考虑气-固-热耦合下页岩流变特性的重复压裂缝开裂特征研究

Under gas-solid-thermal coupling effects，shale presents the rheological behavior, which affects the stress changes of the gas well adjacent area, thereby affecting the initiation characteristic of refracture. In order to analyze the problem, firstly, laboratory tests are carried out to study interactions between stress field, temperature field, adsorbed and free shale gas, reveal the nonlinear law of compressive strength, yield strength, cohesion, internal friction angle, adsorption capacity, permeability coefficient, etc, and explore the mechanism of the gas-solid-thermal coupling effect on shale rheological properties. On this basis, a nonlinear rheological constitutive model of shale is established. Secondly, combination of different numerical software advantages, based on the established rheological constitutive model, the change of local stress field before refracturing is calculated in shale reservoirs, in which containing undulant shale beds and complex non-symmetrical, non-planar hydraulic fractures. Finally, based on the true triaxial hydraulic fracturing tests, using the mathematical statistics methods of multiple-regression model with variable selection and the theory of fracture mechanics, a theoretical model of refracturing direction angle is established, and to conduct law analysis of shale rheological characteristics initial stress, initial fractures, etc. Research results provide a more scientific and reasonable theoretical guidance

气-固-热耦合作用下页岩呈现的流变特性影响气井区应力场变化，进而影响转向重复压裂缝开裂特征。围绕该问题，首先开展多场耦合室内试验，研究页岩气吸附及游离状态、应力场、温度场之间的相互作用关系，揭示多场耦合作用下页岩抗压强度、屈服强度、内聚力、内摩擦角、吸附量、渗透系数等参数的非线性变化规律，探讨气-固-热耦合作用对页岩流变特征的影响机理，建立页岩的非线性流变本构模型。其次，结合多种数值软件优点集成分析和二次编程开发，基于建立的流变本构模型，计算包含起伏页岩储层和复杂的非对称、非平面水力裂缝的重复压裂前应力场变化。最后，基于真三轴水力压裂物理模拟试验，采用多元回归模型与变量筛选的数理统计方法和断裂力学理论，建立重复压裂转向角理论模型，并开展页岩流变特征、初始地应力、初始裂缝等因素的影响规律分析，为页岩气开采提供更加科学合理的理论指导。

为深入研究气-固-热耦合作用下页岩气储层呈现的蠕变特性对应力场和重复压裂特征的影响，首先，开展多场耦合分析，研究应力场、温度场和页岩气吸附及游离状态之间的相互作用关系。从理论层面推导了渗透系数与孔隙率、体积应变的非线性变化关系，建立页岩的非线性流变本构模型。其次，采用有限差分方法，考虑页岩气各向异性渗流特征，计算因页岩气开采而导致的应力场变化。最后，基于水力压裂物理和数值模拟试验，采用多元回归模型与变量筛选的数理统计方法，建立重复压裂转向角理论模型，并开展初始裂缝和应力场特征的影响规律分析，为页岩气开采提供更加科学合理的理论指导。