Absolute adsorption capacity of shale on CO2 and its influencing factors

In order to study the absolute adsorption capacity ( na) of CO2 on shale and its influencing factors in the process of achieving adsorption equilibrium under the action of CO2 ,the calculation model of absolute adsorption capacity of CO2 suitable for “stage equilibrium method” was deduced based on the principle of mass conservation,considering the influence of two factors of adsorption phase volume and shale volume strain on the free space volume of adsorption system. The shale adsorption-deformation test under the pressure of 0-16 MPa CO2 was carried out with the self-developed “high-temperature and high-pressure shale adsorption expander”. The effects of three factors on na were analyzed,including CO2 mass injection quantity, adsorption phase volume and specimen volume strain. The results show that the na value calculated by this model under low CO2 pressure is consistent with the calculation result of conventional empirical formula of absolute adsorption amount,when the CO2 pressure is higher than 4 MPa,the calculated value of absolute adsorption capacity of CO2 is obviously larger than that calculated by the empirical formula,and the difference increases gradually with the increase of equilibrium pressure. The fitting results show that na and CO2 balance pressure satisfy the exponential function relation. The proportion of the adsorption amount of na caused by the CO2 mass injection quantity factor continues to decrease with the increase of equilibrium pressure. The proportion of the adsorption amount of na caused by the adsorption phase volume factor increases with the increase of equilibrium pressure. After the equilibrium pressure is higher than 6 MPa,it reaches more than 50% . The adsorption phase volume becomes the dominant factor affecting na. Adsorption caused by mass injection of CO2 and volume of adsorption phase accounts for more than 95% of absolute adsorption of CO2 ,while the adsorption amount caused by shale volume strain has little influence on na and the proportion of na is always below 5% ,but for other adsorbents such as coal,the volume deformation of CO2 adsorption could reach 10% of the sample volume. At this time,the volume strain factor will have a significant impact on na.