Experimental study on kinetic swelling of organic-rich shale in CO2, CH4 and N2

Abstract

With the increasing demand in natural gas and interest in greenhouse gas sequestration, gas transport mechanism and gas-induced deformation of shale have become important research topics. The shale matrix swells after adsorbing gas, which will impact on the gas transport behaviour in shale. In this paper, the kinetic shale swelling and kinetic gas adsorption in different types of gases at various gas pressures under confining pressure of 20 MPa and temperature of 25 °C were investigated. The gases used in the tests were helium, N2, CH4 and CO2. It was found that shale swelling under this experimental conditions has two components: strain caused by poroelasticity and gas adsorption-induced swelling strain. The experimental results show that gas adsorption induced shale swelling and the absolute adsorption amount in different gases are both about one order of magnitude lower than that of coal and are in a trend line with the results of gas adsorption-induced coal swelling, indicating that shale swelling mechanism may be similar to that of coal. The swelling rate of shale has a positive correlation with the mass rate of gas uptake. Moreover the swelling rate of shale in helium is the highest, and that in CO2 is the lowest due to slow gas diffusion caused by larger CO2 adsorption-induced swelling and CO2 phase transition from vapor state to liquid state at the experimental condition in this study. The swelling rate of shale in CH4 and N2 are almost the same. Helium-induced swelling rate increases with gas pressure due to the change of effective stress, while the swelling rates of shale in CH4, N2 and CO2 are positively correlated to the gas pressure in the early stage and negatively correlated in the later stage due to the different adsorbing gas transport mechanism in macropores and micropores. The phase change of CO2 leads to the change in density and viscosity, resulting in the change of kinetic swelling rate. It was also found in this work that the anisotropy ratio for shale swelling decreases in the order of He, N2, CH4 and CO2. Gas adsorption results in a lower anisotropy ratio, which may be related to the more random distribution of the organic matter and clay minerals, where gas adsorption and its induced swelling occur.