页岩气储层水力压裂扩展有限元模拟方法及应用

页岩气储层低孔低渗,需用水力压裂等方法进行储层改造方可获得经济产能。储层改造中裂缝的形态和分布对体积改造效果至关重要。为了研究压裂裂缝的模拟方法,系统调研和对比了储层水力压裂模拟常用方法,开展了扩展有限元模拟,研究表明:①水力压裂物理模拟实验能够直观观测裂缝的形态及展布特征,但因试样尺寸等问题难以代表实际储层压裂情形;②常用的数值模拟方法有边界元法、非常规裂缝模型、离散化缝网模型和扩展有限元法等,这些方法各有优缺点,需做有针对性的改进才能更好地模拟真实页岩储层压裂情况;③应用扩展有限元法模拟水力压裂和分段顺序压裂过程中裂缝的延伸情况,得到射孔方向与最大水平主应力之间夹角和诱导应力对压裂裂缝的影响,夹角越大,裂缝偏转角度越小,偏转距离越大,初始破裂压力越高,裂缝稳定延伸的压力也越大,而诱导应力的存在会抑制压裂裂缝的延伸。对实际压裂工程中射孔方向的选择和分段压裂射孔间距的设计具有指导意义。

Hydraulic fracturing simulation technology of shale gas reservoir and application of extended finite element method

Low porosity and low permeability of shale gas reservoirs require hydraulic fracturing and other methods to achieve economic productivity. The shape and distribution of fractures are very important to the volume transformation. In order to study the simulation methods of hydraulic fracture,the common methods of reservoir hydraulic fracture simulation are systematically investigated and compared,and the extended finite element simulation is carried out. The results show that:(1)The physical experiment of hydraulic fracturing can visually observe the fracture morphology and distribution characteristics,but it is difficult to represent the actual fracturing situation of the reservoir due to the sample size and other problems.(2)The commonly used numerical simulation methods include boundary element method(BEM),unconventional fracture model(UFM),discrete fracture network(DFN)and extended finite element method(XFEM). These methods have their own advantages and disadvantages,which need to be improved to better simulate the real shale reservoir fracturing.(3)The extended finite element method is used to simulate the fracture extension of hydraulic fracturing and staged sequential fracturing. The influence of the angle between the perforation direction and the direction of the maximum horizontal principal stress and the induced stress on the fracturing pressure is obtained. The larger the angle is,the smaller the fracture deflection angle is,while the larger the fracture deflection distance is. The larger the angle is,the higher the initial fracturing pressure is,and the greater the pressure of stable fracture extension is. The induced stress will hinder the fracture extension. The simulation results have guiding significance for the selection of perforation direction and the design of perforation spacing in staged fracturing in actual engineering.