Editorial: Quantitative characterization and engineering application of pores and fractures of different scales in unconventional reservoirs—Volume II

With the depletion of conventional oil and gas resources in the world, unconventional oil and gas resources have become the focus and hotspot of exploration and development (Li et al., 2019; Yin et al., 2020a; Fan et al., 2020; Li, 2022a). In recent years, a series of unconventional oil and gas resources (e.g., tight sandstone gas, shale gas, coalbed methane, and gas hydrate) have been explored and developed in China, among which tight sandstone gas and shale gas have been commercially exploited (Wu et al., 2022; Xie et al., 2022). Tight sandstone gas is the earliest unconventional gas developed in China, which plays an important role in China’s total natural gas reserves and production, with a total resource of about 17.4 × 10–25.1 × 10 m, and its recoverable resource is about 8.8 × 10–12.1 × 10 m (Zou et al., 2018). In the past decade, China has achieved significant exploration and development achievements in shale gas. Eight shale gas fields have been built in and around the Sichuan Basin (e.g., Fuling, Luzhou, Changning, Weiyuan, and Zhaotong). In 2021, China’s shale gas production reached 230 m × 10 m, mainly from shale formations shallower than 3,500 m. Deep shale gas resources (more than 3,500 m) will be long-term exploration and development targets for more than 80% of the total resources (Li J. et al., 2022a). Both tight sandstone and shale reservoirs have the characteristics of ultra-low porosity and permeability, and shale reservoirs have the worst physical properties (Li J. et al., 2022b; Fan et al., 2022). Therefore, quantitatively characterization of such reservoirs’multi-scale pore and fracture characteristics is of profound significance for tight oil and gas. OPEN ACCESS


Introduction
With the depletion of conventional oil and gas resources in the world, unconventional oil and gas resources have become the focus and hotspot of exploration and development (Li et al., 2019;Yin et al., 2020a;Fan et al., 2020;Li, 2022a). In recent years, a series of unconventional oil and gas resources (e.g., tight sandstone gas, shale gas, coalbed methane, and gas hydrate) have been explored and developed in China, among which tight sandstone gas and shale gas have been commercially exploited Xie et al., 2022). Tight sandstone gas is the earliest unconventional gas developed in China, which plays an important role in China's total natural gas reserves and production, with a total resource of about 17.4 × 10 12 -25.1 × 10 12 m 3 , and its recoverable resource is about 8.8 × 10 12 -12.1 × 10 12 m 3 (Zou et al., 2018).
In the past decade, China has achieved significant exploration and development achievements in shale gas. Eight shale gas fields have been built in and around the Sichuan Basin (e.g., Fuling, Luzhou, Changning, Weiyuan, and Zhaotong). In 2021, China's shale gas production reached 230 m 3 × 10 8 m 3 , mainly from shale formations shallower than 3,500 m. Deep shale gas resources (more than 3,500 m) will be long-term exploration and development targets for more than 80% of the total resources (Li J. et al., 2022a). Both tight sandstone and shale reservoirs have the characteristics of ultra-low porosity and permeability, and shale reservoirs have the worst physical properties (Li J. et al., 2022b;Fan et al., 2022). Therefore, quantitatively characterization of such reservoirs' multi-scale pore and fracture characteristics is of profound significance for tight oil and gas. This Research Topic is Volume II of a series "Quantitative characterization and engineering application of pores and fractures of different scales in unconventional reservoirs." The set of 39 studies in this Research Topic aimed to understand multiple methods for quantitative characterization of the pore and fracture systems, as well as provide a general framework for future research efforts.
Quantitative characterization of multiscale pores in unconventional reservoirs

Tight sandstone reservoirs
The results of qualitative and quantitative characterization of multi-scale pores are abundant, and systematic theoretical methods and characterization techniques have been formed. Guo et al. (ID: 959796) take the Chang 6 Member of the Yanchang Formation in the Huaqing area of the Ordos Basin as an example. The pore-throat heterogeneity of tight sandstone reservoirs and its influence on the fluid occurrence state were studied using thin sections, scanning electron microscope (SEM), X-ray diffraction (XRD), constant velocity Mercury intrusion, and nuclear magnetic resonance

Shale reservoirs
Shale is a kind of fine-grained sedimentary rock denser than conventional sandstone and conglomerate reservoirs. The conventional reservoir characterization techniques are difficult to apply to shale. Multiple methods are generally used to realize the qualitative and quantitative characterization of shale pore characteristics Li J. et al., 2022a;Li J. et al., 2022b). Scholars have made extensive studies on marine, transitional and continental shales. Based on the results of highpressure Mercury intrusion, low-pressure N 2, and CO 2 adsorption and organic geochemical experiments, XRD and SEM observations, In general, since the pores of shale reservoirs are mainly nanoscale, the pore size is composed of nano-and micron-scale. Therefore, it is impossible to rely on a single method to characterize shale pores. Generally, the combination of multiple methods is used to achieve qualitative and quantitative characterization. Almost all of the above studies have adopted this method. In addition, it applies to shale FIGURE 1 Characterization method of natural fracture (Lv et al., 2021;Li, 2022c).

