Deformation Characteristics of Coal and Its Impact on Permeability during Pre-mine Gas Drainage

摘要

Knowledge of the deformation response ofcoal to gas pressure variations aids the mining engineer in developing the method of gas drainage.Coal reservoirs are different from conventional reservoirs because of the unique matrix volume shrinkage effect, which has a significant influence on the stress regime and fracture permeability of the reservoirs.In the study of deformable coal formations, cleat compressibility is used by many researchers to incorporate the stress sensitivity.The permeability varies exponentially with variations in stress.For CBM reservoirs, the hypothesis of constant cleat compressibility is not valid due to the unique shrinkage/swelling phenomenon.This has been proved experimentally and theoretically.An experimental study was carried out to investigate the stress-dependent-permeability.During experimental work, uniaxial strain condition was maintained.This condition requires in the laboratory,or results in the field, in a continuous change in the horizontal stress with depletion.Variations in the effective horizontal stresses are thus affected as a function of reservoir pressure reduction.Cleat permeability was estimated using experimental results and the variation in horizontal stress was also monitored throughout experiment.The experimental results showed that decreasing reservoir pressure results in decreased horizontal stress and increased permeability for methane.The horizontal stress decrease was found to vary linearly for both helium and methane depletion.However, stress reduction for methane was higher than that for helium due to the matrix shrinkage effect.Cleat compressibility was estimated by application of the exponential relationship between changes in effective stress and permeability.The results showed that the cleat compressibility is somewhat constant for helium but varies significantly for methane.The results also showed conclusively that the cleat compressibility is a parameter dependent on gas-type and stress.