页岩脆性的综合评价方法——以四川盆地W区下志留统龙马溪组为例

页岩储层的脆性特性对页岩气的开发效果具有重要影响，脆性页岩有利于天然裂缝的发育和压裂后形成具有一定导流能力的网状复杂裂缝，从而提高页岩产气量。为了探寻页岩脆性特征，弥补单一学科技术评价页岩脆性存在的不足，提出了一种利用X射线衍射、计算弹性参数和室内岩石力学测试3种技术来综合评价页岩脆性的方法，即通过分析页岩矿物组分含量、泊松比、杨氏模量、横纵波比参数和岩石应力、应变性质，对页岩储层的脆性特征进行综合评价。将该方法应用于四川盆地W区下志留统龙马溪组黑色页岩的脆性评价，结果表明：该区龙马溪组二段脆性好于龙马溪组一段，龙马溪组二段在1 535~1 541 m层段时，具有高的石英含量，较低的泊松比、横纵波比和较高的杨氏模量值；深度在1 541 m时岩样的应力、应变曲线负坡较陡，页岩储层的脆性指数大于50%，有利于对页岩储层进行压裂改造。实践证明：该方法具有较高的有效性，与实际压裂测试结果吻合较好，可作为压裂选层的重要依据。

A comprehensive evaluation methodology of shale brittleness:A case study from the Lower Silurian Longmaxi Fm in Block W,Sichuan Basin

The brittleness of shale formation has an important impact on shale gas extraction. Brittle shale is favorable for the formation of natural fractures and the creation of complex fracture networks with good conductivity through fracturing, thus enhancing shale gas productivity. In order to understand shale brittleness, and to overcome the disadvantages of brittleness evaluation with a single technique, we proposed a comprehensive evaluation method that integrates X ray diffraction, calculated elastic parameters and laboratory rock mechanics tests, i.e. comprehensively evaluating shale brittleness based on the analysis of mineral components, Poisson′s ratio, Young′s modulus and S P ratio as well as rock stress and strain characteristics. This comprehensive evaluation method was applied to brittleness evaluation of the black shale in the Lower Silurian Longmaxi Fm in Block W, Sichuan Basin. The following results were obtained. The brittleness of the second member of Longmaxi Fm is better than that of the first member. The shale in the interval of 1535 – 1541 m has a high content of quartz, a low Poisson′s ratio, a low S/P ratio, and a high Young′s modulus. The shale sampled at a burial depth of 1541 m has a relatively steep negative slope in its stress strain curve, and its brittleness index is greater than 50%, being favorable for fracturing stimulation. Practices show that this method is highly effective and the evaluation results are in good consistence with the real fracturing test results, thus shale brittleness can be used as one of the major bases for fracturing optimization.