Using blasting to induce fracture networks within rock mass is one of the effective reservoir stimulation methods for low-permeability sandstone-type uranium deposits. Nonetheless, there remains a deficiency of suitable theoretical methods to investigate the impact of CO2+O2in situ leaching on blasting-stimulated uranium deposits. In this work, a reaction-flow numerical model based on blasting fractures was first established; second, numerical simulations of blasting-induced fractures in the six injection and two extraction well groups were performed. Finally, the entire process of CO2+O2in situ leaching is simulated under various process parameters to predict the leaching effect of CO2+O2 on blasting-stimulated uranium deposits. Results show that there is a trend of first increasing and then decreasing between the blasting peak pressure and uranium recovery rate, reaching its maximum at a blasting peak pressure of 1000 MPa. The CO2+O2in situ leaching effect of blasting-stimulated uranium deposits is influenced by matrix permeability, O2 concentration, HCO3− concentration, injection rate, and average uranium grade. The matrix permeability, O2 concentration, HCO3− concentration, and average uranium grade are positively correlated with the uranium recovery rate, providing sufficient seepage space and required material composition for CO2+O2in situ leaching. However, the injection rate is negatively correlated with the uranium recovery rate because it reduces the leaching reaction time between the leaching agent and uranium deposits. The important ranking of factors affecting the peak uranium concentration and uranium recovery rate at the CO2+O2in situ leaching period of 900 days is the O2 concentration > matrix permeability > injection rate > average uranium grade > HCO3− concentration. This study serves as a reference for selecting and optimizing technology parameters for blasting and CO2+O2in situ leaching during field tests.

Hydraulic fracturing is an effective method for developing unconventional reservoirs. The fracture height is a critical geometric parameter for fracturing design but will be limited by a weak interface. Fracture containment occurs when fracture propagation terminates at layer interfaces that are weaker than the surrounding rock. It always occurs in multilayer formation. Therefore, the mechanism of fracture height containment guides fracture height control in hydraulic fracturing. In order to study the fracture containment mechanism, this paper first calculates the propagation behaviour of the fracture in 3D under the influence of a weak interface through a block discrete element method and analyzes the geometric characteristics of the fracture after it meets the weak interface. Then, the induced stress of the hydraulic fracture on the weak interface is calculated by fracture mechanics theory, and the mechanism of blunting at the fracture tip is explained. Then, two kinds of interface slippage that can lead to blunting of the fracture tip are discussed. Based on the behavior of shear slippage at the interface, a control method for multilayer fracturing in thin sand-mud interbed and pay zone fracturing in shale is proposed. The results show that the fracture height is still limited by the weak interface in the formation without the difference of in-situ stress and rock properties. Interface slippage is the main factor impeding fracture propagation. Fracture height containment can be adjusted and controlled by changing the angle between the hydraulic fracture, the interface, and the stress state to strengthen and stiffen the interface. This study has a certain guiding significance for fracture height control in the design of hydraulic fracturing of shale or thin sand-mud interbed reservoirs.

The formation of the fracture network in shale hydraulic fracturing is the key to the successful development of shale gas. In order to analyze the mechanism of hydraulic fracturing fracture propagation in cemented fractured formations, a numerical simulation about fracture behavior in cemented joints was conducted based firstly on the block discrete element. And the critical pressure of three fracture propagation modes under the intersection of hydraulic fracturing fracture and closed natural fracture is derived, and the parameter analysis is carried out by univariate analysis and the response surface method (RSM). The results show that at a low intersecting angle, hydraulic fractures will turn and move forward at the same time, forming intersecting fractures. At medium angles, the cracks only turn. At high angles, the crack will expand directly forward without turning. In conclusion, low-angle intersecting fractures are more likely to form complex fracture networks, followed by medium-angle intersecting fractures, and high-angle intersecting fractures have more difficulty in forming fracture networks. The research results have important theoretical guiding significance for the hydraulic fracturing design.