Dynamic model for charging tight gas reservoirs: A case study from the Xujiahe Formation in the Sichuan Basin, China

As an important unconventional natural gas resource, the occurrence of tight gas is of great significance for the accumulation of natural gas. However, the charging process of tight gas is complex and diverse. Although previous studies have mainly focused on individual charging parameters, there is a lack of research on the coupling of multiple factors on the control of 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, to elucidate insights by employing physical charging simulation of nuclear magnetic resonance (NMR) coupling displacement combined with physical property analyses, scanning electron microscopy (SEM), X-ray diffraction (XRD), and high-pressure mercury injection (HPMI) experiments. 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, chlorite coatings and dissolution progressively weaken, with pore characteristics (as shown by NMR T₂ spectra) transitioning from a bimodal distribution dominated by macropores to a unimodal distribution characterized by micropores. (2) The charging process of tight gas is divided into three stages: low-pressure rapid charging stage (A), medium-high pressure slow charging stage (B), and late non-continuous slow charging stage (C). Type I reservoirs are mainly charged in stage A, Types II and III reservoirs in stage B, and Type IV reservoirs in stage C. (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) 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. 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.