Experimental observations of matrix swelling area propagation on permeability evolution using natural and reconstituted samples
Many researchers have concluded that the permeability should be measured after the adsorption equilibrium state is reached and in many permeability models, it is normally assumed that pm (pressure in the coal matrix) is equalized with pf (pressure in the fractures). However, when the dynamic balance between pm and pf is broken which exists in most coalbed methane engineering, the differences between the two pressures induced by gas diffusion and matrix shape factor will generate nonuniform matrix swelling which have not been well learned to date, thus influence the permeability evolution. It is of great importance to investigate this issue on account of it will play a more important role in CO2-enhanced coalbed methane and CO2 sequestration engineering as the coal adsorption capacity for CO2 is considerably higher than that for CH4. To achieve this target, a series of novel coal permeability tests and simultaneous strain measurements were conducted using both natural coal and reconstituted coal. In these experiments, the radial strain evolution characteristics and the permeability evolution characteristics as a function of the adsorption equilibrium time and confining pressure were obtained. Based on the experimental results, the effects of the differences between pm and pf on radial strain evolution and coal permeability evolution were discussed, and a new matrix swelling area propagation theory was achieved. In addition, the differences between natural coal and reconstituted coal in terms of their matrix swelling and stress sensitivity properties were obtained.