基于离散元法的水力压裂数值模拟

不同条件下水压裂隙的发展特性对有效开采页岩气具有重要的指导作用。针对岩体在微观上为颗粒和孔隙的结构系统,提出离散元水力压裂数值模拟方法,离散元能量转化和能量守恒计算方法,建立了相应的三维离散元模型。采用自主研发的三维离散元模拟软件Mat DEM3D,通过控制模型的竖向应变与颗粒直径,来模拟地层中的应力与压裂速率的变化。模拟结果表明:(1)水力压裂产生裂隙的数量和方向受岩石的各向异性,压力状态和变化速率所影响。(2)裂隙在压缩波传播时发展,当水压力高速增加时,诱发的裂隙数量增多,并且有效能量(断裂热)百分比也随之增加,压裂作用也变得更明显。(3)当竖向应变为零时,50%的裂隙呈垂直状态,当竖向应变为-1×10-4时,裂隙趋于沿着最大压力方向发展,竖向裂隙的百分率增大。数值模拟和能量分析为定量地研究岩石水力压裂过程提供了一个新的方法。

Numerical Simulation of Hydraulic Fracturing Based on Discrete Element Method

The studies on the characteristics of hydraulic fracturing under different conditions have an important meaning to theeffective shale gas exploration. As rocks are composed of grains and pores at micro scale, the discrete element method is introduced tosimulate hydraulic fracturing processes. The rules of energy calculation and energy conversion in the models are proposed and a 3Ddiscrete element model is built. Furthermore, 3D discrete element numerical simulation software“MatDEM3D”is developed. Thevariations of strata stress and hydraulic speed are simulated by varying the model vertical strain and the particle diameters,respectively. The simulation results indicate: (1) the number and direction of hydraulic fractures are influenced by the anisotropicproperties of rocks, stress state, and hydraulic speed. (2) Fractures are generated when the compressive wave passes. When hydraulicpressure increases at a high rate, the number of induced fractures is much greater. Furthermore, the percentage of effective energy(fracturing energy) is also greater, and as a result, the efficiency of hydraulic fracturing is higher. (3) When the vertical strain is zero,50% of the fractures are vertical. When the vertical strain is -1×10-4, the fractures tend to develop along the direction of the maximumcompressive stress, and the percentages of vertical fractures are greater. The numerical simulation and energy analysis provide a newmethod for researching the hydraulic fracturing processes of rocks.