Stress Sensitivity of High-rank Coalbed Methane Reservoir

Water facies in coal beds has been a critical problem in coal development and has been received a wide attention because it makes more complicated stress sensitivity of coal reservoir.This study analyses characteristic of stress sensitivity of coal beds by an experiment on dry and wet samples from coad beds,and its influence on productivity of gas well using digital modeling.The research shows distinctly irreversible,intensive stress sensitivity of coal reservoir.It was proved that the gas permeability decreased about 90% with efficient pressure increasing from 2 MPa to 10 MPa;the lower the initial pressure,the more intensive stress sensitivity.The permeability of coal rock could not be restored completely even if efficient pressure was lowered.Existence of water facies makes stress sensitivity much intensive.The experiment shows that the permeability of the wet samples decreased 66.0%,50.4% and 58.5% with efficient pressure increasing from 2 MPa to 3 MPa,while the permeability of a couple of the dry samples decreased averagely by less than 50%.Meanwhile,stress sensitivity was continuously intensive with increasing of water saturation.The sensitivity of coal beds greatly affects output of oil well:the worse the permeability of original reservoir is,the greater the sensitivity influences over output of coalbed gas.Therefore,during the production of coal beds,especially at the beginning of discharge and mining,increasing pressure difference doesn't advance the output of coalbed gas.