Study On The Composition,pore Structure Characterization Of Shales And Their Effects On Adsorption Characteristics

As one of the replacement resources for conventional oil and gas resources,shale gas development is of great significance for alleviating the contradiction between energy supply-demand and ensuring energy security.By utilizing the competitive adsorption characteristics of shale gas and CO₂,CO₂ can be injected into shale reservoirs to achieve shale gas production and CO₂ geological storage.In order to evaluate the development potential of shale reservoirs and the ability to seal CO₂,as well as the safety of shale gas production and CO₂ storage,it is important to study the adsorption characteristics of CH₄and CO₂ on shales.The shale adsorption characteristics are controlled by intrinsic factors such as compositions and pore structure,so it is important to explore the componential and pore structure of shales and their effects on adsorption characteristics.In this thesis,taking the marine shales from the Tongren Longmaxi Formation,the Sichuan Basin Wufeng Formation,and the Shaanxi Yanchang Formation continental shales as research objects,characterization of components,pore structures and isothermal adsorption experiments of CH₄&CO₂ were carried out.An improved adsorption model is established.Based on the experimental results,combining fractal and thermodynamic theory,the main relations between shale compositions,pore structures and adsorption characteristics are as follows:(1)The testing results such as R_O,TOC,XRD,XRF,FTIR and the pore structures characterization of low temperature N₂ adsorption and mercury porosimetry show that compared to continental shales,marine shales have higher TOC,richer Nano-sized pores,larger specific surface area,micro and mesopore structures are more complex and rich in clay minerals,which is more conducive to the occurrence of gas.The continental shales has more macropore content and more complex pore structure than the marine shales.(2)Isothermal adsorption experiments of CH₄&CO₂ at different temperatures(298.15K,318.15 K,338.15 K)were carried out.The results show that the adsorption capacity of CH₄&CO₂ increases with the equilibrium pressure increases at the same temperature.It will decrease with the increase of temperature at the same pressure.Under the same conditions,the adsorption capacity of CO₂ is significantly higher than that of CH₄.In addition,the absolute adsorption capacity of CH₄ and CO₂ on shales is larger than the excess adsorption capacity,and the difference between them is affected by temperature,pressure and gas type.(3)Using the Langmuir model,three-constant BET model,and D-R model to fitting analysis the experimental data.The results show that the Langmuir model is more suitable for fitting the adsorption isotherms of CH₄&CO₂ on shales.The selectivity coefficient?_CO2?CH4 of the shale adsorption obtained by the fitting parameters of the Langmuir model are greater than 1,indicates that the feasibility of CO₂ enhanced shale gas recovery.The calculation results of isosteric hest of adsorption and entropy change of shale samples explain from thermodynamic theory that the adsorption capacity of CO₂ is greater than that of CH₄ on shales.Based on the results of thermodynamic analysis and the Langmuir model,an improved shale adsorption model considering temperature was established,and the D-R model was used to distinguish between micropore,mesopore and macropore adsorption modes.The improved model is better than the Langmuir model in fitting CH₄and CO₂ isothermal adsorption characteristics,and has better prediction effect.(4)TOC is the decisive factor for the development of shale micropores and the most important control factor affecting the adsorption of CH₄ on shales.The Clay mineral content controls the development of mesopores and is also the most important control factor affecting the adsorption capacity of CO₂ on shales.The pore structure is the direct factor controlling adsorption capacity.The specific surface area and volume of micropores and mesopores are significantly related to the maximum adsorption capacity of CH₄&CO₂.The fractal dimension also shows a certain correlation,therefore,it can be used as one of the potential parameters to evaluate the shale gas adsorption capacity.The selectivity coefficients of CH₄&CO₂ are positively correlated with TOC,and negatively correlated with the quartz mineral content.Although there is no significant correlation with the clay mineral content,the selectivity coefficient of the terrestrial shale with high kaolinite content is greater.