Weakening-Relaxation Mechanism And Fluid-Solid Coupling Property Of Low-Permeability Coal Sujected To Hydraulic Slotting

Coal and gas resources are abundant in China. But the low gas permeablitiy is a key constraint for coal and gas co-exploitation with high efficiency in majority of Chinese coal mines. Hydraulic slotting is an effective way to increase the coal permeability and thus prominently enhance the gas drainage efficiency, which is widely adopted in underground coal mines. Presently, the study of this technique is focused on field test and numerical analysis and limited research is conducted on rupture behaviors of slotted coal under the coupling effect of dynamic-static load and fluid-solid coupling property of gas-bearing slotted coal. In this paper, based on the engineering background of hydraulic slotting and low-permeablity reconstruction, the weakening-relaxation mechanism and fluid-solid coupling property of low-permeability coal seam sujected to hydraulic slotting were expounded from four aspects:weakening characteristics and mechanism of slotted coal, coal relaxation mechanism under coupling effect of water-jet impact and crustal stress, fluid-solid coupling property of gas-bearing slotted coal and macro-micro coal parameters variation mechanism after hydraulic slotting and gas drainage, by combining laboratory experiments, numerical simulation, theoretical analysis and field test. The main research achievements are as follows:(1) The multi-parameters and multi-objectives optimization method was proposed by combining grey Taguchi, response surface method and Mahalanobis distance to provide a feasible way for fast physical and numerical low-permeablity coal reconstruction. The main mesoscopic parameters, controlling the macro-parameters (compressive strength, elastic modulus and poisson’s ratio) are obtained by factor screening experiment. The macro-mesoscopic models were established by response surface method. Based on those models and constraint conditions, the meso-parameter set can be fast matched.(2) The slotting patterns were summarized according to the combination of dill pipe rotation and drawing. The slot inclination and ratio of borehole radius to slot length were adopted to describe the pressure relief space forms under various slotting patterns. The variation rules of coal compressive strength, elastic modulus and poisson’s ratio with slot inclination and ratio of borehole radius were obtained by establishing forty geometric models of pressure relief space. The "slot weakening degree" is proposed to quantitatively describe the weakening degree of slotted coal mechanical properties and slot weakening degree maps were obtained by linear interpolation based on different indices. The corresponding numerical models were established via particle flow code to reveal the mesoscopic ruputure patterns and mesoscopic parameters variations of slotted coal. It is pointed out that the slot changes the macro-parameters by controlling the mesoscopic charameteristics, leading to the weakening phenomenon.(3) Physical simulation of coal relaxation under coupling effect of water-jet impact and crustal stress was realized. The detected vibration acceleration signals had better consistency with water-jet impact process and sample relaxation process. The vibration acceleration effective value induced by 7MPa water-jet is 13.15 times larger than that induced by drilling, which indicated that water-jet disturbance performance is much larger than that of drilling. The average amplitude and centroid frequency are introduced to make the quantitative description of the frequency domain characteristics of vibration acceleration signal. The water-jet disturbance degree is defined by average amplitude and adopted to quantitatively describe the disturbance action of the water-jet impact on loaded coal. Based on the axial and lateral pressure variations of loaded coal in the process of the water-jet impact, the axial and lateral relaxation degree are defined to quantitatively describe coal relaxation phenomenon. In addition, the influence of the water-jet impact angle and lateral pressure coefficient on axial and lateral relaxation degree was analyzed and the coal relaxation mechanism under coupling effect of water-jet impact and crustal stress was elaborated.(4) Physical simulation of hydraulic slotting in gas-bearing coal under real crustal stress circumstances was realized. The gas pressure at the monitoring point is closely related to coal deformation. The slot-relaxation induced deformation and desorption induced contraction deformation were found in time-varying curves of vertical and parallel strains. The final deformation before slotting, ultimate deformation and gas diffusion parameters are used to quantitatively describe the coal-gas coupling characteristics and the effect of gas pressure and slot radius on fluid-solid coupling property of gas-bearing coal.(5) Based on the sensitivity and accuracy of test methods, a pore structure characterization method was put forward by combining N2GA and MIP to illustrate the variations in pore size distribution and pore volume proportions of the coal samples in the various distances between the slotted borehole and sampling boreholes, and to reveal change characteristic of coal adsorption property after hydraulic slotting and gas drainage. Additionally, three obvious partition zones (significant variation, transitional variation and insignificant variation) were observed around the slotted borehole. Furthermaore, the variation rules of residual gas content, initial gas diffusion velocity and coal firmness coefficient were revealed after hydraulic slotting and gas drainage. The results of grey relation analysis indicate that this technique dominates coal microscopic parameters to change coal macroscopic parameters.(6) Based on primitive gas content and pressure of coal seam, a new judgment index is put forward to determine effective gas extraction radius. Through this index, the effective gas extraction radius of filed test was determined. Compared with the original borehole distribution spacing, the pure flows of slotted boreholes with borehole distribution spacing of 6m,7m and 8m increased by 3.86,2.04 and 3.86 times, respectively. Large coal disturbance range and sufficient crack initiation and coalescence can be obtained via hydraulic slotting, which significantly improves coal permeability and gas drainage efficiency.These above-mentioned research achievements provide significant theoretical and practical guidance significance for selection of slot pattern, slot inclination angle and the ratio of borehole radius to slot length; nozzle design; implementation of hydraulic slotting under various gas pressure and crustal stresses; revelation of variation in gas flux after slotting and in-depth illustration of the permeability-enhancing mechanism.During the completion of the thesis, fifteen research papers were published, in which eight ones were the first author identity (four ones indexed by SCI-Expanded database and two ones were indexed by El database). Twenty-one invention patents were applied for, in which ten ones were authorized. Three provincial and ministerial awards were obtained.