Fracture Propagation Patterns of Pre-CO2 Fracturing in Shale Oil Reservoirs
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摘要: CO2作为压裂液可提高储层改造效果，并实现碳封存。前置CO2压裂技术兼具CO2压裂和水基压裂的优势，是提升页岩储层压裂效果的重要途径。建立了热-流-固-损伤（THMD）耦合的前置CO2压裂裂缝扩展模型，通过数值模拟，分析前置CO2压裂过程中应力、温度、流体压力及损伤的演化规律，探究CO2注入方式、注入量和排量对裂缝扩展的影响。结果表明，热应力与流体压力共同驱动裂缝扩展，其中热应力的影响主要局限于井筒附近约5 m范围内，但其作用不可忽略。先注入CO2后注入滑溜水的注入模式更有利于促进裂缝扩展。CO2注入量与储层改造效果呈非单调关系，过量注入CO2可能抑制损伤演化。当CO2注入量为500 m3时，改造效果最优，损伤面积为 629.91 m2，较常规水力压裂增加41.82 m2。此外，随CO2排量增大，损伤面积和压裂效率均有所增加。研究结果可为页岩储层前置CO2压裂参数优化及CO2高效利用与地质封存提供理论依据。Abstract: Using CO2 as a fracturing fluid can enhance reservoir stimulation effect and achieve carbon sequestration. The CO2 pre-pad fracturing technology combines the advantages of both CO2 fracturing and water-based fracturing, serving as an important approach to improving fracturing performance in shale reservoirs. In this study, a thermal-hydro-mechanical-damage (THMD) coupled fracture propagation model was established for CO2 pre-pad fracturing. Through numerical simulations, the evolution patterns of stress, temperature, fluid pressure, and damage during the CO2 pre-pad fracturing process were analyzed, and the effects of CO2 injection mode, injection volume, and injection rate on fracture propagation were investigated. The results indicated that fracture propagation was jointly driven by thermal stress and fluid pressure. The influence of thermal stress was primarily confined to a range of approximately 5 m near the wellbore, but its effect could not be ignored. The injection mode of injecting CO2 first followed by slickwater proved more conducive to promoting fracture propagation. A non-monotonic relationship was observed between the CO2 injection volume and the reservoir stimulation effect, and excessive CO2 injection might inhibit damage evolution. When the CO2 injection volume was 500 m3, the stimulation effect was optimal with a damage area of 629.91 m2, which was 41.82 m2 larger than that of conventional hydraulic fracturing. Furthermore, as the CO2 injection rate increased, both the damage area and fracturing efficiency increased. These findings provided a theoretical basis for optimizing CO2 pre-pad fracturing parameters in shale reservoirs and the efficient utilization and geological sequestration of CO2.
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表 1 页岩前置CO2压裂数值模型参数
岩石属性 页岩基质 天然裂缝 ρ/（kg·m−3） 2600 2200 杨氏模量/GPa 40 10 泊松比/无量纲 0.26 0.2 抗拉强度/MPa 10 2.5 抗压强度/MPa 210 100 孔隙度/% 10 7 渗透率/mD 0.01 0.5 内摩擦角/(°) 30 15 热膨胀系数/(K−1) 2.5×10−6 4×10−6 比热容/（J·(kg·K)−1） 1000 800 导热系数/（W·(m·K)−1） 3 4 
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