Study On Pore Structure Characterization Of Organic-rich Shale In Sichuan Basin And Shale Gas Flow And Transport Simulations

With the global energy market switching its interest to low-carbon sources,shale gas,which is of high quality and clean,will play a critical role in energy consumption and environmental protection.As a primary feature,the pore system in shale matrices not only provides the space for gas storage and the flow channels for gas production but also exerts dominant control upon the enrichment and transport of shale gas.This study focuses on the complex pore structure in shale,and tries to answer the following urgent but still unresolved questions:What are the limitations in existing methods for the characterization of the pore structure,and how to ease them?Since Longmaxi Formation shale is currently the only target for shale gas production in Sichuan Basin,what are the principal features of the pore structure?Niutitang Formation shale has been explored very early but showed low gas output.What are the differences between such two formations?What are the primary controlling factors on the pore structure for highly mature shales?How to develop efficient numerical models for accurate prediction of shale gas production and for better understanding the mechanisms in shale gas flow and transport?To answer the above questions,Lower Silurian Longmaxi Formation shales and Upper Cambrian Niutitang Formation shales were analyzed using the methods of organic geochemistry,low-pressure gas adsorption,two-and three-dimensional imaging techniques,and numerical simulations.Conclusions are drawn as follows:（1）An improved method for accurately characterizing the multiscale pore structure of shale is developed,which includes four aspects.First,the range of particle size for the gas adsorption experiment is 60-120 mesh.Second,extraction treatment is only necessary for immature and marginally mature samples.Then,the adsorptive can be the combination of CO₂ and N₂ or CO₂ and Ar.Meanwhile,DFT and NLDFT models are recommended for adsorption data analysis.（2）Different kinds of pores are well developed in shales from Longmaxi Formation and Niutitang Formation.The enrichment and connectivity（especially the local connectivity）of pores in Longmaxi Formation shales are better than that in the Niutitang Formation.The primary difference in nanopore development between the Longmaxi Formation and Niutitang Formation shales lies in the abundance of micropores and fine mesopores（i.e.,<10 nm）.And the amount,pore volume,and specific surface area of micropores and fine mesopores decrease with the increasing maturity.（3）The primary controlling factor for the development of micropores and fine mesopore is the organic matter,and both are well correlated with total organic matter.Besides,the burial depth also influences the micropores and fine mesopore development.In terms of the development of medium mesopore,coarse mesopore and macropore,organic matter,minerals,and burial depth are among the primary factors.（4）Given the multiple length scales of pore structure,an image-based continuum-pore network model for gas flow in shale rock and an image-based nano-continuum-pore network model for gas flow in organic matter are developed.Numerical case studies and applications were conducted to validate the reliability and practicability of the continuum-pore network model.The continuum-pore network models are still in the preliminary numerically experimental stage using the artificial samples.At the end of the dissertation,the novelties and contribution of this work were summarized,and the future development of this work was also proposed.