Experimental investigation of effects of CO2 injection on enhanced methane recovery in coal seam reservoirs

Bifunctional Co3O4/g-C3N4 Hetrostructures for Photoelectrochemical Water Splitting

This study explored the synergistic potential of photoelectrochemical water splitting through bifunctional Co3O4/g-C3N4 heterostructures. This novel approach merged solar panel technology with electrochemical cell technology, obviating the need for external voltage from batteries. Scanning electron microscopy and X-ray diffraction were utilized to confirm the surface morphology and crystal structure of fabricated nanocomposites; Co3O4, Co3O4/g-C3N4, and Co3O4/Cg-C3N4. The incorporation of carbon into g-C3N4 resulted in improved catalytic activity and charge transport properties during the visible light-driven hydrogen evolution reaction and oxygen evolution reaction. Optical properties were examined using UV-visible spectroscopy, revealing a maximum absorption edge at 650 nm corresponding to a band gap of 1.31 eV for Co3O4/Cg-C3N4 resulting in enhanced light absorption. Among the three fabricated electrodes, Co3O4/Cg-C3N4 exhibited a significantly lower overpotential of 30 mV and a minimum Tafel slope of 112 mV/dec This enhanced photoelectrochemical efficiency was found due to the established Z scheme heterojunction between Co3O4 and gC3N4. This heterojunction reduced the recombination of photogenerated electron-hole pairs and thus promoted charge separation by extending visible light absorption range chronoamperometric measurements confirmed the steady current flow over time under constant potential from the solar cell, and thus it provided the effective utilization of bifunctional Co3O4/g-C3N4 heterostructures for efficient solar-driven water splitting.

Experimental Investigation of Supercritical CO2-Rock-Water Interactions in a Tight Formation with the Pore Scale during CO2-EOR and Sequestration

In recent years, gas injection, especially CO₂ injection, has been acknowledged as a promising approach for enhanced oil recovery (EOR) and CO₂ capture and storage (CCS), especially for tight reservoirs. However, when CO₂ is injected into the oil reservoirs, it can disturb the equilibrium of the system and lead to chemical reactions between CO₂, formation water, and reservoir rocks. The reactions will alter some geochemical and physicochemical characteristics of the target reservoirs. However, the reactions still lack quantitative characterization at the pore scale, especially under reservoir conditions. Herein, we conducted an experimental study of the interactions between CO₂, brine, and rocks in the Mahu oilfield at 20 MPa and 70 °C. The low-field nuclear magnetic resonance (LF-NMR) measurements showed that the incremental amplitude for tight cores of CO₂-rock-water tests was larger than that for CO₂-rock tests, and the amplitude alteration presented significant differences corresponding to different types of minerals and pores. Furthermore, the interplanar spacing of the core samples was increased with the increase of reaction time in the CO₂-rock experiments but still lower than that in CO₂-rock-water tests. This research demonstrated evident changes in the geochemistry in tight reservoirs caused by CO₂, brine, and rock reactions. The results of this study may provide a significant reference for the exploration of similar reservoirs in the field of CO₂-EOR and CO₂ sequestration.

New Technology of Mechanical Cavitation in a Coal Seam to Promote Gas Extraction

Realizing efficient gas drainage in low permeability coal seams has always been a difficult problem for coal miners. Based on this, this paper proposes a new technology of large-diameter mechanical cave-making to promote gas extraction in a coal seam. This technology mainly uses the pressure of a water injection pump to control the automatic opening and closing of a mechanical reaming device to realize mechanical cavitation, and the hole diameter can reach up to 500 mm. The gas drainage effect of mechanical cavitation drilling is analyzed by a numerical simulation, which shows that under the condition of the same drainage time, the larger the cavitation radius is, the larger the effective influence radius of gas drainage is. According to the field test results, the time of single cave-making is about 5 min, and the speed of cave-making is fast. The coal output of a single cave is 0.42 t/m, and the pressure relief effect is obvious. Compared with ordinary drilling, the gas drainage concentration of mechanical cavitation drilling is increased by 2 times and the net amount of drainage is increased by 1.8 times. Large-diameter mechanical cavitation technology can better improve the gas drainage effect of outburst coal seams with low permeability and has a good application prospect.

