Hydrogen generation through serpentinization reactions in peridotite formations under high temperature conditions represents a promising avenue for subsurface hydrogen production. However, the thermal energy in the formation environment have not been sufficiently considered. To integrate hydrogen production with thermal energy development, this study develops a thermo-hydro-chemical coupled numerical model, which is used to investigate hydrogen-thermal co-development in hydraulically fractured peridotite. Given that the hydrogen-thermal co-development process involves fluid flow, heat transfer and serpentinization reactions, the governing equations are formulated based on Darcy flow and energy conservation equations, with serpentinization kinetics incorporated through the reactive source terms. To evaluate hydrogen production and thermal energy recovery under varying formation and injection fluid temperatures, the coupled system is solved numerically. The results show that under high geothermal temperature conditions, continuous high-temperature injection combined with moderate natural fracture development can sustain stable high production during long-term operation. When the contribution of thermal energy is neglected, the total system energy output decreases significantly, highlighting the necessity of hydrogen-thermal co-development. This study further identifies the optimal injection temperature range under high geothermal conditions. Under normal geothermal conditions, hydrogen production is limited by reaction temperature, and high-level production cannot be maintained solely through thermal stimulation. Sensitivity analysis reveals that reaction kinetics are the dominant factor controlling system hydrogen productivity, and enhancing them can increase hydrogen production by nearly an order of magnitude. This work establishes a quantitative framework for artificial hydrogen generation and provides theoretical guidance for engineering design and the operational parameter optimization of hydrogen-thermal co-development systems.

Supercritical CO₂ fracturing, as a waterless fracturing technology, is attracting increasing attention in the shale oil reservoir development industry. In recent years, a novel CO₂ hybrid fracturing method has been proposed to integrate the advantages of both CO₂ fracturing and hydraulic fracturing. However, the specific effects of different pre-injection CO₂ conditions on the physical properties, mechanical characteristics, and crack propagation behavior of shale reservoirs remain unclear. This study utilized Chang-7 shale samples from the Ordos Basin and conducted CO₂ hybrid fracturing experiments under simulated high-temperature and high-pressure reservoir conditions, employing a self-developed experimental apparatus. Quantitative analysis of fracture propagation patterns under the influence of CO₂ pre-injection was performed based on CT scanning results. The findings reveal that: (1) Among different fracturing fluid systems, conventional hydraulic fracturing exhibits the highest breakdown pressure, pure CO₂ fracturing is intermediate, while CO₂ hybrid fracturing significantly reduces the breakdown pressure by 36.2% compared to hydraulic fracturing. (2) Employing CO₂ hybrid fracturing not only effectively increases fracture dimensions (length, width) but also substantially enhances fracture network complexity. (3) The CO₂ pre-injection soaking time significantly influences fracture morphology, with both fracture dimensions and structural complexity showing marked increases as soaking time extends. (4) Increasing formation pore pressure promotes the activation of bedding planes with relatively weaker mechanical strength, leading to significant enhancements in fracture length and complexity, but simultaneously restricts the widening of fracture apertures. The outcomes of this research provide a theoretical foundation for optimizing the design of operational parameters in CO₂ hybrid fracturing for shale oil reservoirs.

High pressure water jets have strong erosion ability and have achieved excellent results in drilling engineering. Cavitation phenomena occur in the process of jetting, which is accompanied by high temperature and high pressure, and is one of the key factors affecting the erosion ability of jets. The structure of the nozzle has a significant influence on the cavitation ability of the jet. Analysis of the relationship between the flow field characteristics of the cavitation jet and the nozzle structure is an important aspect of the study of high pressure water jets. In this paper, we conducted visualization experiments and we 3D-printed organ-pipe self-resonant cavitating nozzles. The characteristics of the cavitation jet flow field at the nozzle outlet, especially the morphological changes of the cavitation cloud in the flow field were captured with high-speed photography. After image processing, the impact of nozzle structure changes on cavitation generation capacity was analyzed. We used the proper orthogonal decomposition (POD) method to obtain the time-averaged characteristics of the flow field structure. The results show that the resonator is an essential structure that affects cavitation. Increasing the length and diameter of the resonator within a certain range ensure the occurrence of cavitation and the structural stability of the flow field. However excessive size affects the self-resonance of the nozzle and makes it difficult to create resonance. In this paper, the optimal values of nozzle outlet diameter and extension angle are twice the outlet diameter and 40°, respectively. The stability difference of water jets were analyzed by comparing the time-averaged characteristics under different nozzle structures and jet hydraulic parameters. The results show that the resonant cavity in the nozzle structure is the main part affecting the cavitation generation ability. Increasing the length and diameter of the resonant cavity within a certain range is conducive to enhancing the cavitation generation ability of the nozzle and improving the structural stability of the flow field at the nozzle outlet. However, excessive length and diameter of the resonant cavity will affect the self-vibration effect of the nozzle, making it more difficult for the fluid to form resonance when passing through the nozzle, and making the cavitation ability of the nozzle drop sharply. In this paper, the optimal values of nozzle outlet diameter and extension angle are twice the outlet diameter and 40°, respectively. This research provides a better way to study optimization of self-resonant nozzles and cavitating-jet characteristics, which is intuitive and can be a validation for other approaches.

