超临界CO_2岩石致裂机制分析

Analysis of mechanisms of supercritical CO_2 fracturing

超临界CO_2是一种介于气体和液体之间的特殊状态的CO_2流体,具有低黏、高扩散性和零表面张力等独特的性质。利用超临界CO_2作为压裂液,有助于裂缝的起裂和扩展,同时可避免储层伤害。通过研究超临界CO_2射流破岩和压裂特性,分析得到了超临界CO_2岩石致裂机制。研究结果表明,超临界CO_2低黏等特性使其更容易进入岩石微孔和微缝之中,在岩石内部建立大小不一的流体压力系统,使岩石发生拉伸和剪切破坏;常规流体压裂起裂压力较高,裂缝一般为单条或多条平直裂缝,大多沿着同一方向贯穿强度较高的胶结颗粒,且裂缝断面光滑、平整;超临界CO_2压裂起裂压力相比于常规流体压裂低,在岩石中形成的裂缝网络较为复杂,裂缝互相连通,一般沿着强度较低的胶结物开裂,较少贯穿胶结颗粒,裂缝断面较为粗糙。该研究结果可为超临界CO_2压裂技术的实施提供理论支撑。

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.