The geochemical characteristics of water produced from coalbed methane (CBM) wells contain rich information about the associated geology, environment, and production. This study was conducted in the Southern Qinshui Basin, where produced water samples were collected from 10 typical CBM wells and their ionic compositions and water quality parameters were tested. The differences in the chemical characteristics of the produced water between different producing coal seams and between single-seam production wells (SPWs) and multi-seam co-production wells (MCWs) were compared, and the geochemical formation process of the produced water was revealed. The following conclusions were obtained: (1) the water produced samples that were mainly Na-HCO₃-type and were generally weakly alkaline and moderately mineralized. The water produced from No. 15 coal seam was more enriched in SO₄, Ca, and Mg compared to that of No. 3 coal seam, and the variations were more intense, reflecting a more complex water chemistry formation. (2) The ionic data of the water produced from MCWs do not lie between the coal seams of SPWs, nor do they satisfy the linear relationship between the ionic compositions of SPWs, reflecting the differences in the water sources between MCWs and SPWs. Water from MCWs tends to communicate with active water sources outside the coal seams, and the produced water contains small amounts of Cl and total dissolved solids, thus inhibiting the pressure reduction efficiency and limiting the effect of CBM co-production. (3) Based on a principal component analysis of the ionic compositions, two characteristic components were extracted, and these represented two types of hydrochemical formation processes. The first type is pyrite oxidation and carbonate dissolution, and its opposite represents sulfate reduction. The second type reflects the groundwater retention and confinement characteristics, and its opposite represents active groundwater or stronger recharge conditions. (4) A geochemical formation model of the water produced from CBM wells in the study area was constructed. Cation exchange adsorption and sulfate reduction were found to be the main water–rock interactions in the coal measure, and they determine the overall water quality of the produced water. Recharge has a relatively significant influence on water produced from MCWs. Pyrite oxidation exists in the water produced from No. 15 coal seam of the Taiyuan Formation, and the higher sulfur content in the coal contributes to this reaction. The results of the study will assist in deepening our understanding of the geochemical formation mechanisms of water produced from CBM wells, and they provide the main reasons for the poor CBM co-production effect from the Shanxi and Taiyuan Formations.

Underground coal mining of CH₄ gas-rich tectonic coal seams often induces methane outburst disasters. Investigating gas permeability evolution in pores of the tectonic coal is vital to understanding the mechanism of gas outburst disasters. In this study, the triaxial loading–unloading stresses induced gas permeability evolutions in the briquette tectonic coal samples, which were studied by employing the triaxial-loading–gas-seepage test system. Specifically, effects of loading paths and initial gas pressures on the gas permeability of coal samples were analyzed. The results showed the following: (1) The gas permeability evolution of coal samples was correlated with the volumetric strain change during triaxial compression scenarios. In the initial compaction and elastic deformation stages, pores and cracks in the coal were compacted, resulting in a reduction in gas permeability in the coal body. However, after the yield stage, the gas permeability could be enhanced due to sample failure. (2) The gas permeability of the tectonic coal decreased as a negative exponential function with the increase in initial gas pressure, in which the permeability was decreased by 67.32% as the initial gas pressure increased from 0.3 MPa to 1.5 MPa. (3) Coal samples underwent a period of strain development before they began to fail during confining pressure releasing. After the stress releasing-induced yield stage, the coal sample was deformed and cracked, resulting in a quickly increase in gas permeability. With a further releasing process, failure of the sample occurred, and thus induced rapidly increasing gas permeability. These obtained results could provide foundations for gas outburst prevention in mining gas-rich tectonic coal seams.

Deep coalbed methane (CBM) is widely distributed in China and is mainly commercially exploited in the Qinshui basin. The in situ stress and moisture content are key factors affecting the permeability of CH₄-containing coal samples. Therefore, considering the coupled effects of compressing and infiltrating on the gas permeability of coal could be more accurate to reveal the CH₄ gas seepage characteristics in CBM reservoirs. In this study, coal samples sourced from Tunlan coalmine were employed to conduct the triaxial loading and gas seepage tests. Several findings were concluded: (1) In this triaxial test, the effect of confining stress on the permeability of gas-containing coal samples is greater than that of axial stress. (2) The permeability versus gas pressure curve of coal presents a ‘V’ shape evolution trend, in which the minimum gas permeability was obtained at a gas pressure of 1.1MPa. (3) The gas permeability of coal samples decreased exponentially with increasing moisture content. Specifically, as the moisture content increasing from 0.18% to 3.15%, the gas permeability decreased by about 70%. These results are expected to provide a foundation for the efficient exploitation of CBM in Qinshui basin.

The low permeability of coal seams has always been the main bottleneck restricting coalbed gas drainage. To improve the permeability of a coal seam, a high-energy gas fracturing technology is proposed. Firstly, based on the high-energy gas fracturing mechanism and gas production principle of fracturing agent, a fracturing agent applicable to coal reservoirs was developed, and its performance and sensitivity were tested. Then, a high-energy gas-fracturing simulated coal sample test was conducted, and the drilling wall pressure and strain of the simulated coal sample were tested. The results show that high-energy gas fracturing technology is a safe and efficient technical means for improving the permeability of coal reservoirs. The pressure–time curve of the borehole wall under the action of high-energy gas can be divided into three stages, namely, the rapid pressure-rising stage, steady pressure stage, and falling stage; the maximum pressure in the borehole is about several hundred MPa, and the pressure distribution in the borehole is not uniform. Compared with explosives blasting, the stress wave intensity in coal caused by the action of high-energy gases is low, the duration is short, and the peak stress attenuates slowly with increasing distance. Under the action of high-energy gas, no crush zone is generated near the borehole; the number of radial cracks produced is small but long. The extent of the fracture zone depends mainly on the quasi-static splitting wedge effect of the high-energy gas.

The presence of phenol in coal-chemical engineering wastewater is considered a serious environmental issue. The use of enzymes to degrade phenol has received wide attention. G-quadruplex can combine with hemin to form G-quadruplex/hemin, which shows peroxidase-like activity and can be used to degrade phenol. In this work, a novel CNTs-NH₂@G-quadruplex/hemin was designed using a single-stranded DNA (ssDNA) that contained G-rich fragments and T-rich fragments. The G-rich fragments assembled with the hemin to form G-quadruplex/hemin; the pyrimidine group of the T-rich fragment could form stable chemical bonds with the amino group, which could be used for immobilization. Characterization analysis of FTIR, XRD and TEM showed that the CNTs-NH₂@G-quadruplex/hemin was successfully prepared. Meanwhile, the activity and phenol degradation ability for CNTs-NH₂@G-quadruplex/hemin were studied in detail, and the effects of pH, temperature, initial phenol concentration, H₂O₂ concentration and reusability on the degradation efficiency of phenol were also explored. The results showed that when immobilized, the CNTs-NH₂@G-quadruplex/hemin presented good activity, especially under conditions of temperature 30 °C and pH 7.0. Notably, the maximum reaction velocity of CNTs-NH₂@G-quadruplex/hemin was increased from 0.016 mM mg⁻¹ min⁻¹ (free enzyme) to 0.056 mM mg⁻¹ min⁻¹. For phenol degradation ability, it was observed that under the conditions of temperature 30 °C, pH 7.0, initial phenol concentration 40 mg L⁻¹, H₂O₂ concentration 120 mM, enzyme dose 40 mg L⁻¹, more than 90% of phenol could be degraded within 600 min. In addition, the CNTs-NH₂@G-quadruplex/hemin still maintained high activity after 10 cycles of phenol removal, demonstrating its appealing potential for practical applications.