加载速率对不同岩性岩石Kaiser效应影响的试验研究

EXPERIMENTAL STUDY OF INFLUENCE OF LOADING RATE ON KAISER EFFECT OF DIFFERENT LITHOLOGICAL ROCKS

当加载速率较快时,砂岩、粗砂岩、泥岩等岩石相对于加载速度较慢的测试,Kaiser点对应的应力值增大,但对于灰岩等脆性岩石,加载速率对其Kaiser效应影响甚微。因此,需要具体分析加载速率对不同岩性岩石Kaiser效应的影响。试验发现,不同加载速率导致声发射累积次数随应力的变化曲线存在显著差异。这是因为当加载速率较慢时,岩心内部较大和较小的裂缝都会发生错动和扩展,致使声发射信号比较明显,得到的Kaiser点对应的应力值较小;而当加载速率较快时,只有较大的裂缝才会发生错动和扩展,Kaiser点对应的应力值相对较大。当深度大于2500m的岩心在声发射测量过程中,Kaiser点往往出现在岩石破坏点之后,因此必须模拟地层条件采用围压下的Kaiser效应进行测量,测得相应地层的地应力大小。针对不同的岩性岩石选取不同的加载速率进行加载研究,试验结果与现场试验结果基本一致。结论为Kaiser效应试验测地应力提供理论依据和借鉴。

Different loading rates on rock samples may lead to considerable differences among the cumulative AE hit-stress curves. Kaiser stress values of sandstone, gritstone and mudstone increase with relatively faster loading rate, while for brittle rock-like limestone, loading rate has hardly any influence on Kaiser effect. Hence, a more in-depth analysis of the influence of loading rate on Kaiser effect of different lithological rock is very necessary. On one hand, all cracks in rock samples, regardless of their sizes, slip and propagate when loading rate is relatively smaller, which leads to a clearer acoustic emission signal. As a result, a smaller stress value is achieved. On the other hand, only larger cracks slip and propagate when loading rate is speeding up, which results in larger stress value. As for different lithological rocks, distinct loading rates are needed during the tests. In addition, the research shows that Kaiser effect point of the core below 2 500 m appears after its collapse point, wnlcn ctemonstrates mat the measurement of Kaiser effect under confining pressure is needed in order to simulate formation situation to achieve corresponding stress value. The results of the test are applied to oil field tests. The experimental results in laboratory test are in largely accordance with those in oilfield. It provides theoretical basis and reference for testing in-situ stress by Kaiser effect laboratory tests