基于室内试验的页岩脆性特征评价方法

Evaluation Method of Shale Brittleness Based on Indoor Experiments

基于全应力应变曲线,提出了包含多参数的综合脆性指标Bd1和Bd2.从扩容拐点(峰前曲线)、残余强度(峰后曲线)、能量分析(全过程曲线)等多角度对脆性指标进行了诠释,通过页岩单三轴实验并结合破坏模式对其适用性进行了验证.结果表明,该指标较好地反应了页岩的脆性,Bd1可根据不同的目的需求选择不同的参数,对脆性评价具有侧重性,且Bd1与围压具有较好的拟合关系,拟合曲线为指数函数;Bd2综合性强,可作为脆性的综合评价指标,但围压较高时对围压不够敏感;90°取芯角度下,随着围压的增加,脆性指数降低,岩石有脆性向延性转化的趋势;低围压下脆性指数对围压比较敏感,降低速度较快,高围压下则相反.

Comprehensive brittleness indexes Bd1 and Bd2including multi-parameters were proposed on the basis of the stress-strain curve. The brittleness characteristics were interpreted through the inflection point of dilatancy（ the pre peak curve）,the residual strength（ the post peak curve）,and energy analysis（ the complete stress-strain curve）. Axial and triaxial compression tests were conducted to examine the newindexes corresponding to failure mode. The results showed that the two evaluation indexes can preferably reflect the brittle characteristics of shale.For Bd1,related parameters can be selected freely according to different purpose,which can pay particular attention to any parameters in the process of brittleness evaluation. Bd1 can be better fitted by confining pressure with an exponential function. Bd2 can be used as a comprehensive evaluation index of brittleness evaluation,but Bd2 is not sensitive to high confining pressure.Under the coring angle of 90°,the evaluation indexes of brittleness decrease with the increase of confining pressure,and shale exhibits a tendency from brittle at lowconfining pressure to ductile at high confining pressure. The evaluation indexes are sensitive to confining pressure under lowconfining pressure and reduce quickly,and vice versa under high confining pressure.