Analysis of coal permeability evolution mechanism considering the effect of temperature and pore pressure

In order to explore the influence of temperature and pore pressure on the variation of coal permeability dur- ing the process of coalbed methane extraction,the original coal samples from the Qianbei coalfield of Guizhou were in- vestigated in this study. The experimental instrument is a positive pressure triaxial seepage device developed independ- ently. Three different differential pressure conditions were set by controlling the inlet and outlet gas pressure. Then the seepage tests were carried out,in which the pore pressure at different temperatures was varied. On the basis of the S&D model,one of the current typical permeability models,combined with the effects of temperature-induced adsorption deformation,thermal expansion and slippage effects,a new permeability model considering temperature and constant ex- ternal stress was newly established. The rationality was verified by ex-periment and model comparison. Quantitative analysis was also made on the calculated values of permeability considering slippage effect and those without consider- ing slippage effect. The results show that ① during the seepage test,the permeability of coal decreases with the in- crease of pore pressure,and the decrease is affected by the increase of temperature. With the increase of pore pres- sure,the sensitivity of coal permeability to temperature decreases gradually. ② Under the pressure point of each pore of the test,the temperature increase causes the permeability of coal to decrease;under each temperature state,the per- meability of coal decreases with the increase of pressure difference. ③ After quantitative analysis,it is found that the modified model is more consistent with the experimental results than the other two models,and the value of slip factor of the modified model increases with the increase of temperature,which proves the rationality of the model theoretical- ly. ④ The permeability curve of coal with slippage effect is more consistent with the test results than that without slip- page effect. The calculated value of the former is higher than that of the latter at different temperatures. With the in- crease of pore pressure,the variation of permeability caused by slip effect reduces gradually.