基于不同断裂准则下水力压裂裂纹起裂规律研究

Research on the crack initiation patterns of hydraulic fracturing cracks under different fracture criteria

为研究煤矿顶板定向水力压裂裂纹扩展机理,探究不同断裂准则之间的关系,基于断裂力学理论构建钻孔水力压裂力学模型,推导不同断裂准则下的起裂角和临界水压计算公式,分析不同变量对起裂角和临界水压的影响效果。结果表明:①注入水压的增大会显著降低起裂角,且预制裂纹倾角与起裂角曲线极值会随着水压的增大而向45°附近移动;随着应力差的增加,起裂角逐渐增加;起裂角随预制裂纹长度与钻孔半径比值的减小而增大;泊松比对起裂角的影响效果有限。②临界水压随着预制裂纹倾角和主应力差的增大而增大,但在低预制裂纹倾角条件下,σ准则计算出的临界水压会随着主应力差的增大而减小;钻孔半径和预制裂纹长度的增加会显著减小临界水压。③对比临界水压的理论计算结果与试验结果知,二者有较好的拟合性。研究结果可指导水力压裂现场施工。

This study aims to investigate the crack propagation mechanism of directional hydraulic fracturing of coal mine roof and explore the relationship between fracture criteria.By drawing on fracture mechanics,we propose a hydraulic fracturing mechanical model of drilling and derive the calculation formulas of crack initiation angle and critical water pressure under different fracture criteria to analyze the impact of different variables on crack initiation angle and critical water pressure.Results show that:①the increase of stress difference leads to a gradual increase of the crack initiation angle;②the effect of prefab crack length and borehole radius on crack initiation angle is caused by two factors.The crack initiation angle increases with the decrease in the ratio of these two factors;③Poissons ratio has limited effect on crack initiation angle;④under high stress difference and low prefab crack inclination angle,the critical water pressure decreases with the increase of stress difference and inclination angle,and vice versa;⑤the increase of borehole radius and prefabricated crack length would significantly reduce the critical water pressure.The two criteria yield different calculation results as①prefab crack length and borehole radius would impact crack initiation angle;②principal stress difference and prefab crack angle would impact critical water pressure.The theoretical calculation results and the experimental results show a good fit.The present study offers reference to the hydraulic fracturing site construction.