基于能量释放率理论的垂直裂缝穿层机理及扩展特征

The thin and interbedded reservoir usually has multiple thin layers, which have many complicated conditions, such as rock mechanics parameters difference, geo-stress field difference and the interface effect. So, the current researches of vertical fracture propagation in multiple layers have many limitations. Therefore, developing the numerical simulation method of hydraulic fracture penetrating the layered formation and establishing a mathematical-physical model for simulation the propagation of hydraulic fracture in layered formation are the key to resolve the problem. Aimed at this, this project plans to research on penetration mechanism and propagation features of vertical fracture using methods of experimental analysis, theoretical calculation and numerical simulation based on the theory of energy release rate. Using linear spring model to simulate the interface effect, this project will calculate the energy release rate of vertical fracture penetration/deflection at the interfaces using virtual crack closure technique(VCCT). The fracture propagation path predicted by this method can be compared with the experimental data. Then, the test of three-point bending will be conducted to verify the prediction results, which forms a prediction method of vertical fracture penetration/deflection at the interfaces. On this basis, a mathematic-physical model to calculate fracure geometric parameters will be established considering interfacial effects and fracturing fluid leakoff. This research will be helpful in developing and perfecting propagation theory of hydraulic fracture in layered formation.

薄互储层具有层多且薄的特征，各层间存在岩石力学差异、地应力差异及层间界面效应等复杂情况，使得现有垂直裂缝的层间扩展研究存在很大的局限性。形成垂直裂缝在层间界面扩展路径选择的判断方法，建立垂直裂缝在层状地层扩展的数学物理模型，是解决这一问题的关键。本项目拟基于能量释放率理论，采用实验分析、理论计算与数值模拟相结合的方法，系统研究垂直裂缝的穿层机理及扩展特征。以线弹簧模型模拟层间界面效应，利用虚拟裂纹闭合法（VCCT）计算垂直裂缝穿过界面及沿界面扩展的能量释放率，结合实验获取的岩石及界面临界断裂能量释放率，判断裂缝扩展路径，通过三点弯曲试验进行验证，形成垂直裂缝界面扩展路径选择的数值模拟方法；考虑界面效应及压裂液滤失对缝内压力分布的影响，建立垂直裂缝在层状地层扩展的数学物理模型，计算其层间几何参数，分析各因素对裂缝扩展的影响。该研究有助于进一步发展和完善水力裂缝层间扩展理论。

薄互储层具有层多且薄的特征，各层间存在岩石力学差异、地应力差异及层间界面效应等复杂情况，使得现有垂直裂缝的层间扩展研究存在很大的局限性。本研究通过模拟层状岩石试件及页岩试件本体力学参数及界面力学参数的测试实验，形成了相关实验测试方法；采用三点弯曲试验测试了不同的天然页岩岩心的力学参数，得到了不同的裂缝扩展路径；建立了垂直裂缝扩展路径选择的有限元数值模型，基于三点弯曲试验与数字图像法开展了裂缝扩展路径选择的物理模拟实验，验证了数值模型的准确性，形成了基于能量释放率理论的裂缝扩展路径选择数值模拟方法；建立了垂直裂缝穿层的数学物理模型，形成了层间裂缝扩展几何参数计算方法，编制了弱界面层间裂缝扩展几何参数计算软件。该研究成果有助于进一步发展和完善水力裂缝层间扩展理论。