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 has the potential to induce complex fracture networks, avoid formation damage, and eliminate water consumption. However, the flow and heat transfer of nitrogen in fractures and its effect on the fracture generation during LN2 fracturing have not been studied, and the fracturing mechanism remains unclear. In this paper, the nitrogen flow during LN2 fracturing in a straight fracture was simulated. First, a 3D unsteady-state fluid flow and heat transfer model for LN2 fracturing in coalbed methane (CBM) reservoir was developed, which considered the phase transition of nitrogen, thermophysical properties variation of coal and the heat transfer between nitrogen and formation. This model was then validated against published analytical solutions. Subsequently, the model was applied to elucidate the phase distribution of nitrogen and its influence on fracture generation. Finally, the factors that affect the flow and heat transfer of nitrogen were analyzed. The results showed that the nitrogen at the fracture tip was in a supercritical state. Thermal stress had minor effects on the propagation of the main fracture. In addition, fracture aperture, injection velocity, reservoir temperature, injection fluid temperature, fracture propagation pressure and coal cleat porosity could affect the effectiveness of LN2 fracturing in a coal seam. The main findings of this study are the keys to the research of liquid nitrogen fracturing mechanisms in CBM reservoirs.