Continuum-discontinuum analysis of permeability enhancement of shale with Supercritical CO2 fracturing

中国和德国的页岩气开发预计潜力巨大。然而，传统的水基压裂处理对于许多页岩气储层可能效果不佳。超临界CO2（SC-CO2）压裂技术可以克服水力压裂的一系列缺点，具有巨大的工业化前景。在本研究中，首先，通过对泥页岩样品的CT扫描来表征其细观结构。通过对圆柱形页岩样品进行超临界CO2压裂试验，研究了预存裂纹对压裂过程的影响。其次，基于SC-CO2状态方程，考虑流体流动与页岩变形损伤的耦合作用，建立了SC-CO2压裂过程的流固耦合模型。第三，将中国团队开发的基于连续介质力学的流固模型与德国团队开发的基于非连续介质力学的模型相结合，形成了一种新的适用于整个压裂过程的连续-非连续介质模型。新模型的实验结果进行了验证。最后，以实际工程为背景，建立了水平威尔斯井多级压裂的力学模型，数值分析了天然裂缝分布密度、天然裂缝发育程度、裂缝面力学参数和压裂液注入量对压裂裂缝及压裂液增渗效果的影响。多裂缝相互作用和SC-CO2压裂引起的体积渗透率提高分析可为压裂工艺设计提供理论支持。

The shale gas development in China and Germany is expected to have great potential. However, conventional water-based fracturing treatments may not work well for many shale gas reservoirs. The supercritical CO2 (SC-CO2) fracturing technology can overcome a series of shortcomings of hydraulic fracturing and has great industrial prospects. In this study, first of all, CT scanning of shale specimen is employed to characterize the meso-structure. Supercritical CO2 fracturing tests on cylindrical shale samples are conducted to investigate the effect of pre-existing crack on fracturing process. Secondly, based on the equation-of-state of SC-CO2 and the coupling effect among fluid flow and shale deformation and damage, the coupled flow-solid model for SC-CO2 fracturing process is established. Thirdly, we form a new continuum-discontinuum model for the whole fracturing process by combining the continuum mechanics based fluid-solid model developed by the Chinese team with the discontinuum mechanics based model developed by the German team. The new model is validated against experimental results. Finally, based on a practical engineering background, the mechanical model for multi-stage fracturing in horizontal wells is built, and the effects of the distribution density of natural fractures, degree of natural fracture development, mechanical parameters of fracture surface, and fracturing fluid injection rate on stimulated fractures and resultant permeability enhancement are analyzed numerically. The analysis of multiple-fractures interactions and the volumetric permeability enhancement induced by SC-CO2 fracturing may provide theoretical support for the design of fracturing process.