Temperature Effect For Injected Nitrogen Flow In Low-permeability Coal Seam And Mechanism Of ECBM

Methane extraction is the main technical measure to prevent coal mine gas disasters. However, more than 70% of coal seam permeability in China is less than 1m D and as mining depth increases gradually, coal permeability will become lower. Therefore, the effects of gas drainage through boreholes are unsatisfactory. On the basis of this and taking the increase of gas extraction rate of low-permeability coal seams as the breakthrough point, this thesis studies temperature effect for injected nitrogen flow in low-permeability coal seam and mechanism of ECBM. The main achievements are summarized as follows:Energy evolution mechanism in failure processes of different coal-rank coals was studied through the method of laboratory test:(1) Under cyclic loading and unloading, the maximal deviatoric stress of coking coal is 41.3MPa and that of anthracite coal is 120.8MPa, whose corresponding strain are 14.4 × 10-3 and 10.8 × 10-3, respectively.(2) The storage limit of anthracite coal is 2.69 times that of coking coal and the maximal dissipated energy density of anthracite coal is 3.19 times that of coking coal.(3) The initial energy release of anthracite coal is 92.8% while that of coking coal is only 39.5%. Based on this, nitrogen injection programs of different coal-rank coals are determined.The flow characteristics of low-permeability coking coal were studied by using triaxial servo-controlled seepage equipment:(1) Triaxial compressive strength, residual strength and elastic modulus of coal decline as the temperature and pressure of nitrogen increase, but the trends differs.(2) In the process of conventional nitrogen injection, initially, the permeability appears to decrease to some extent over time and then after it abruptly increases to an extreme value, it remains stable. The higher nitrogen temperature is, the greater the nitrogen pressure is and the greater permeability maximal ratio of coal mass becomes, the more obvious temperature-pressure effects of nitrogen are.(3) Multi-cycle nitrogen injection is in favor of the re-expansion of coal fractures, and the effects of improving coal permeability are better than effects of conventional nitrogen injection.The flow characteristics of low-permeability anthracite coal were studied by using uniaxial pneumatic fracturing equipment:(1) Under nitrogen injection with continuous room temperature, the greater the axial pressure is, the greater initiation pressure of coal mass becomes and the time that initiation pressure needs is less.(2) Under graded nitrogen injection, two cases may happen on resistivity: in one case, it increases after the first decrease with time and then jumps at last while in the other case, before the final jump, it increases slowly at first. The initiation pressures of coal samples decrease linearly with the increase of nitrogen temperature.(3) Nitrogen adsorption by coal can reduce the resistivity of coal mass and the resistivity decreases as the temperature rises. Fracture expansion can cut off the conducting channel of coal mass effectively and increase the resistivity of coal.(4) During the seepage process of nitrogen in coal, the initial decrease of resistivity is because the effects of resistivity decline caused by integrated effects of coal’s adsorption for nitrogen and coal heating are greater than the effects of resistivity increase caused by coal’s fracturing by nitrogen; the slow rise of coal resistivity is quite the contrary; the jumps of resistivity are the result of impediment to conducting channel caused by coal ruptures from top to down.The thermal- hydrologic-solid coupling mathematical model of nitrogen seepage process in coal was established. Taking crossing boreholes of Pingdingshan Coalfiled for example, the effects of various factors of injected nitrogen on temperature-effect are studied. The resules are: nitrogen temperature, gas injection time and borehole diameter have a notably significant effect on temperature effect while nitrogen pressure and the initial permeability of coal have a little effect on temperature effect.Numerical simulation of injected nitrogen displacing methane effects on standard coal samples(φ50mm×100mm) was conduct:(1) The higher nitrogen temperature is, the longer high concentration methane lasts and the faster methane recovery rate increases, the greater the recovery becomes.(2) The greater the nitrogen injection pressure is, the greater extreme of methane production is, the faster the recovery rate increases over time.(3) Considering the safety of coal mining and the economy of gas utilization, the best ratio of nitrogen temperature and pressure in this case is obtained. These research results are beneficial to guide the field application of 60~100℃ nitrogen injection in coal seams.