Abstract CO2-based fracturing was widely introduced to stimulate shale oil reservoirs for its multiple advantages. However, fracture height containment may become noticeable as the low-viscosity CO2 used in the shale with massive mechanically weak bedding planes (BPs). To address this issue, a novel CO2-gel fracturing design is introduced in this study. The design consists of initially pumping gel to break through the BPs at the near-wellbore zone and then injecting supercritical CO2 (Sc-CO2) to further extend these gel-induced hydraulic fractures (HFs) and to reactivate far-wellbore BPs. In this paper, the design feasibility was demonstrated through laboratory multi-stage fracturing experiment results, and their comparisons to single-phase Sc-CO2, slickwater, and gel fracturing results. Computerized tomography scanning and acoustic emission monitoring were employed to reveal the fracture geometry and the tension, shear and compression failure mechanisms. Fracturing with low-viscosity Sc-CO2 and slickwater encountered the problem of fracture height containment caused by high infiltration ability of these two fluids, which would lead to opening of wellbore-connected BPs dominated by tensile failure. High-viscosity gel fracturing can create multiple transverse HFs but with a few fractures on BPs. When CO2-gel fracturing fluid was used, the fracture height is enhanced and more far-wellbore BPs are activated mostly in slip. The isolation effect of gel on BPs can lower the leak-off amount of Sc-CO2. Consequently, CO2-gel fracturing is relatively far to the leak-off dominated regime, which means more elastic energy of high-compressibility Sc-CO2 could be attained for fracturing the reservoir rocks. CO2-gel fracturing can provide a useful alternative in improving the vertical fracture height during the fracturing treatment.

中文翻译：

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超临界CO2-凝胶压裂增产页岩油可行性试验研究

摘要 CO2 基压裂技术以其多重优势被广泛应用于页岩油藏改造。然而，由于在具有大量机械弱层理面 (BPs) 的页岩中使用低粘度 CO2，裂缝高度控制可能会变得明显。为了解决这个问题，本研究引入了一种新型的 CO2 凝胶压裂设计。该设计包括首先泵送凝胶以突破近井眼区域的 BP，然后注入超临界 CO2 (Sc-CO2) 以进一步扩展这些凝胶诱导的水力压裂 (HF) 并重新激活远井眼 BP。在本文中，通过实验室多级压裂实验结果及其与单相 Sc-CO2、滑溜水和凝胶压裂结果的比较，证明了设计可行性。采用计算机断层扫描和声发射监测来揭示裂缝几何形状和拉伸、剪切和压缩破坏机制。低黏度Sc-CO2和滑溜水压裂遇到两种流体的高渗透能力导致的裂缝高度围堵问题，这将导致以拉伸破坏为主的井筒BPs打开。高粘度凝胶压裂可以产生多个横向 HF，但在 BP 上有一些裂缝。当使用 CO2 凝胶压裂液时，裂缝高度增加，更多的远井 BP 主要在滑移中被激活。凝胶对BPs的隔离作用可以降低Sc-CO2的泄漏量。因此，CO2-凝胶压裂相对于渗漏为主的区域，这意味着压裂储层岩石可以获得更多的高压缩性Sc-CO2的弹性能量。CO2-凝胶压裂可以提供一种有用的替代方法，可以在压裂处理过程中提高垂直裂缝高度。