Research On The Rule Of Coal Adsorption Swelling Deformation Under Supercritical Carbon Dioxide Action

Although the coalbed methane reserves are rich in China,the characteristics of low permeability for coalbed methane result in low extraction rate and commercial development difficulties.The method of increasing the permeability of low permeability on coal seam by injecting supercritical CO2 has been proved by experiments.However,the coal adsorption swelling under supercritical CO2 have an adverse effect on the permeability of reservoir.CO2 has characteristics that are different from noncritical states.In order to reveal the law of coal adsorption swelling under supercritical CO2,the paper adopts the combination of test,theoretical analysis and numerical simulation.The main research work and achievements are as follows:(1)In order to overcome the problem that the strain gauge is used to measure the coal swelling under the action of supercritical CO2,such as the measurement range is small,the test result is large,the strain piece is easy to break away from the damage,and strain gauge of the swelling volume with the applied thermal flow load function is independently developed,the deformation law of coal on different temperature and pore pressure under supercritical CO2 was studied.The results show the change law that the volume strain of coal increases with the time of supercritical CO2,then tends to be stable.When the pore pressure is constant,the volume strain increases with the supercritical temperature,when the temperature is higher,the longer the time required to achieve the stable swelling.When the temperature is constant,the swelling volume strain increases with the supercritical pore pressure,but the time required to achieve the steady swelling increases first and then decreases with the increase of the pore pressure.Under supercritical CO2,the volumetric strain of the coal with the temperature and pore pressure showed a change of the "S-type" Logistic function.The rate of change of the volumetric strain for the change of the supercritical temperature and the pore pressure was partitioned,and the rate of change followed by:near critical area>transcritical zone>high critical area.(2)Based on the Elliott-Suresh-Donohue equation of state and the simplified local density model,the excess adsorption of coal under supercritical carbon dioxide was determined to solve the problem that the theoretical model established on the basis of the absolute adsorption capacity of the traditional adsorption theory was no longer applicable in the supercritical state.Based on the thermodynamic theory and the internal energy relationship of the adsorption system,the swelling volume strain of the supercritical carbon dioxide adsorbed by the coal body is obtained.Considering the volume strain of the coal under the action of temperature and pore pressure,the theoretical mode on the coal swelling is established under supercritical carbon dioxide.The numerical solution of the theoretical model under different temperature and pressure conditions is calculated and compared with the experimental results.It is proved that the theoretical model has good agreement with the experimental data,and the theoretical model based on the excess adsorption capacity can be better reflect the law of supercritical carbon dioxide under different temperature and pressure conditions.(3)The swelling of the coal on supercritical carbon dioxide was simulated under different load conditions by ABAQUS,the results show that under the action of supercritical CO2.The volume strain of coal is changed by the "S-type" Logistic function in the case of constant temperature and other factors.The experimental data,theoretical model calculation and simulation results of coal swelling volume strain under supercritical CO2 are compared and analyzed,and the advantages and disadvantages of the three methods are analyzed,the swelling deformation laws obtained by the three methods are basically the same.(4)Based on the established swelling deformation model,the change law of adsorption swelling deformation and permeability in the low permeability coal seam during the process of supercritical carbon dioxide injection is analyzed.The results show that when the temperature is constant,the "S" Logistic function changes regularly which coal swelling volume strain change with the increase of injection pressure is demonstrated.The permeability of coal seam decrease with the increase of injection pressure.The effect of coal seam permeability on the change of adsorption swelling is analyzed under different supercritical CO2 injection spacing conditions.In order to avoid the loss of permeability due to the superposition effect between the holes,the pore spacing should be selected reasonably in the process of supercritical CO2 injection in coal seam.