深层页岩储层CO₂复合压裂裂缝扩展机理研究

Study on Fracture Propagation Mechanism of CO₂ Hybrid Fracturing in Deep Shale Reservoirs

摘要: 深层页岩储层地应力水平高，裂缝扩展受地应力与岩体非均质性共同影响。在CO₂前置压裂与后续水力压裂过程中，前置裂缝的形成及其对后续流体通道的改变，使裂缝扩展同时受到流体性质、注入条件和已有裂缝分布的影响。为了明确前置CO₂造缝与后续水力扩展之间的关系，本文基于有限–离散元方法建立二维热流固耦合模型，通过重启动开展两阶段连续计算，研究固定总注入量下不同排量及水平地应力差条件下的裂缝扩展规律。结果表明，CO₂压裂较水压裂形成更多近井及主裂缝附近的分支，后续注水沿前置主缝和部分支缝继续扩展；提高CO₂排量使终态裂缝长度与开度增加，其中开度变化更为明显；较低地应力差有利于多方向分支发育，较高地应力差下优势主缝延伸更加突出，前置分支仍对后续扩展产生影响。研究结果可为深层页岩储层CO₂前置注入与后续水力压裂的参数选择提供依据。

Abstract: High in-situ stress and rock heterogeneity jointly affect fracture propagation in deep shale reservoirs. During CO₂ prepad and subsequent hydraulic fracturing, the fractures created in the prepad stage alter the flow paths for subsequent injection, linking fracture growth to fluid properties, injection conditions and pre-existing fracture geometry. To clarify the connection between the two stages, a two-dimensional thermal-hydraulic-mechanical model is established using the finite-discrete element method. A restart procedure enables continuous simulation of CO₂ prepad and subsequent water injection. Fracture propagation is examined at different injection rates under a fixed total injection volume and at different horizontal stress differences. The results show that CO₂ fracturing produces more branches near the injection point and main fractures than water fracturing. Subsequent water injection extends the prepad main fractures and some branches. Increasing CO₂ injection rate increases final fracture length and aperture, with a more pronounced change in aperture. Lower stress differences favor multidirectional branching, whereas higher differences produce more distinct main-fracture paths while retaining the influence of prepad branches. The findings provide a basis for selecting injection parameters for CO₂ prepad and subsequent hydraulic fracturing in deep shale reservoirs.