Study On Permeability And Damage Characteristics Of Coal Under Different Phase Carbon Dioxide Adsorption

As global energy demand continues to rise,the consumption of fossil fuels has led to a significant increase in carbon dioxide emissions.Geologic carbon storage technology is widely recognized as one of the key measures to mitigate CO₂emissions by storing CO₂underground for a long period of time.However,underground coal seams have a non-continuous structure,and injecting gas can cause an increase in internal pore pressure,changes in the flow field,and deterioration of the coal seam due to the stored gas,leading to large-scale structural movements and the risk of CO₂leakage.Therefore,this paper focuses on the geological storage of CO₂in difficult-to-mine coal seams,as well as the flow characteristics and damage evolution laws of coal.Firstly,based on the difference in burial depth of the coal seam,a self-made triaxial flow system is used to simulate the real stress and temperature environment of the reservoir,control the injection pressure of CO₂,and conduct adsorption experiments on coal samples with different phases of CO₂.Then,flow tests are carried out,starting with ultrasonic wave velocity measurements of the coal samples,characterizing the change in internal flow channels of CO₂in coal rock to explore the degree of deterioration of coal rock caused by CO₂based on the fracture spacing effect,followed by permeability measurements to analyze the relationship between the permeability of coal samples adsorbed with CO₂and the variation of effective stress.Finally,uniaxial compression tests are conducted to investigate the change in mechanical properties such as peak strength,stress-strain relationship,and elastic modulus of coal specimens before and after CO₂adsorption.The damage state and trend of the total acoustic emission energy change of coal specimens after CO₂adsorption are analyzed in depth using acoustic emission monitoring methods,and the damage energy zone is defined based on the original coal energy value.The specific results are as follows:（1）By comparing the acoustic wave velocity of coal samples before and after adsorption,it is found that the internal cracks of the coal samples expand and connect when CO₂is absorbed,resulting in a negative correlation between the propagation velocity of acoustic waves in coal samples and the injection pressure of CO₂.At the same time,as the phase of CO₂changes from gas to liquid to supercritical state,the attenuation rate of acoustic waves in coal samples gradually increases,and the internal flow channels are improved.（2）The permeability of coal samples is affected by the different phases of adsorbed CO₂and the variation of effective stress.The permeability gradually increases as the phase of adsorbed CO₂changes from gas to supercritical state.The increase in pore pressure leads to a decrease in effective stress,and the permeability of coal samples gradually increases.The permeability has a negative exponential relationship with effective stress.（3）CO₂adsorption can cause damage to coal samples,reduce their mechanical parameters,change their macroscopic failure characteristics,and deteriorate their mechanical properties.As the injection pressure of CO₂gradually increases,the compressive strength and elastic modulus of coal samples decrease.The overall reduction in uniaxial compressive strength ranges from11.72%to 70.89%,the difference in elastic modulus ranges from 0.34 GPa to 1.74 GPa,and the damage factor increases from 0.084 to 0.431.In addition,the effect of liquid and supercritical CO₂on the mechanical properties of coal samples is more significant than that of gaseous CO₂.（4）The acoustic emission energy value undergoes a sudden change when it reaches the later stage of non-stable crack development and before the post-peak fracture stage,and the distribution of acoustic emission energy points is dense.After CO₂adsorption,the total acoustic emission energy value of coal samples during uniaxial compression is in the high-energy zone.Moreover,as the phase of CO₂changes from gas to liquid to supercritical state,the maximum value of acoustic emission energy shows an increasing trend.There are 54 figures,9 tables,and 101 references in this paper.