Anisotropic creep characteristics and mechanism of shale under elevated deviatoric stress
Anisotropic creep characteristics and mechanism of shale under elevated deviatoric stress
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高偏应力下页岩各向异性蠕变特征及机理
DOI：
10.1016/j.petrol.2019.106670
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发表时间：
2020
影响因子：
--
通讯作者：
Heping Xie
中科院分区：
文献类型：
--
作者：
Cunbao Li;Jun Wang;Heping Xie
各向异性泥页岩的时变特征对于准确预测储层在长时间段内的动态是重要的。为了研究页岩的横观各向同性蠕变特性，采用4种不同层理方位（0°、45°、75°和90°）的页岩试件，在多水平偏应力作用下进行了一系列的蠕变试验。实验结果表明，即使在偏应力较低的情况下，泥页岩也表现出蠕变行为。各向异性对蠕变变形和稳态蠕变速率有显著影响。当层面倾角为45°时，在相同的偏应力作用下，蠕变变形量和稳态蠕变速率最大;当层面方位为90°时，蠕变变形量和稳态蠕变速率最小。当偏应力达到裂纹损伤门槛值时，随时间变化的变形出现第三蠕变阶段，表明裂纹损伤应力可以作为页岩的长期强度。当偏应力大于裂纹萌生门槛值时，稳态蠕变速率随偏应力的增加呈指数增长。评价了经验蠕变规律的合理性。得出经验蠕变模型只能拟合现有的蠕变数据，而几乎不可能预测蠕变变形的结论。详细分析了泥页岩各向异性蠕变行为的产生机理，并根据应力张量分量的作用机理提出了泥页岩蠕变的3种基本模式。最后提出了一种提出各向异性蠕变模型的一般方法。
The time-dependent characteristics of anisotropic shale are important for accurately predicting a reservoir's behavior over a long period of time. To study the transverse isotropic creep characteristics of shale, a series of shale creep tests using specimens with 4 different bedding layer orientations (0°, 45°, 75° and 90°) under multiple levels of deviatoric stress were conducted. The experimental results indicate that shale presents creep behavior even when the deviatoric stress is relatively low. Anisotropy has significant influences on the creep deformation and steady creep rate. When the bedding plane inclined angle is 45° and under the same deviatoric stress, the creep deformation and steady creep rate are the highest, while the smallest creep deformation and steady creep rate are generated when the bedding plane orientation is 90°. Time-dependent deformation appears the tertiary creep stage when the deviatoric stress is crack damage threshold, which implies that crack damage stress can be regarded as the shale's long term strength. The steady creep rate increases exponentially with increasing deviatoric stress as the stress is larger than the crack initiation threshold. The rationality of the empirical creep law is evaluated. It is concluded that the empirical creep model can only fit the existing creep data, but it is almost impossible to predict the creep deformation. The mechanism for generating anisotropic creep behavior of shale is analyzed in detail, and three basic creep patterns for shale are proposed based on the action mechanism of the stress tensor components. A general methodology for proposing the anisotropic creep model is finally suggested.