Pore structure and reservoir physical properties for effective development of tight sandstone gas: A case study from the Central Sichuan Basin, China

Several large gas fields have been found in the Central Sichuan Basin in recent years, with proven reserves of more than 100 x 10 (9) m(3). Among these, the tight sandstone gas in the Late Triassic Xujiahe Formation is a key target in the Sichuan Basin. The key to the discovery of high-yield areas rich in tight sandstone gas is to determine the pore structures and the lower limits of effective physical properties of reservoirs that control natural gas accumulation. In this study, we investigate the Late Triassic second Member of the Xujiahe Formation (hereinafter referred to as the Xu2 Member) in the Central Sichuan Basin as an example. We analyse the distribution characteristics of pores and throats in the reservoirs and identify that the reservoir in the study area is mainly characterized by small pores and narrow throats. The physical properties of reservoirs in the area are mainly controlled by pore-throat radius and the permeability-porosity ratio. Nuclear magnetic resonance analysis indicates that the irreducible water saturation is mostly more than 50% and is inversely correlated with the physical properties of reservoirs in the study area. Meanwhile, with an increase in the physical properties of reservoirs, the proportion of large pores increases and the irreducible water saturation rapidly decreases. According to the gas-water two-phase seepage experiment, for reservoirs with good physical properties, the gas relative permeability significantly increases with an increase in gas saturation and the reservoir permeability has more significant effects. Where the reservoir permeability is less than 0.4 mD, the permeability of both gas phase and water phases is only less than 0.1, and the mobility of both gas and water is poor. Only when the reservoir permeability is more than 0.4 mD, can the gas relative permeability reach more than 0.3, and the water mobility is significantly inhibited. In this case, favourable results can be achieved from the natural gas development. As verified by the development of the tight sandstone reservoirs in the Xu2 Member in Central Sichuan Basin, the lower limits of physical properties allowing for effective natural gas development include the reservoir permeability of more than 0.1 mD (preferably more than 0.2 mD) and the reservoir porosity of greater than 6%-7%.