Frontiers in Earth Science
frontiersin.org reservoirs and tight sandstone, carbonate, volcanic, and other tight reservoirs.

Quantitative characterization of multiscale fractures in unconventional reservoirs
Natural fractures can improve the porosity of tight reservoirs and control the seepage system of such reservoirs. The fracture development is the key to the high and stable production of tight oil and gas (Yin et al., 2020b;. However, they may also communicate with the upper strata, which is not conducive to preserving tight reservoirs. In recent years, quantitative characterization of natural fractures has been established based on the outcrop, core, multi-type thin sections, image/conventional logging, and seismic data (Figure 1) (Yin and Ding, 2019;Yin and Gao, 2019;Li et al., 2020;Li et al., 2021;Forstner and Laubach, 2022;. According to this research method, scholars have done many studies. The paleo-tectonic stress field controls tectonic fractures. Zhang et al. (ID: 1024748) used the finite element numerical simulation technology to analyze the distribution of the paleo-tectonic stress field of the Longmaxi Formation in the Lintanchang area. Then they predicted the fracture development areas under the superposition of two stages of tectonic stress. It is found that the comprehensive fracture coefficients of the anticline core and fault areas are both greater than 1.1, which are the areas with the most developed tectonic fractures, and these areas have poor shale gas preservation conditions. The shale is in a state of "breaking without cracking," and shale gas can be well preserved. The preservation of shale gas is closely related to tectonic fractures, the structural preservation conditions are the key factors in controlling the rich and integrated shale gas reservoirs in the Wuxi area of Chongqing City. Li et al. (ID: 1032597) found that there are three structural preservation modes of shale gas in the study area, i.e., lost destruction type, lost residual type, and trap preservation type. The trap preservation type is more conducive to the preservation of shale gas, which is the most favorable structural mode for shale gas exploration. Frackability is of great significance to shale gas development, Guo et al. (ID: 993829) take the Longmaxi Formation shale in the Changning and Luzhou Blocks in the southern Sichuan Basin as an example, and the Analytic Hierarchy Process (AHP) method was adopted to establish a comprehensive evaluation index of shale fracturability. The modulus brittleness index, mechanical brittleness index, in-situ stress difference coefficient, rock compressive strength, and TOC content are the most important evaluation indexs.
For the engineering application, Yang et al. (ID: 970719) carried out the experimental evaluation and numerical simulation research on the adaptability of CO 2 flooding in beach-bar sand reservoirs of the Shahejie Formation in the Dongying Sag. Yang et al. (ID: 1029309) also conducted indoor water flooding experiments after polymer gel injection using artificial cores, and a set of criteria for determining reservoir-matched polymer gel profile control agents was proposed. Liu et al. (ID: 1037532) put forward a coupled CFD-DEM method to simulate the particle plugging process of propagating fracture, and the effects of positive pressure difference, fracture roughness, particle concentration, and particle shape on the plugging mechanism were examined.
Among these 39 studies, we received a paper about the Lunpola Basin and the southern East China sea shelf. Zhang et al. (ID: 1034069) summarized the oil and gas accumulation conditions of the Lunpola Basin and pointed out the favorable zones with the most exploration potential. Wang et al. (ID: 1015832) analyzed the burial history of wells and simulation wells in SECS by the data of drilling, seismic, and source rocks. Moreover, favorable oil-gas accumulation modes in Lower-Middle Jurassic were proposed by combining them with studies on oil-gas accumulation elements and conditions.

Summary
In summary, the 39 high-quality papers on this Research Topic represent a step forward in understanding the quantitative characterization and engineering application of pores and fractures of different scales in unconventional reservoirs. We appreciate the opportunity to present this hot Research Topic and hope that readers will benefit from the breadth and scope of the research. More importantly, we hope these studies offer new directions for future research that will guide future scientific community efforts. Many researchers have contributed to the hot Research Topic, and we have applied for Volume III. We hope more scholars will be involved in the Research Topic, especially on the quantitative characterization of multi-scale fractures. It will offer new directions for future research that guide the quantitative characterization of multi-scale pores and fractures of unconventional reservoirs in the future.
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