| With the rapid development of China’s economy,the demand for oil and gas resources is increasing.The Upper Ordovician Wufeng Formation and Lower Silurian Longmaxi Formations in the Sichuan Basin and its surrounding areas are the important shale gas exploration and development area in China,but the gas-producing layers are mainly distributed in medium and shallow shale,while research on deep shale gas with great resource potential is relatively weak.Therefore,this thesis takes the shale from the Wufeng-Longmaxi Formations in the Dongxi area（the southeast Sichuan Basin）as the research object,and many methods such as geochemical analysis,field emission scanning electron microscopy,low-pressure gas adsorption experiment（CO2and N2）,high-pressure mercury intrusion,low frequency nuclear magnetic resonance and high-temperature and high-pressure methane adsorption are conducted.First,the pore structure characteristics and methane adsorption characteristics of shale in the study area were systematically studied.Then the supercritical methane adsorption mechanism on this basis of shale gas was revealed.The following understandings have been achieved:1.Four shale lithofacies are recognized in the Wufeng-Longmaxi Formation shale in Dongxi area,including silica-rich argillaceous shale,argillaceous-siliceous mixed shale,clay-rich siliceous shale and siliceous shale,among which clay-rich siliceous shale and siliceous shale are high-quality lithofacies to form favorable reservoirs.Organic pores,inorganic pores（intragranular and intergranular pores）and microfractures are commonly developed in the pores of organic-rich shale in Dongxi area.Organic matter（OM）pores are relatively developed,often in irregular and oval,and some intragranular pores on the surface of local brittle minerals is scattered distribution and poorly connected.The quantitative characterization results showed that the pores of the Wufeng-Longmaxi Formation in the Dongxi area were mainly composed of micropores and mesopores,but macropores are less and strong heterogeneity.The pore structure characteristics of shale are influenced by the sedimentary environment,mineral composition and tectonic movement in the study area,among which OM is the material basis for the development of shale pores,and the rigid frame formed by brittle minerals such as quartz is the guarantee that shale pores are not damaged by compaction.2.Methane adsorption tests were carried out to systematically analyze the methane adsorption characteristics in the supercritical state,model optimization,causes of"abnormal adsorption"and the main controlling factors.It is found that the main reason for the excess adsorption characteristics of shale methane adsorption curve is the density difference between the free phase and the adsorbed phase.Compared with different supercritical methane adsorption models,the SDR model is more suitable for calculating methane adsorption capacity of shale,while the S-Langmuir model is more suitable for studying the thermokinetic characteristics of methane adsorption.The key influencing factors of shale methane adsorption capacity in the study area mainly include OM abundance,mineral composition,temperature,pressure and pore structure characteristics.The influence of Pressure,quartz and organic matter abundance on the methane adsorption capacity is favorable,while temperature and clay minerals are unfavorable to methane adsorption capacity.The micropores and mesopores can provide sufficient adsorption space for methane,which is conducive to methane adsorption and storage.3.The methane adsorption form of shale in Dongxi area was analyzed through various adsorption theories,shale gas should be both micropore filling and monolayer adsorption under supercritical geological conditions.When the pressure is low,the methane molecules will preferentially adsorb in the micropores,and the agglomeration and filling will occur.with filling saturated in the micropores,methane molecules will begin monolayer adsorption in the mesopores-macropores structure until complete adsorption.With the increase of temperature,the activity capacity of methane molecules becomes stronger,the upper limit of the scale of microporous filling will be reduced,and methane molecules will begin monolayer adsorption in the pore surface.4.Based on methane adsorption experiments at different temperatures and combined with a variety of theoretical models,the thermodynamic characteristics of methane adsorption in shale under supercritical conditions were clarified.The size of the adsorption potential on the surface of shale is related to the degree of micropore development in shale,and generally the more microporous it develops,the greater the adsorption potential of shale.With the increase in pressure,the variation of surface free energy of the shale matrix gradually increased,while the surface free energy reduction gradually decrease.As the temperature increased,the difference in surface free energy between the shales decreased,indicating that the increase in temperature inhibits the progress of methane adsorption.In the shale methane adsorption process,if the effect of the adsorption phase volume is neglected,the actual adsorption heat for methane adsorption will be overestimated.Therefore,it is particularly important to consider the effect of the adsorption volume on the actual adsorption capacity. |
