PORE-THROAT STRUCTURE AND FRACTAL CHARACTERISTICS OF SHIHEZI FORMATION TIGHT GAS SANDSTONE IN THE ORDOS BASIN, CHINA

In order to better understand the impact of fractal features of pore-throat structures on effective physical properties of tight gas sandstones, this paper carried out constant-rate mercury injection tests, gas-water permeability analyses, helium-based porosity and nitrogen pulse decay permeability measurements, and SEM and casting thin-section analyses on 20 sandstone samples of the Shihezi Formation from 16 wells of the Sulige Gas Field in the Ordos Basin, China. The fractal dimensions of pores corresponding to two pore radius ranges, namely fractal dimensions D-p2 and D-p3, and those of throats also corresponding to two throat radius ranges, namely fractal dimensions D-t1 and D-t2, were then calculated using the constant-rate mercury injection data. Fractal dimensions D-p2 and D-p3 are negatively correlated with the average pore radius and pore volume as well as quartz content, and they are positively related to contents of clay minerals and lithic fragments. Compared with pore fractal dimensions, throat fractal dimensions have lower correlations with throat structural parameters, and show vague relations with mineral compositions. Although the effects imposed by mineral compositions upon throat structures are similar to those upon pore structures, the flake-like and curved shapes of throats result in irregularity of fractal dimensions. Tight gas sandstones, with smaller fractal dimensions D-p2 and D-p3, have larger effective porosity, indicating that more gas volumes can be replaced by liquids. As for tight gas sandstones with lower fractal dimensions D-p3 and D-t2, the effective gas permeability is relatively high. In such cases, gas can flow more easily through the reservoir.