基于XSite的钻孔起裂水力裂缝三维扩展研究
Three-dimensional simulation of hydraulic fracture from a borehole using XSite
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摘要: 为了认识近井筒水力裂缝三维扩展特征,采用基于离散格点理论和合成岩体方法研发的数值模拟软件XSite,分析了井筒方位和地应力对水力裂缝三维扩展的影响。研究结果表明:裸眼井筒起裂条件下,井筒方位变化导致裂缝三维形态产生差异,但整体上裂缝趋向沿垂直于最小主应力的平面扩展。中间主应力的增加抑制了垂直于本方向的裂缝扩展,裂缝在中间主应力方向的扩展轨迹变得更加平直。低应力比(最大主应力与最小主应力之比)条件下,井筒上产生多条裂缝分支,趋向于形成空间缝网。随应力比增加,裂缝分支扩展受到抑制,主裂缝趋于沿最大主应力方向扩展。对于预制切槽压裂情况,裂缝沿切槽方向扩展后会逐渐偏转至垂直于最小主应力的平面,裂缝并未产生多条分支。水力裂缝最终扩展方向不受起裂方式的控制,而是由地应力主导。研究结果可为优化水力压裂设计和施工提供参考。Abstract: To recognize the three-dimensional near-wellbore propagation of hydraulic fracture, XSite, a simulator developed based on the lattice method and the synthetic rock mass, is used to analyze the influences of borehole orientation and in-situ stress on the propagation of hydraulic fracture. The results show that the variation of borehole orientation leads to the difference of fracture geometry, and the fracture tends to propagate along the plane perpendicular to the minimum principal stress. The increase of the intermediate principal stress will restrain the propagation of fracture perpendicular to this direction, and the hydraulic fracture parallel to this direction becomes straighter. Under a low stress ratio (ratio of the maximum to the minimum principal stresses), multiple fracture branches are induced on the borehole and tend to form a spatial fracture network. The increase in stress ratio will inhibit fracture branching. Under the high stress ratio, fractures tend to propagate along the direction of the maximum principal stress. As the fluid injects into the notch, the fracture will gradually deflect to the plane perpendicular to the minimum principal stress after propagating along the direction of the notch. There are no branches of the fracture that can be observed. The initiation mode does not change the dominating propagation direction of the fracture, but the stress field controls the propagation direction of the hydraulic fracture. The research results can provide references for the design and operation of hydraulic fracture.