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基于微米力学实验的页岩I型断裂韧度表征

STUDY ON FRACTURE TOUGHNESS OF MODE I OF SHALE BASED ON MICRO-MECHANICAL TEST

  • 摘要: 页岩断裂韧度(K_IC)是页岩气储层水力压裂设计的基础参数之一,由于组成的非均质性,常规宏观力学测量方法存在制样困难、力学解释参数不连续、精度偏低等问题. 如何及时获取页岩的断裂特性,确保安全高效的工程施工,是当前面临的一大问题. 因此,提出了基于微米力学实验的页岩Ⅰ型断裂韧度分析方法,可用于页岩微裂纹起裂、发育直至形成宏观裂纹的机理研究,进行页岩宏观Ⅰ型断裂韧度预测. 基于页岩多尺度组成分析,开展了维氏压头和玻氏压头的页岩微米力学实验,分析了页岩残余压痕与压头间的相似关系、有效测试载荷以及压头参数的优化与选择. 分析了不同压入载荷下的页岩细观断裂韧度分布特征,开展了宏观巴西圆盘实验,验证页岩微米力学测试方法的适用性. 研究结果表明,在有效载荷范围内的页岩细观Ⅰ型断裂韧度波动性较小,当压入载荷过大时,由于岩样压痕区域出现局部剥落导致断裂韧度测量值偏小. 与宏观实验的比对分析显示,微米力学实验的K_IC平均值为0.86 MPa\cdot \sqrtm,直槽切缝巴西圆盘实验得到的K_IC平均值为0.92 MPa\cdot \sqrtm,两类方法的统计平均值较为接近,页岩局部组成的非均质性使得微米力学测量结果较宏观测试更为分散. 研究结果可用于页岩宏观Ⅰ型断裂韧度预测,为有效解决页岩气储层水力压裂参数评价提供新的思路和方法.

     

    Abstract: Fracture toughness of mode I (K_IC) is one of the important mechanical parameters for hydraulic fracturing of shale gas reservoir. Due to the heterogeneity of shale composition, the conventional mechanical measurement has some problems such as large sample volume, discontinuous mechanical interpretation parameters, and low interpretation accuracy. One of the challenges is to obtain the fracture characteristics of shale in time to ensure the safety and efficiency of engineering construction. In this paper, research on fracture toughness of mode I of shale is performed based on micro-indentation. It can be used to study the mechanism of shale micro-crack initiation, development and formation of macro-crack, and to predict the macro-parameters of shale. Based on the analysis of multi-scale composition of shale, the fracture toughness tests with pyramid indenter (Vickers indenter and Berkovich indenter) were performed by micro-indentation. The relationship between residual indentation and indenter was evaluated, and the effect of experimental load on shale micro-fracture was analyzed. The optimization of indenter parameter also was discussed. The fracture toughness of shale is evaluated at meso-scale. The applicability of the micro-indentation test was evaluated, based on a comparative analysis with the results of the Brazil disc test. The results show that t the fluctuation of fracture toughness obtained by micro-indentation is slight when load is within the effective range. When load is too large, the fracture toughness of meso-scale is gradually reduced due to local drop-cuts on the indentation area. The average value of K_IC obtained by micro-indentation is 0.86 MPa\cdot \sqrtm,and the average value obtained by Brazilian disc test is 0.92 MPa\cdot\sqrtm. The heterogeneity of shale composition results in more dispersed meso-mechancial measurement than macroscopic measurement. Micro-indentation test can be used to characterize shale fracture toughness of mode I and perform macroscopic prediction. It provides a new method for effectively solving shale gas hydraulic fracturing.

     

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