Experimental Study of Gas Adsorption-induced Coal Swelling and its Influence on Permeability

Coal permeability is a key parameter to control gas flow in coalbed. Adsorption of gas on coal will induce coal matrix swelling and desorption of gas may induce coal matrix shrinkage. Under reservoir conditions these strain changes affect the cleat porosity and thus permeability. Therefore it is significant to investigate coal swelling caused by different gases (CO2, N-2, CH4, etc.), adsorption on coal and its influence on coal permeability, for the purpose of studying on the technologies of coalbed methane (CBM) exploitation and CO2 sequestration in coalbed and enhanced coalbed methane (ECBM) recovery. In order to study the changes of coal swelling and permeability caused by different gases adsorption the experiments for simultaneously measuring the adsorption adsorption-induced swelling strain and permeability of coal had been carried out using CO2, N-2, CH4, respectively, by use of a new method and device for simultaneously measuring adsorption-strain-permeability. The experimental results show that while maintaining a constant effective stress (3MPa in this work), coal swellings caused by three gases adsorption increased with increasing gas pressures and the relationship of swellings caused by different gases is: epsilon(CO2) > epsilon(CH4) > epsilon(N2); coal permeabilities of three gases all decreased with increasing coal swellings, the permeability decrease of CO2 was the most obvious which nearly an order of magnitude, followed by CH4, the permeability decrease of N-2 was very small. These results then are used to explain the mechanism of why CO2 causes coal permeability reduction but N-2 enhances coal permeability in the process of ECBM operation.