Characterization of Rock Fabric and Its Influence on Water Occurrence in Tight Gas Reservoirs

Abstract

After hydraulic fracturing, the flowback rates of some tight gas reservoirs are less than 50%, which seriously affects tight gas production. Regarding the fracturing fluid retention mechanism in tight gas reservoirs, it is still unclear. This research focused on the rock fabric and water occurrence in tight sandstones selected from the Upper Paleozoic Taiyuan and Shihezi Formations in the eastern Ordos Basin. The experiments include X-ray diffraction, porosity, permeability, casting thin section (CTS), scanning electron microscopy (SEM), microcomputed tomography (micro-CT), nuclear magnetic resonance (NMR), and high-speed centrifugation. Particles with concavo-convex contact are a typical characteristic in both formations, and mica shows obvious deformation, which indicates that these formations have undergone severe compression. There are many microscale cracks in the Shihezi Formation, some of which pass through complex pore zones, and the pore filled with clay can be as large as 271.7 μm, and a crack width can reach 2.8 μm. The pore size can be as large as 48.9 μm in the Taiyuan Formation, and the crack width can reach 2.4 μm. At the microscale, the average pore size, throat size, and throat length for the Shihezi Formation are 10.6, 6, and 9.4 μm, respectively, while those for the Taiyuan Formation are 16.6, 7.5, and 15.6 μm, respectively. The water occurrence was investigated using NMR and high-speed centrifugation. Using NMR T1-T2 (2D) maps to monitor the dry, centrifugated, and saturated states of samples, the crystal water, irreducible water, and movable water were identified. The average movable water saturations of the Shihezi and Taiyuan Formations are 51.9% and 63.6%, respectively, which indicates that the Taiyuan Formation has a higher movable water saturation. Movable water saturation is mainly controlled by porosity, permeability, quartz, feldspar, carbonite, clay, and average throat size. Overall, this research contributes to the understanding of rock fabric and water occurrence in tight reservoirs.