Geothermal energy storage technology is a kind of technology using injected and subsurface in-situ fluid as heat carrier and underground porous media as storage space to store energy, and exploiting it to the ground for comprehensive utilization when necessary. The technology has been continuously developed since the 1960s to keep balance between energy consumption and emission of different industries, and thus establish a technical system based on different heat carriers, scales and energy storage methods. In the process of technological innovation, the geothermal energy storage concept has realized the transformation from a single energy storage form of "Earth Battery" to a multi-energy complementary storage/energy supply system of "Earth Charge and Geothermal Storage", and made full use of the characteristics of geothermal energy storage technology "large scale, wide application, cross-season and low cost", with the advantages of large heat storage space, high heat utilization efficiency, safety, green and low carbon, etc. At present, there are a number of projects around the world to test the geothermal storage of industrial waste heat and renewable energy, and which has achieved good results. It shows better technical practicability and broad development space. It has great significance to the stable supply and efficient utilization of energy. The main mechanisms of geothermal energy storage and heat extraction include heat conduction, convective heat transfer, heat dispersion, thermosiphon effect and physicochemical interaction, etc. At the same time, energy is stored, transferred and converted underground through the heat-fluid-solid coupling effect between fluid and rock. Therefore, the effect of geothermal energy storage depends on the fluid-rock interaction and the way of geothermal energy storage. And the more fluid types in the reservoir, the more complicated the mechanism involved. This paper first described the developing history of geothermal energy storage technology at home and abroad, summarized the heat transfer and energy storage mechanism based on fluid-rock interaction in the process of geothermal energy storage, and analyzed the key technical problems and research status in the process of geothermal reservoir location, aquifer depth selection and energy storage carrier selection on the basis of summarizing previous work. At the same time, the overview and operation status of major geothermal energy storage projects around the world were sorted out and summarized. It was concluded that the porosity, permeability, thickness, anisotropy and heterogeneity of the thermal reservoir have a great influence on its thermal storage efficiency and scale, and the properties of thermal reservoir and heat carrier, and the matching degree with the ground heat source should be considered comprehensively in the selection process. On this basis, this paper looked forward to the application prospect of geothermal energy storage technology, and pointed out a series of challenges that the technology may face from the perspective of heat storage mechanism. It was believed that the breakthrough point of geothermal energy storage technology in the future lies in the joint storage and utilization of carbon capture, utilization and storage technology, sustainable energy such as wind, light and electricity, searching for underground space with good thermal insulation performance, development and utilization of high-performance thermal energy carriers and anti-blocking and corrosion technology. As a further efficient use of the existing energy system and beneficial supplement, with its unique advantages in peak cutting and valley filling, energy conservation and emission reduction and comprehensive utilization of energy, geothermal energy storage has huge potential resources and market potential, and is the future direction of low-carbon geological energy development.

There are significant reserves of shale oil in China, which has substantial potential. The efficient development of these resources is crucial for ensuring national energy security and accelerating strategic transformation in the industry. This study introduces an innovative extraction technique that combines horizontal wells with radial water jet drilling, aimed at optimizing the recovery of shale oil. The approach involves drilling radially distributed boreholes around the horizontal well to penetrate the adjacent contamination zone, which can increase the contact area with the reservoir and establishing highly conductive pathways to enhance production and injection performance. A coupled flow model considering formation damage was developed to enable accurate predictions of productivity from shale oil reservoirs developed via horizontal and radial jet drilling. The model, employing dual refined grids, accurately describes the pressure dynamics within shale oil reservoirs and analyzes the impact of radial well parameters such as length, diameter, initial angle, number of laterals, drilling locations, and cluster numbers on productivity. Comparative analyses between reservoirs developed under identical conditions by radial jet drilling versus hydraulic fracturing were conducted. Findings reveal that radial wells significantly mitigate near-well fluid flow resistance and expand the effective drainage area of the formation. Enhancements in radial well dimensions, including length, number of branches, clusters, and diameter, are shown to increase cumulative shale oil production. Although increasing the number of clusters in radial wells expands the drainage area and benefits oil production, the interference between wells intensifies with more clusters, leading to a diminishing trend in the increase of cumulative oil production. The initial angle of radial wells, influenced by anisotropic permeability and gravity, is crucial for targeting vertical formations. Longer radial wells demonstrate that opening positions have a minimal impact on output. Under equal cluster conditions, a radial well with four 30 m laterals slightly outperforms a hydraulically fractured well with 15 m fracture height; when combined with a 15 m hydraulic fracture, the output slightly exceeds that of a 25 m fracture height. This study reveals that the horizontal plus radial well model not only serves as an effective alternative or supplement to hydraulic fracturing techniques but also facilitates more efficient, cost-effective shale oil extraction. The technique proposed herein combines the horizontal well and radial jet drilling technology and has higher economic feasibility, which can serve as a supplementary technology for the hydraulic fracturing approach and provide reference for efficient development of shale oil resources.

