火山岩力学特性试验研究

对取自松南气田营城组的流纹岩和凝灰岩开展室内岩石力学试验研究,得到了流纹岩不同亚相和凝灰岩在不同围压下的应力-应变曲线;在此基础上分析火山岩变形破坏的特征。火山岩的CT图像显示,随着流纹岩亚相的变化,岩石内的孔隙和微裂纹发育程度发生变化,上部亚相气孔较发育、中部亚相次之、下部亚相气孔发育较少。试验结果表明上部亚相、中部亚相和下部亚相的单轴抗压强度、弹性模量依次增大。凝灰岩在围压下的岩石力学特性遵循脆性岩石的普遍规律;但是由于非均质性、各向异性的影响,凝灰岩的抗压强度和弹性模量随着围压的增大出现波动。流纹岩在单轴条件下表现为沿轴向的劈裂破坏;凝灰岩在围压下的微裂纹起裂方向与轴向存在夹角,最终形成与轴向存在一定夹角的宏观破坏面。火山岩在压缩破坏过程中所产生的应变很小;而且岩石强度和弹性模量较高,这会造成水力压裂过程中的破裂压力梯度高,难以形成裂缝。

Experimental study on rock mechanics of volcanic rock

The rock mechanics tests studies of rhyolite and tuff rocks from SongNan gas field were made in laboratory. The stress - strain curves of rhyolite in different subfacies and tuff under different confining pressures were obtained. Meanwhile, this paper analyzes the deformation and failure characteristics of volcanic rocks. The CT images of volcanic rocks show that the pores and microcracks existing in rocks changes in different rhyolite subfacies. The pores of upper subfacies develop well, followed by middle subfacies, while the lower subfacies have few pores. These differences have a great impact on rock mechanics properties. The test results show that the uniaxial compressive strength and elasticity modulus of the rhyolite increases in the sequence of upper subfacies, middle subfacies, and lower subfacies. The rock mechanics properties of tuff under confining pressure follow the universal law of brittle rock. However, due to heterogeneity, anisotropy of the tuff, the compressive strength and modulus elasticity of tuff show fluctuations as the confining pressure increases. The failure of rhyolite under uniaxial conditions is the splitting failure along the axial of core, while failure surface of tuff under confining pressure have a certain angle with the axial of core. The strains of the volcanic rocks during the compression tests are very small, and the rock strength and modulus elasticity are very high, which will cause high fracture gradient in hydraulic fracturing with the result of difficult to create fracture in the volcanic rocks.