基于离散裂缝模型的复杂裂缝系统水平井动态分析

针对目前人工压裂裂缝存在倾斜、分支及不对称分布等情况,提出复杂裂缝系统下水平井渗流数值计算方法。首先对裂缝系统进行显式处理,分别建立基岩和复杂裂缝系统的数学模型,利用有限元方法对模型进行求解,并基于离散裂缝模型对裂缝系统进行显式降维处理,以减少数值计算量。与Odeh和Zerzar模型对比验证数值算法的正确性,并对复杂裂缝系统下的水平井进行计算。结果表明:压裂水平井除常见的4种流动形态(不包括外边界),早期还可能存在裂缝内的径向流动;定产量生产时早期裂缝内径向流动不再出现,且外部裂缝产量所占比例增大;水平井定井底流压生产时,裂缝分支增加了储层改造体积,能大幅度提高压裂水平井产量。

Dynamic analysis of horizontal wells with complex fractures based on a discrete-fracture model

In a hydraulically fractured horizontal well, a complex fracture system can be formed, in which inclined and bifurcated fractures can be generated, and their distribution is asymmetric. A numerical method was developed for the dynamic analysis of horizontal wells with complex fractures. Firstly, the mathematic models were established to describe micro-fractures in matrix and the complex fractures hydraulically generated respectively. The models were solved using a finite element method. Then based on a discrete-fracture model (DFM), the dimension of the fracture system was reduced from 3D to 2D, in order to reduce the load in numerical calculation. The accuracy of the new method was verified by comparing the results obtained using the Odeh and Zerzar analytical solution. Finally, a dynamic analysis for the flow in a horizontal well with the complex fractures system was conducted. The results show that radical flows within the fractures, along with the four common flow forms (i.e. line flow in fracture, radical flow in fracture, line flow in formation and radical flow in formation, not including flow from outside the simulated reservoir volume (SRV)), may appear at the early stage of the production. But the early radical flow in fractures will disappear when the well is restricted at a constant production rate, and the contribution to the productivity from the fractures outside the SRV increases. When the bottom hole pressure is kept constant, the productivity of the horizontal well can be greatly enhanced due to the branched fractures that can increase the SRV.