Controlling Factors And Evaluation Of Gas-bearing Properties Of Deep Shale Of Longmaxi Formation In Southern Sichuan

The deep shale of the Longmaxi Formation in the southern Sichuan area shows great potential for exploration,but the degree of exploration and development of deep shale gas is low.At present,the research on the gas-bearing properties of deep shale is still limited.The geological and geochemical characteristics of deep shale,the characteristics of reservoir and the main controlling factors of gas-bearing properties needs to be improved,and it is still unclear whether the application results of the existing gas-bearing properties prediction models in deep shale are accurate.Aiming at the problem of controlling factors for gas-bearing properties of deep shale,seven deep shale gas wells in Luzhou and West Changning areas were taken as the research objects,and various methods were used to analyze the deep shale gas areas of the Longmaxi Formation in southern Sichuan.The source-reservoir characteristics of the deep shale in the Longmaxi Formation reveal the gas adsorption mechanism of the deep shale,determine the main controlling factors of the gas-bearing property of the deep shale,and establish a quantitative evaluation model for the gas-bearing property of the deep shale.Through TOC content test,laser raman experiment,gas adsorption experiment,high-pressure mercury intrusion,scanning electron microscope,spontaneous imbibition and other technical means,it is concluded that the deep shale gas in the Longmaxi Formation in southern Sichuan has a good material basis,and the pore types are organic pores,organic matter and other minerals(clay,pyrite)are dominated by composite pores and intergranular pores.The development of pores in the Luzhou area is better than that in the West Changning area.More than 50%of the pore volume is provided by mesopores,and more than 90%of the specific surface area is provided by the mesopores.Microporous and mesoporous available.Influenced by the high degree of evolution in the West Changning area,the connectivity of the oil-wet pores in the vertical direction is stronger than that of the oil-wet pores in the Luzhou block,which plays a stronger role in the escape of hydrocarbons,which is not conducive to the sealing of shale gas.Based on methane adsorption isotherm experiments,model comparison,and ideal gas equation of state,it is found that the adsorption law of deep shale gas conforms to the supercritical BET model,which is multi-layer adsorption,and the number of adsorption layers can reach 1.236～2.122.The average maximum adsorption gas volume was 2.375m~3/t,and the average free gas volume was 3.71m~3/t.Through correlation analysis and factor analysis optimization,it is obtained that the main controlling factors of gas adsorption in deep shale are TOC,water saturation,formation temperature and formation pressure;the main controlling factors of free gas quantity are porosity,water saturation and formation pressure.On this basis,using IBM-SPSS software to carry out multiple regression analysis on the amount of adsorbed gas and free gas,a quantitative evaluation model for different occurrence states of deep shale gas was established,and the model was tested with the help of field analytical gas content data,which is smaller and more reliable.Under deep conditions,the amount of adsorbed gas maintains a steady growth trend.The total gas content is mainly controlled by the amount of free gas,and the ratio of adsorbed gas to free gas is about 4:6 high feature.