Effects of pore structures on the movable fluid saturation in tight sandstones: A He8 formation example in Sulige Gasfield, Ordos Basin, China

Abstract

A complex pore structure is a crucial factor influencing the movable fluid saturation in tight sandstones, which poses a significant challenge to the exploration and development of the He8 formation in the Sulige Gasfield. To confront this challenge, various experimental methods, including, thin section, scanning electron microscopy (SEM), cathode luminescence (CL), high pressure mercury injection (HPMI), constant rate mercury injection and nuclear magnetic resonance (NMR), were introduced to investigate the factors that affect the movable fluid saturation. The results showed that coarse to medium-grained quartz sandstone and lithic quartz sandstone were the dominant sandstone types. The main pore types were intercrystalline pores and dissolution pores. The mercury injection and NMR test results showed that the pore structure can be divided into three main types with different curve morphologies and different peak amplitudes. The pore structures were highly correlated with movable fluid saturation. Multiple factors were shown to influence the movable fluid saturation, including pore radius, throat radius, pore-throat radius ratio, sorting coefficient, pore mercury saturation and throat mercury saturation. Among these factors, throat radius and pore mercury saturation were dominant in influencing the movable fluid saturation in all three types. The throat mercury saturation had the greatest effect on type 3, demonstrating that the throat volume strongly contributed to the total volume in tight sandstones. The pore-throat radius ratio, and the sorting coefficient, had a degree of influence on the movable fluid saturation. Meanwhile, strong diagenesis was the internal reason for the complex pore structure that further influenced the movable fluid saturation. Mechanical compaction and cementation led to the rearrangement and compactness of detrital grains, which resulted in the reduction of effective space and the decline in the movable fluid saturation. Feldspar dissolution and debris dissolution provided significant storage space and enhanced the seepage capacity.