Rock Fabric of Tight Sandstone and Its Influence on Irreducible Water Saturation in Eastern Ordos Basin

After flowback, the residual fracturing fluid will reduce the gas seepage space and influence natural gas production, which attracts widespread attention. In this study, the irreducible water saturation was investigated, and its controlling factors were clarified. We target the Upper Paleozoic Taiyuan and Shihezi Formations, which belong to a tight gas reservoir in the eastern Ordos Basin. The main experiments include porosity, permeability, mineral composition, nitrogen adsorption, mercury intrusion porosimetry, nuclear magnetic resonance, and high-speed centrifugation. The specific surface area is very low and varies from 0.95 to 4.03 m(2)/g, and the median pore-throat diameter ranges from 28.6 to 698.6 nm. Through the T-2 cutoff value, the water saturation can be divided into movable water saturation (S-mov) and irreducible water saturation (S-irr). Furthermore, the S-irr can be divided into water saturation in large pores controlled by the small throat (S-irrl) and water saturation controlled by the capillary force (S-irrc). In both formations, the S-irr has a negative relationship with porosity, permeability, and average pore diameter and exhibits a positive relationship with the specific surface area. The S-irrl has a positive relationship with the median pore-throat diameter in Taiyuan Formation, but the S-irrl has a weak relationship with the median pore-throat diameter in Shihezi Formation. The Sirrc has a negative relationship with porosity, permeability, and average pore diameter and displays a positive relationship with the specific surface area in Taiyuan Formation, but the S-irrc has a weak relationship with these parameters in Shihezi Formation. The relationship difference between Taiyuan and Shihezi Formations was mainly caused by the pore structure, demonstrated by the amplitude ratio in three peaks. Based on the above analysis, this study is conducive to understanding the mechanism of water occurrence and its controlling factors.

Automated summary

In this study, the irreducible water saturation and its controlling factors in the Upper Paleozoic Taiyuan and Shihezi Formations were investigated. The results showed that the saturation was related to porosity, permeability, average pore diameter, and specific surface area in both formations. Furthermore, the saturation can be further analyzed by dividing it into saturation controlled by small throat or capillary force. Overall, this study improves our understanding of the mechanism and controlling factors of water occurrence.