Ionic composition data of coalbed methane (CBM) produced water from five CBM blocks in China were used to investigate the variability in and the geological controls on water quality across geologic basins and the resulting environmental impacts. The assessment tools included the Schoeller diagram, the Na-K-Mg ternary diagram and the Cl-Na-HCO₃ ternary diagram. Techniques such as principal component analysis (PCA) and R-mode hierarchical cluster analysis were also applied to the ionic database to reveal the relationships between the ions. The results from this study indicate that three major ions (Na, Cl, and HCO₃) and two primary processes (sulfate reduction and mixing of the coal seam water with the fracturing fluid) affect the ionic compositions of CBM produced water. The burial depth of the coal seam, the fresh water influence and the fracturing fluid contamination jointly control the water quality variability of produced water. The Na-K-Mg ternary diagram provides a potential method to evaluate the origin of CBM produced water, such as coal seam water in the partially equilibrated region and exotic water (including the fracturing fluid or shallow cold groundwater) in the immature water region. Most of the CBM produced water in China has high concentrations of Na, HCO₃, Cl and TDS compared to the standards for drinking water and irrigation water. The produced waters from deep formations (>1000 m), or those produced during the early drainage periods, are always saline, “Na-Cl” type waters. Appropriate treatment methods and regulations for CBM produced water in China are urgently needed.

By using proximate analysis, X-ray diffraction mineral analysis, scanning electron microscope, contact angle measurement, and settlement simulation experiment, the coal fines produced from the coalbed methane wells of Binchang area were used to study the characteristics including particle size distribution, composition, morphology, wettability, and settleability. The results show that the particle size of coal fines produced from coalbed methane wells are mainly 20 mesh, ranging of 1-400μm, and the particle size distribution curve is mainly dominated by the main-secondary bimodal type, with the main peak of 30-300μm. The particle size from large to small is drill cutting coal fines, flowback coal fines, bailing coal fines, and pipeline filter coal fines. In terms of ash content, coal fines are higher than coal seam, and drilling cuttings are higher than bailing coal fines, while the fixed carbon content of the former is lower than that of the latter. The minerals of coal fines are mainly kaolinite, illite, quartz, and other 6 minerals, and the mineral types of drilling coal fines are the most abundant, while the bailing coal fines only contain illite and quartz. The roundness of coal fine particles ranges from excellent to poor in the order of bailing coal fines, pipeline filter coal fines, flowback coal fines, and drilling cuttings. However, the sorting of drilling cuttings is excellent, and the particle edges are straight, neat, and smooth, while the sorting of bailing coal fines is poor, and the particle edges are curved, uneven, and rough. The contact angles of coal fines are 40.25-69.5, indicating hydrophilous. The wettability of bailing coal fines is better than that of drilling cuttings. The particle size has a negative correlation with the wettability effect. The more obvious the modification effect of positive wetting agent is, the worse the modification effect of negative wetting agent is. The modification of surfactant has nothing to do with the particle size of the coal fines, but is closely related to organic components and minerals. The larger the coal particle size, the higher the settling rate, and the higher the ash content and the lower the fixed carbon content, the faster the settling rate. With the dividing point 150 mesh, the settling rate of large particles is mainly affected by particle size, while that of small particles is affected by the composition.

The heterogeneity of pore structure in coal reservoir is extremely complex. In this paper, mercury intrusion porosimetry (MIP) and liquid nitrogen adsorption (LNA) were used to describe pore characteristics of macro-coal components, and the fractal characteristics of pores and their relationship with adsorption and desorption were discussed. The findings revealed that there were obvious differences in pore characteristics of different macro-coal components at different pore sizes. The total pore volume of vitrain and durain was equivalent, and the total specific surface area was larger than durain, indicating that the micropores in vitrain were more developed, while the macropores in durain were more developed, indicating that the specific surface area was smaller. Fractal results indicated that the pore structure of coal was more complex with the increase of pore diameter. The Da1 and Da2 of vitrain and durain were affected by larger specific surface area and pore volume. The Ds of vitrain increased first and then decreased with the content of vitrinite, while that of durain was the opposite. The relationship between the adsorption capacity of vitrain and fractal dimension Da1 was binomial distribution, and it was positively correlated with Da2. The adsorption capacity of durain samples increased first and then decreased with Da1. With the increase of fractal dimension Ds, the theoretical desorption rate and recovery rate of durain had a downward trend, that is, the more complex the pore structure, the poor the desorption efficiency.

As the co-exploration and co-production of unconventional gas have become a new trend of the unconventional gas exploration and development in the eastern margin of the Ordos Basin, this study focuses on the distribution, pore characteristics, geochemical properties, and mineral composition of the mud shale in the coal-bearing strata. The results show that the mud shale in the coal-bearing strata is distributed relatively stably with an average cumulative thickness of 77.60 m and a buried depth of 200–2400 m, increasing from east to west. The total organic carbon content of mud shale is relatively high, ranging from 0.14% to 39.9%, and the thermal maturity ranges from 0.23% to 2.49%, affected by both buried depth and magmatic intrusion. The organic matter type of the mud shale is dominated by type III, favorable for gas generation. The mineral composition of mud shale is mainly clay minerals and quartz, with low brittle minerals content (averaging 30.34%) and high kaolinite content (averaging 59.23%). Pores and micro-fissures are relatively developed in the mud shale, dominated by intergranular pores, organic pores, and the micro-fissures developed between brittle mineral crystals, within crystals and between clay mineral particles, with a porosity of 1.05%–1.59% and a permeability of 0.001–0.142 (×10⁻³µm²). The specific surface area of pores in mud shale is 12.31–28.99 m²/g, and the total pore volume is 0.0146–0.0483 cm³/g. Mesopores and micropores (mainly slit shaped pores) provide the main pore volume of mud shale, and micropores contribute most of the specific surface area. The controlling factors on pore development include the total organic carbon, thermal maturity, and inorganic minerals composition. The total amount of shale gas resources is 2.49 × 10¹² m³, and the Linxian-Xingxian area and Shilou-Daning area are the potential favorable zones for shale gas exploration and development.

Geothermal anomaly has gradually become a prominent issue affecting the efficient mining of coal with the depth of coal mining increasing in eastern Chenghe mining area of Weibei coalfield. Here, we comprehensively investigated the distribution characteristics of the present geothermal field, analyzed the main controlling factors, and constructed forming mode of geothermal anomaly by the temperature measurement in surface borehole and underground water, combined with coal-rock thermal conductivity test. The results show that the geothermal gradient ranged from 25.7C/km to 54.3C/km. The areas with geothermal gradient greater than 30C/km accounted for 88.31, and there was the highest gradient value in the southeast F₁ fault zone. The heat flow was between 66.81mW/m² to 128.49mW/m², which belonged to the obvious high heat flow area. Under the action of the main controlling factors such as fault, fold, coal-rock thermal conductivity, and groundwater activity in the region, the geothermal gradient and geothermal heat flow values showed an increasing trend from northwest to southeast. According to the distribution characteristics of heat flow and the action mechanism of main controlling factors, the geothermal anomaly in the study area was finally divided into two forming modes, i.e., fault-deep circulating hot water uplifting type and coal seam heat resistance-fold type. The research provides guidance for the geothermal hazard prevention of coal mine and the rational development and utilization of geothermal resources.