川南地区志留系龙马溪组页岩力学性质及微观破裂机理

Mechanical properties of the Silurian Longmaxi Formation shale,southern Sichuan Basin and its microfracturing mechanisms

为揭示页岩力学性质和微观破裂机理,对川南地区志留系龙马溪组页岩开展了X射线衍射、三轴压缩、微米CT扫描和扫描电镜测试。研究发现:页岩矿物成分和围压对其力学性质和储层物性具有显著影响。随着脆性矿物含量的增加,页岩弹性模量和峰值应力增大,指示两者具有正相关关系。随着黏土矿物含量的增加,页岩塑性增强,岩石强度降低。随着围压的增加,页岩中的裂缝逐渐闭合、孔隙变形收缩、孔隙度减小,围压越大,压缩越严重。页岩在破坏过程中主要发育矿物颗粒边界裂缝和矿物颗粒内部裂缝两种裂缝。对于力学性质相近的页岩,随着围压的增大,页岩从矿物颗粒边界裂缝占主导演变为矿物颗粒内部裂缝占主导。围压增大导致矿物颗粒内部和颗粒边界裂缝向破碎带演变,页岩发育更多的裂缝。

To reveal the mechanical properties and microfracturing mechanisms of shales,we investigate shales in the Silurian Longmaxi Formation in the southern Sichuan Basin using X-ray diffraction(XRD)analysis,together with triaxial compression,micro-computed tomography(micro-CT)and scanning electron microscopy(SEM)tests.The results indicate that the mechanical properties and reservoir physical properties of shale are significantly influenced by its mineral composition and confining pressure.Specifically,an increase in the brittle mineral content enhances its elastic modulus and peak stress,indicating positive correlations between these mechanical properties and the brittle mineral content.In contrast,higher clay mineral content increases the shale plasticity while reducing its rock strength.As confining pressure increases,fractures in the shale gradually close,and the pores deform and contract,leading to reduced porosity.A higher confining pressure results in greater compression.Two dominant types of fractures form during shale damage:boundary fractures occurring at interfaces between mineral grains and internal fractures occurring within grains.For shales with similar mechanical properties,increasing confining pressure shifts their dominant fracture type from boundary to internal fractures.Additionally,higher confining pressure causes the boundary and internal fractures to evolve into fracture zones,leading to an increased fracture density in the shale.