利用超声尾波干涉评价页岩应力敏感性

岩石应力敏感性是岩石内部微裂隙发育的一种反映，对于致密储层物性和成缝能力（可压性）评价具有重要作用。由于页岩油气储层基质模量高，对应力的敏感性相对较弱，需要寻求新的高灵敏度测试方法。尾波干涉是运用介质中的散射波来监测介质微弱变化的一种测量方法，对岩石应力变化具有较高的敏感性。借助GCTS RTR-1500高温、高压三轴测试试验系统，对重庆城口县下寒武统鲁家坪组露头页岩开展尾波干涉试验，研究了页岩尾波发育规律及其对应力的响应，分析了页岩在不同围压下的应力敏感性。结果表明：页岩尾波应力敏感性大于直达波，S波应力敏感性大于P波；尾波波速相对变化率随应力的变化能够较好地反映页岩内部裂隙的产生，与扩容点有很好的对应关系；围压使部分裂隙闭合，天然裂隙密度减小，页岩应力敏感性系数降低。尾波干涉可作为页岩的应力敏感性和成缝能力评价的方法。

Using ultrasonic coda wave interferometry to evaluate shale stress sensitivity

Rock stress sensitivity is a reflection of the development of microcracks in the rock, which plays an important role in the evaluation of the physical properties and the fracture forming ability of tight reservoirs. Due to the high modulus of the matrix and relatively weak sensitivity to stress of shale，a new test method of high sensitivity is required. Coda wave interferometry is a measurement method for monitoring weak variation based on scattered wave of media, and it has a high sensitivity to the change of rock stress. In this paper, coda wave interferometry tests are conducted on the Lower Cambrain Lujiaping Formation shale from Chengkou, Chongqing using GCTS RTR-1500 high temperature and pressure triaxial testing system, the development and response to stress of coda are studied, and the stress sensitivity of shale is analyzed under different confining pressures. The results show that the shale stress sensitivity to coda wave is greater than to the direct wave; S-wave stress sensitivity is greater than P-wave. The relative variation ratio of the coda wave velocity varied with the stress can reflect the generating of internal fissures, and it has a good relationship with the corresponding expansion point; confining pressure leads part of the fractures to close and natural fracture density to decrease, which results in a reduction in sensitivity coefficient of shale stress. Coda wave interferometry is available as evaluation of shale stress sensitivity and fracturability.