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.

Organic phase-change materials (PCMs) offer great promise in addressing challenges in thermal energy storage and heat management, but their applications are greatly limited by low energy density and a rigid phase transition temperature. Herein, by introducing carbon dots (CDs) with abundant oxygen-related groups, we develop a novel kind of erythritol (ET)-based composite PCMs (CD-ETs) featuring an enhanced latent heat storage capacity and a reduced degree of supercooling compared to pure ETs. The optimally formulated CD-ETs increase the latent heat storage capacity from 377.3 to 410.2 J·g⁻¹ and the heat release capacity from 209.0 to 240.2 J·g⁻¹ compared to the pristine ETs. Moreover, the subcooled degree of CD-ETs is more than 30 °C lower than that of pristine ETs. By successively encapsulating CD-ETs and CD-containing polyethylene glycol (PEG) with a low melting point in a reduced graphene oxide-modified melamine sponge, the resultant shape-stabilized system not only prevents leakage of molten PCMs but also allows for a wide response temperature window and promotes the heat transfer ability of melted PEG in close contact with solid CD-ETs. Stepped melting and crystallization guarantee phase changes in high-melting-point ETs via solar heating, Joule heating or a combination thereof. Specifically, the melting enthalpy of this system is as high as 306.5 J·g⁻¹, and its cold crystallization enthalpy reaches 196.5 J·g⁻¹, surpassing numerous organic PCMs. This work provides a facile and efficient strategy for the design of ideal thermal energy storage materials to meet the needs of application scenarios in a cost-effective manner.

This study investigates pore-size-dependent competitive CH₄/CO₂ adsorption in coal-measure shale organic-matter nanopores represented by HRTEM-informed short-chain aromatic fragments. Coal-measure shale from the eastern Yushe Block of the Qinshui Basin was selected as the research object. High-resolution transmission electron microscopy was employed to quantitatively characterize the aromatic lamellar structure of the organic matter. Based on the obtained structural parameters, organic-matter pore models with pore sizes ranging from 1 to 4 nm were constructed. Grand canonical Monte Carlo simulations were subsequently performed to investigate the single-component and competitive adsorption behaviors of CH₄ and CO₂ at temperatures of 288–308 K and pressures of 0.25–8 MPa. The results indicate that the aromatic structure of the organic matter is characterized by short chains and a low degree of condensation, while the pore system is dominated by micropores and small mesopores. CH₄ exhibited its highest nominal-pore-volume-normalized adsorption in the 1 nm pore, consistent with micropore filling, whereas CO₂ reached its maximum in the nominal 2–3 nm pore models. These maxima may be influenced by model geometry and normalization because the probe-accessible pore volumes and surface areas after annealing were not quantified. In the smaller 1 and 2 nm pore models, CO₂ exhibits a pronounced competitive adsorption advantage. As the pore size increases to 3–4 nm, the relative adsorption capacity of CH₄ increases, thereby narrowing the adsorption difference between CH₄ and CO₂. These findings provide molecular-scale insights into pore-size-dependent equilibrium CH₄/CO₂ adsorption in experimentally informed organic-matter nanopores, offering a reference for future evaluations of CO₂-enhanced gas recovery and storage in coal-measure shale.