不同应变速率下泥页岩力学特性试验研究

为了揭示泥页岩在不同应变速率下的力学特征,为页岩气的开采提供科学指导,分别在5×10-4/s、1×10-4/s、1×10-5/s、1×10-6/s四种不同应变速率下对页岩开展单轴压缩力学试验。研究表明,应变速率对页岩的弹性模量、峰值强度、破裂形态等具有显著影响,应变速率从高到低,弹性模量和峰值强度都呈逐渐降低趋势,但下降速度逐渐放缓,两者与应变速率负对数大致呈幂函数关系,而采用平均值拟合时则相关性较高,相对而言,弹性模量的应变率效应更显著;应变速率对破裂形态的影响极为明显,在高应变速率下页岩迅速劈裂为较少的大块,整体构架丧失,随着应变速率降低,破裂过渡为劈裂为主,伴随局部剪切破坏,达到最低应变速率时主要呈劈裂破坏,但同时形成较多横向分布裂纹,即低应变速率下破坏均匀且形成纵横切割试样的网状裂纹,页岩的应变速率在整体上对破裂方式影响最为显著,主要表现为低应变速率下的破坏更均匀、更易形成裂纹网。因此,该特性对于确立页岩力学参数及设计压裂方案具有重要启发作用。

Experimental research on effects of strain rate on mechanical properties of shale

To investigate the mechanical properties of shale under different strain rates, then to provide scientific guidance for the exploration of shale gas, the uniaxial compression experiments of shale samples have been conducted under strain rates of 5×10-4/s, 1×10-4/s, 1×10-5/s and 1×10-6/s, respectively. The experimental results show that the strain rates affect obviously on elastic modulus, peak strength and fracture morphologies, etc. of shale. Elastic modulus and peak strength gradually decrease along with the inclining of strain rate from high to low, but the decreasing speed becomes gradually smaller and smaller. The fitting relationship of elastic modulus (also peak strength) versus minus logarithm of strain rate largely adapts to the power function, whereas a higher fitting degree occurs if the mean values are implemented. Relatively speaking, the elastic modulus has a higher strain rate reaction. The fracture modes are obviously affected by the strain rate: under high strain rate, sample rapidly split into several large pieces and the entire skeleton ruptured; with the decreasing of strain rate, the fracture mode gradually transfers to main splitting failure with local shear failure; under the lowest strain rate, the main fracture mode is still splitting fracture but with many transverse cracks inserting the lateral surface; that is to say, the fracture mode under lowest strain is characterized by occurrence of cracks-net. In a word, the strain-rates affect the fracture morphologies most obviously, which performs mainly the relatively mean fracture and cracks net. Thus, these features of strain rate effect can provide important enlightenment for the parameters determination and hydro-fracture plan-design of shale.