Study On The Influence Of Super-critical CO₂ Treatment On The Microstructure And CH₄/CO₂ Adsorption Characteristics Of Shale

As the economic development of China,the demand of natural gas resources is growing,and the dependence of natural gas consumption on foreign import is increased year by year,which is a serious threat to the energy security of China.As one of the important successor resources of conventional oil and gas resources,China's shale gas recoverable resources are of 25 trillion cubic meters,the efficient development of shale gas to ease the contradiction between China's energy supply and demand,to ensure energy security is of great significance.At present,hydraulic fracturing and horizontal well technology is the main technology in shale gas development,but the technology is has some shortages,such as the consumption of large water resources,clay mineral in shale swelling caused by water,results in poor production of shale gas.The super-critical CO₂ enhanced shale gas recovery technology has shown a very good application prospect.The basic idea is to use super-critical CO₂ as substitute of water for shale gas reservoir stimulation,and displacing shale gas to improve recovery,and simultaneously sequestrate CO₂.After super-critical CO₂ injection into reservoirs,the influence of shale-super-critical CO₂ interaction on the microstructure and CO₂ and CH₄ adsorption characteristics in shale is unclear,which is very important for the prediction of shale gas production and CO₂ storage potential.Therefore,in this paper,through experimental and theoretical research,adsorption characteristics of CH₄/ CO₂ in different types of shale?marine shale of Longmaxi and Wufeng Formation from Sichuan Basin,continental shale of Yanchang Formation from Ordos Basin?,and the changes of microstructure and adsorption characteristics of shale after super-critical CO₂-shale interaction were studied.The main results are as follows:?1?The analysis results of TOC content measurement,XRD,FE-SEM scanning electron microscopy low temperature N2 adsorption pore structure characterization show that: Compared to continental shales,the marine shale has a higher TOC content,larger specific surface area and pore volume,and higher brittle minerals content and lower clay mineral content.?2?CH₄ and CO₂ adsorption experiments at different temperatures?35?,45?and 55??showed that the average adsorption capacity of marine shale was much higher than that of continental shale.The comparison of Langmuir model,Ono-Kondo model,D-R model and D-A model fitting results of adsorption experimental data of gaseous CH₄ and CO₂ were conducted.The results showed that the D-A model had the highest fitting precision,Langmuir model is also a better description of the adsorption behavior of shale.For the super-critical CO₂ adsorption,the Ono-Kondo model has the highest accuracy.The results of the equal adsorption heat of shale obtained by the adsorption data showed that the adsorption mechanism of CH₄ and CO₂ in shale were mainly of physical adsorption.The average equal adsorption heat of CO₂ is 3.72⁴.86 k J/mol higher than that of CH₄.?3?The main factors influencing shale adsorption capacity were discussed thorough langmuir parameter.It was found that there was a significant positive correlation between shale adsorption capacity and TOC content,total specific surface area,total pore volume,micro-pore specific surface area,micro-pore volume and experimental pressure.While temperature is negatively correlated the adsorption capacity of shale.The correlation between clay mineral content and shale adsorption capacity is not obvious.The adsorption capacity of CO₂ in the same shale sample is about 1.76-8.61 times higher than that of CH₄.?4?The low temperature N2 adsorption,KBr-FTIR infrared spectroscopy and CO₂ and CH₄ adsorption experiments of shale before and after super-critical CO₂?8,12,16 MPa?treatment showed that:with the increase of super-critical CO₂ pressure,there is a significant decrease in CH₄ and CO₂ adsorption capacity for all the shale samples.The main reason is that after the super-critical CO₂ treatment,due to the dissolution of some of the organic matter and minerals in shale,and CO₂ adsorption induced swelling effect,pore structure of the shale is changed.after super-critical CO₂ treatment,total specific surface,micro-pore specific surface area and the micro-pore volume of shale were decreased,total pore volume and the average pore size were increased,and the fractal dimension of the pore structure was slightly reduced,therefore,the adsorption capacity in shale was changed.