Natural gas hydrate is a potential alternative energy source for the future, and the low mining efficiency severely restricts its commercial utilization. The technology of radial wells is a newly proposed method for increasing the productivity of natural gas hydrates. To evaluate the effectiveness of this method, experimental studies were carried out to simulate the extraction of hydrate sediments in water-rich and gas-rich environments by vertical well depressurization and radial well depressurization. Results indicate that: for the extraction of gas-rich hydrate samples by depressurization, gas production behavior of vertical well and radial well is almost the same; for the small experimental scale and relatively high permeability of samples the pressure propagation patterns are similar when vertical wells and radial wells are applied. For the extraction of water-rich hydrate samples by depressurization: compared with that of vertical wells, the cumulative gas production and cumulative water production of radial wells at the end of the production cycle were increased by 20.16% and 38.98%, respectively; as a high conductivity channel, the radial well can increase the drainage area in the hydrate sediments and promote the propagation of pressure drop to the inside of the reactor, and ultimately accelerating the decomposition rate of hydrate; besides, the temperature drop is more significant when the radial well is used, which provides more sensible heat for the decomposition of hydrates and further improves the efficiency of hydrate production.

Accurate identification of fracture geometry in hydraulic fracturing is essential for understanding fracture propagation, optimizing stimulation design, and predicting production performance. Distributed acoustic sensing, as a high-resolution near-wellbore monitoring technique, provides rich spatiotemporal data for real-time observation of fracture responses. However, reconstructing fracture geometry from distributed acoustic sensing measurements remains challenging due to high model dimensionality, ill-posed inversion processes and substantial computational costs. This study presents a fracture geometry inversion framework based on radial basis function, in which the fracture width distribution is represented using a small number of radial basis function modes. Owing to the intrinsic smoothness and symmetry of radial basis function, the method eliminates the need for explicit regularization terms, thereby simplifying the objective function and improving inversion stability. This approach significantly reduces the number of inversion parameters while enhancing both accuracy and physical consistency. Applications to a synthetic benchmark model and real field data from the hydraulic fracturing test site demonstrate that the radial basis function-based method consistently outperforms conventional fullparameter inversion approaches, in terms of fitting accuracy and computational efficiency. The proposed method provides a structurally informed and computationally efficient modeling framework for high-dimensional fracture inversion, offering a promising solution for real-time fracture monitoring and parameter estimation in hydraulic fracturing operations.

CO₂ injection into oil reservoirs is expected to achieve enhanced oil recovery along with the benefit of carbon storage, while the application potential of this strategy for shale reservoirs is unclear. In this work, a numerical model for multiphase flow in shale oil reservoirs is developed to investigate the impacts of nano-confinement and oil composition on shale oil recovery and CO₂ storage efficiency. Two shale oils with different maturity levels are selected, with the higher-maturity shale oil containing lighter components. The results indicate that the saturation pressure of the lower-maturity shale oil continues to increase with increasing CO₂ injection, while that of the higher-maturity shale oil continues to decrease. The recovery factor and CO₂ storage rate for higher-maturity shale oil after CO₂ huff-n-puff are 12.02% and 44.76%, respectively, while for lower-maturity shale oil, these are 4.41% and 69.33%, respectively. These data confirm the potential of enhanced oil recovery in conjunction with carbon storage in shale oil reservoirs. Under the nano-confinement impact, a decrease in the oil saturation in the matrix during production is reduced, which leads to a significant increase in oil production and a significant decrease in gas production. The oil production of the two kinds of shale oil is comparable, but the gas production of higher-maturity shale oil is significantly higher. Nano-confinement shows a greater impact on the bubble point pressure of higher-maturity shale oil and a more pronounced impact on the production of lower-maturity shale oil.

