页岩单轴压缩应力-应变特征及能量各向异性

Stress-strain characteristics and anisotropy energy of shale under uniaxial compression

层理对页岩力学性质和应变能的积聚和耗散具有重要影响,以不同层理面角度下龙马溪组页岩为研究对象,开展电镜扫描试验和单轴压缩试验,研究起裂、扩容和峰值特征点的应力-应变、弹性模量和泊松比的各向异性特征,分析其页岩变形破坏过程中输入应变能、可释放弹性应变能和耗散应变能的变化规律,揭示输入应变能与层理面角度和抗压强度的关系。结果表明:龙马溪组页岩脆性矿物含量达到72.58%,微观结构各向异性明显;随层理面角度增加,起裂、扩容和峰值特征点的应力和应变都先减少后增大,在θ=30°时均达到一个最低值,总体上呈现两边高、中间低的U型变化规律;随层理面角度增加,起裂、扩容和峰值特征点的输入应变能、可释放弹性应变能和耗散应变能也先减少后增大,在θ=30°时均达到一个最低值;各特征点的应力、应变和应变能各向异性敏感性明显,0°≤θ≤30°和30°≤θ≤60°内各向异性的敏感性大于60°≤θ≤90°;起裂应力和扩容应力均与峰值应力呈线性相关,同时峰值应变能与抗压强度存在相应的二次非线性关系,这为页岩气钻井、储层压裂改造和井壁稳定性预测预警提供了根据和参考。

The bedding plane has an important effect on mechanical properties of shale and its accumulation and dissipation of strain energy. In this study, the scanning electron microscope（SEM） tests and uniaxial compression tests were carried out on Longmaxi Formation shale with different bedding plane angles. Then, this study investigated anisotropy characteristics of elastic modulus, Poisson＇s ratio and the stress-strain of initial cracking, critical dilatancy and peak characteristic points. The evolutions of input strain energy, releasable elastic strain energy and dissipated strain energy during the deformation and failure of shale were analysed, and the relationship among the input strain energy, bedding angles and compressive strength was revealed. The results showed that the brittle mineral content of Longmaxi Formation shale was found to be as high as 72.58%, and its microstructure was distinctly anisotropic. With the increase of bedding plane angle, the stress and strain of initial cracking, critical dilatancy and peak characteristic points were all decreased first and then increased. A minimum value was found at θ=30°, and the curves generally presented U-shape. With the bedding plane angle increasing, the input strain energy, releasable strain energy and dissipated strain energy of initial cracking, critical dilatancy and peak characteristic points were also decreased first and then increased, reaching a minimum value at θ=30°. Stress-strain and strain energy of each feature point showed significant anisotropy sensitivity, and the anisotropic sensitivities at 0°≤θ ≤30° and 30°≤θ ≤60° were greater than that at 60°≤θ ≤90°. The peak stress had correlations linearly with both the initial cracking stress and critical dilatancy stress, while there was a corresponding quadratic nonlinear relationship between the input strain energy and compressive strength. This study provides references for shale gas drilling, reservoir fracturing and wellbore stability prediction and warning.