Dynamic charging mechanism of tight gas reservoirs based on experimental and numerical simulation

As an important unconventional natural gas resource, the charging mechanism of tight gas is of great significance for the accumulation of natural gas. Although previous studies have mainly focused on qualitative evaluation, there is a lack of quantitative evaluation research on the charging process of tight gas. Consequently, this paper uses an example from the tight sandstones of the Upper Triassic Xujiahe Formation, Sichuan Basin, China, by employing physical charging simulation of nuclear magnetic resonance (NMR) coupling displacement, physical property analyses, scanning electron microscopy (SEM), X-ray diffraction (XRD), and high-pressure mercury injection (HPMI) experiments, combined with numerical simulation methods, reveals the tight gas charging mechanism. The principal findings are: (1) The tight reservoirs of the Xujiahe Formation can be classified into four types based on the differences in pore structure. From Type I to IV reservoirs, the distribution of pore sizes (as shown by NMR T₂ spectra) gradually transitions from a bimodal shape dominated by large pores to a single peak shape dominated by small pores. (2) Through multi-factor analysis, a tight gas saturation evaluation model is established that considers reservoir types and pressure and can predict the tight gas charging process and gas saturation in different types of tight reservoirs. (3) The charging process of tight gas is controlled by a combination of charging pressure, pore structure, and water film. Higher charging pressure has a significant impact on the gas content of poor reservoirs. Under the same charging pressure, the gas saturation decreases with the decrease in of pore size. As the charging pressure increases, the influence of the water film diminishes. (4) Based on the principles of mechanical equilibrium and material balance, a numerical model for tight gas charging and reservoir formation is established for three types of source-reservoir combinations: “lower-generation and upper-storage type”, “upper-generation and lower-storage type”, and “interlayer reservoir type”. In the “lower-generation and upper-storage” type, the gas saturation gradually improves from bottom to top. As the thickness of the source rock increases, the gas saturation in the middle and lower parts increases rapidly. The thickness of high-quality source rock has a significant impact on the gas-bearing properties of Type I and Type II reservoirs. In the “upper-generation and lower-storage” type, as the thickness of the source rock increases, the gas-bearing stable zone grows until it becomes stable. For the “interlayer reservoir type”, with the increase in the thickness of the interlayer, the gas saturation of the sand bodies in the middle and lower parts of Type I and Type II reservoirs exhibits a downward tendency, and the gas-bearing capacity of the thick interlayer is lower than that of the thin interlayer. This research not only aids in understanding the accumulation process of tight gas but also provides a theoretical foundation for the accurate prediction of tight gas sweet spots.