Deformation characteristics and model of coal adsorption/desorption on CO2

The adsorption/desorption characteristics of coals on CO2 is an important indicator to evaluate the potential of CO2 geological storage,and the deformation of coals during the process of CO2 adsorption/desorption is an important factor affecting the injectability of CO2 in a coal seam. The self developed high pressure adsorption and desorption (deformation) test system of the coal seam was used to carry out simultaneous experiments on the adsorption and deformation of the three different rank coals. The CO2 adsorption/desorption deformation characteristics of coal samples were analyzed,the strain adsorption relationship model and the adsorption/desorption deformation model in the adsorption/desorption process of coal samples were established,and the CO2 injectability of different rank coals and the applicability of CO2 storage technology were discussed. The results show that there are obvious desorption hysteresis characteristics of three different rank coals in the desorption process,and the desorption hysteresis characteristics of coal samples gradually weaken with the increase of coal rank,which causes the change of desorption strain curve from “hysteresis” to “advanced”. There is anisotropy in the adsorption/desorption deformation of the three different rank coals. The adsorption/desorption deformation of gas coal and 1/3 coking coal shows large discreteness,while lean coal is small. The discreteness of adsorption/desorption deformation of each coal sample decreases gradually with the increase of equilibrium pressure. Under the same adsorption capacity,the adsorption/desorption volumetric strain relation of coal samples is gas coal>1/3 coking coal>lean coal,however,due to the difference in the adsorption capacity of three kinds of coal samples,the adsorption/desorption volumetric strain relation of coal samples is lean coal>1/3 coking coal>gas coal under the same adsorption equilibrium pressure. It can be seen that the strain of coal samples is determined by the adsorption capacity and the swelling capacity. The strain and adsorption capacity of coal samples show a cubic function relationship in the adsorption/desorption process. Combined with the isothermal adsorption model of coal (D-A equation),the adsorption/desorption deformation model of coal samples is established,which can accurately fit the deformation data of coal adsorption/desorption CO2,reflect the relationship between adsorption/desorption deformation and adsorption capacity,also the relationship between adsorption/desorption deformation and equilibrium pressure of coal samples. According to the comparative analysis of the adsorption/desorption characteristics and deformation characteristics of gas coal,1/3 coking coal,and lean coal,lean coal has the best CO2 injectability,which is more suitable for intermittent injection to improve coal seam injectability,while gas coal is more conducive to long term storage of CO2 in a coal seam.