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

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.