Damage And Permeability Evolution Mechanism Of Dual-porosity Coal Under Multiple Stress Paths And Its Application

In recent years, as the major coal mining area in eastern China has entered into the deep mining state, the gas extraction in the deep coal seam has encountered a lot of bottlenecks, and the basic mechanical behavior and permeability evolution characteristics of coal under deep mining stress condition still need further research. The mechanics and permeability evolution tests under multiple stress paths which are obtained based on the actual mining stress conditions have been carried out systematically in this paper. The mechanical properties, expansion and damage rule and the permeability evolution inner mechanics of coal have been studied by integrated use of rock mass mechanics, seepage flow mechanics, and C.D. Martin fracture strain model. A mathematical model of coupled gas migration and coal deformation has been developed based on the test results. The mining stress distribution and gas migration characteristic have been simulated in l ab and engineering scale, respectively. The main research results are as follows:(1) Confining pressure unloading will lead coal strength degradation. Under the loading condition, the cohesion of conventional reconstituted coal sample and natural soft coal sample are 1.72 MPa and 1.18 MPa, the internal friction angle of the two type of coal sample are both 35.06°; With the confining pressure unloading, the cohesion of conventional reconstituted coal sample and natural soft coal sample are 0.95 MPa and 0.83 MPa, the internal friction angle of the two type of coal sample are 38.03° and 34.74°. With the confining pressure unloading, the cohesion degradation of conventional reconstituted coal sample and natural soft coal sample are about 44.77% and 29.66%, respectively.(2) The progressive failure process of coal is formed by gradually compaction of native fractures, extension of native fractures, generation of new fractures, then developed, connection, and form the macroscopic fractures. Quantitatively describe of the development of coal fractures has been achieved and the four corresponding characteristic stress has been obtained by using the C. D. Martin fracture strain model and the stress-strain curve. When the coal’s stress level reaches damage expansion stress cds, it will produce plastic deformation and begin to expand. Based on the permeability evolution test results, it can be find that the relationship between permeability and deviatoric stress conforms to 0 1 3k =kexp[ A(s -s)] before the damage and expansion of coal; the rapid growth of the coal sample permeability reflects the approximate linear trend after the damage and expansion of coal.(3) The permeability results of natural coal and reconstituted coal obtained under hydrostatic pressure condition showed that, the pressure in the fractures can reach to the target pressure fast, however, the pressure in the coal matrix changed very slowly. Permeability evolution during the adsorption equilibrium is the competition results of the effective stress and sorption-induced deformation; The equilibration time which the natural coal needs is about 2.7 times than that of reconstituted coal; The coal sample frame compresses, crack and permeability decreases with the increase of confining pressure. The relationship between permeability and effective stress conforms to 0 0exp[ 3()]fk =k -c s -s. The fracture compressibility constants for the reconstituted coal samples and natural coal sample are 2.34′10-4 psia-1, 1.96′10-4 psia-1 and 1.09′10-3 psia-1, respectively. It can be concluded that the stress sensitivity of the natural coal sample exceeds that of the reconstituted coal samples, and the two reconstituted coal samples were similar to each other in terms of stress sensitivity.(4) Lab and engineering scale simulation were conducted to stud y the gas migration characteristic by using COMSOL Multiphysics and calling the mathematical model of coupled gas migration and coal deformation established in this paper. Simulation results show that there is a big difference of response speed to the target balance status between the pressures in fractures and in matrix. The pressure in the coal matrix obtained at an equilibration time of 12 h is less than the pressure in the coal fractures obtained at the equilibration time of 1h. The impacts of the gas diffusion effect and Klinkenberg effect are of the same order of magnitude. Without considering the gas diffusion effect, the gas pressure in the coal matrix will be underestimated.(5) The common failure modes of bedding borehole are zero collapse, local collapse, arch collapse and overall collapse. The stress disturbance zones around the borehole are peak crushing zone, damage and expansion zone and elastic deformation zone. Comparing to the damage and expansion yield criterion, it is "conservative" by using the MohrCoulomb criterion to judge whether the plastic deformation appeared or not. The test results show that the biggest uniaxial strength of the 2 inch, 1.5 inch and 1 inch PVC screen are 105 k N/m, 61 k N/m and 49 k N/m, respectively. The PVC screen will damage only when the borehole can’t form the steady pressure arch and the in-situ stress passed to the screen is greater than its biggest uniaxial strength.(6) The distribution of mining stress, coal permeability and pressure in fractures of 1331(1) working face(Ding Ji coal mine) were studied by using COMSOL Multiphysics and calling the mathematical model of coupled gas migration and coal deformation established in this paper. The stress disturbance zones of borehole and roadway are 0.27 m and 18 m, respectively. The maximum permeability in the stress disturbance zones of borehole and roadway are 1550 times and 2000 times of the initial permeability, the corresponding minimum permeability are 0.5 times and 0.2 times of the initial permeability. Mining stress distribution shows that the boreholes will form the steady pressure arch after local collapse. The distribution of the pressure in the fractures shows that the stress disturbance around a borehole will weaken the extraction effect. However, using PVC screens to protect a borehole will not weaken the extraction effect even when the borehole can remain stable. The rationality of the theoretical and simulation results was proved by comparing the extraction effect between the traditional method and the new efficient method.