液氮对页岩的致裂效应及在压裂中应用分析

Thermal cracking effect of liquid nitrogen on shale and its application analysis in hydraulic fracturing

为研究液氮对页岩的致裂效应,对同一页岩岩样在液氮冷却处理前后进行超声波和渗透率测试,并对比岩样表面的变化情况,分析液氮对岩石的致裂机制和影响因素,并讨论液氮压裂技术在页岩气开发上具有的优势及其应用前景。试验结果表明:经过液氮冷却后,页岩表面有明显的热力裂缝产生,液氮能够对页岩产生显著的致裂效果;液氮低温作用能够促进页岩内部微裂纹的扩展,提高孔隙结构的连通性,使页岩波速降低、渗透率增加;试验中页岩的波速最大降幅为10.43%,渗透率最大增幅为177.27%。

In order to investigate the effect of liquid nitrogen on thermal cracking of shale rocks,ultrasonic and permeability tests were performed on the shale rock samples before and after being soaked in liquid nitrogen. The cracking mechanism of liquid nitrogen and its influencing factors were analyzed. The advantages and prospects for liquid nitrogen to be used as a fracturing fluid in shale gas development were discussed. The experimental results indicate that obvious thermal cracks can be observed on the surface of the shale samples after the soaking in liquid nitrogen. The rapid cooling effect of liquid nitrogen can promote the growth of micro cracks inside the shale samples,resulting in the reduction of sound velocity and increase in permeability. The reduction in sound velocity was up to 10. 43% while the increase in permeability can be over 177. 27%.