龙马溪组页岩脆性特征试验研究
Experimental study on the brittle characteristics of Longmaxi formation shale
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摘要: 基于应力应变曲线,采用反映脆性破坏难易程度的脆性跌落系数R、反映脆性强弱的应力降系数P和软化模量M,建立了脆性评价综合指标Bd1和Bd2。通过页岩的单三轴试验,验证了Bd1和Bd2的适用性,该指标可以有效反映页岩在破坏前后抵抗非弹性变形的能力和丧失承载力的情况,基于该指标讨论了取芯角度为0°,30°,60°和90°页岩的脆性各向异性,并与破坏模式相对应,得到如下结论:1Bd1侧重性强,可自由选择参数,Bd2综合性强,对围压比较敏感,与围压拟合为指数函数关系;2页岩脆性随围压的增加整体呈减弱势,低围压下下降速度快,高围压下下降速度慢,同围压下,90°时脆性指数最大,30°时最小;3单轴压缩下,破坏模式和脆性指标密切相关,0°和90°脆性指数最高,60°次之,30°最小;220 MPa围压下,0°时脆性指数下降速率最快;2030 MPa围压下,0°,30°和60°取芯页岩脆性指数出现了"上扬"现象,90°未出现上扬;4围压通过对侧向位移限制和对层理面的压实效应来影响脆性指数,低围压下,侧向位移限制占主导,脆性减小;高围压下,压实效应占主导,脆性稍增。Abstract: Two evaluation indexes of brittleness ( Bd1 and Bd2 ),which can synthetically reflect the ability of resisting inelastic deformation and the situation of losing bearing capacity,are established based on stress-strain curve,in which the brittleness-drop coefficient R can reflect the difficulty of brittle failure,the stress landing coefficient P and soften- ing modulus M can stand for the strength of brittleness. This paper verifies the applicability of Bd1 and Bd2 by doing u- niaxial and triaxial experiments. In the end,corresponding to failure mode,the authors discuss the brittle anisotropy properties of Longmaxi formation shale under the angle of 0°,30°,60°and 90°and obtain the results as follows:① For Bd1 ,the related parameters can be selected freely according to different purposes. Bd2 is comprehensive and sensitive to the confining pressure. The fitting relationship between Bd2 and confining pressure is quadratic parabola. ② The brittle- ness index of shale decreases with the increase of confining pressure and it drops more quickly under low confining pressure than under high confining pressure. It is bigger under the angle of 90° than any other angles and it is relatively small under the angle of 30°,while the confining pressure is same. ③ Under the uniaxial compression test,the brit- tleness index is closely related to failure mode. The index accesses to maximum while the angle is 0° or 90° and rea- ches minimum while the angle is 30°. Under the confining pressure of 2-20 MPa,the lowering speed of index is su- preme while the angle is 0°. Under the confining pressure of 20-30 MPa,the brittleness index all show a rising trend except 90°. ④ The confining pressure affects the brittleness index mainly by limiting the radial strain and compacting bedding plane. Under the low confining pressure,the brittleness index of shale decreases for the former is dominant un- der the high confining pressure,the case is contrary.