The role of sorption-induced coal matrix shrinkage on permeability and stress evolutions under replicated in situ condition for CBM reservoirs

Matrix shrinkage is a unique property of coal which dictates the sorption induced strain in the constitutive relation. Both permeability and stress profiles are passively modified with continuous depletion and they are intrinsically controlled by both poroelastic and matrix shrinkage properties. The models for quantifying matrix shrinkage related effects including horizontal stress loss, vertical strain variation and permeability evolution were proposed. The matrix shrinkage property was also coupled into the poroelastic relationships to study the potential possibility of local failure. Experimental study was performed to measure the permeability and its dynamic applied horizontal stress under replicated in situ uniaxial strain condition. The effects of sorption induced horizontal stress evolution varies with gas types depending on the sorption intensity. The vertical strain of coal bulk under uniaxial strain condition consists of the sorption induced matrix shrinkage/swelling strain, cleat volume strain and matrix mechanical strain due to changes of pore pressure and external stresses, which was validated against with the experiment data. As gas depletion from 4.14 MPa to 0.55 MPa, matrix shrinkage effects can lead to the permeability ratio nonlinearly increases from 1 to 2.27 (~127%) for methane and from 1 to 4.58 (~358%) for carbon dioxide, respectively. Additionally, the magnitude of the vertical effective stress always increases with continuous gas depletion, but the horizontal effective stress may increase or decrease depending on the intensity of sorption-induced matrix shrinkage. The local deviatoric effective stress tends to increase by considering matrix shrinkage effect, which potentially increase the possibility of local coal failure.