Hydraulic fracturing is an efficiency approach to improve underground gas drainage. Although the interaction of fluid and coal has been comprehensively investigated in fracturing process and gas drainage process, fewer scholars have combined these two processes together and taken the gas-water two-phase flow into account, which brought a large deviation for design of hydraulic fracturing enhancing underground gas drainage. In this paper, we proposed a fully coupled hydraulic stress damage mathematical model considering gas-water two-phase flow, which can be used to simulate the whole process of hydraulic fracturing enhancing underground gas drainage. The coal seam is simplified as a dual-porosity single-permeability elastic media with elastic modulus reduce and permeability increase when encountered damage. The permeability and porosity serving as the coupling term is a function of stress, water/gas pressure, gas ad/desorption, and damage value. The proposed model was first verified by showing that the modeled gas flux agrees with the field data. The evolution laws of permeability and gas pressure during hydraulic fracturing enhancing underground gas drainage were studied and several influence factors were analyzed by accomplishing a series of simulations. Gas drainage can be effectively enhanced only when the hydraulic fracturing induced damage zone is breakthrough at drainage hole. After the coal seam is effectively fractured, the gas flux has a decline-incline-decline tendency with increasing of drainage time. The breakthrough time of damage zone increases linearly
The fully mechanized top-coal caving mining method was used to mine the 15 m thick No. 3–5 seam at a depth of 450 m in Tongxin mine, located in the Datong area, China. The seam was overlain with strong strata that are hard to cave and four partially mined coal seams. Excessive mine roadway closure and severe ground conditions at the longwall face and the tailgate were induced by periodic weighting and remnant coal pillars left in the four mined coal seams above. These ground conditions were studied using physical modelling and microseismic monitoring. The results indicated that hard rock strata above the coal seam did not cave readily, causing minor and major face loading events. The ground conditions deteriorated when mining was undertaken below the remnant pillars left in the four upper Jurassic seams. In addition, the physical model showed overlying longwall goaf collapse reaching the upper goafs and causing global movement in the strata. The data from the physical model and the underground microseismic survey presented here agreed closely with the visual observations and reported events that occurred at the underground longwall face in Tongxin coal mine. Overall, this study demonstrated the mechanisms of the overburden strata movement, longwall weighting occurrences and deteriorating ground conditions along the longwall face and the tailgate roadway when mining was undertaken under the overlapped Jurassic remnant coal pillars. Methods to minimize these bad ground conditions were suggested, using stress relief hydraulic fracturing methods and blasting, such as pre-splitting and softening of coal pillars and roof strata in key places. A design of small coal pillars <3–5 m in size were recommended for overlying seams to minimize stress concentrations in seams below.
Understanding the dynamic mechanical behaviors and microstructural properties of outburst-prone coal is significant for preventing coal and gas outbursts during underground mining. In this paper, the split Hopkinson pressure bar (SHPB) tests were completed to study the strength and micro-structures of outburst-prone coal subjected to compressive impact loading. Two suites of coals—outburst-prone and outburst-resistant—were selected as the experimental specimens. The characteristics of dynamic strength, failure processes, fragment distribution, and microstructure evolution were analyzed based on the obtained stress-strain curves, failed fragments, and scanning electron microscopy (SEM) and nuclear magnetic resonance (NMR) images. Results showed that the dynamic compressive strength inclined linearly with the applied strain rate approximately. The obtained dynamic stress-strain responses could be represented by a typical curve with stages of compression, linear elasticity, microcrack evolution, unstable crack propagation, and rapid rapture. When the loading rate was relatively low, fragments fell in tension. With an increase in loading rates, the fragments fell predominantly in shear. The equivalent particle size of coal fragments decreased with the applied strain rate. The Uniaxial compressive strength (UCS) of outburst-prone coal was smaller than that of resistant coal, resulting in its smaller equivalent particle size of coal fragments. Moreover, the impact loading accelerated the propagation of fractures within the specimen, which enhanced the connectivity within the porous coal. The outburst-prone coal with behaviors of low strength and sudden increase of permeability could easily initiate gas outbursts.
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