The critical buoyancy threshold for tight sandstone gas entrapment: Physical simulation, interpretation, and implications to the Upper Paleozoic Ordos Basin

Tight sandstone gas is regarded as a significant unconventional resource. One intriguing problem for tight sandstone gas accumulation is how gas was trapped in the tight-sandstone reservoirs. In this paper, we attempted to look for a reasonable explanation for this, which has attached great theoretical and practical significances. In this study, a dynamic equilibrium equation characterizing the critical buoyancy threshold (CBT) was established based on physical experiments, and the formation mechanism of tight gas and CBT were discussed. The explanations were further testified through the tight-gas reservoirs of the Upper Paleozoic Ordos Basin. A dynamical CBT occurs during the charge process in tight-gas reservoirs, and water- gas inversion with separating phases is dominated before natural gas reaches the CBT. At this stage, the driving force for gas movement is abnormal gas pressure, and almost no buoyancy exists. The pore throats, smaller than those corresponding to the CBT, enable the formation water to resist the gas to move upward. As a result, with no water-gas displacements, the gas migrates upward in a piston-like style forming continuous gas reservoir. The corresponding pore throat and depth of the CBT are geologically significant, which indicates a critical condition. However, the CBT condition is not constant and varies significantly according to the geologic setting. The case study of the tight-gas reservoirs of the Upper Paleozoic Ordos Basin indicates that the critical pore-throat radius was 1.77 µm by the end of the Early Cretaceous, with the corresponding critical permeability and porosity value of 0.686 mD and 10.6%, respectively. In contrast, its current counterparts are 1.33 µm, 0.39 mD, and 8.6%, respectively, which indicates that the original tight sandstone gas reservoir has shrunk during the geologic history.