The brittleness index (BI) is a key parameter used to identify the desirable fracturing intervals of shale gas reservoirs. Its correlation with fracability is still controversial. There have been a variety of methods proposed that can estimate BI. The brittleness evaluation method based on stress-strain curves according to the energy-balanced law is the most suitable and reliable in this study. Triaxial compression test, optical microscopy and scanning electron microscopy (SEM) observation, and X-ray diffraction analysis (XRD) were performed on nine drill core samples from well SY3 located in the peripheral regions of Sichuan Basin, China. These tests further evaluated several commonly used methods (brittleness indices based on rock elastic parameters, rock mineral compositions) and determined the relationship between brittleness, rock elastic parameters, and the content of minerals. The results obtained indicate that for sedimentary rocks, a higher Young’s modulus reduces the brittleness of rock, and Poisson’s ratio weakly correlates with brittleness. Excessive amounts of quartz or carbonate minerals can increase the cohesiveness of rock, leading to poor brittleness. Furthermore, the most suitable fracturing layers possess a high brittleness index and low minimum horizontal stress.

A suite of shale samples from the Lower Cambrian Niutitang Formation in northwestern Hunan Province, China, were investigated to better understand the pore structure and fractal characteristics of marine shale. Organic geochemistry, mineralogy by X-ray diffraction, porosity, permeability, mercury intrusion and nitrogen adsorption and methane adsorption experiments were conducted for each sample. Fractal dimension D was obtained from the nitrogen adsorption data using the fractal Frenkel-Halsey-Hill (FHH) model. The relationships between total organic carbon (TOC) content, mineral compositions, pore structure parameters and fractal dimension are discussed, along with the contributions of fractal dimension to shale gas reservoir evaluation. Analysis of the results showed that Niutitang shale samples featured high TOC content (2.51% on average), high thermal maturity (3.0% on average), low permeability and complex pore structures, which are highly fractal. TOC content and mineral compositions are two major factors affecting pore structure but they have different impacts on the fractal dimension. Shale samples with higher TOC content had a larger specific surface area (SSA), pore volume (PV) and fractal dimension, which enhanced the heterogeneity of the pore structure. Quartz content had a relatively weak influence on shale pore structure, whereas SSA, PV and fractal dimension decreased with increasing clay mineral content. Shale with a higher clay content weakened pore structure heterogeneity. The permeability and Langmuir volume of methane adsorption were affected by fractal dimension. Shale samples with higher fractal dimension had higher adsorption capacity but lower permeability, which is favorable for shale gas adsorption but adverse to shale gas seepage and diffusion.

Fluorine, a hazard that is associated with coal, has resulted in serious environmental issues during the production and utilization of coal. In this paper, we provide a detailed review of fluorine in Chinese coal, including the distribution, concentration, modes of occurrence, genetic factors, and environmental effects. The average concentration of fluorine in Chinese coal is 130.0 mg/kg, which is slightly higher than coal worldwide (88.0 mg/kg). The enrichment of fluorine in Chinese coal varies across different coal deposit regions, and it is especially high in Inner Mongolia (Junger coalfield, Daqingshan coalfield) and southwest China (coal mining regions in Yunnan, Guizhou province). The fluorine distribution is uneven, with a relatively high content in southwest coal (including Yunnan, Guizhou, Chongqing, and Sichuan provinces), very high content in the coal of North China (Inner Mongolia) and South China (Guangxi), and is occasionally found in the northwest (Qinghai). Fluorine occurs in various forms in coal, such as independent minerals (fluorine exists as fluorapatite or fluorite in coal from Muli of Qinghai, Taoshuping of Yunnan, Guiding of Guizhou, and Daqingshan of Inner Mongolia), adsorption on minerals (fluorine in coal from Nantong, Songzao of Chongqing, Guxu of Sichuan, and Shengli, Daqingshan, and Junger from Inner Mongolia), substitution in minerals (Wuda coal, Inner Mongolia), and a water-soluble form (Haerwusu coal, Inner Mongolia). The enrichment of fluorine is mainly attributed to the weathering of source rock and hydrothermal fluids; in addition to that, volcanic ash, marine water influence, and groundwater affect the fluorine enrichment in some cases. Some environmental and human health problems are related to fluorine in coal, such as damage to the surrounding environment and husbandry (poisoning of livestock) during the coal combustion process, and many people have suffered from fluorosis due to the burning of coal (endemic fluorosis in southwest China).

Coalbed methane is a major unconventional resource that has been exploited commercially for decades in the southern Qinshui Basin of China. The hydrogeochemical characteristics of coal reservoir water play a key role in the exploration and development of coalbed methane resources. In view of this, a detailed study was performed on coalbed methane co-produced water collected from the Shizhuangnan block to assess water–rock interactions and biogeochemical processes. Water samples were analyzed to establish major ions, isotopic compositions and perform 16S rRNA sequencing. Results suggest that the hydrochemistry was controlled by water–rock processes and that methane was consumed by sulfate reduction through calculation. Meanwhile, the isotopic compositions of water samples indicated that they had a predominantly meteoric origin and were influenced by microbial activity. The 16S rRNA sequencing results of bacteria and archaea provide an important foundation for understanding the activity of sulfate-reducing bacteria and methanogens at different hydraulic heads, which was consistent with isotopic analysis. Carbonates containing calcite and dolomite were found to be distributed at different hydraulic head due to the biogeochemical characteristics and associated water–rock interactions.