Classification Evaluation of Gas Shales Based on High-Pressure Mercury Injection: A Case Study on Wufeng and Longmaxi Formations in Southeast Sichuan, China

Abstract

The physical properties of gas shales, especially the structural features of connected pore-fractures, are key parameters for evaluating shale gas resource potential and effective production. In this study, four major techniques, including mercury intrusion capillary pressure (MICP), low-temperature nitrogen adsorption/desorption (LT-NA/D) tests, scanning electron microscopy (SEM), and fractal theory, were adopted to establish a suitable pore size classification for gas shales from the Wufeng–Longmaxi formations in the southeastern Sichuan Basin. On the basis of the inflection points of the mercury intrusion curves, a quantitative classification for connected pore-fractures of gas shales in the study area was proposed: micropores (<10 nm), ostioles (10–25 nm), mesopores (25–100 nm), macropores (100 nm–5000 nm), and fractures (>5000 nm). This classification method was verified using fractal geometry theory and was reasonable as compared to other pore size classification methods of unconventional reservoir. The controlling factors of pore-fractures systems were also investigated according to shale compositions and SEM images. The results show that gas shales mainly developed organic matter (OM) pores, interparticle (interP) pores, and intraparticle (intraP) pores. Among them, the OM pores are composed mainly of micropores and ostioles, which can effectively increase the gas storage capacity of shale. The increase of pore content with diameter greater than 25 nm may increase the seepage capacity of gas shales. The quality of shale gas reservoirs is usually affected by multiparameter such as microscopic pore-fractures content, pore size distribution, specific surface area, porosity, and shale compositions (including minerals and OM). The gas shales could be classified into three types (i.e., types I, II, and III). Type III shale is a high-quality shale gas reservoir, which is rich in micropores (35.90–87.83 vol %), has a high total organic carbon (TOC) content (2.55–3.39%), has a high porosity (10.96–16.01%), and has good fracturing ability (the average content of quartz is 53.88%). Type I shale is a poor shale gas reservoir, but it can be used as a good gas seepage channel due to the high content of fractures and macropores (mean 39.48 vol % and 18.89 vol %). Type II shale is a transitional shale gas reservoir.

Characterizing pore size distribution of high maturity shales and main controlling factors analysis: A case study of Permian Gufeng Formation in the Lower Yangtze region, China

“…These organic matter pores were illustrated as ellipsoid to irregular polygons with diameters from several nanometres to hundreds of nanometres. Wei et al (2021) observed the organic matter pores were widespread and clustered in the organic‐rich samples cored from Wufeng and Longmaxi formations in south‐east Sichuan Basin, which were characterized by sponge‐like, elliptical and alveolate shapes with pore diameters ranging from 5 to 50 nm. Li et al (2021) showed the dominant pore shape in undeformed Wufeng‐Longmaxi shales were bubble‐like pores, spongy pores, static pores and randomly distributed pores.…”

“…It is also rapidly developed to investigate shale oil and shale gas resource distribution and strategic development in China (Chen et al, 2021; Liu et al, 2017; Wang, 2020; Wang et al, 2021). Spatial distribution, sedimentary evolution, organic geochemical characteristics, reservoir characteristics and shale gas accumulation mechanism of Palaeozoic marine shale have been widely studied in southern China (Cao et al, 2016; Liu et al, 2022; Wang et al, 2021; Wei et al, 2021; Zhang et al, 2022). Although Permian shale is also widely distributed in the south of China, its shale gas resource is rarely studied in the previous literature owing to unclear industrial value.…”

Research Progress of Microscopic Pore–Throat Classification and Grading Evaluation of Shale Reservoirs: A Minireview

“…On the basis of this, the grading evaluation criteria and the lower limit of formation of shale reservoirs are established and applied to the evaluation of target reservoirs. Combined with the research results of the author’s team in recent years, − ,− ,,− this paper reviews and summarizes the latest progress and development trends in this area.…”

Classification and Control Factors of Reservoir Types in Hybrid Sedimentary Rocks: A Case Study on Lucaogou Formation in the Jimusaer Sag, Junggar Basin

“…The pore size classification proposed by the International Union of Pure and Applied Chemistry in 1972 was widely accepted for chemical products and coal . However, it has limited applicability to HSR reservoirs, necessitating the establishment of an appropriate pore size classification standard.…”