Fractal characteristics analysis of pore structure in tight sandstone reservoirs of the He 8 member, Shihezi formation: eastern Ordos Basin

Tight sandstone reservoirs, characterized by low porosity and permeability, have long posed significant challenges for oil and gas exploration and development. This study focuses on the reservoirs in the Chang 6–Chang 8 section of the Yanchang Formation, located in the Heshui area of the Ordos Basin. These reservoirs, with their complex pore structures and diverse genesis, represent typical examples of tight sandstone reservoirs. The primary aim of this study is to systematically investigate the pore structures of these reservoirs, elucidating the mechanisms that govern permeability and thereby providing a scientific basis and technical support for the efficient development of tight sandstone oil and gas resources. Utilizing high-pressure mercury injection and nuclear magnetic resonance techniques, the study emphasizes the crucial role of pore fractal properties in controlling reservoir permeability. The results indicate that the complexity of pore structures directly influences both permeability and mobile fluid saturation. Specifically, smaller pores exhibit lower fractal dimensions, while larger pores show higher fractal dimensions, with significant impacts on both permeability and mobile fluid saturation. Additionally, the study examines the effects of different sedimentary facies and diagenetic stages on the pore structures of sandstone reservoirs. The findings reveal distinct evolutionary patterns of pore structures across various depositional environments within the braided river delta system. Furthermore, diagenetic processes and cementation play essential roles in shaping the diversity of pore structures in these reservoirs. In conclusion, this study offers new insights into the relationship between pore structure and permeability in tight sandstone reservoirs, providing valuable theoretical guidance for the exploration and development of oil and gas resources.

The complex pore system in tight sandstone reservoirs controls the storage and transport of natural gas. Thus, quantitatively characterizing the micro-nanopore structure of tight sandstone reservoirs is of great significance to determining the accumulation and distribution of tight gas. The pore structure of reservoirs was determined through polarizing microscopy, scanning electron microscopy (SEM), and the combination of mercury injection capillary pressure (MICP) and nuclear magnetic resonance (NMR) experiments on Late Paleozoic conventional and tight sandstone samples from the Linxing Block, Ordos Basin. The results show that in contrast to conventional sandstone, dissolution pores, with diameters less than 8 μm, are the main contributors to the gas storage space of tight sandstone reservoirs. The pore size distribution derived from the MICP experiment demonstrates that the main peak of tight sandstones corresponds to a pore radius in the range of 247 nm to 371 nm, while the secondary peak usually corresponds to 18 nm. The results of the NMR test illustrate that the T₂ spectra of tight sandstones are unimodal, bimodal and multimodal, and the main NMR peak is highly related to the MICP peak. Fractal theory was proposed to quantitatively characterize the complex pore structure and rough porous surface. The sandstones show fractal characteristics including nanopore fractal dimension DN obtained from the MICP and large pore fractal dimension DL obtained from the NMR experiment. Both DN and DL are positively correlated with porosity and negatively correlated with permeability, demonstrating that complex and heterogeneous pore structure could increase the gas storage space and reduce the connectivity.

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In tight sandstone reservoirs, diagenesis has a significant impact on the development of reservoirs and pore structures. To clarify the effect of diagenesis on the pore structure of tight sandstone, 12 samples of the Yanchang Formation in the basin were studied based on experiments such as high-pressure mercury intrusion and low-temperature nitrogen adsorption. The diagenetic facies in the study area are divided into two categories: strong cementation facies of carbonate minerals and strong compaction facies of soft component minerals, which are relatively unfavorable diagenetic facies, and stable facies of felsic minerals and strong dissolution facies of feldspar minerals, which are dominant diagenetic facies. The pore structure of the Chang 63 reservoir in the study area has obvious fractal characteristics, with a fractal dimension D 1 greater than D 2 and a greater heterogeneity of large pore throats. Compared to compaction and cementation, dissolution has a stronger controlling effect on the pore structure of reservoirs. In tight sandstone reservoirs with low porosity and permeability, dissolution has a more important impact on reservoir transformation and development. The intensity of different types of diagenesis in the Chang 63 reservoir affects reservoir heterogeneity, and the level of the reservoir heterogeneity affects the complexity of reservoir pore structure. In tight sandstone reservoirs, cementation has a stronger controlling effect on the structural complexity of large pores, while dissolution has a stronger controlling effect on the structural complexity of small pores. The dissolution has a strong control effect on the physical properties of the reservoir. This study provides insights into the relationships among the diagenetic facies, reservoir quality, and pore structure of tight sandstone reservoirs. This study has reference significance for the exploration and development of tight oil in the research area.

