Proppants are widely employed in coalbed methane extraction. The use of proppant effectively mitigates the closure of hydro-fractures during production, thereby maintaining efficient gas flow pathways. The transport distance and accumulation morphology of proppants within hydro-fractures are critical factors influencing coalbed methane production; however, their quantitative and comprehensive evaluation remains insufficiently explored in coal reservoirs. In this study, a Box-Behnken design was adopted to establish a four-factor, four-level experimental framework for investigating the influence of multiple variables on dune parameters within secondary hydro-fractures through a coupled computational fluid dynamics-discrete element method approach. Response surface methodology and statistical significance testing were employed to quantify the effects of multiple parameters and to establish an empirical predictive model of proppant dune characteristics. The adequacy and significance of the proposed model were verified through analysis of variance. The results demonstrated that both the transport distance and accumulation morphology of proppant within hydro-fractures are jointly controlled by the coupled influence of multiple parameters. Four basic variables, including injection rate, proppant size, proppant density and sand carrying fluid viscosity, were selected, and their influences on sand dune parameters were ranked. The model predictions revealed that dune height may reach up to 79.7% of the hydro-fracture height, while the horizontal dune length can extend up to 15 times the hydro-fracture height. These findings elucidate the mechanisms governing proppant transport and deposition under diverse conditions, offering valuable insights and optimization strategies for proppant selection and injection parameter design in hydraulic fracturing in coalbed methane reservoirs.

With the demand for peak-shaving of renewable energy and the approach of carbon peaking and carbon neutrality goals, salt caverns are expected to play a more effective role in oil and gas storage, compressed air energy storage, large-scale hydrogen storage, and temporary carbon dioxide storage. In order to effectively utilize the underground space of salt mines on a sound scientific basis, the construction of salt caverns for energy storage should implement the maximum utilization of salt layers, improve the cavern construction efficiency, shorten the construction period, and ensure cavern safety. In this work, built upon design experience and on-site practice in salt cavern gas storage, the four pivotal construction stages – conceptual design, solution mining simulation, tightness assessment, and stability evaluation – have been thoroughly enhanced, strengthening the technical framework for salt cavern energy storage.

With the promotion of China’s carbon peaking and carbon neutrality goals, the energy industry is transforming from traditional fossil energy to renewable energy, which is sustainable, clean and safe. The development of renewable energy is not only an important measure to achieve the above goals but also a significant factor to alleviate the global energy crisis. Salt caverns, with good air tightness, have been considered as the best choice for large-scale underground energy storage. To elaborate on the research and future development of salt cavern compressed air energy storage technology in China, this paper analyzes the mode and characteristics of compressed air energy storage, explores the current development, key technologies and engineering experience of the construction of underground salt caverns for compressed air energy storage at home and abroad. Focusing on salt cavern compressed air energy storage technology, this paper provides a deep analysis of large-diameter drilling and completion, solution mining and morphology control, and evaluates the factors affecting cavern tightness and wellbore integrity. The future development and challenges of underground salt caverns for compressed air energy storage in China are discussed, and the prospects for the three key technologies of large-diameter drilling and completion and wellbore integrity, solution mining morphology control and detection, and tubing corrosion and control are considered. This paper aims to provide a useful reference for the development of underground salt cavern compressed air energy storage technology, the transformation of green and renewable energy, and the realization of carbon neutral vision.