页岩巴西劈裂试验的能量分布与临界特征分析

为了清晰了解具有脆性破坏特征的页岩在拉伸破坏下的能量统计分布特征,研究整体破坏能量分布与局部应力下的能量统计分布规律,从而为现场压裂机制提供理论依据。选取四川盆地南部地区龙马溪组钙质页岩进行巴西劈裂(间接拉伸)的声发射试验,加载速率分别为0.005,0.05,0.5 mm/min。结果表明:(1)在各个加载速率下岩石破坏过程的能量概率密度函数符合幂定律分布,并遵循时间上的幂律无尺度分布。(2)局部应力下的能量概率密度函数也符合幂定律分布,与幂值分布图变化规律形成良好对应,各局部应力阶段下的能量概率分布曲线向着整体破坏的能量概率分布曲线流动。(3)当应力σ达到0.9σmax,系统进入临界态,能量分布曲线与幂值分布曲线接近于整体破坏过程中的能量分布曲线与幂值分布曲线,并逐渐重合。(4)试验结果与纤维束模型数值模拟结果相符,当应力σ达到试件破坏应力的90%后,分叉率ξ急剧增大,直至岩石破坏,能良好验证临界态特性。

Energy distribution and criticality characteristics analysis of shale Brazilian splitting test

In order to clearly understand the energy statistical distribution of brittle shale during tensile test，a research has been conducted to obtain the distribution law of the destruction energy and energy statistics in the local stress,thus providing a theoretical foundation for the in-situ fracturing mechanism.This paper selects the calcareous shale from the Longmaxi Formation of the Southern Sichuan Basin to carry out Brazilian splitting (indirect tensile) test detected by the acoustic emission probe (AE).There are three settled loading rates which are 0.005,0.05 and 0.5 mm/min respectively.The results are as follows：① The probability density function of energy conforms to the power-law distribution under different loading rates, that is and it follows the time scale-free distribution function as power-law form.② The probability density function of energy in the local stress also conforms to the power-law distribution，which have a good fit of this distribution to the power-law distribution.The energy probability distribution curves under different stress stages flow towards overall energy probability distribution.③ The system comes into a critical state when the applied stress reaches about the 0.9σmax.The energy distribution curve and exponent distribution curve are close to the energy distribution curve and exponent value distribution curve at this stage during the whole loading process,and a gradual overlap takes place.④ The test results are in conformity with the numerical simulation results of fiber bundle model.When the applied stress σ reaches to 90% of the specimen failure stress,the branching ratio ξ increases sharply until the rock failures.The results above can well validate the criticality characteristics.