Characterization of Closed Pores in Longmaxi Shale by Synchrotron Small-Angle X-ray Scattering

The characterization of closed and open pore structures is key to evaluating the migration mechanism of gas in shales. In this study, we combined small-angle X-ray scattering (SAXS) with two fluid intrusion methods (low-pressure N-2 physisorption and mercury intrusion capillary pressure) to systematically characterize the closed pores of samples from the Longmaxi Formation shale in the Sichuan Basin, China. The total porosity (including connected pores and closed pores) of the shale was measured by SAXS, and the connected pores of the shale (the part of the pore system intruded mercury and nitrogen) were measured by the fluid intrusion methods. Characterization of closed pores in the shale was based on similarities and differences between the two methods. This study also introduces the calibration process of SAXS from measuring scattering intensity to absolute scattering intensity. Study deductions include the development of closed pores being affected by factors, which include TOC content and mineral composition, an indirect relationship between shale matrix permeability, and the fraction of closed pores. Furthermore, fractal dimension of the system affected by the closed pores is also deduced from manifestation of decrease in connectivity. These results indicate that further opening of the closed system during shale gas development could increase oil and gas production and recovery rates.

Automated summary

The study systematically characterized the closed pores of Longmaxi Formation shale in Sichuan Basin by combining small-angle X-ray scattering with two fluid intrusion methods. Factors such as TOC content and mineral composition were found to affect the development of closed pores in the shale, while a relationship between shale matrix permeability and the fraction of closed pores was indirectly observed. Additionally, the fractal dimension of the system was deduced to be influenced by the closed pores, indicating potential for increased oil and gas production and recovery rates through further opening of the closed system during shale gas development.