黑色页岩巴西劈裂破坏的层理效应研究及能量分析

以重庆龙马溪组黑色页岩为试样,对黑色页岩进行了不同层理角度的巴西劈裂试验,研究了其抗拉强度和破坏形态的各向异性特征,分析了其变形破坏过程中的吸收能变化规律,揭示了层理角度、抗拉强度、AE能量与最终吸收能之间的关系。结果表明:1黑色页岩的抗拉强度受层理影响明显,90°时,抗拉强度最大,最小值则出现在30°附近。在0°≤θ≤30°和60°≤θ≤90°,抗拉强度值变化较小,而30°≤θ≤60°,其值随着层理角度增加而迅速增大;2试样破坏形态可以分为三类典型的裂纹,且低层理角度的试样仅出现单一种类的裂纹,而高层理角度的试样存在多种裂纹形态;3吸收能随着载荷增加呈非线性增长,根据吸收能增长速率的大小,发现黑色页岩发生拉伸滑移、滑移拉伸、纯拉伸、滑移和压张型拉伸破坏的剧烈程度依次减弱;4低角度试样的最终吸收能小,高角度试样的最终吸收能大,在30°≤θ≤60°,最终吸收能变化跨度最大,其与抗拉强度和AE能量均存在相应的二次非线性关系,为页岩气勘探开发提供了参考。

Effect of bedding orientation on failure of black shale under Brazilian tests and energy analysis

The anisotropic characteristics of tensile strength and failure mode of black shale from Longmaxi Formation are investigated by using Brazilian tests. The evolution of absorbed energy during the deformation and failure of black shale is analyzed, and the relationship among the ultimated absorbed energy, bedding angles, tensile strength and AE energy is revealed. The results show that: (1) The tensile strength of black shale is obviously influenced by bedding orientations. The maximum tensile strength appears at angle of 90°, while the minimum tensile strength occurs at near 30°. In the ranges 0°≤ θ≤30°and 60°≤θ≤ 90°, variation of the tensile strength with bedding angles is small, however, it grows rapidly with the increase of bedding angles in the range 30°≤θ≤60°. (2) The failure modes of specimens can be divided into three typical fractures. The specimens with low bedding angles only show single variety fracture and more than one variety fractures are found in samples with high bedding angles. (3) The absorbed energy grows nonlinearly with the increase of load. The growth rate of absorbed energy reflects the violent extent of specimen failure, and the violent extent of failure decreases in turn for all types of failures as follows: tensile-slip failure, slip-tensile failure, tensile failure, sliding failure and pressure tension destruction. (4) The ultimated absorbed energy is small at low bedding angles and is large at high bedding angles. In the range 30°≤θ≤60°, variation of the ultimated absorbed energy is very large. The corresponding quadratic nonlinear relationship presents itself between the ultimated absorbed energy and the tensile strength, AE energy, respectively, which may provide references for the exploitation and development of shale gas.