砂岩钻孔轴向预制裂缝定向压裂试验研究
Experimental study of directional fracturing in sandstones with prefabricated cracks in the axial direction of borehole
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摘要: 针对煤矿坚硬难垮顶板带来的冲击灾害，提出沿钻孔轴向预制裂缝定向水力压裂缩短悬顶长度的降冲技术。选用尺寸为300 mm×300 mm×300 mm的紫砂岩试样，在试件中心打通孔，孔径为25 mm，沿孔壁不同方位预制裂缝长度为10 mm的对称裂缝，共制备试件16块，利用真三轴压裂试验平台与恒流恒压注液泵开展水力定向压裂试验，监测获得注液压力-注液时间的变化曲线，统计压裂裂缝路径形态，并引入裂缝偏转角表征裂缝路径的偏转特征，研究了不同预制裂缝角、水平应力差异系数及注液速率对水力裂缝的起裂、扩展规律的影响，并结合Hubbert-Willis弹性水力压裂模型与Ⅰ-Ⅱ型复合裂纹水力压裂模型分别讨论了试验结果中水力裂缝的起裂机理与扩展机制。试验结果表明:预制裂缝定向压裂均形成单一裂缝，且裂缝形态可归纳为两类:转向裂缝和平直裂缝。起裂压力随预制裂缝角的增大而增大，随着水平应力差异系数的增加而减小，随着注液速率的增加而增大;具有定向作用的预制裂缝角度随水平应力差异系数增大而减小。在相同偏转角下裂缝的扩展长度随预制裂缝角的增大先减小后增加，在45°时为最小;随着水平应力差异系数的增加而减小;随着注液速率的增加而增大。结合试验结果与理论模型讨论可知:预制裂缝具有定向作用时，可用Ⅰ-Ⅱ型复合裂纹水力压裂模型预测起裂压力和裂缝偏转规律;预制裂缝定向作用失效时，适用Hubbert-Willis弹性水力压裂模型求解起裂压力。研究结果为现场预制裂缝定向压裂参数设计提供参照。Abstract: To address the potential disaster caused by the un-stability of hard roof in coal mines,the directional hy- draulic fracturing technology using prefabricated cracks along the borehole axial direction is proposed for reducing the length of suspended roof. Sixteen specimens were prepared using purple sandstone samples with the size of 300 mm× 300 mm×300 mm. In each specimen,the authors prefabricated symmetrical cracks with the diameter of 25 mm and the length of 10 mm along the borehole with different orientations. Hydraulic directional fracturing tests were carried out u- sing a true triaxial fracturing test platform and a constant-flow / constant-pressure injection pump. The variation curve of injection pressure against injection time was recorded and the crack propagation path was monitored. The effect of pre- fabricated crack inclination angles, horizontal stress difference coefficients and injection rates on the initiation and propagation of hydraulic fractures was studied. The initiation and propagation mechanism of hydraulic fracture in test results were discussed by combining Hubbert-Willis elastic hydraulic fracturing model and hydraulic fracturing model of mixed crack type I-II. The test results show that a single fracture is formed,and the fracture morphology can be clas- sified into two types:propagating along the maximum horizontal stress direction or deflecting. The crack initiation pres- sure increases with the increase of prefabricated fracture angle or injection rates,and decreases with the increase of horizontal stress difference coefficient. At the same deflection angle,the crack propagation length decreases first and then increases with the increase of prefabricated crack inclination angle (attaining the smallest values at 45 degrees), decreases with the increase of horizontal stress difference coefficient,and increases with the increase of injection rate. Based on the experimental results and theoretical model analysis,it is concluded that the hydraulic fracturing model of mixed crack type I-II can well predict the initiation pressure and crack deflection law when the prefabricated fracture comes into play,and the Hubbert-Willis elastic hydraulic fracturing model can be used to compute the crack initiation pressure when the prefabricated fracture fails to control the crack propagation direction. The research results provide a reference for the parameter design of the technology of directional fracturing with prefabricated cracks in the field ap- plication.
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