Movable Fluid Distribution Characteristics and Microscopic Mechanism of Tight Reservoir in Yanchang Formation, Ordos Basin

Abstract

Tight sandstone reservoirs have smaller pore throats, complex structures, strong heterogeneity in the pore throat system, and significant differences in fluid distribution. To reveal the occurrence characteristics of movable fluids in tight sandstone reservoirs, the typical tight sandstone core samples from the Chang 4+5 reservoir of the Upper Triassic Yanchang Formation of the Ordos Basin were selected and the micro pore throat classification standard was established by using casting thin slices, field emission scanning electron microscopy (FE-SEM), high-pressure mercury injection (HPMI), and nuclear magnetic resonance (NMR) technology. The test analysis results showed that the Chang 4+5 tight sandstone reservoir can be divided into Type I, II, and III reservoirs by the different pore throat structure. Type I reservoirs have well-developed pore throats, good connectivity, low threshold pressure (0.42 MPa on average), high movable fluid saturation (46.82% on average), and a minimum pore throat radius of movable fluid (0.056 μm on average). The pore throat structure and fluid production degree of the other two reservoir types gradually deteriorated. Note that the tight sandstone reservoirs of the Chang 4+5 reservoirs have smaller pore throats but high movable fluid content in smaller pore throats (17% on average). With the deterioration of the pore throat structure in reservoir, the degree of fluid utilization in larger pore throats decreases (74.33%–57.33%), whereas the degree of fluid utilization in smaller pore throats does not change significantly (25.67%–13.82%). Many factors affect movable fluid parameters. The movable fluid parameters of tight sandstone reservoirs in the Chang 4+5 reservoir have a good positive correlation with permeability (R2 = 0.85) and sorting coefficients (R2 = 0.88). The movable fluid parameters of smaller and larger pore throats have no obvious correlation with a single factor. Multiple factors affect the fluid occurrence characteristics of different scale pore throats.

Pore Structure Characteristics and Main Control Factors of Sandstone in the Jurassic Zhiluo Formation in the Northern Ordos Basin

“…The Ordos Basin is located in the North China Craton. In the western part, a multitectonic system containing multiple sedimentary types, depression migration, and significantly distorted multicycle superimposed oil-and gas-bearing basins have developed [39,40]. The research area is located on the Yimeng Uplift in the northern part of the Ordos Basin and has been one of the main areas for analyzing sandstone-type uranium deposits in recent years (Figure 1).…”

“…In a study on the sandstone pore structure in the Ordos Basin, the Yanchang Formation was found to be relatively mature, and scholars have systematically studied the pore structure characteristics of the sandstone in different layers of the Yanchang Formation [36][37][38][39][40]. With the increasing demand for oil and the increasing difficulty of oil extraction, researchers have noted the low permeability pores of the Yanchang Formation [41][42][43].…”

A Characterization of Tight Sandstone: Effect of Clay Mineralogy on Pore-Framework

“…We characterized the micro-pore systems in tight rocks by using image analysis with petrographical texture quantification on thin sections; SEM/Micro-CT/NMR imaging, and complementary MICP, and XRD analyses (Akin et al, 2003;Pan et al, 2015;Gao et al, 2016). Each technique provides independently unique spatial and mineralogical information and the insight into the distribution behavior within micro-pore system results (Dullien 1979;Wang et al, 2021;Wu et al, 2022).…”

The Power of Characterizing Pore-Fluid Distribution for Microscopic CO2 Injection Studies in Tight Sandstones

“…This further implies that a higher movable fluid proportion corresponds to more interconnected pores [62,63]. Both movable and bound-fluid volumes can be determined using both centrifuge and NMR measurements [64][65][66]. In these methods, movable fluid is discharged from interconnected pores using a high-speed centrifuge, while bound fluid is confined inside the micropore system due to the clay presence and capillarity [66,67].…”