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.