Supercritical CO₂ (SC-CO₂) fracturing, being a waterless fracturing technology, has garnered increasing attention in the shale oil reservoir exploitation industry. Recently, a novel pre-SC-CO₂ hybrid fracturing method has been proposed, which combines the advantages of SC-CO₂ fracturing and hydraulic fracturing. However, the specific impacts of different pre–SC-CO₂ injection conditions on the physical parameters, mechanical properties, and crack propagation behavior of shale reservoirs remain unclear. In this study, we utilize a newly developed “pre-SC-CO₂ injection → water-based fracturing” integrated experimental device. Through experimentation under in-situ conditions, the impact of pre-SC-CO₂ injection displacement and volume on the shale mineral composition, mechanical parameters, and fracture propagation behavior are investigated. The findings of the study demonstrate that the pre-injection SC-CO₂ leads to a reduction in clay and carbonate mineral content, while increasing the quartz content. The correlation between quartz content and SC-CO₂ injection volume is positive, while a negative correlation is observed with injection displacement. The elastic modulus and compressive strength exhibit a declining trend, while Poisson's ratio shows an increasing trend. The weakening of shale mechanics caused by pre-injection of SC-CO₂ is positively correlated with the injection displacement and volume. Additionally, pre-injection of SC-CO₂ enhances the plastic deformation behavior of shale, and its breakdown pressure is 16.6% lower than that of hydraulic fracturing. The breakdown pressure demonstrates a non-linear downward trend with the gradual increase of pre-SC-CO₂ injection parameters. Unlike hydraulic fracturing, which typically generates primary fractures along the direction of the maximum principal stress, pre-SC-CO₂ hybrid fracturing leads to a more complex fracture network. With increasing pre-SC-CO₂ injection displacement, intersecting double Y-shaped complex fractures are formed along the vertical axis. On the other hand, increasing the injection rate generates secondary fractures along the direction of non-principal stress. The insights gained from this study are valuable for guiding the design of preSC-CO₂ hybrid fracturing in shale oil reservoirs.

Digital rock analysis is a promising approach for visualizing geological microstructures and understanding transport mechanisms for underground geo-energy resources exploitation. Accurate image reconstruction methods are vital for capturing the diverse features and variability in digital rock samples. Stable diffusion, a cutting-edge artificial intelligence model, has revolutionized computer vision by creating realistic images. However, its application in digital rock analysis is still emerging. This study explores the applications of stable diffusion in digital rock analysis, including enhancing image resolution, improving quality with denoising and deblurring, segmenting images, filling missing sections, extending images with outpainting, and reconstructing three-dimensional rocks from two-dimensional images. The powerful image generation capability of diffusion models shed light on digital rock analysis, showing potential in filling missing parts of rock images, lithologic discrimination, and generating network parameters. In addition, limitations in existing stable diffusion models are also discussed, including the lack of real digital rock images, and not being able to fully understand the mechanisms behind physical processes. Therefore, it is suggested to develop new models tailored to digital rock images for further progress. In sum, the integration of stable diffusion into digital core analysis presents immense research opportunities and holds the potential to transform the field, ushering in groundbreaking advances.

Radial borehole fracturing that combines radial boreholes with hydraulic fracturing is anticipated to improve the output of tight oil and gas reservoirs. This paper aims to investigate fracture propagation and pressure characteristics of radial borehole fracturing in multiple layers. A series of laboratory experiments with artificial rock samples (395 mm × 395 mm × 395 mm) was conducted using a true triaxial fracturing device. Three crucial factors corresponding to the vertical distance of adjacent radial borehole layers (vertical distance), the azimuth and diameter of the radial borehole are examined. Experimental results show that radial borehole fracturing in multiple layers generates diverse fracture geometries. Four types of fractures are identified based on the connectivity between hydraulic fractures and radial boreholes. The vertical distance significantly influences fracture propagation perpendicular to the radial borehole axis. An increase in the vertical distance impedes fracture connection across multiple radial borehole layers and reduces the fracture propagation distance along the radial borehole axis. The azimuth also influences fracture propagation along the radial borehole axis. Increasing the azimuth reduces the guiding ability of radial boreholes, which makes the fracture quickly curve to the maximum horizontal stress direction. The breakdown pressure correlates with diverse fracture geometries observed. When the fractures connect multi-layer radial boreholes, increasing the vertical distance decreases the breakdown pressure. Decreasing the azimuth and increasing the diameter also decrease the breakdown pressure. The extrusion force exists between the adjacent fractures generated in radial boreholes in multiple rows, which plays a crucial role in enhancing the guiding ability of radial boreholes and results in higher breakdown pressure. The research provides valuable theoretical insights for the field application of radial borehole fracturing technology in tight oil and gas reservoirs.