Characterization of pore structure of tight sandstone reservoirs based on fractal analysis of NMR echo data

Quantitative evaluation of pore structure from mineralogical and diagenetic information extracted from well logs in tight sandstone reservoirs

Experimental study of oil recovery from pore of different sizes in tight sandstone reservoirs during CO2 flooding

Pore structure is the key element of tight sandstone reservoir, which restricts the accumulation and flow of oil and gas in the reservoir. At present, reservoir pore structure is the focus and difficulty of unconventional oil and gas exploration and development research. The tight sandstone reservoir in the Chang 4 + 5 member of the Upper Triassic Yanchang Formation is the main reservoir for oil and gas exploration in G area. At present, there is little research on its pore structure and fractal characteristics, which to some extent affects the progress of exploration and development. This paper selects the tight core samples of the Chang 4 + 5 member in the southern edge of the Ordos Basin, and based on the high-pressure mercury intrusion experiment, uses fractal theory to study the pore structure and fractal characteristics of the reservoir in the study area, thus providing theoretical basis for the evaluation and exploration and development of the Chang 4 + 5 tight reservoir in the G area. The research results show that the lithology of the Chang 4 + 5 tight sandstone reservoir in the southern edge of the Ordos Basin is mainly feldspathic sandstone, with the highest feldspar content, followed by quartz, and the clay mineral is mainly chlorite. The reservoir has poor physical properties and strong heterogeneity. There are three main fractal characteristics in Chang 4 + 5 reservoir in G area: the fractal curve of Type I reservoir sample is in two segments, the relatively large pore has certain fractal characteristics, the pore structure is relatively regular, and the heterogeneity is weak; Relatively small pores have no fractal characteristics and pore structure is irregular. The fractal curve of Type II reservoir samples shows a three-segment pattern, and each pore size range has certain fractal characteristics, and it gradually gets better with the increase of pore size. The fractal curve of Type III reservoir samples presents a similar one-segment pattern, and the fractal dimension exceeds the upper limit of 3. It is considered that the full pore size of this type of reservoir does not have fractal characteristics, the pore throat is completely irregular or the surface is rough, and the heterogeneity is very strong.

Pore-throat size distribution is a key factor controlling the storage capacity and percolation potential of the tight sandstone reservoirs. However, the complexity and strong heterogeneity make it difficult to investigate the pore structure of tight sandstone reservoirs by using conventional methods. In this study, integrated methods of casting thin section, scanning electron microscopy, high-pressure mercury intrusion (HPMI), and constant-pressure mercury intrusion (CPMI) were conducted to study the pore-throat size distribution and its effect on petrophysical properties of the Shanxi Formation tight sandstones in the northern Ordos Basin (China). Results show that pore types of the Shanxi tight sandstone reservoirs include intergranular pores, dissolution pores, intercrystalline micropores, and microfracture, while the throats are dominated by sheet-like and tube-shaped throats. The HPMI-derived pore-throat size ranges from 0.006 to 10 μm, and the pore-throats with a radius larger than 10 μm were less frequent. The pore body size obtained from CPMI shows similar characteristics with radii ranging from 100 to 525 μm, while the throat size varies greatly with radii ranging from 0.5 to 11.5 µm, resulting in a wide range of pore-throat radius ratio. The full range of pore size distribution curves obtained from the combination of HPMI and CPMI displays multimodal with radii ranging from 0.006 to 525 µm. Permeability of the tight sandstone reservoirs is primarily controlled by relatively larger pore throats with small proportions, and the permeability decreases as the proportions of smaller pore-throats increase. The pervading nanopores in the tight gas sandstone reservoirs contribute little to the permeability but play an important role in the reservoir storage capacity. A new empirical equation obtained by multiple regression indicates that r15 (pore-throat size corresponding to 15% mercury saturation) is the best permeability estimator for tight gas sandstone reservoirs, which yields the highest correlation coefficient of 0.9629 with permeability and porosity.

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 m2/g, and the median pore-throat diameter ranges from 28.6 to 698.6 nm. Through the T2 cutoff value, the water saturation can be divided into movable water saturation (Smov) and irreducible water saturation (Sirr). Furthermore, the Sirr can be divided into water saturation in large pores controlled by the small throat (Sirrl) and water saturation controlled by the capillary force (Sirrc). In both formations, the Sirr has a negative relationship with porosity, permeability, and average pore diameter and exhibits a positive relationship with the specific surface area. The Sirrl has a positive relationship with the median pore-throat diameter in Taiyuan Formation, but the Sirrl 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 Sirrc 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.

