Fractal Characteristics and Model Applicability for Pores in Tight Gas Sandstone Reservoirs: A Case Study of the Upper Paleozoic in Ordos Basin

Thirty-eight samples from tight sandstone reservoir in the upper Paleozoic layer of Ordos Basin, China were examined. The micropore structure of the reservoir was observed by casting thin sections, which were analyzed via scanning electron microscopy. The pore size distribution characteristics of the reservoir were studied via high-pressure mercury injection. The fit of five different models to the fractal characteristics of the tight gas sandstone reservoir were analyzed, and the fractal characteristics of pores in tight sandstone reservoirs were further revealed. Pores were divided into microscale macropores (P-1) (>10 mu m), microscale micropores (P-2) (1-10 mu m), submicron pores (P-3) (100 nm-1 mu m), and nanopores (P-4) (2-100 nm). The results show that P 2 account for 63.24% of the total pore volume in type I reservoirs. P 3 are dominant in type II reservoirs, accounting for 51.74% of the total pore volume. Type III reservoirs are mainly composed of P-3 and P-4, accounting for 48.96% and 41.88% of pore volume, respectively. In type IV reservoirs, P-4 is the main pore size followed by P-3, accounting for 54.98% and 36.96%, respectively. The Brooks-Corey model can characterize the pore fractal characteristics of the tight sandstone reservoir effectively, while other models presented some limitations. The evaluation of fractal characteristics showed that the three-segment fractal fitting curve was closely related to the inflection point of the S-Hg/P-c-S-Hg curve. The fractal dimensions of P-1, P-2, P-3, and P-4 were 2.904-2.998, 2.384-2.999, 2.155-2.951, and 2.151-2.911, with average values of 2.9826, 2.951, 2.69, and 2.604, respectively. The correlation between the stage pore fractal dimensions and reservoir parameters showed that the fractal dimensions of P-3 and P-4 better reflected the complexity of the pore throats and were more suitable for pore throat heterogeneity characterization. In tight gas sandstone reservoirs, porosity shows relatively low sensitivity to fractal dimensions, while permeability is controlled by the fractal dimensions of P-1 to a great extent. Thus, reservoirs featuring regular macropores are favorable target areas for oil and gas filling.