Conventional hydraulic fracturing generally produces high breakdown pressure, results only in single major fracture morphology and increases the risk of seismic events during the stimulation of dry hot rock (HDR) reservoirs. Aiming at addressing the above bottlenecks during hydraulic fracturing, a new reservoir stimulation method, known as cyclic liquid nitrogen (LN₂) fracturing, based on cyclic soft stimulation (CSS) and LN₂ fracturing was explored in this paper. In cyclic LN₂ fracturing, low-temperature LN₂ was injected in a cyclic manner, i.e. alternating high-injection-rate and low-injection-rate (or stop injection). Hence, formation rocks would be subjected to fatigue damage under the combined action of alternating thermal stress and fluid pressure, which was expected to promote fracture initiation, propagation and bifurcation to form complex fracture networks and improve the stimulated reservoir volume. However, the research on LN₂ fracturing was mainly focused on the mechanisms of cyclic or single cooling treatment on rock and the fracturing performance of LN₂ fracturing. No works on the cyclic LN₂ fracturing performance, especially subjected to in-situ stresses were published as far as we know. To verify the feasibility of developing HDR by cyclic LN₂ fracturing, the fracture initiation and morphology of cyclic LN₂ fracturing were revealed by using Polymethyl Methacrylate (PMMA) based on the self-developed true-triaxial experiment of cyclic LN₂ fracturing. The effects of horizontal stress difference ratio and the number of cycles on cyclic LN₂ fracturing performance were studied. Cyclic water fracturing experiments were also conducted as a comparison. The results show that cyclic LN₂ fracturing can significantly reduce the breakdown pressure, with 47.1%~71.7% lower than cyclic water fracturing. Under the combined action of alternating thermal stress and fluid pressure, cyclic LN₂ fracturing tends to form a complex fracture network characterized by “thermally-induced fractures + major fractures”. The fracture initiation and morphology of cyclic LN₂ fracturing are not easily affected by the horizontal stress difference ratio. Complex fracture networks can still be produced by the cyclic LN₂ fracturing under a larger horizontal stress difference ratio. Increasing the number of cycles can reduce more breakdown pressure and generate more pronounced complex fracture networks. When high-pressure LN₂ was injected into the wellbore, the breakdown pressure was even higher than that of water fracturing, which indicated that cyclic LN₂ cooling pretreatment was the key to enhancing the LN₂ fracturing performance. In general, cyclic LN₂ fracturing can achieve better fracturing performance with a relatively lower number of cycles and cyclic pretreat pressure compared with cyclic water fracturing. Cyclic LN₂ fracturing was expected to provide a new way for the green, economic and efficient development of HDR. The results were expected to provide a theoretical and experimental basis for the development of HDR by using cyclic LN₂ fracturing.

As the primary conduit for oil and gas seepage, the pore and fracture structure of oil shale undergoes intricate changes during pyrolysis. However, the lack of understanding regarding its variations hinders effective reservoir classification and development. Therefore, it is important to investigate the influence of oil shale pyrolysis on pore structure at different temperatures and organic matter content. This paper proposes a new method based on 3D digital core reconstruction to quantify fractures and pore structures. The methodology employs parameters such as fracture width, fracture orientation, fractal dimension, and porosity to assess changes in pore and fracture structures throughout the pyrolysis process. The findings are as follows: (1) The proposed digital rock processing method and midplane extraction algorithm for fractured rock can accurately calculate fracture width and fracture orientation distribution. (2) As pyrolysis temperature increased, the porosity of oil shale samples rose from 18.3% to 20.9%, with a corresponding increase in fracture width, indicating improvements in the physical properties of the samples. However, the rate of change in pore and fracture structures decreased after pyrolysis (3) For fractured oil shale samples, porosity increased from 12.1% to 29.3% with higher organic matter content at consistent temperatures. However, Fractal analysis shows that pyrolysis fracture complexity is governed by both the content and, importantly, the initial spatial distribution of organic matter. This provides a new perspective for the precise characterization of pore structure after fracturing oil shale.

