Research On Huff And Puff Of Shale Gas CO₂ Injection

Energy transition and low-carbon emission reduction are issues that my country has to solve in recent years.Injecting CO₂ that causes the greenhouse effect into shale reservoirs can not only increase the production of shale gas,solve the problem of energy transition,but also sequester CO₂.Realize environmental benefits in the stratum.In this paper,the Longmaxi Formation shale is used as the research object,and the adsorption kinetics of CH₄ and CO₂ on shale is carried out under different experimental conditions by volume method.The results show that increasing the pressure will significantly increase the CO₂ and CH₄ on shale.Adsorption capacity,increasing the temperature will reduce the adsorption capacity of CO₂ and CH₄ on shale.Based on the experimentally obtained adsorption capacity-time curve,the diffusion coefficient of CO₂and CH₄ on shale was calculated using a single-hole diffusion model.The diffusion coefficient of CO₂ on shale is positively correlated with pressure and temperature,while the diffusion coefficient of CH₄ on shale is negatively correlated with pressure and temperature.Compared with the quasi-first-order kinetic model and Bangham model,the quasi-second-order kinetic model can better fit the experimental data.In the rapid adsorption stage,the adsorption rate of CO₂ continues to decrease,and the adsorption rate of CH₄ first increases and then decreases.In the adsorption equilibrium stage,the adsorption rate of the two gases on shale is zero.Increasing pressure and temperature will reduce the adsorption rate constants of CO₂ and CH₄ on shale.In order to explore the effect of CO₂ huff and puff gas injection on increasing shale gas production and CO₂ storage,three CO₂ injection pressures of 5MPa,7.5MPa,and10.5MPa were carried out for Yanchang Formation shale,at three temperatures of 305K,307K,and 318K,0min,30min Experimental study of CO₂ huff and puff gas injection to displace shale gas under three well-simmering times of,60min.The Coats-Smith two-zone model was used to model the CO₂ penetration curve.The results show that the Coats-Smith two-zone model is very sufficient.A good description of the migration mechanism of CO₂ and CH₄ on shale during competitive adsorption.The increase in CO₂ injection pressure mainly increases the mass transfer driving force for CO₂ diffusion and adsorption to the shale matrix,increases the CO₂ diffusion coefficient,reduces the CH₄ diffusion coefficient,and leads to a decrease in the recovery rate of CH₄.The retention rate has increased.Therefore,reducing the pressure is beneficial to increase the recovery rate of CH₄,and can also increase the recovery rate of pure CH₄.As the temperature increases,the recovery rate of CH₄ and the storage rate of CO₂ increase,but the increase is very small.Therefore,the appropriate experimental temperature should be selected according to the characteristics of the shale reservoir to maximize the benefits.The increase in simmering time accelerates the adsorption and analysis of CO₂ and CH₄ on shale and shortens the time for CO₂ and CH₄ to reach adsorption equilibrium on shale.CO₂ has different effects on the recovery rate of CH₄and the storage rate of CO₂ under different conditions.When the injected CO₂ is in the gaseous state,as the simmering time increases,the recovery rate of CH₄ increases,and the storage rate of CO₂ decreases;when CO₂ is in the supercritical state In the state,the recovery rate of CH₄ and the storage rate of CO₂ decrease with the increase of the simmering time.