Insight into the Pore Structure of Tight Gas Sandstones: A Case Study in the Ordos Basin, NW China

• Published in: Energy & fuels 2017-12, Vol.31 (12), p.13159-13178
• Main Authors: Wu, Hao, Ji, Youliang, Liu, Ruie, Zhang, Chunlin, Chen, Sheng
• Format: Article
• Language: English
• ISSN: 0887-0624; 1520-5029
• DOI: 10.1021/acs.energyfuels.7b01816

A wide spectrum of pore size distributions (PSD) exists in tight gas sandstones, ranging from several nanometers to several hundred micrometers in radius, which controls both the physical rock-flow capacity and storage capacity. Thin-section, scanning electron microscope, field emission scanning electron microscope, high-pressure mercury intrusion (HPMI), constant-rate mercury intrusion (CRMI), and microcomputed tomography scanning experiments are performed on tight sandstone samples from the eighth member of the Middle Permian Shihezi Formation (P2h8) in the Ordos Basin to better understand the pore system characteristics of a tight gas sandstone. The results of this case study show that various types of pores exist in the P2h8 sandstones: residual intergranular pores, intraparticle dissolution pores, intercrystalline pores, and small microcracks are observed. We combine HPMI and CRMI to determine the PSD; the pore sizes range from 3.7 nm to 600 μm in radius. The multimodal PSD is characterized by two broad peaks. The right peak with radii between 50 and 600 μm acts as a pore body and is associated with residual intergranular pores and partial-dissolution pores in grains. The left peak, which corresponds to the throat, shows notable fluctuations and ranges from 3.7 nm to 50 μm; the pores within this size range are mostly associated with dissolution pores and intercrystalline pores. The permeability is mainly controlled by relatively large throats with a lower percentage. When the permeability is less than 1.0 mD, it is dominated by nanopores and micropores; in contrast, higher permeability is almost solely dominated by micropores. Additionally, nanopores are increasingly important in reservoir storage capabilities with decreasing permeability. A new empirical equation to estimate the permeability indicates that the pore throat radius of r 30, which is the optimal representative for the permeability estimation of tight gas sandstones, generates the strongest correlation with the porosity and permeability.