Nanoscale Pore Structure Evolution And Shale Gas Occurrence Of Longmaxi Formation In Upper Yangtze Area

Nanoscale pore structure is the key to quantitative characterization of shale-gas reservoirs,meanwhile it is the connection between microscopic accumulation mechanism and macroscopic enrichment rule.This thesis focuses on the nanoscale pore structure evolution and microscopic gas accumulation of Lower Silurian Longmaxi Formation shale in Upper Yangtze area.By using field investigation,experimental test,numerical simulation,and systematic theory analysis,following conclusions have been drawn:The organic-rich shale of Longmaxi Formation is widely developed in the study area.Based on the field profile,borehole statistics and other research results,the isopach map of Longmaxi Formation black shale was drawn.Four lithofacies types of Longmaxi Formation were identified including black carbonaceous shale,calcareous shale,silty shale and muddy siltstone.Meanwhile,geochemical characteristics and mineralogy composition of Longmaxi Formation shale were quantitatively characterized.It was found that the vertical differentiation of total organic carbon(TOC)content in Longmaxi Formation was obvious,and the TOC content of the lower section was high.The mineral composition of shale was complex with high heterogeneity,mainly including quartz,clay,feldspar and carbonate.Based on the field emission scanning electron microscope(FE-SEM),different types of pore morphology were characterized.The preservation mechanism of skeleton mineral rigid frame on organic matter pore was proposed.We also found that the bedding microfractures were important for the shale gas occurrence and migration.Moreover,high pressure mercury intrusion,low temperature N2 and CO2 adsorption were carried out to achieve full-scale quantitative characterization of shale pore structure,the result show that pore size distribution(PSD)is bimodal with micropore(< 2 nm)dominated and the specific surface area is mainly contributed by pores smaller than 10 nm while pore volume is contributed by pores smaller than 200 nm.Based on the high temperature and high pressure of situ formation thermal simulation experiment combined with FE-SEM,the dynamic evolution characteristics of pore morphology of organic pores,dissolution pores and clay pores were revealed.Also,dynamic evolution characteristics of micropore,mesopore and macropore structures(specific surface area,pore volume,PSD)were quantitatively characterized by using high pressure mercury intrusion,low temperature N2 and CO2 adsorption.Based on WX2 well sedimentary burial history,the pore dynamic evolution of Longmaxi Formation was revealed and a four-stages evolution mechanism of pore network with thermal evolution was established.Based on the high pressure methane isothermal adsorption experiment,we explained the ?inverse adsorption? phenomenon by the definition of Gibbs adsorption capacity.The excess adsorption capacity was converted to absolute adsorption capacity by using supercritical micropore filling model with adding excess adsorption correction term.Correlational analysis between the geochemical parameters,mineral composition,pore structure and absolute adsorption capacity was performed to reveal the main control factors to the adsorption capacity.Finally,based on the theory of molecular dynamics,using Grand Canonical Monte Carlo(GCMC)simulation,the microscopic occurrence mechanism of shale gas in nanoscale pore was revealed.It was found that with the increasing of pore size,the adsorption capacity of per unit specific area show stepwise increments and would become steady after the pore size was higher than 2nm.It is also revealed that the methane stay in the form of adsorbed state in micropore and PSD is the crucial factor of gas content.