The effects of gas pressure and adsorption in coal permeability under uniaxial strain conditions

Abstract

Coal permeability evolution related to gas pressure is important to coal mining and coalbed methane exploitation. To apply the exponential model of coal permeability, we simplified the stress of coal reservoirs to the uniaxial strain condition. To investigate the permeability of coal samples changed in gas pressure under strain-controlled conditions, we measured the strain and permeability of coal samples collected from 12-1# coal seam in Hongyang No. 3 Mine in Shenyang, China, during gas pressure reduction under strain-controlled and constant temperature conditions using the Gas seepage measurement system in State Key Laboratory of Coal Resources and Safe Mining of China University of Mining & Technology (Beijing). The permeability experiment was performed under uniaxial strain conditions using adsorptive gas (carbon dioxide) and non-adsorptive gas (helium). The experimental results show that with decrease in gas pressure, the permeability ratio, k/k 0, of coal samples with helium and carbon dioxide firstly changed slightly with gas pressure from 4.0 to 1.5 MPa and then increased sharply with a gas pressure of <1.5 MPa. Coal samples shrank with the reduction of gas pressure under the uniaxial strain condition. The gas pressure of 1.5 MPa can be considered as the turning point of the volumetric strain change. When gas pressure was <1.5 MPa, the volumetric strain of samples sharply changed because of carbon dioxide gas desorbed from coal when the outlet was open. As the gas pressure decreased, the radial stresses of coal samples with carbon dioxide were less than those of samples with helium. Therefore, coal matrix shrinkage because of gas desorption has an important influence on the radial stress of coal samples under the uniaxial strain condition.