With increase in the number of operations involving relief wells, radial wells, U-shaped wells, and other complex well structures, challenges such as collision prevention, obstacle bypassing, and adjacent-well connectivity achievement during drilling have become inevitable. These challenges necessitate a technology that can accurately detect adjacent wells in real time during drilling operations. As current borehole acoustic reflection imaging technology heavily relies on cable-based logging, it cannot perform real-time detection of adjacent wells during drilling, thereby limiting the drilling efficiency. This study proposes a new adjacent-well-acoustic-detection-while-drilling method that integrates wireline borehole acoustic reflection imaging with drilling technology, along with an adjacent-well imaging method based on compressed sensing (CS). Together, these methods enable high-resolution, real-time detection of the adjacent target wells during drilling, ensuring safe and efficient underground drilling operations. The finite-difference method was used to simulate three-dimensional numerical models under drilling conditions for two scenarios—with and without target wells adjacent to the drilling well. Experimental validation was conducted in a water tank using an adjacent-well-acoustic-detection-while-drilling tool. The simulated target well was imaged using the CS method, and the imaging results were compared with those obtained from numerical and physical simulations, thereby validating the feasibility of the proposed acoustic detection and imaging methods. The results demonstrate that as the radial distance from the target well increases, the PP echo exhibits delayed arrival times and approaches a plane wave while exhibiting amplitude attenuation. Conversely, a linear increase in the target well diameter advances the PP echo arrival time and enhances its amplitude proportionally. When the target and drilling wells are approximately parallel with a small intersection angle, PP echoes yield better detection results than SS echoes; when the wells are coplanar with a large intersection angle, SS echoes provide better detection results. The receiver element aligned with the target well's azimuth detects all echo modes with the earliest arrival times and highest amplitudes. The adjacent-well imaging method based on CS offers very high spatial resolution, with target wells appearing as local amplitude maxima. This feature enables the precise determination of their azimuth and inclination relative to the drilling wells. The findings offer a solid physical and methodological foundation for real-time detection of adjacent wells during drilling operations and demonstrate enormous theoretical and engineering application potential.

Exploring the interaction between hydraulic fractures and complex geological conditions is critical for multilayered commingling production in the laminated continental shale oil reservoirs. In this paper, a 2D hydro-mechanical coupling numerical model is developed to investigate the fracture propagation behavior affected by the joint interaction of multi-lithologic stack and shale anisotropy. The model adopts a smear approach to reproduce the mechanical anisotropy of shale observed from laboratory experiments with powerful finite-discrete element method (FDEM) to precisely capture the transition from elastic deformation to failure during fluid injection in layered heterogeneous media. The results indicate that shale anisotropy affects hydraulic fracture initiation and propagation behavior. The estimated breakdown pressure is 15% higher than that in horizontal homogeneous shale oil reservoirs. The elastic anisotropy alters the stress trajectory and magnitudes, while the strength anisotropy affects the failure mode and fracture morphology. Under the combined two factors, the established fracture network reveals potent cross-layer abilities with less activation of weak planes. Additionally, the sedimentary structure of thin interlayers hinders fracture height extension, resulting in a limited stimulated reservoir volume (SRV). Optimization of engineering and geological parameters could mitigate this limitation and efficiently co-develop the multiple sweet-spot pay zones. For field application, it is proposed to select a modest stress difference formation (K_v around 0.75–1.00) for stimulation. Then, an alternated high/low injection rate can be employed to improve the cross-layer ability and activate the underlying weak planes, finally realizing an ideal SRV. The key findings are expected to provide new insights into the fracture propagation mechanism and guide reservoir stimulation in continental shale oil.

The accurate evaluation of hydraulic fracturing performance is essential for the iterative optimization of unconventional reservoir development. In this aspect, fracturing pressure diagnostics has been recognized as a non-invasive technique that significantly reduces operational time and cost. However, pressure-based diagnostics lack a unified workflow for the evaluation of fracture complexity and area and cannot provide sufficient guidance for design optimization. Thus, this paper proposes an integrated diagnostic framework, constructed by pressure interpretation and data mining, from which the hydraulic fracture complexity and fracture area can be quantified. The normalized fracture complexity index is defined by propagation events and energy intensity extracted from wavelet-transformed pressure signals, and the fracture area is evaluated from pressure falloff analysis. Data mining is then used to optimize the fracturing parameters based on these two indices. The results show that the proposed framework effectively characterizes the stimulated fracture area and complexity and reveals their relationships with fracturing parameters and geological factors on the basis of multi-stage data from three horizontal coalbed methane wells. The stimulated fracture area is primarily determined by the fracturing fluid volume and pumping rate, while the fracture complexity is strongly regulated by the pumping rate and compressive strength of the rock. A negative correlation was detected between the fracture complexity and the main fracture area. To balance the main area and complexity of fractures, it is necessary to optimize the key fracturing parameters. This study provides a low-cost tool that can diagnose hydraulic fracturing performance and effectively optimize unconventional completion.

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.

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.

Cavitation jet drilling has been extensively employed for the exploitation of geo-energy resources. The dynamics of cavitation bubbles in close proximity to the solid boundary have been a subject of great interest during jet drilling, as they play a crucial role in determining the cavitation performance. In present work, the dynamics of a single cavitation bubble near a solid surface is numerically investigated by using the axisymmetric Navier-Stokes equations and the volume of fluid method with considering the surface tension of gas-liquid interface, liquid viscosity and compressibility of gas in bubble. The simulated profiles are qualitatively and quantitatively consistent with the experimental images, which proves the reliability of employed numerical model. The effects of stand-off distance on the bubble profiles, bubble volume and collapse time have been analysed. Moreover, the cavitation erosion patterns towards the solid wall are also revealed for different dimensionless stand-off distances. The simulation results reveal two distinct collapse patterns for the bubble profiles. The solid wall significantly impedes the shrinkage rate of the bubble, resulting in the longest collapse time when the dimensionless stand-off distance is 1.0. Three erosion patterns of cavitation bubbles towards the solid wall are observed, with the shock wave and micro-jet both contributing significantly to the damage caused by cavitation erosion. The shock wave sweeps the wall resulting in circular corrosion pits with a severely eroded centre, while the micro jet penetrates the wall leading to small spot corrosion pits.