基于能量释放率的不同赋存深度砂岩脆性指数研究

Brittleness index of sandstones from different buried depths based on energy release rate

为建立表征不同赋存深度砂岩脆性指数,以布尔台矿不同赋存深度砂岩试样为研究对象,开展单轴压缩试验,针对不同赋存深度试样应力–应变曲线特征,从应力–应变正负相关性出发,构建不同赋存深度试样应力–应变曲线能量演化模型。基于能量释放率,建立满足不同赋存深度试样脆性指数,并进行理论验证。结果表明:不同赋存深度试样对应4种曲线形态,Ⅰ与Ⅳ类曲线位于最浅和最深赋存深度,分别为101.6~203.2和509.8~589.3 m,Ⅱ类曲线分布于各个深度,Ⅲ类曲线分布主要集中于406.4~589.3 m。不同赋存深度下不同曲线形态对应4种能量演化方式,总应变能整体呈"S"型曲线增长,弹性能演化曲线与应力–应变曲线相似,Ⅱ与Ⅲ类曲线的耗散能随应变增加呈台阶式增长,Ⅳ类曲线耗散能随应变增加呈"L"型增长,其中表面能和释放弹性能两者与塑性能随应变增加交替产生。脆性指数整体上随赋存深度的增加而增大,且与赋存深度呈二次函数关系,对应平均值分别为0.77,1.01,0.93,3.66,9.94,11.55,Ⅳ类曲线的最大能量释放率与脆性指数远大于其他3种曲线形态,平均值分别为555.27,30.98。同时,不同赋存深度砂岩脆性分区有较大差异,赋存深度为101.6,317.3 m时,砂岩层为弱脆性;赋存深度为203.24 m时,砂岩层为中脆性;赋存深度为406.42,589.35 m时,砂岩层为强脆性。

To establish brittleness index of sandstone at different buried depths,the uniaxial compression test was carried out on the rock samples from different depths in Buertai coal mine. According to the stress-strain curve characteristics of rock samples at different buried depths,an energy evolution model of the stress-strain curve at different buried depths was constructed based on the positive and negative correlation of stress and strain. Based on the energy release rate,the brittleness index of rock samples at different buried depths was established and verified theoretically. The results show that rock samples at different buried depths correspond to four types of curves. Types Ⅰ and Ⅳ curves are located at the shallowest and deepest buried depths,101.6–203.2 m and 509.8–589.3 m respectively. Type Ⅱ curve is distributed at various depths,and type Ⅲ curve is mainly distributed at 406.4–589.3 m. Different curve shapes at different depths correspond to four energy evolution models. The total strain energy increases in a"S"shape,and the elastic energy evolution curve is similar to the stress-strain curve. The dissipation energy of types Ⅱ and Ⅲ curves increases step by step with increasing the strain,and the dissipation energy of type Ⅳ curve increases in a"L" shape with increasing the strain,in which both surface energy and released elastic energy alternate with plastic energy while the strain increases. The brittleness index increases with increasing the buried depth, showing a quadratic function relationship. The corresponding average values are 0.77,1.01,0.93,3.66,9.94 and 11.55 respectively. The maximum energy release rate and brittleness index of type Ⅳ curve with average values of 555.27 and 30.98 respectively are much higher than those of the other three curves. In addition,brittleness index partitions of sandstones from different buried depths are quite different. The sandstone layer is weak brittleness when the buried depths are 101.6 m and 317.3 m,medium brittleness for 203.24 m depth and strong brittleness at the buried depths of 406.42 m and 589.35 m.