低孔隙度泥页岩应力依赖的各向异性裂纹演化特性研究

本文在实验室对所获取的东营地区层理发育的低孔隙度页岩和泥岩的各向异性裂纹演化特性进行了研究,获得了各向同性条件下泥页岩的力学与超声波响应特性,分析了应力幅度对于页岩声波速度和各向异性的影响.主要结论包括:(1)泥页岩在循环载荷下存在滞后效应,表明其经历了去压实或油气产生导致的超压;(2)泥岩和页岩具有不同程度的各向异性,随着各向同性压力的增高微裂隙逐渐闭合,样品的各向异性程度减弱;(3)分析了岩石韧度和裂纹损伤参数随压力的变化特征,相比泥岩,页岩各向异性程度更高,随压力变化更明显,其裂纹导致的附加各向异性更强;(4)分析了各向异性岩石的动态弹性模量特征,由于软裂隙空间的闭合,动态弹性模量在低压条件下都随着围压的增加有硬化趋势.

Stress-dependent anisotropy of mudstone and shale with low porosity

Taking mudstone and shale from the Shengli oilfield as an example, the stress-dependent anisotropic characteristics of mudstone and shale are studied. This work obtained the velocity and anisotropy response characteristics under different conditions by laboratory measurement, which permits further research of the effects of stress on the velocity and anisotropy of these two rocks.#br#The measured samples are cut into cylinders along three different directions, parallel(perpendicular to the symmetry axis), vertical to the bedding(parallel to the symmetry axis)and a certain angle(45°)to the symmetry axis. The diameter of samples is 25 mm, and both ends are polished and buffed. The hysteresis curve is obtained by the stress-strain tests under cyclic loading. An ultrasonic detection instrument measures both P and S wave velocity under different confining pressures. Then we take three cores with different angles to get the five independent elastic parameters in the VTI medium. The samples' dynamic elastic modulus and compliance matrix changes with the increasing confining pressure are calculated. The change characteristics of damage parameters under confining pressure are analyzed combining pore deformation theory.(1)Mudstone and shale have hysteresis effects under cyclic loading because of the unsynchronized phases of stress and strain. The strain phase may lag behind, equal to, or exceed to the stress phase in loading stage. While the strain phases are all lagging behind the stress phase in the unloading stage.(2)When the confining pressure is increased from 1 MPa to 40 MPa with isotropic stress, P and S wave velocities in various directions increase. Compared with P wave velocity parallel to the bedding plane(V_Ph), the P wave velocity vertical to the bedding plane(V_Pv)changes less. Also, V_Ph is greater than V_Pv, indicating that mudstone and shale have P wave anisotropy. P wave velocity with 45 degrees to the symmetry axis(V_P45)is fairly close to V_Pv. S wave velocity in the direction parallel to the bedding plane(V_Sh)increases slightly with the increasing stress. Each component of the stiffness matrix also increases with the rise of confining pressure. But P and S wave anisotropy parameters characterized by Thomsen parameters decrease.(3)The dynamic Young's modulus increases with the increase of confining pressure and the amplitude is relatively larger under the low pressure. It will tend to a stable value when the confining pressure is greater than 40 MPa. Similarly, dynamical poisson's ratio also reaches a stable value above 40 MPa. Dynamic shear modulus increases with the growth of confining pressure, but its change rate decreases gradually.(4)The calculated crack damage parameters of mudstone α₃₃ and α₁₁ drop gradually with the increasing confining pressure. α₃₃ is the maximum and α₁₁ is smaller. The crack damage parameters of shale are larger than that of mudstone. α₃₃ decreases with the increase of pressure, showing that the crack number parallel to the bedding plane is the most and the increase of pressure leads to the crack closure.#br#Mudstone and shale have hysteresis effects under cyclic loading, indicating that they have experienced compaction or overpressure caused by oil and gas generation. The both rocks have a high anisotropy. Microcracks are gradually closed with the increase of the isotropic pressure which will weaken the degree of anisotropy in the sample. As soft fracture space is closed, the dynamic elastic modulus under low pressure has a hardening trend with increasing confining pressure. Variation characteristics of rock compliance and crack damage parameters with the pressure show that shale has a higher degree of anisotropy compared to mudstone. It changes more obviously with pressure and the additional anisotropy caused by cracks is stronger.