Classifications of the Reservoir Space of Tight Sandstone Based on Pore Structure, Connectivity, and Fractal Character: A Case Study from the Chang 7 Member of the Triassic Yanchang Formation in the Ordos Basin, China

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Tight sandstone reservoirs have become important areas for unconventional reservoir development, and their pore network is a key feature for identifying tight sandstone, which affects fluid migration path and reservoir development efficiency. However, the connectivity characteristics of the pore network at different scales have remained unclear owing to the numerous pores and uneven pore shape. Here, using pore size distributions from many hundreds of tight sandstone samples and subsequent topological data analysis, we construct the topological structure of the pore network in the Yanchang Formation tight sandstone of the Ordos Basin in China and visualize the topological characteristics of the pore network with distances. We show that there are three connected groups within the pore structure of the tight sandstone. The topology of the pore network resides on a trident ring manifold, suggesting that the pore network in the tight sandstone encompasses three obvious dominant connection paths. One prominent bar on the H 0 dimension in the barcode indicates a twopoint connection from nanoscale to microscale in the pore network. Three prominent bars with varying durations on the H 1 dimension indicate the presence of three separate multipoint connections within a limited extent in the pore network. Connectivity of combined pores is good and controlled by the topological structure of the pore network. This demonstration of pore connections on a trident ring manifold provides a population-level visualization of the pore network in the tight sandstone.

Section: Discussionmentioning

confidence: 99%

Topological Characteristics of the Pore Network in the Tight Sandstone Using Persistent Homology

Wang,

Yan,

Wang

et al. 2024

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“…The fractal theory is an essential method for examining the characteristics of the intricate pore system . The fractal dimension ( D ) can characterize the fractal characteristics and build a connection between the morphological features and connectivity of microstructures and physical properties. , Thus, the fractal dimension is a powerful tool to demonstrate the features of the microstructure . The pore types can be divided according to the corresponding fractal dimension. − The pore structure changes and rock–fluid interactions in tight sandstone during CO 2 flooding can be better depicted by combining the fractal methods with the NMR experimental data.…”

Section: Introductionmentioning

confidence: 99%

“… 26 The fractal dimension ( D ) can characterize the fractal characteristics and build a connection between the morphological features and connectivity of microstructures and physical properties. 27 , 28 Thus, the fractal dimension is a powerful tool to demonstrate the features of the microstructure. 29 The pore types can be divided according to the corresponding fractal dimension.…”

Section: Introductionmentioning

confidence: 99%

Effect of CO₂–Brine–Rock Interactions on the Pore Structure of the Tight Sandstone during CO₂ Flooding: A Case Study of Chang 7 Member of the Triassic Yanchang Formation in the Ordos Basin, China

Wang

Li²,

Zhen³

et al. 2023

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CO 2 flooding is an effective technique to enhance oil recovery from tight sandstone reservoirs. The CO 2 -rich fluid reacts with the in situ minerals and results in the corrosion and precipitation of minerals in the sandstone, affecting the connectivity and morphological features of pores. However, the diagenesis of the tight sandstone is complex, and the structural modification behavior and mechanism in different lithofacies of tight sandstone during CO 2 flooding are still unclear. This study combined CO 2 flooding, thin-section casting, scanning electron microscopy, nuclear magnetic resonance, X-ray diffraction, and fractal analysis methods to investigate the changes in the pore structure of tight sandstone after CO 2 flooding. The results show that the cement of the tight sandstone is complex and diverse. The tight sandstone can be divided into two types of lithofacies: clay cementation (CL) and ferrocalcite cementation (CA). The mineralogical alterations occur differently in each lithofacies of tight sandstone. Alterations in the cement minerals affect the pores morphology of tight sandstone depending on their mineralogical structure and texture, and the CO 2 flooding mainly changes the micromorphology and heterogeneity of large pores. Clay minerals dominate the cement in the CL lithofacies of tight sandstone. The dissolution mainly occurs in small pores because the precipitation of new minerals and exfoliation of skeleton particles partially block large pores and transform them into small pores. Thus, the number of small pores of CL lithofacies increases while the number of large pores decreases. Such a phenomenon hinders the increase in porosity and permeability. On the other side, in the CA lithofacies of tight sandstone, the cement is dominated by ferrocalcite, and the dissolution of ferrocalcite is the primary mechanism of mineralogical alteration. As the ferrocalcite dissolution expands, it creates new flow paths, improving the connectivity of pores. The petrophysical properties of CA lithofacies improve significantly after CO 2 flooding. There is a crucial need to study the changes in diagenetic characteristics of tight sandstone due to CO 2 flooding. Such type of study provides insights related to the improvement and evaluation of the development of tight sandstone reservoirs during CO 2 flooding.

“…Several techniques have been used to depict the fractal dimensions of porous rocks, such as scanning electron microscopy (SEM), thin sections, rate-controlled porosimetry, high-pressure porosimetry, X-ray computed tomography, and nuclear magnetic resonance (NMR) measurements. − Most of these techniques frequently utilize a single pore model. For instance, high-pressure porosimetry considers the pore throats as cylindrical models, while rate-controlled porosimetry simulates pores as spherical models, which makes it difficult to determine the differences in the microscopic structure of the various pore types.…”

Section: Introductionmentioning

confidence: 99%

Pore Structure and Fractal Characteristics of Tight Sandstones Based on Nuclear Magnetic Resonance: A Case Study in the Triassic Yanchang Formation of the Ordos Basin, China

Wang

Wang²,

Wang³

et al. 2023

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The characterization of the pore structure of tight sandstones is of great importance for the exploration and development of tight oil reservoirs. However, little attention has been given to the geometrical features of pores with various scales, which implies that the effect of pores on the fluid flow and storage capacity is still ambiguous and presents a significant challenge to the risk assessment of tight oil reservoirs. This study investigates the pore structure characteristics of tight sandstones by applying thin section petrography, scanning electron microscopy, nuclear magnetic resonance, fractal theory, and geometric analysis. The results indicate that the tight sandstones have a binary pore system, consisting of small pores and combine pores. A shuttlecock model expresses the shape of the small pore. The radius of the small pore is comparable to the throat radius, and the connectivity of the small pore is poor. A spiny spherical model describes the shape of the combine pore. The connectivity of the combine pore is good, and the pore radius is larger than the throat radius. The most significant contribution to the storage space of the tight sandstones is attributed to the small pores, while permeability is primarily controlled by the combine pores. The heterogeneity of the combine pore has a strong positive correlation with flow capacity, which is associated with the multiple throats of the combine pores that developed during diagenesis. Therefore, the sandstones that are dominated by combine pores and are located near the source rocks represent the most favorable area for the exploitation and development of tight sandstone reservoirs.