Swelling Characteristics and Interaction Mechanism of High-Rank Coal during CO2 Injection: A Molecular Simulation Study

In CO₂-enhanced coalbed methane (CO₂-ECBM) engineering, accurate knowledge of the interaction mechanism of CO₂ and coal matrix is crucial for improving the recovery of CH₄ and contributing to the geological sequestration of CO₂. This study is performed to prove the accuracy of molecular simulation and calculate the variation characteristics of pore structure, volumetric strain, mechanical properties, Fourier transform infrared (FT-IR) spectra, and the system free energy by molecular dynamics (MD) and grand canonical Monte Carlo (GCMC) methods. According to the obtained results, a relationship between pore structure, swelling strain, mechanical properties, chemical structure, and surface free energy was established. Then, the correlation of various coal change characteristics was analyzed to elucidate the interaction mechanism between CO₂ and coal. The results showed that (1) the molecular simulation method was able to estimate the swelling mechanism of CO₂ and coal. However, because the adsorption capacity of the molecular simulate is greater than that of the experiment and the raw coal is softer than the macromolecular structure, the molecular results are slightly better than the experimental results. (2) As pressure increased from 0 to 4 MPa, the intramolecular pores and sorption-induced strain changed significantly, whereas when the pressure increased from 4 to 8 MPa (especially at 6-8 Mpa), there was an increase of the intermolecular pores and mechanical properties and transition from elastic to plastic. In addition, when the pressure was >8 MPa, the coal matrix changed slightly. ScCO₂ with a higher adsorption capacity results in greater damage and causes larger alterations of coal mechanical properties. (3) The change of the coal matrix is essentially controlled by the surface free energy of the molecular system. E _valence affects the aromatic structure and changes the volume of the intramolecular pores, thus affecting the sorption-induced strain change rate. E _non affects the length of side chains and the disorder degree of coal molecules and changes the volume of the intramolecular pores, thus affecting the mechanical property change rate. Our findings shed light on the dynamic process of coal swelling and provide a theoretical basis for CO₂ enhancing the recovery of CH₄ gas in coal.

Experimental Study of CO2-ECBM by Injection Liquid CO2

Coal mine gas disasters have severely restricted production safety. Improving gas extraction efficiency can effectively reduce disasters. Scholars have confirmed that CO2 successfully displaces coal seam CH4. This study conducted displacement and in situ experiments and compared gas drainage under different injection pressures. The displacement experiments indicated that CH4 production rates increased under increased pressures while the displacement ratios decreased. The pressure had a positive effect on sweep efficiency. The in situ experiment showed that CH4 and CO2 concentration trends in the inspection hole remained consistent. Through observing the data of the original and inspection holes, the average gas drainage concentration during low- and medium-pressure injections increased by 0.61 times and 1.17 times, respectively. The low-pressure average gas drainage scalar was increased by 1.08 times. During the medium-pressure injection, the average gas drainage purity increased by 1.94 times. The diffusion ranges of CO2 under low- and medium-pressure injections were 20–25 m and 25–30 m, respectively. The sweep efficiency of medium-pressure injection was 26% better than that of the low-pressure injection, with average pressures of 2.8 MPa and 1.4 MPa, respectively, for sweep efficiency. This study proposes an effective method for improving coal mine gas drainage efficiency.

Carbon Peak and Carbon Neutrality in the Building Sector: A Bibliometric Review

Due to large energy consumption and carbon emissions (ECCE) in the building sector, there is huge potential for carbon emission reduction, and this will strongly influence peak carbon emissions and carbon neutrality in the future. To get a better sense of the current research situation and future trends and to provide a valuable reference and guidance for subsequent research, this study presents a summary of carbon peak and carbon neutrality (CPCN) in buildings using a bibliometric approach. Three areas are addressed in the review through the analysis of 364 articles published from 1990–2021: (1) Which countries, institutions, and individuals have conducted extensive and in-depth research on CPCN in buildings, and what is the status quo of their collaboration and contributions? (2) What subjects and topics have aroused wide interest and enthusiasm among scholars, and what are their time trajectories? (3) What journals and authors have grabbed the attention of many scholars, and what are the research directions related to them? Moreover, we propose future research directions. Filling these gaps will enrich the research body of CPCN and overcome current limitations by developing more methods and exploring other practical applications.