Fracture networks, fluid flow and heat extraction within fractures constitute pivotal aspects of enhanced geothermal system advancement. Conventional hydraulic fracturing in dry hot rock reservoirs typically requires high breakdown pressure and only produces a single major fracture morphology. Thus, it is imperative to explore better fracturing methods and consider more reasonable coupling mechanisms to improve the prediction efficiency. Cyclic fracturing using liquid nitrogen instead of water can generate more complex fracture networks and improve the fracturing performance. The simulation of fluid flow and heat transfer processes in the fracture network is crucial for an enhanced geothermal system, which requires a more comprehensive coupled thermo-hydro-mechanical-chemical model for matching, especially the characterization of coupling mechanism between the chemical and mechanical field. Based on the results of field engineering, laboratory experiments and numerical simulation, the optimum engineering scheme can be obtained by a multi-objective optimization and decision-making method. Furthermore, combining it with the deep-learning-based proxy model to achieve dynamic optimization with time is a meaningful future research direction.

Liquid nitrogen (LN₂) fracturing is a kind of non-aqueous fracturing technology, which is expected to provide a new and efficient way for coalbed methane (CBM) development. The mechanical properties of coal under LN₂ freezing are very important for studying the mechanism of LN₂ fracturing. However, most of the current research is limited to studying mechanical properties of rocks after being frozen by LN₂ and returned to room temperature. In this paper, the effect of LN₂ freezing on the mechanical properties of coal was studied. Uniaxial strength tests and Brazil tests were carried out for dry and water-saturated coal samples with different types and bedding directions. In addition, standard electron microscopy (standard SEM) and cryo-electron microscopy (Cryo-SEM) were used to compare the fracture morphology of coal samples at room temperature and LN₂ temperature. The results showed that LN₂ freezing can damage and improve the mechanical properties of coal simultaneously. The strength of saturated coal under freezing is higher than that of dry coal, and the filling of ice can enhance the mechanical strength of coal. In addition, the mechanical properties of coal with higher porosity are enhanced more than that of coal with lower porosity under LN₂ freezing. The main findings of this study are the keys to the research of LN₂ fracturing mechanisms in CBM reservoirs.

Geothermal energy is a kind of renewable, sustainable and clean energy resource. Geothermal energy is abundant in carbonate reservoirs. However, low matrix permeability limits its exploitation. The supercritical carbon dioxide (SC–CO₂) jet fracturing is expected to efficiently stimulate the carbonate geothermal reservoirs and achieve the storage of CO₂ simultaneously. In this paper, we established a transient seepage and fluid-thermo-mechanical coupled model to analyze the impact performance of SC-CO₂ jet fracturing. The mesh-based parallel code coupling interface was employed to couple the fluid and solid domains by exchanging the data through the mesh interface. The physical properties change of SC-CO₂ with temperature were considered in the numerical model. Results showed that SC-CO₂ jet fracturing is superior to water-jet fracturing with respect to jetting velocity, particle trajectory and penetrability. Besides, stress distribution on the carbonate rock showed that the tensile and shear failure would more easily occur by SC-CO₂ jet than that by water jet. Moreover, pressure and temperature control the jet field and seepage field of SC-CO₂ simultaneously. Increasing the jet temperature can effectively enhance the impingement effect and seepage process by decreasing the viscosity and density of SC-CO₂. The key findings are expected to provide a theoretical basis and design reference for applying SC-CO₂ jet fracturing in carbonate geothermal reservoirs.

Producing complex fracture networks in a safe way plays a critical role in the hot dry rock (HDR) geothermal energy exploitation. However, conventional hydraulic fracturing (HF) generally produces high breakdown pressure and results only in single main fracture morphology. Furthermore, HF has also other problems such as the increased risk of seismic events and consuption of large amount of water. In this work, a new stimulation method based on cyclic soft stimulation (CSS) and liquid nitrogen (LN₂) fracturing, known as cyclic LN₂ fracturing is explored, which we believe has the potential to solve the above issues related to HF. The fracturing performances including breakdown pressure and fracture morphology on granites under true-triaxial stresses are investigated and compared with cyclic water fracturing. Cryo-scanning electron microscopy (Cryo-SEM) tests and X-ray computed tomography (CT) scanning tests were used for quantitative characterization of fracture parameters and to evaluate the cyclic LN₂ fracturing performances. The results demonstrate that the cyclic LN₂ fracturing results in reduced breakdown pressure, with between 21% and 67% lower pressure compared with using cyclic water fracturing. Cyclic LN₂ fracturing tends to produce more complex and branched fractures, whereas cyclic water fracturing usually produces a single main fracture under a low number of cycles and pressure levels. Thermally-induced fractures mostly occur around the interfaces of different particles. This study shows the potential benefits of cyclic LN₂ fracturing on HDR. It is expected to provide theoretical guidance for the cyclic LN₂ fracturing application in HDR reservoirs.