围压对横观各向同性砂岩弹性参数的影响

通过层理面角度为0°、45°和90°的长方体砂岩试样的单轴和三轴试验,直接测量了不同围压下砂岩的5个独立的横观各向同性弹性参数:弹性模量E,E′;泊松比ν,ν′;剪切模量G′。测得了前人难以测量的不同围压下的ν和G′值,得到了横观各向同性弹性参数与围压之间的函数关系,并揭示了弹性参数随围压变化的力学机制。试验结果表明,随着围压增大,弹性模量增大,泊松比减小,剪切模量增大。当围压超过30 MPa时,2个垂直方向的弹性模量、泊松比和剪切模量分别趋于相同值,表现为各向同性。由此可以推断:该砂岩的横观各向同性由内部微裂纹在平行于和垂直于层理面方向分布不均引起,围压升高造成裂纹闭合,最终导致各向异性消失。岩石的裂纹闭合效应可以用裂纹密度来定量评价与描述。

Effect of Confining Pressure on the Transversely Isotropic Elastic Parameters for Sandstone

Uniaxial and triaxial compression tests were performed on prismatic specimens with bedding angles of 0°, 45°and 90° to directly measure the five independent transversely isotropic elastic parameters, i.e., Young’s modulus E, E′, Poisson’s ratio ν, ν′, and shear modulus G′, under different confining pressures. Particularly, ν and G′ under different confining pressures have never been measured by former researchers. The functional relationships between the confining pressure and the transversely isotropic elastic parameters were obtained, and the mechanism of confining pressure dependency of elastic parameters was explained. Experimental results show that as confining pressure increases, Young’s moduli increases, Poisson’s ratios decreases, and shear moduli increases. Moreover, Young’s moduli, Poisson’s ratios, and shear moduli in two perpendicular directions tend to become the same constant value respectively when confining pressure is beyond 30 MPa, implying that the sandstone exhibits isotropic behavior. It is inferred that the transversely isotropy of the sandstone is due to the different distributions of microcracks in two directions of parallel and perpendicular to bedding, and the anisotropy is vanishing as more microcracks are closed, with the increase of confining pressure. The crack closure effect of rock can be quantitatively characterized and described by the concept of crack density.