Experimental study of coal matrix-cleat interaction under constant volume boundary condition

Interaction of coal cleat and matrix plays an important role in determining the dynamic change of coal pore structure and its permeability. Majority of the experimental studies on the cleat-matrix interaction are carried out by measuring the correlation between coal permeability and pore pressure. Under this condition, it is commonly believed that the cleat pressure is equalized with the matrix pressure, in which the permeability is available only for gas flow/adsorption reaching equilibrium state. In this study, the coal cleat-matrix interaction is to uncover during helium unsteady flow from the cleat network to matrix blocks. This objective is achieved by measuring local deformation of coal sample and its permeability under a constant volume boundary. The results show that the incremental of coal strain firstly increases from 184 με for 2.0 MPa to 440 με for 4.5 MPa, resulting from the opening of the fractures and the compression of the matrix blocks, and then recovers to 105 με for 2.0 MPa and 222 με for 4.5 MPa, due to expansion of the coal matrix. Such a transition of the coal deformation reveals that the gas injection process generates a dynamically imbalance pressure between the cleat and matrix, and then gas diffusion around the vicinity of the cleat causes non-uniform expansion of the coal matrix. Recovery ratio of 0.5 for the matrix strain was observed, quantifying the contribution of matrix expansion to the cleat aperture. Through comparing the measured permeability data with two predicted permeability by the constant-volume model and the matchstick model, it is found that the variation of coal permeability is controlled not only by the change of cleat aperture, but also affected by the matrix expansion process. This work offers a new direct observation into the dynamics of gas mass transfer from the cleat to the matrix and a new understanding of coal permeability evolution in response of the transition. It sheds light on the development of new permeability model that incorporate the fracture-matrix interaction.