CO2破岩机理及压裂工艺技术研究

为了明确不同介质注入过程中有效应力的变化规律，揭示超临界CO₂压裂的起裂压力低、穿透距离远、裂缝密度广的力学机理，基于线弹性多孔介质模型，线性分解井筒平面各向应力，引入井筒增压速率，对孔隙压力与附加周向应力进行修正。结合长庆气田致密气特征，集CO₂破岩增压与滑溜水体积压裂双重优势，改进气藏地质储量容积差值法，优化CO₂注入量，根据井下压力计监测数据分析动态滤失平衡点，优化CO₂施工排量，研发防冻隔离液，开发单机组作业流程，攻关形成前置CO₂蓄能压裂技术。计算结果表明：液态CO₂压裂的起裂压力降低了69.2%，超临界CO₂压裂的起裂压力降低了75.5%。在鄂尔多斯盆地东部开展先导性试验6口井，一次喷通率100%，平均试气产量7.59万m³/d，为长庆气田探索出了新的技术增产途径。

Rock breaking mechanism of CO2 and fracturing technology

In order to make clear the change rule of effective stress during the injection processes with different media and reveal the mechanical mechanism of low break down pressure, long penetrate distance and high fracture distribution density during super-critical CO₂ fracturing, based on linear elastic porous medium model and linear decomposition of anisotropic stress on wellbore plane, pressurization rate was introduced to carry out correction of pore pressure and additional circumferential stress. In view of the characteristics of tight gas reservoir in Changqing gas field, with the dual advantages of CO₂ rock breaking, pressurizing and slick water volume fracturing, geological reserve volume difference method of gas reservoir was improved to optimizing the CO₂ injection quantity. According to the monitoring data of down hole pressure gauge, the dynamic filtration loss equilibrium point was analyzed, and the CO₂ construction displacement was obtained. Thanks to anti-freeze isolating liquid, a single fracturing unit operation process was formed. Finally, CO₂ volume fracturing technology was formed. The result shows that fracture initiation pressure dropped by 69.2% in the case of liquid CO₂ and 75.5% in the case of super-critical CO₂. Pilot tests were carried out on six wells in eastern Ordos Basin, 100% cleanup with no nitrogen lift, and the average single layer initial gas rate of subject wells is 75 900 m³/d, which indicates that CO₂ fracturing techniques is expected to be a new stimulation method for Changqing gas field.