Study On The Law Of CO₂ Gas Backflow In Low Permeability Sandstone Type Uranium Mine After Blasting

The sandstone-type uranium reservoirs in China are mainly low permeability.CO₂ phase change fracturing technology is an important means to improve the permeability of uranium reservoirs,and its permeability enhancement effect is closely related to the flowback rate of CO₂ gas in fractures.Therefore,how to maximize the flowback of CO₂ gas in the fracture after liquid CO₂ phase transition is the main problem of current research.In this thesis,the gas flowback after CO₂ phase change fracturing of low permeability sandstone-type uranium deposits is taken as the starting point.Based on the theory of gas-liquid two-phase flow in porous media,the flowback law of CO₂ gas in core scale and macro scale is analyzed by establishing fracture network analysis model.The specific contents and conclusions are as follows:（1）Split Hopkinson pressure bar（SHPB）impact load was used to simulate the blasting stress wave,and the acoustic wave velocity damage definition method was used to determine the rock sample after impact.The results show that the fracture development of gray sandstone is mainly shear fracture and tensile fracture.The CT scan and image recognition of the rock samples after the impact were carried out.The three-dimensional reconstruction software was used to analyze the fracture spatial morphology,and a numerical fracture network analysis model based on the experimental results was established.（2）On the basis of fracture network analysis model,CO₂gas backflow analysis model of core fracture is established by using gas-liquid two-phase flow law and phase field method.By tracking the two-phase flow interface,the temporal and spatial evolution of the shape and position of the water-air interface in the fracture is analyzed,and the flowback law of gas in the core fracture is obtained.Based on the response surface method,the significance of the three factors of displacement pressure difference,fracture aperture and contact angle on CO₂ backflow is analyzed.The primary and secondary order is:displacement pressure difference>fracture aperture>contact angle.（3）The fracture network model after blasting around the well is established,and the influence of parameters such as closing time,contact angle,fracture complexity and wellbore radius on gas flowback law after blasting is explored.The results show that the longer the shut-in time after blasting,the greater the gas flowback speed.When the contact angle is 75°,the gas lock effect is the strongest and the gas backflow speed is the smallest.Under the condition of single fracture,the more the number of fracture bifurcations,the greater the gas backflow rate;under the condition of fracture network,the simpler the fracture at the same time,the larger the flowback volume and the higher the flowback rate;when the wellbore radius is 8 cm,the gas volume fraction in the fracture is the smallest and the flowback rate is the largest.The influencing factors of wellbore fracture flowback under different contact angle conditions were analyzed by multi-factor analysis.The order of influencing gas volume fraction is:shut-in time>fracture complexity>wellbore radius.（4）Based on the obtained gas flowback law after blasting,the analysis process of the best gas flowback scheme is established for the new well and the existing well.