Understanding complex pore structures is important for evaluating tight oil reservoir performance and predicting favorable pore structure. However, quantitative characterization of pore structure in tight sandstones by combining different methods is still poorly understood. Using the Upper Triassic Yanchang Formation in Ordos Basin, China as a case study, we first introduce a new method to quantitatively characterize full-range pore-throat size distribution (PSD) through multifractal dimension analysis of integrated pressure-controlled porosimetry (PCP) and rate-controlled porosimetry (RCP). Second, we propose a technique using helium porosity and nitrogen permeability to obtain multifractal dimensions in an attempt to predict favorable pore structure in tight oil reservoirs. In the new method of obtaining full-range PSD, PCP and RCP data were merged at various positions instead of the same position for each sample. Multifractal dimension curves derived from full-range pores are divided into four segments as D₁, D₂, D₃, and D₄, corresponding to the fractal characteristics of large pores, large pore throats, small pores, and small pore throats, respectively. Among them, the fractal dimension D₂ of large pore throats and D₄ of small pore throats from the combination of PCP and RCP significantly control petrophysical properties (porosity and permeability). The multifractal dimensions obtained using porosity and permeability data input through a back-propagation (BP) neural network method show that the relatively large D₂ and the relatively small D₄ correspond to favorable pore structure and good reservoir quality. The results of this research significantly improve our understanding of complex pore characteristics and prediction of favorable pore structure in tight reservoirs, thus enhancing hydrocarbon exploration and production.

The Carboniferous-Permian coal measures in China contain abundant natural gas resources. Shale, coal and tight sandstone reservoirs are developed in coal measures, and the quantitative characterization of the pore structures of different types of reservoirs can provide scientific guidance for the sweet spot prediction of tight reservoirs. In this study, taking the Shan 2 Member coal measure of the Shanxi Formation in the eastern Ordos Basin as an example, the pore structures of shale, coal rock and tight sandstone were systematically studied based on organic geochemistry, scanning electron microscopy, high-pressure mercury injection, and low-temperature N2 and CO2 adsorption experiments. The results show that the microscopic pore structures of different types of reservoirs in the Shan 2 Member coal measures are quite different. Shale and tight sandstone mainly develop clay mineral pores at mesopore scale, followed by intragranular and dissolution pores developed in quartz and feldspar minerals, while organic pores are rarely developed. A large number of macro-scale clay mineral pores and micro-fractures are developed in tight sandstone, meanwhile the pore connectivity of tight sandstone is better than that of shale. A large number of micro to nano-scale organic pores are developed in coal, and the specific surface area of micropores in coal is much larger than that of mesopores in shale and tight sandstone. Sandstone, shale, and coal are frequently interbedded in coal measure strata. Tight sandstone provide the main storage space for free gas, and pores in shale and coal absorb a large amount of natural gas. Sandstone-shale-coal assemblages and sandstone-coal assemblages are the key targets for the exploration of hydrocarbons in the Shanxi Formation coal measures in the study area.

Chapter 4 - Multifractal Analysis of Pore Structure of Tight Oil Reservoirs Using Low-Field NMR Measurements

Reservoir petrofacies — A tool for characterization of reservoir quality and pore structures in a tight sandstone reservoir: A study from the sixth member of Upper Triassic Yanchang Formation, Ordos Basin, China

Due to the influence of a sedimentary environment, sandstone characteristics, diagenesis, and geological structure, the complexity and heterogeneity of the pore structure in tight sandstone reservoirs act as key barriers for accurate characterization of the influence of different pore microparameters on reservoir physical properties. This paper obtained different scale microscopic pore–throat parameters through mercury intrusion porosimetry (MIP) and digital core reconstruction models. According to the connectivity of different scale pores, connected pore structure parameters, and pore fractal dimension, the pore structure characteristics of tight sandstone reservoirs were evaluated. Subsequently, through partial correlation analysis, the contribution rate of different scale connected pore parameters to permeability and porosity was clarified. Then, a multiparameter fitting equation for the absolute permeability and porosity of the rock was obtained through multiple regression analysis. The analysis results show that (1) the connected pores of tight sandstone reservoirs are mainly mesopores, with the pore radius distribution between 1 and 3 μm, and throat radius distribution between 0 and 2 μm. (2) The complexity of the pore structure in tight sandstone reservoirs is most strongly correlated with the fractal dimension of the pore structure at the mesoporous scale. (3) The pore radius and throat length at the mesoporous scale have the strongest correlation with the absolute permeability of the rock, and the pore radius at the mesoporous scale has the strongest correlation with the porosity of the rock. (4) The multiparameter fitting equation established by multiple regression analysis quantitatively and qualitatively analyzed the impact of microscopic parameters of the pore–throat structure at different scales on reservoir properties, achieving the purpose of predicting the absolute permeability and porosity of the tight sandstone reservoir. It provides guidance for the study of the pore structure and permeability characteristics of tight sandstones.