Damage&Permeability Development Of Unloading Coal Body During Mining The Protective Coal Seam

In present, extraction of protective coal seam and pressure relief gas drainage is thepreferred regional gas control measures. In recent years, although the theoretical study ofmining the protective coal seam has made great breakthrough, however, there are stillinsufficient, at the same time, the current study results play more emphasis on macrophenomena and laws. By using the theory of rock mechanics, unloading rock mass mechanics,damage mechanics and fluid mechanics, together with combining theoretical research,laboratory experiments, numerical simulation and engineering research, the damage&permeability development during unloading in coal and permeability distributioncharacteristics of the protected coal seam were studied. At last, the results were applied togas drainage drilling optimization of Panyi coal mine in Huainan coalfield. The mainconclusions of this paper are as follows.(1) According to the forcing process that the coal body of protected coal seam will beloaded firstly, then unloaded with the three dimensional stress being reduced simultaneously,the fixing axial displacement with unloading confining pressure and fixing deviatoric stresswith unloading confining pressure stress paths were choosed for laboratory experiments, byusing the reconstituted coal with small discrete and similarly mechanical properties comparedto the raw coal samples, on the other hand, taking the principle of effective stress andequivalent mechanical path, the influence of damage on permeality during unloading wasstudied by coupling the results of the CT experiments and permeability experiments, also, theconsistency of stress-strain curve from two different experiments verified the method ofcoupling was feasible.(2) The CT tests denoted that the damage development was low at the initial unloadingstage; the damage growth has accelerated after the confining pressure was unloaded to5MPaunder the experimental conditions in this paper; The internal destroy cracks was invertedconeand, also, the failure occurs firstly at the low-density side, developing toword thehigh-density side; To the same sample, the smaller the density, the faster the damagedeveloped, at the same time, fixing deviatoric stress with unloading confining pressure stresspaths had greater damage to sample.(3) The permeability tests denoted that with the increases of the initial confining pressure,to the stress paths of the fixing axial displacement with unloading confining pressure andfixing deviatoric stress with unloading confining pressure, at the increasing point of thedecreasing rate about the deviatoric stress, the value of the confining pressure became bigger;The relarion between permeality and axial strain before the unloading point isk A B11e, and also, after the unloading point, the relarion isk A2eB2.(4) The damage was gotten CT values. According to the CT values during unloading, thedamage variable changes were gotten, together with the results of permeability tests under thequivalent stress path, this paper gave the influence of damage on permeability that at theinitial stage, the damage and permeability were all small, however, the increases of damageand permeability speeded with the effective confining pressure being unloaded to5MPa underthe fixing axial displacement with unloading confining pressure and fixing deviatoric stresswith unloading confining pressure stress paths.(5) According to the expansion&deformation of the protected coal seam and therelation between permeability and axial strain derived from permeability tests, thepermeability distribution characteristics was gotten by the help of Surf fution in Matlabsoftware. The permeability of protected coal seam could be divided into the originalpermeability district, permeability diminishing district, permeability increasing district1,permeability increasing district2. Based on this, from cut eye and closing to the inside of theprotected coal seam,5m×5m,10m×10m,40m×40m drilling arrangement was used. At last,the gas parameters of mining the wokface vefied the rationality about the drillingarrangement.