基于位移不连续方法的页岩气藏压裂缝网扩展全三维数值模拟方法研究

The existence of multi-scale weak planes, such as beddings and natural fractures, is one of the main causes of the formation of complex fracture networks. It is of critical importance to figure out the interactions between hydraulic fracture and natural discontinuities to guide the design and optimization of hydraulic fracturing. Currently, two dimensional numerical models have been well developed. However, few numerical investigations have been conducted about the three dimensional hydraulic fracturing process, resulting in the lack of knowledge about the roles of the naturally weak planes’ height, inclination, and the distribution of proppant within the fracture network. In this project, the displacement discontinuity method based on triangular elements will be used to calculate the deformation of fracture surfaces with high accuracy while effectively capture the complex geometry of the three dimensional fractures. A novel grid optimization strategy is proposed to obtain high quality grids for numerical calculation. To model the two phase solid-fluid flow within fractures, an coupling method which iteratively calculate the fluid flow with finite volume method and the proppant transport with smoothed particle hydrodynamics is constructed. Combing the criteria for fracture initiation and propagation, activation of natural fractures, and fracture interaction between artificial and natural discontinuities, and the fracture propagation and fluid transportation models described above enables us to model the complex fracture propagation in three dimension. The fracturing model will be validated against the true tri-axial fracturing experimental data. The goal of this project is to establish an effective three dimensional hydraulic fracturing model so as to promote the development of fracking simulation technologies, provide a fundamental basis for the high-efficiency fracturing treatment and enhancement of shale gas production.

页岩储层中层理、天然裂缝等多尺度弱面的广泛分布是诱导压裂缝网形成的主要原因之一，厘清人工裂缝与天然弱面的作用方式是水力压裂设计与优化的关键。目前二维压裂模型已较为完善，但对三维缝网形态的研究仍相对缺乏，支撑剂在三维缝网中的铺置情况也未明确。项目利用基于三角形单元的位移不连续方法和新型网格优化策略，实现裂缝力学变形的高精度计算与复杂裂缝三维形态的刻画；建立欧拉-拉格朗日混合坐标系下的新型固液两相运移模型，实现缝内（尤其是相交处）压裂液非达西流动与支撑剂运移的精细描述；结合裂缝起裂、扩展、天然弱面激活与裂缝交叉准则，耦合裂缝扩展与流-固运移模型，实现对页岩气藏三维压裂缝网形成过程的模拟；采用真三轴压裂实验结果，对所建模型实施验证；形成有效的三维缝网模拟方法，完善压裂模拟理论与方法，为页岩气高效压裂与产能提升提供理论基础与模拟手段。

本项目以非常规油气资源的高效开发为背景，以提高压裂后储层中人工裂缝系统形态预测准确性及压裂改造效果为目标，重点关注水力压裂全三维压裂缝网演化过程及其数值模拟方法，针对位移不连续方法模拟全三维压裂缝网时的相交准则及网格、算法难题、矿场尺度缝内支撑剂运移模拟方法等开展技术攻关，并应用本项目压裂缝网预测模型进一步研究其他非常规储层（如玛湖砾岩储层、致密气储层等）中压裂缝多尺度形态及模拟方法以及实际井区压裂井距、段、簇自动组合优化算法等问题。项目研究成果表明，全三维压裂缝扩展形态较低维度模型预测结果在裂缝扩展速度、缝间干扰强度等方面有显著区别，全三维空间裂缝相交呈现模式更为复杂多样，对于密切割、天然裂缝发育的致密油气藏，有必要采用全三维模拟手段进行缝网形态预测与分析。本项目发展并创新了全三维压裂缝网扩展模型，相关研究成果在四川盆地页岩气及新疆玛湖砾岩油藏压裂施工参数优化等方面开展了现场应用，并取得了良好的应用效果。