Fluid Injection Experiments in Shale at Elevated Confining Pressures: Determination of Flaw Sizes From Mechanical Experiments
Fluid Injection Experiments in Shale at Elevated Confining Pressures: Determination of Flaw Sizes From Mechanical Experiments
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DOI：
10.1029/2018jb017207
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发表时间：
2019-06-01
影响因子：
3.9
通讯作者：
Lee, Peter
中科院分区：
文献类型：
--
作者：
Chandler, Michael R.;Mecklenburgh, Julian;Lee, Peter
对Mancos页岩、Whitby泥岩、Penron板岩和彭南特砂岩进行了围压达100 Mpa的三轴实验和直接注液实验。实验采用平行和垂直于材料分层的样本轴进行。在三轴破坏过程中，解理方向与最大主应力平行的板岩强度较大，但两个方向的页岩破坏应力基本相同。利用翼裂模型由三轴资料计算出初始缺陷尺寸约为40微米，页岩的初始缺陷比非页岩的初始缺陷短。在直接注入流体的过程中，破裂速度很快，裂缝起始和破裂之间没有明显的间隙。破裂压力随围压呈线性增加，但对围压的敏感性低于现有模型的预期。提出了一种基于断裂力学的模型来确定注塑实验中导致破裂的初始缺陷尺寸。用这种方法确定的裂隙大小与低围压下非页岩三轴资料确定的裂隙大小基本一致。随着围压的升高，达到一个临界值，超过这个临界值，注液实验表明初始缺陷长度较低，约为10微米。这个临界值被解释为由于缺陷的部分闭合。在页岩中，在所有围压下确定的初始缺陷长度约为10微米，与通过三轴实验确定的长度合理地一致。
Triaxial experiments and direct fluid injection experiments have been conducted at confining pressures up to 100MPa on Mancos shale, Whitby mudstone, Penrhyn slate, and Pennant sandstone. Experiments were conducted with sample axes lying both parallel and perpendicular to layering in the materials. During triaxial failure Penrhyn slate was stronger for samples with cleavage parallel to maximum principal stress, but the two orientations in the shales displayed similar failure stresses. Initial flaw sizes of around 40 mu m were calculated from the triaxial data using the wing crack model, with the shales having shorter initial flaws than the nonshales. During direct fluid injection, breakdown was rapid, with no discernible gap between fracture initiation and breakdown. Breakdown pressure increased linearly with confining pressure but was less sensitive to confining pressure than expected from existing models. A fracture mechanics-based model is proposed to determine the initial flaw size responsible for breakdown in injection experiments. Flaw sizes determined in this way agree reasonably with those determined from the triaxial data in the nonshales at low confining pressures. As confining pressure rises, a threshold is reached, above which the fluid injection experiments suggest a lower initial flaw length of around 10 mu m. This threshold is interpreted as being due to the partial closure of flaws. In the shales an initial flaw length of around 10 mu m was determined at all confining pressures, agreeing reasonably with those determined through the triaxial experiments.