156 / 2023-07-05 16:10:12

Experimental study on influence of strain and permeability characteristics of gas hydrate bearing coal under loading-unloading conditions

coal and gas outburst,gas hydrate bearing coal,loading and unloading conditions,strain,permeability

Dynamic disasters such as coal and gas outburst have become more frequent and intense due to significant effect of some special conditions including high geotemperature, high hydraulic pressure, high ground stress and mining-induced, which caused by the increases in mining depth. Coal and gas outburst prevention technology based on the hydrate method has led to the reduced the mine gas pressure and the enhanced mechanical performance of coal simultaneously, demonstrating that this approach is effective for preventing coal and gas outburst prevention. Addittionally，During the process of coal mining, the coal near the coal face experiences vertical stresses incereasing and rehorizontal stresses decreasing, significantly altering the deformation and permeability of the coal. Con-sidering the characteristics of the actual coal seam mining method, the effect of hydrate formation on coal permeability , the deformation and permeability of gas hydrate bearing coal during loading and unloading is a key issue in hydrate method application in coal and gas outburst prevention. Therefore, we first conducted the permability tests of gas bearing coal and subsequently the three axial compressions and permeability tests of gas hydrate bearing coal (three particle sizes and three saturations) under the axial loading and confining pressure unloading control methods, by using triaxial testing machine for coupling action of stress, seepage, and chemical effect. Then, the relation are analysised relating to the effect of hydrate formation, strain and deviator stress on coal permeability. The results show that the coal permeability decreased significantly after the formation of gas hydrate, with the percentage decrease ranging from 58.3% to 98.3%. The percentage reduction in coal permeability after gas hydrate formation tends to increase with increasing saturation. The permeability of gas hydrate bearing coal shows an overall decrease with increasing saturation, with a decrease of 55.6% to 86.1%. The axial strain of gas hydrate bearing coal can be divided into three stages: stable stage, slowly increasing stage and rapidly increasing stage, The stable stage is mainly dominated by the elastic deformation of the coal body, and the change of axial strain is small; the slowly increasing stage is mainly dominated by plastic deformation, and the axial strain increases slowly; the rapidly increasing stage is when the coal body enters into yield-breaking ring deformation, and the axial strain increases rapidly; the permeability of gas hydrate bearing coal shows three trends with increasing partial stress, which are increasing, decreasing, increasing and then decreasing respectively; The exponential function fitting can not only describe the trend of the permeability of gas hydrate bearing coal with partial stress better, but also has good prediction accuracy for a few points; The rate of loss of gas hydrate permeability tends to increase with increasing partial stress. There is a correlation between permeability and strain in gas hydrate-bearing coal. Changes in permeability of gas hydrate bearing coal bodies generally follow larger changes in strain. At lower initial permeability of gas hydrate bearing coal, the permeability is less affected by the effects of increased deviator stress. With increasing saturation, the permeability loss rate of 20-40 mesh and 60-80 mesh gas hydrate bearing coal tends to decrease, while the permeability loss rate of 60-80 mesh gas hydrate bearing coal tends to increase. This test is expected to provide data reference for hydrate method application in coal and gas outburst prevention.