Study Of CO₂ Enhancing Shale Gas Recovery Based On Competitive Adsorption Theory

As we all know,shale gas is an important part of unconventional natural gas.It is an environment-friendly energy which has abundant reserves and high combustion value.Effective development and utilization of shale gas can not only effectively improve China’s energy structure and alleviate China’s energy crisis,but also play an important role in the transformation of China’s energy system to the sustainable development mode.Using carbon dioxide injection to exploit shale gas can not only increase the production of shale gas,but also realize the resource utilization and long-term storage of carbon dioxide,so as to slow down the greenhouse effect.Under the condition of formation temperature and pressure,both CH₄ and CO₂ are in supercritical state,which can not be simply considered as normal gas state.Therefore,the studies on the adsorption/desorption laws and competitive adsorption phenomenon of supercritical CH₄ and CO₂ in shale are the key to improve the recovery of shale gas reservoir by CO₂ injection.Based on the phenomenon of competitive adsorption of methane and carbon dioxide,the mechanism of enhancing shale gas recovery by carbon dioxide injection was studied.Based on the determination of shale physical parameters and microstructure,the adsorption and desorption laws of shale gas and carbon dioxide in the shale under high temperature and high pressure was studied.The feasibility of enhancing shale gas recovery by injecting carbon dioxide was confirmed by physical experiments.On the basis of the existing isothermal adsorption models,an optimal combination model of shale isothermal adsorption under high temperature and high pressure was proposed.And the micro distribution of methane in nanopores was studied by molecular simulation.The adsorption and diffusion capacity of methane and carbon dioxide in nanopores were compared.By using nuclear magnetic resonance technology,the migration,conversion and recovery of methane in the process of shale gas depressurization and enhancing shale gas recovery by carbon dioxide injection were compared.And the competitive adsorption mechanism and applicable conditions of CO₂ injection to improve recovery of shale gas were revealed,which has a guiding role for shale gas reservoir development.The results shows that the Cengong block has high organic matter content and good organic maturity,which maens this block has a good development potential.However,the formation is very dense with the ultra low porosity and permeability.The pores and fractures in the formation are micron and nano scale,so the migration ability of shale is limited.Moreover,the proportion of adsorbed gas in Cengong block is very high,so the output of adsorbed gas plays a crucial role in improving shale gas recovery.With the increase of pressure,the isothermal adsorption curves of CH₄ and CO₂ show a downward trend after the peak value,and the decrease of CO₂ is lower than that of CH₄.High temperature is not conducive to the adsorption / desorption of CH₄ and CO₂ in shale.However,when the temperature is higher than80℃,the effect of temperature on adsorption/desorption of CH₄ is obviously weakened.In the high-pressure stage,the desorption amount of CO₂ increases gradually with the decrease of pressure.After a certain pressure point,the desorption amount of CO₂ increases rapidly with the s decrease of pressure.The pressure value of the turning point ranges from 7 MPa to 7.8MPa.Langmuir model,BET model and D-A model cannot well characterize the decline of shale gas adsorption capacity under high pressure,while the optimal combination model of supercritical shale gas isothermal adsorption can accurately describe the isothermal adsorption law of supercritical shale gas with a high fitting accuracy.CH₄ is not uniform distributed in the shale nanopore,and most of CH₄ are adsorbed on the surface of the nano pore which cause an adsorption layer.With the increase of pressure,there will be multi-layer adsorption,and the first adsorption layer is less affected by temperature and pressure.The adsorption layer has a certain thickness and density,which has influence on the assessment of shale gas reserves.When shale gas enters into shale,it first enters into the small pores of shale in the form of adsorption state.When the adsorption capacity reaches a certain degree,it will appear in the form of free state.In the early stage of shale gas depressurization,the main gas produced is the gas in the macropores,and the gas recovery degree in the small pores is very low.And when the formation pressure is low,shale gas in small pores begins to produce in large quantities.However,when the formation pressure is low,the diffusion capacity of shale gas in the reservoir will decrease greatly.And after the formation pressure is reduced to a certain value,it is impossible to continue to promote the production of adsorbed gas by reducing pressure.Therefore,a large amount of adsorbed gas remains after shale gas depressurization,resulting in a low recovery.Compared with shale gas,more CO₂ can be stored in the same stratigraphic space,which is conducive to CO₂ storage.Shale has stronger adsorption capacity for CO₂.In shale,CO₂ can replace shale gas adsorbed on the pore surface,which makes the adsorption shale gas desorption into free state.CO₂ has the best displacement effect on shale gas in pores with diameter of 15.04 nm-21.62 nm,and it is almost impossible to replace shale gas in pores with diameter of less than 1.08 nm.Depending on the competitive adsorption of CO₂ and shale gas,it can effectively promote the further desorption of residual adsorbed gas after depressurization,and significantly improve the cumulative gas production of shale gas,so as to improve the ultimate recovery of shale gas reservoir.And in the shale gas reservoir with high formation pressure,low formation temperature and high organic matter content,the effect of injecting CO₂ to improve shale gas recovery will be better.