页岩二氧化碳压裂裂缝扩展机制及工艺研究

针对目前不同状态二氧化碳压裂裂缝扩展机制及二氧化碳压裂最优模式研究欠缺的问题，设计并开展了不同状态二氧化碳压裂物理模拟实验，同时进行了相关数值模拟，研究了不同状态二氧化碳对裂缝起裂及扩展的影响，对比分析了不同工艺下的压裂效果，并对二氧化碳压裂相关工艺的参数进行了优化。研究结果表明，二氧化碳作为压裂介质可降低破裂压力，提高裂缝复杂度；破裂压力由小到大排序为：超临界二氧化碳 ＜液态二氧化碳 ＜二氧化碳泡沫滑溜水 ＜滑溜水；二氧化碳复合压裂方式有利于增产、稳产，前置液 ＋后半程伴注液态二氧化碳／二氧化碳泡沫复合压裂工艺有助于提高改造效果；二氧化碳复合压裂工艺中起泡基液黏度应控制在3~6 mPa·s，泡沫质量应大于 75％，施工后半程泵入泡沫段塞更有利于提高改造体积。

Research on Fracture Propagation Mechanism and Technology of Carbon Dioxide Fracturing in Shale Gas

In view of the lack of research on the optimal model of carbon dioxide fracturing at present, physical simulation experiments of CO₂ fracturing in different states were designed and carried out, and related numerical simulations were carried out at the same time to study the influence of CO₂ fracturing in different states on fracture initiation and propagation and the fracturing effect under different technologies. The results show that CO₂ as a fracturing medium can reduce fracture initiation pressure and increase fracture complexity. The sequence of fracture initiation pressure is: supercritical CO₂ < liquid CO₂ < CO₂ foam slippery water < slippery water. Composite CO₂ fracturing method is conducive to increasing production and stabilizing production. Pad fluid followed by injection of liquid CO₂ /CO₂ foam composite fracturing process can enhance production. The base fluid viscosity should be controlled at 3～6 mPa· s for foaming process, and the foam quality should be greater than 75% ;it is more advantageous to pump the foam slug after second half of fracturing.