Influence of water on elastic deformation of coal and its control on permeability in coalbed methane production

Abstract

The permeability of the coal reservoir is affected by the stress caused by water production and matrix shrinkage caused by methane desorption, and its change rate is mainly affected by the elastic parameters of coal, namely Young's modulus (E) and Poisson's ratio (v). E and v will change with the change of coal water saturation, which will affect the reservoir permeability, but there is little research on it. Therefore, in this study, triaxial compression experiments were carried out on high volatility bituminous coal samples with different water saturation to obtain the variation law of E and v with water saturation. Combined with gas permeability experiments of coal samples with different water saturation and finite element method, the influence of E and v on permeability change were analyzed based on the P&M permeability evolution model. The results showed that E decreases linearly with the increase of water saturation, while v increases linearly, and the change rate was affected by the fracture of coal and stress. The existence of fractures makes the mechanical properties of coal show great heterogeneity, resulting in the reduction of water action, and the stress will close the fractures to be as a whole, more prone to elastic deformation and greater deformation. The change of E and v will lead to a decrease of permeability faster or slower, which depends on the initial E and v and their change rate. Because the effects of E and v on permeability are different and nonlinear, the control effects of E and v on permeability are also different, depending on the E and v value. The quantitative numerical analysis showed that the variation of E can increase permeability change range by at least 32%. When the E is less than 3000 MPa and greater than 4000 MPa, the influence degree is greater, more than 60%, which means that the change of E has a significant control effect on permeability change. The change of v has little effect on the permeability change range with the maximum permeability change range is only 31%. The permeability of coal reservoir generally exceeds 3000 MPa, which means that more attention should be paid to the influence of E and v change with water production on permeability, especially the change of E.