Abstract: Coal deformation induced by gas migration is significant to investigate coal bed methane recovery and geological sequestration of greenhouse gas. Generally, the variations of effective stress result in the shrinkage of geo-materials. However, the relationship between coal permeability and effective stress or pore pressure is nonlinear from extensive experimental results. Therefore, experiments are performed to study coal deformation caused by the flow of injected pure helium gas under hydrostatic pressure and triaxial stress conditions, respectively. Experimental results show that the coal sample undergoes a transition from shrinkage to recovery under hydrostatic pressure. Although both the coal shrinkage and recovery are proportional to the pressure of injected gas, the magnitude of shrinkage is greater than that of recovery. Under triaxial stress conditions, the coal sample rapidly expands at the beginning of helium injection. As the gas injection approaches equilibrium, the coal deformation is significantly controlled by the boundary condition. The coal expansion rate changes slowly under stress controlled condition, while the coal transits from expansion to shrinkage under displacement controlled condition. The above results indicate that the gas pressure difference between coal matrices and cleats is able to compress the matrix volume, and such compressed coal also could recover due to gas diffusion. In addition, the coal deformation is controlled by the interaction between cleats and matrix. It can be explained that coal matrices and cleats expand freely under the stress controlled boundary, while the coal matrix expansion induced by gas diffusion only narrow the aperture of cleats under displacement controlled condition. In conclusion, this study demonstrates the deformation evolution of coal induced by gas injection based on experimental results, which is particularly significant for deeply understanding the coal permeability.

Key words: gas injection, cleat and matrix, boundary condition