Study On Numerical Simulation Of Liquid Carbon Dioxide Flow In Coal Seam And Its Effect On Permeability

As a green and clean energy,gas has huge reserves in my country.The development and utilization of gas can optimize my country's energy structure.However,my country's gas is stored in coal seams with low permeability,which is difficult to extract.Therefore,research on technical methods to improve coal seam permeability has become the key.In recent years,my country 's carbon emission reduction pressure has increased,and coal seams have a stronger carbon dioxide adsorption capacity than gas.Therefore,cracking coal seams with liquid carbon dioxide can not only increase coal seam permeability,but also store carbon dioxide underground,reducing emissions.Exhaust pressure and slow down the greenhouse effect.Analyzed from multiple angles,the research on gas displacement from liquid carbon dioxide cracked coal seams is of great significance.Based on the physical and chemical properties of coal and the gas migration mechanism in the coal seam,this paper uses a self-developed experimental platform to study the change law of coal permeability under different gas injection pressures and different environmental back pressures,and conducts carbon dioxide flow flooding.Numerical simulation of multi-field coupling of heat,fluid and solid in the gas process has obtained the temporal and spatial evolution law of parameters such as carbon dioxide and gas pressure,temperature and coal seam permeability.The main conclusions of this paper are as follows:(1)As the injection pressure increases,the permeability of the coal sample also increases.Liquid carbon dioxide has obvious cracking and anti-reflection effects on coal samples,which can effectively improve the permeability of coal samples.With the increase of environmental back pressure,the permeability of coal samples gradually decreases,but the rate of decrease becomes slower and slower.(2)There is the Klinbenberg effect in the coal permeability experiment.Under the condition of low gas injection pressure,the permeability of the coal sample will decrease with the increase of gas injection pressure,and then increase with the increase of pressure after reaching a certain level.This is mainly due to the phenomenon of slippage between the gas molecules and some of the tiny pores in the coal.(3)In the whole flow process,the cracks play a leading role in the initial stage,and the pressure transmission basically coincides with the crack network.When carbon dioxide fills the cracks,the coal pores play a leading role,and the pressure begins to spread around.(4)After gas injection,the free gas in the fissures is quickly replaced by carbon dioxide.The displacement process of carbon dioxide and gas in the pores is slow,and there is a partial pressure effect in the adsorption analysis of the two,which conforms to the Langmuir adsorption equation and is consistent with the actual situation.(5)Due to the uneven distribution of fissures,the effect of gas displacement in different coal seam areas is also very different.In areas with more fissures,as the time of carbon dioxide injection increases,the gas pressure drops significantly.In areas with fewer fissures,gas displacement is more difficult and takes longer.(6)The temperature transfer in the flow process is dominated by fissure transfer in the initial stage,and convective heat transfer is the dominant factor.In the middle and late stages of heat transfer,the matrix heat transfer is the dominant factor,and heat conduction and heat radiation are the dominant factors.The cracks have little effect on the temperature field.Due to the large difference in specific heat capacity between carbon dioxide and coal,the effect of temperature is difficult to transmit to the entire coal seam.(7)In the entire flow displacement process,the pressure of carbon dioxide is the main influence among the influencing factors of matrix permeability.