基于BEM的水力裂缝起裂与扩展数值模拟

水力压裂是对低渗透油气田进行储层改造的重要技术,现已在国内外得到广泛应用.但井壁上原有的裂缝会造成水力裂缝在近井筒区域发生转向,诱发提前脱砂、砂堵等现象,甚至导致压裂作业失败.本文以边界元方法、流体力学为理论依据,考虑了近井筒处裂缝的转向行为,建立了含裂缝的井壁起裂及水力裂缝扩展的流固耦合模型.研究发现:当压裂液黏度较小时,裂缝迅速转向至垂直于最小水平主应力的方向;随着黏度的逐渐增大,井底注入压力不断升高,裂缝转向半径亦逐渐增大;压裂液排量对井壁裂缝延伸的影响和黏度对其的影响相似;井壁上原有裂缝的长度对井壁破裂压力有着显著的影响,其长度越长,井壁破裂压力越小;随着水平主应力差值的增大,裂缝的延伸路径会发生明显的转变,所需的延伸压力不断增大;井壁原有裂缝与最大水平主应力方向间的夹角越小,所需的破裂压力及延伸压力也越小.

Numerical simulation of hydraulic fracture initiation and propagation based on BEM

Hydraulic fracturing is an important technology for reservoir reconstruction in low permeability oil and gas fields, which has been widely used at home and abroad. However, the initial fracture on the borehole results in the hydraulic fracture deflection in the area near the borehole, which causes the sand screen-out in advance, sand plugging and even failure of fracturing process. Based on the theory of boundary element method and fluid mechanics, a fluid-solid coupling model, which takes into account the deflection behavior of fractures near wellbore, to simulate the initiation and propagation of hydraulic fracture from the borehole with a pre-existing crack has been established. It is found that when the viscosity of fracturing fluid is small, the fracture rapidly turns to the direction perpendicular to the minimum horizontal principal stress. With the increase of viscosity, the injection pressure at the well bottom increases and the deflection velocity of hydraulic fracture becomes slow. And the effects of fracturing fluid injection rate on the fracture propagation are similar to that of viscosity. The length of the pre-existing crack on the borehole dramatically affects the breakdown pressure. The breakdown pressure will decrease sharply with the increase of crack length. With the decrease of the angle between the pre-existing crack and the direction of the maximum horizontal principal stress, the breakdown pressure and the extension pressure will also decrease. When the difference of horizontal in-situ stresses is small, the deviation between the direction of pre-existing crack and maximum horizontal principal stress has little effect on the propagation of hydraulic fracture. However, when the difference of horizontal in-situ stress is large, the direction deviation will easily lead to a sharp deflection of hydraulic fracture, which not only increases the extension pressure and the difficulty of fracturing, but also causes sand plugging at the turning point, and even leads to the failure of fracturing operation. Therefore, it is helpful to find out the direction of the maximum horizontal principal stress before fracturing operation. In the initial stage of fracturing, increasing the length of oriented perforation and adopting smaller injection rate or viscosity can significantly reduce the difficulty of fracturing.