Coupling model of stress-dependent permeability in high-rank coal reservoir and its control mechanism

Stress sensitivity of the coal reservoir is one of the geological factors affecting the production of CBM well.Targeting on the high-rank coal reservoirs in Southeast margin of Ordos Basin,based on field measured data and through the stress sensitivity experiment of coal samples,the correlations and their models between permeability and stress of high-rank coal reservoir were established,and the control mechanism of permeability change was studied.Research results show that the permeability decreases according to negative exponential function with the increase of stress.When stress is less than 5 MPa,the coal reservoir permeability decreases rapidly with the increase of stress,the stress sensitivity of the coal samples is the highest;when stress is between 5 MPa and 10 MPa,the stress sensitivity is medium;when stress bigger than 10 MPa,the decline of the permeability slows down,the stress sensitivity is weak.Compared with the experimental results of high-rank coal in Southern Qinshui Basin,the stress sensitivity of the NO.2 coal seam of Shanxi formation in the research area is smaller than the NO.3 coal seam′s in Southern Qinshui Basin.The compressive deformation of fracture induced by stress in coal reservoir leads to the sharp decrease of fracture aperture then resulted in stress sensitivity of coal reservoir.Controlling the production rate especially the drop liquid rate in early stage has practical significance to prevent the stress sensitivity in coal reservoir and improve recovery.