体积压裂中三维应力场演化特征的可视化与定量表征研究

The hydraulic fracture presents a typical 3D fracturing behavior in the volume fracturing, and prefers to deflect or branch to form a complicated fracture network due to the effect of in situ stress and pre-existing natural fracture inside the rock. Accurately understanding and characterizing the dynamic evolution of the stress state of the fracturing zone is the key issue to reveal the mechanism of initiation and propagation of hydraulic fracture. For the difficulties on in situ detecting, numerical simulation is the most popular approach used to study the evolution of 3D stress field. However, reasonable simplification is generally required in the numerical simulation, which make it difficult to accurately obtain and quantitatively characterize the evolution of 3D stress field. In this project, a stress visualization method in hydraulic fracturing was developed. We will fabricate the transparent specimens based on 3D printing technique to accurately recognize and record the 3D stress field evolution by means of experimental approaches which combines the frozen-stress technique with the triaxial hydraulic fracturing experiment. The photo-elastic method was improved and utilized to directly reveal and quantitatively characterize the space distribution of the 3D stress field. This proposal provides a new way that we could reveal the mechanism and the influence of stress field on 3D fracture propagation, and establish the criterion for initiation and propagation of hydraulic fracture.

体积压裂中形成的压裂裂缝呈现典型的三维扩展模式，在扩展过程中会受到原岩应力以及岩体内天然裂缝等的影响，容易发生偏转、分叉等现象，形成复杂的裂缝网络。准确认识和表征压裂裂缝扩展区应力场的动态演化规律是准确认识压裂裂缝起裂和扩展规律的核心科学问题。由于原位探测具有很大的困难，现阶段主要依靠数值模拟的方法来研究三维应力场的演化。然而，数值模拟中需要对问题进行简化，难以准确获取和定量表征三维应力场的演化特征。本项目提出一种压裂过程中三维应力场可视化方法，基于3D打印技术制作透明的类岩石模型，将真三轴水力压裂实验和三维应力场“冻结”技术相结合，准确识别和捕捉体积压裂过程中三维应力场的演化过程。发展和利用三维应力场的光弹分析方法，直观展示和定量分析实验中捕捉到的三维应力场的空间展布特征，为揭示三维裂缝扩展机理及应力场的控制作用，建立压裂裂缝起裂与扩展准则提供一个新的思路。

准确获取和定量表征压裂过程中三维应力场的动态演化特征，是揭示复杂压裂缝网形成机理，科学实施低渗透煤岩压裂改造的核心科学问题。借助自主设计研发的真三轴压裂与应力场可视化实验系统，通过精确的真三轴加载和线性的升降温控制，实现不同压裂阶段的三维应力场冻结，发展了三维应力场计算机自动提取算法，定量表征和分析了压裂过程中模型内部三维应力场的空间展布；为了揭示复杂三维缝网形成机理，基于透明模型的特殊性质，发展了复杂裂缝扩展过程中三维应力场透明解析方法，开展了岩石内部三维裂纹扩展过程中全场应力的透明显示与定量解析研究，分析了岩石内部三维裂纹起裂、扩展和桥接过程的应力场控制机理，提出了裂纹起裂、扩展与桥接的判别准则，为揭示压裂过程中三维复杂缝网形成机理提供了理论基础。项目执行期间，在高水平学术期刊上发表SCI论文5篇（JCR Q1区4篇、Q2区1篇）。授权发明专利6件，其中：美国发明专利3件、英国发明专利1件、澳大利亚专利1件、中国发明专利1件。项目负责人获批国家重点实验室开放课题1项、参与科技部十四五国家重点研发计划课题1项。协助指导博士研究生2名；培养硕士研究生7名。