Research On Anisotropic Mechanical And Seepage Properties Of Coal Under True Triaxial Stress Conditions

Deep coal masses are generally under the complex geological conditions of high geostress,high gas pressure and high temperature.Understanding how to accurately evaluate the mechanical and seepage characteristics of deep coal masses is a scientific problem that needs to be solved to achieve the co-production of deep coal and gas.In this thesis,cubic coal samples were mainly used in this study.Through a combination of laboratory tests,theoretical analysis and engineering applications,the anisotropic mechanics and seepage characteristics of coal under true triaxial stress conditions were systematically analyzed and discussed.By using a multi-functional true-triaxial fluid-solid coupling test system,together with three-dimensional fracture CT scanning reconstruction and acoustic emission signal acquisition and analysis methods,the true triaxial anisotropic mechanical behaviors of coal with different fracture geometries,different mining stress paths,and different water-saturated conditions were investigated.An anisotropic permeability prediction model and anisotropic failure conceptual model of coal applicable for true triaxial stress conditions were established.The coal permeability enhancement results of introducing artificial macro-fractures in low permeability coal seam by using carbon dioxide fracturing technology were examined.The main research results of this thesis are as follows:(1)By testing the uniaxial compressive strength,tensile strength,and shear strength of coal and ultrasonic response characteristics,the results showed that the physical and mechanical parameters and ultrasonic response characteristics of coal exhibited significant anisotropy.By analyzing the characteristics of natural cleats in coal at multi-scales,the results showed that natural cleats in coal were in a form of mesh distribution,some mineral fillings might exist inside the cleats.Moreover,bedding planes,face cleats and butt cleats were approximately perpendicular to each other.The face cleats were approximately parallel to each other,with continuous distribution and relatively large length,while the butt cleats were approximately parallel to each other with discontinuous distribution,and relatively short length.(2)Anisotropic seepage characteristics tests of intact coal under true triaxial stress conditions were carried out.The permeability of intact coal had a negative correlation with the intermediate principal stress coefficient.Under the conditions of true triaxial stress dominated by tectonic stress(σ_H>σ_h>σ_v),the permeability of intact coal decreased more significantly.The permeability of coal in the butt cleat direction decreased the most during the loading of each principal stress,and the stress-sensitivity was higher.An anisotropic permeability model of coal applicable for true triaxial stress conditions was established,and it fitted well with the anisotropic coal permeability data tested under true triaxial stress conditions.(3)By introducing smooth and rough artificial fractures in the center of cubic coal samples,anisotropic seepage characteristics tests of fractured coal under true triaxial stress conditions were carried out.The introduction of artificial fractures increased the permeability of coal by about 1 to 2 orders of magnitude.Compared with intact coal,the permeability anisotropy of coal with artificial fractures was greater,but the anisotropic order of permeability was the same,that is,the permeability in the face cleat direction>the permeability in the butt cleat direction>the permeability in the bedding plane.Due to the swelling of the coal matrix induced by water-absorption,the cleats and artificial fractures inside the coal were further close,and the permeability of the water-saturated coal was reduced by up to about an order of magnitude.The hysteresis effect of coal permeability measured under conventional triaxial stress conditions was more significant than that measured under true triaxial stress conditions.(4)By designing differnt true triaxial stress paths to simulate different stress states of coal masses during underground coal mining,an experimental test on anisotropic mechanics and seepage characteristics of mining-induced coal under true triaxial stress conditions was conducted.The unloading of unidirectional or bidirectional horizontal stress would make the failure progress of mining-induced coal during the residual strength loading stage more sudden,and would increase the permeability nadir during stress loading and increased the number of fractures after failure.The increase of permeability after the failure of mining-induced coal were within an order of magnitude.The strength of mining-induced coal was anisotropic,and the strength of mining-induced coal in butt cleat direction was lower than that in bedding plane and face cleat direction.The anisotropic strength of mining-induced coal was well fitted by a linear function relationship between octahedral shear stress and average effective normal stress.(5)Considering water-saturated conditions of the coal seam in the field,experimental tests on the anisotropic mechanical properties and failure behaviors of dry and saturated coal under true triaxial stress conditions were carried out.The types of failure of dry and saturated coal belonged to Class I brittle failure.The brittleness of coal measured when the maximum principal stress was loaded along the vertical bedding plane direction was greater than the maximum principal stress along the direction of parallel face cleat and butt cleat planes.The failure modes of dry and saturated coal were shear failure or axial split-shear composite failure.When the maximum principal stress was loaded parallel to the butt cleat direction,the failure mode of coal was mainly shear failure.When the maximum principal stress was loaded along the face cleat direction or perpendicular to the bedding plane,the failure mode of coal was shear failure or axial split-shear composite failure.By using acoustic emission characteristic curves,two typical failure modes of dry and saturated coal under true triaxial stress loading could be identified and distinguished.(6)The carbon dioxide fracturing was carried out in the low-permeability coal seam of Nantong underground coal mine.By arranging observation boreholes around the fracturing borehole,the pure gas flow and concentration data in the observation boreholes were analyzed.The pure gas flow and gas concentration of gas extraction have increased to varying degrees,and after three weeks of continuous extraction,the pure gas flow and gas concentration in each observation borehole have not decreased significantly.After carbon dioxide fracturing,the average gas drainage pure flow rate and gas concentration in the observation boreholes increased by 19.25%and 15.15%,respectively.The shear artificial fractures introduced by carbon dioxide double-borehole fracturing can extend to the 8-9 m area and the gas drainage results of coal mass near this area was better.According to anisotropic seepage characteristics of intact and fractured coal at laboratory scale,an optimized conceptual model of carbon dioxide fracturing applicable for low permeability coal seam was proposed.