Study On Percolation Mechanism In Carboniferous Shale Of Eastern Qaidam Basin

Shale percolation is related to many resources and environmental problems.The study of gas flowcharacteristics in shale is of great practical significance forthe exploration and development of shale gas and the safety evaluation of nuclear waste underground and the performance evaluation of geological storage of greenhouse gas such as C02、CH4.Based on the extensive study of the existing research results,this thesis selects the Carboniferous shale of eastern Qaidam Basin for study.We used the self-designed laboratory equipment,which can simulate temperature and pressure conditions of actual formation,to explore the complex gas percolation mechanism in the nano-pore of shale by conducting percolation experiments of CH4,N2 and He.According to the relevant national testing standards,the TOC content of shales in the study area are almost greater than 1%,which reaches the standard of gas production and indicate strong hydrocarbon generating ability.However,the thermal maturity is relatively low and the kerogen type is dominated by type II.The main mineral compositions include detrital minerals(quartz),carbonate minerals(dolomite),clay mineral(illite / smectite).The methods of mercury intrusion porosimetry,nitrogen adsorption and carbon dioxide adsorption were used to measure the pore size of shale.The results show that the macroporeof the shale in this study area is mainly distributed in 50nm-100 nm,and the proportion is 35%-60%.The proportion of micropores is less than 10%,and the proportion of mesopore is 37%-54%.Based on the Knudsen number(Kn),the flow state of gas in the nanometer pore of the shale can be divided into Darcy flow,slippage flow and diffusion flow,and a mathematical model of gas flow in the nano-pore of shale is established.The model shows that the apparent permeability of the shale nano-pores is positively proportional to the reciprocal of the mean pressure.According to the results of the shale permeability tests,it is found that the Darcy permeability does not change with the change of pressure.And the contribution of slip flow and diffusion flow to the apparent permeability decreases with the increased pressure.It is also found that for the same shale sample,the rank in the apparent permeability of the three gases is helium >methane>nitrogen.Besides,we found that the apparent permeability of shale sample which parallel to the formation is larger than the that of shale sample which perpendicular to the formation.Finally,we comprehensively analyzed the influences of pressure,pore size and shale lithology on the apparent permeability,and we concluded that the apparent permeability decreases with the increase of pressure.Moreover,we found that the apparent permeability is related to the pore size distribution and the mineral composition of shale,as well as the average free path of the gas molecules.