The coal inter-particle stress and porosity-permeability evolution characteristics of the confined coal by the applied loading relief effects

Coalbed methane (CBM) is a fast-growing source of clean energy throughout the world. However, the low permeability of the gas-containing coal beds has limited the efficiency and economic CBM drainage. Stress relief technology is an effective means to improve the permeability of the low-permeability coal beds. Herein, the confined coal with different coal particles was prepared, and then the inter-particle stress and porosity-permeability evolution characteristics by the applied loading relief effects were discussed. It is revealed that as the applied loading on the confined coal is relieved, the coal inter-particle stress evolves in a negative exponential form according to the time. With the elapse of time, the accumulated elastic deformation energy in the confined coal releases rapidly at the initial moment, when the inter-particle stress of the confined coal decreases sharply. As the elastic deformation energy decreases to an extent that is not enough to overcome the friction resistance among the coal particles, the residual inter-particle stress remains in the confined coal. It is the coal inter-particle stress releasing effect that makes the larger coal particle composition be a supporting skeleton for increasing the porosity and permeability of the confined coal. The confined coal that consists of larger coal particle composition has a more than 2-fold increment in the coal porosity. It is the structural deformation of the coal inter-particles pore space that determines the permeability of the confined coal increases in an exponential form. The more large particle composition contained in the confined coal, the more permeability increment can be achieved. As multiple loadings are applied on the coal, due to the irreversible deformation behavior of coal inter-particle pore space for each of the applied loading relief effects, the permeability of the confined coal becomes sensitive and decreases dramatically for the later applied loading.