slow down the rate of gas release, and the established hydraulic-mechanical coupling model can accurately reveal the law of gas extraction by drilling and punching boreholes. After adopting the gas extraction technology of drilling and hydraulic punching from the floor roadway, the remaining gas pressure and gas content are reduced to lower than 0.5 MPa and 5.68 m3/t, respectively. The achievements set a theoretical foundation to the application of drilling and punching integrated technology to enhance gas extraction.
Hydraulic flushing can increase the efficiency of gas extraction by artificially modifying the coal reservoir. Considering the plastic failure of coal mass, an improved gas–liquid–solid coupling model for hydraulic flushing and gas extraction is constructed. The parameter evolution in the hydraulic flushing process was numerically investigated to determine the optimal borehole arrangement of hydraulic flushing. The results show that the relative permeability of gas gradually increases with the initial dewatering. The gas rates of both regular extraction and hydraulic flushing enhanced extraction show an increasing–decreasing trend. An increased and delayed peak gas rate is observed comparing with the regular extraction, caused by the hydraulic flushing induced new fractures. The area around of borehole is divided into the failure zone, the plastic softening zone, and the elastic zone after hydraulic flushing. The failure zone has the greatest increase in coal permeability, followed by the plastic softening zone, while the elastic zone keeps no significant change. The larger difference between the horizontal stress and vertical stress, the more obvious the elliptical shape of the permeability change area near the borehole, as well as the pressure drop in the elliptical zone. With the increase in the hydraulic flushing radius, the permeability increasing zone and gas pressure decreasing zone gradually increase. Subsequently, the equivalent effective radius and equivalent influencing radius were obtained, as well as the optimal borehole spacing for hydraulic flushing by cross-layer drilling. Finally, the optimal borehole spacing is obtained for different borehole diameters and efficient extraction times. These provide a theoretical guidance for field application of hydraulic flushing in a low-permeable coal seam.
The sediment bedding direction and loading-unloading have significant effect on permeability of coal sample. The coal samples taken from Zhangcun Coal Mine in Shanxi were used to measure coal permeability with different sediment beddings under the effect of loading and unloading by the triaxial coal-rock seepage experimental apparatus. The influence of gas pressure, loading-unloading, and sediment bedding direction on the permeability of coal samples was analyzed, and the functional relationship among these parameters was recovered by fitting. The results show that the permeability of all vertical, dip, and horizontal bedding coal samples decreases exponentially with the increase of effective stress during stress loading, while the permeability of three kinds of bedding coal samples increases exponentially with the decrease of effective stress during stress unloading. Under the same gas pressure, the fracture space of coal samples with vertical and dip bedding is more likely to be compressed and closed at the initial stage of loading, resulting in great decrease of the permeability. In the initial stage of unloading, the microcracks and natural beddings in coals gradually expand and connect. Due to the well development of fractures, the permeability of vertical bedding coal samples increases greatly, while the permeability of dip and horizontal bedding coal samples increases slightly. In the loading process, the permeability of coal sample is vertical bedding > dip bedding > horizontal bedding in order. For the tested samples, the permeability of vertical bedding coal samples is 1.3 to 2.8 times that of dip or horizontal bedding coal samples. In the unloading process, the permeability of the vertical bedding coal samples was 2.8 to 3.3 times that of the dip or horizontal bedding coal samples.
In order to improve efficiency of gas drainage, the reasonable layout parameters for borehole on roadway head are investigated. Assuming that the coal mass has a dual pore structure with fractures and pores, a multiphysical field coupling mathematical model is proposed for gas drainage. The governing equations of gas adsorption, seepage, and diffusion are considered. The process of gas preextraction and excavating-extraction collaboration on roadway heading face in 2606 haulage roadway of Zhangcun mine is modeled by finite element (FE) method. The effects of gas drainage under different drilling arrangements are analyzed. The results show that the fastest decreasing rate of gas content occurs at the beginning of drainage, and decreasing rate of gas content tends to be stable in the later stage. After removing the predraining on roadway head, the gas content at the center of coal wall on the roadway heading face will rebound. The included angle between the borehole and the roadway middle line determines the spatial area of gas drainage. Too small included angle will reduce the area of gas content reduction, while too large included angle will result in a blind area on both sides of the roadway. The change in borehole spacing affects the decreasing rate of gas content in the rock and coal around the roadway. Large spacing may cause an inadequate decreasing rate in the middle of the roadway, while small spacing may cause an inadequate decreasing rate in both sides of the roadway. In the field application, the pure gas flow and gas concentration of drainage show a downward trend in whole. The gas concentration decreased from 13.2% to 10.8%, and the pure gas flow decreased from 10.6 m3/min to 8.15 m3/min. Research achievements can provide a basis for gas drainage in underground roadway.
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