Characterization On The Microcosmic Adsorption Mechanism Of Methane In The Shale Complex Medium Space

The composition media of shale gas reservoirs is complex,especially in marine-lacustrine transitional shale,which has strong inhomogeneity and diversity in many aspects such as pore size distribution,pore media surface combination and pore morphology.The pore space between shale media is the site for shale gas generation,transportation and enrichment,in which the adsorption of methane is the key space for storaging shale gas.Therefore,accurately characterizing the pore structure in the complex media of marine-lacustrine transitional shale gas reservoirs and pinpointing its internal methane distribution pattern and adsorption behavior are the important aspects for providing guidances in shale gas reservoir exploration and production.The Longtan Formation marine-lacustrine transitional shale in southeast Sichuan Basin,currently regard as one of the important backup reservoir of shale gas,is used in the thesis as study object.A comprehensive investigation of the pore structure in the shale formation is presented,and a multi-component,multi-scale complex media surface combination pore models are established based on the actual pore structure characteristics.In this thesis,combining molecular simulation,experimental validation,and theoretical model calculations,the dynamic adsorption behaviors of methane in shale pores under burial conditions are revealed.Meanwhile,the quantitative prediction and evaluation of gas-bearing properties and enhanced recovery of shale gas in the Longtan Formation are investigated,which can provide important advice in theoretical and practical for shale gas exploration and development.The main conclusions are listed as follows,(1)For the complex pore structure characteristics of organic-rich shale in Longtan Formation,an improved multi-view and multi-scale scanning electron microscope pore identification techniques is used to analyze the pore structure characteristics in quantity.The main pore structures of the shale are organic matter pores,microfractures and clay mineral pores.The morphology is mainly slit-like pores,round bubble pores and ellipsoidal bubble pores,and the pores in clay minerals are mostly distributed in a directional or semi-directional manner with a wide range of pore size distribution.The results of the analysis indicate that nanoscale-microscale organic matter and clay mineral pores are heavily developed.Based on the actual pore structure development parameters of Longtan Formation shale,the multi-scale,multi-component,complex medium pore models are established according to the molecular structure of organic matter and clay minerals.(2)Under the supercritical temperature and pressure conditions,the total methane adsorption capacity in the space of different media surfaces show as sequence of organic matter(graphene)>montmorillonite>illite>kaolinite,organic matter and clay mineral surfaces have strong adsorption capacity for methane.The cations exchange at the interface of montmorillonite and illite make the surface have a higher interparticle electric power,which can significantly enhance the degree of methane adsorption phase aggregation.Methane molecules can form a low density multilayer adsorption with the increasing pressure,and the arrangement of adsorption methane molecules on the media surface is more tight and regular.In the small size pore,methane molecules are subject to strong repulsive forces,and it is difficult for the adsorbent methane molecules to enter the space.As the scale of the interconnected pores increases,the repulsive forces weaken to negligible.Fluid adsorption in the pores is a dynamic process,and the adsorption heat reaches a minimum when the excess adsorption amount reaches a maximum.In the complex system of multi-scale media surfaces,the adsorption of methane molecules is simultaneously affected by the composite effects of media properties,pore size distribution,pore morphology,and temperature and pressure conditions,making the methane adsorption characteristics and mechanisms in shale complex and variable.(3)The causes of errors in shale isothermal adsorption are revealed,and a correction method for shale internal adsorption results is proposed based on molecular simulations.The errors in isothermal adsorption experiment were corrected by directly calibrating the intrusive molecules using CH₄ molecules,the pore size was classified into three ranges according to the volume difference ratio,and correcting them with corresponding correction coefficients.The organic and inorganic clay minerals with strong adsorption capacity in the shale are considered for correct excess sorption.After correction,the excess sorption volume can have faster growth at low pressure,which proves again that the high degree of shale micropore-mesopore development from the perspective of sorption.The decreasing trend of corrected excess sorption volume at high pressure is significantly weaker,which indicates the strong adsorption of pore space composed of complex media surface of shale.(4)Fast prediction of excess adsorption by building an extended Simplified Local Density(E-SLD)function applicable to cylindrical pore structures.The results show that the characteristics of adsorbed and free gas distribution can be clearly identified and distinguished,and the discrete density distribution from the pore wall to the center of carbon nanotubes can be separated out by multi methane molecular layers in the high-pressure case.Meanwhile,the excess methane adsorption in the cylindrical medium space calculated by E-SLD model can better match the GCMC molecular simulation as well as isothermal adsorption results.(5)A comprehensive prediction model of storage capacity rating applicable to the Longtan Formation shale reservoir was established.According to the gas content correction process and evaluation method,the prediction results show that the maximum excess adsorption capacity of the Longtan Formation shale in CLD-1 well is1.99-2.39 cm³/g.The shale is rich in shale gas accumulation with high gas content and has development potential based on good preservation conditions.The results show that the gas content of the Longtan Formation shale is affected by burial temperature,burial pressure conditions,and burial depth,and the areas with high excess gas content are mainly located in the Gulin-Xishui area and Yongchuan-Rongchang area.(6)Based on molecular simulation of CO₂ injection,sequestration and enhance shale gas recovery,the variation schema and influences of hydrocarbon gas recovery are investigated,and the quantitative evaluation of enhanced recovery is conducted for Longtan Formation shale.The clay mineral media surface has a stronger adsorption force for CO₂ compared with CH₄,which enables CO₂ to displace CH₄ and occupy the adsorption space.Under different temperature,pressure,and multi-component ratios,the enhanced recovery rate,replacement efficiency and CO₂-CH₄ replacement efficiency are deeply quantified and evaluated.The enhanced CH₄ recovery rate by CO₂injection is closely related to the formation burial conditions,CO₂ injection volume and injection timing,which requires comprehensive design in various aspects to achieve the recovery results.