关键词: 超临界CO2, 压裂, 破岩, 裂缝, 致裂机制

Abstract: Supercritical CO2 is a kind of fluid, which is in a special state between gas and liquid, with unique properties such as low viscosity, high diffusivity, and zero surface tension. Thus, supercritical CO2 can be used as fracturing liquid to assist crack initiation and propagation without leading to the damage of reservoir zones. In this study, the mechanisms of supercritical CO2 fracturing were obtained by studying and analysing the characteristics of rock breaking and fracturing with supercritical CO2. The results indicate that, owing to its lower viscosity, supercritical CO2 can easily penetrate into micro-pores and micro-cracks and build fluid pressure systems with varied magnitudes in rocks, which results in tensile and shear failure. In conventional hydraulic fracturing, the initiation pressure is high, and the fractures are single or multiple straight cracks. Most fractures penetrate through mineral grains with high strength along the same direction, and the fracture sections are smooth and flat. However, the initiation pressure of supercritical CO2 fracturing is lower than that of conventional fluid fracturing methods; the fracture network is complex, and fractures are connected with each other. Generally, the induced fracture majorly initiates along the lower-strength grain boundaries, but seldom penetrates the mineral grains. Moreover, the fracture planes are rough. This study provides theoretical support for implementation of supercritical CO2 fracturing technology.

Key words: supercritical CO2, fracturing, rock breaking, fracture, fracturing mechanism