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Theoretical and Experimental Study on Physical Mechanism behind the Formation of Drilling Induced Tensile Wall Fracture (DTF) for Borehole Stabilization

钻孔诱发张拉墙断裂（DTF）形成的物理机制的理论与实验研究

基本信息

批准号：
13650982
负责人：
ITO Takatoshi
金额：
$ 2.18万
依托单位：
Tohoku University
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (C)
财政年份：
2001
资助国家：
日本
起止时间：
2001 至 2002
项目状态：
已结题

项目摘要

The advanced logging tools such as FMI and BHTV/UBI tools allow us to see the structure of borehole wall in very detail. The results show that there appear frequently the multiple fractures in an en-echelon pattern on opposite sides of the borehole wall. They are called as the Drilling Induced Tensife Wall Fractures, DTF for short. It is known that they are formed in the borehole wall while drilling, however, it has been not clarified yet how they are formed. In this paper, we investigate the mechanism behind the formation or DTF. To this end, we developed theoreticai models to estimate the critical borehole pressure necessary to initiate inclined fractures at the borehole wall, P_<frac>, and that necessary to link them near the borheole wall, P_<link>. The models are taken into account of thermal stress caused by the difference in temperature of rock and borehole fluid, Dt. Our modeling shows that P_<frac> and P_<link> are changed with borehole orientation, in-situ stresses and Dt. P_<link> can become to be larger than P_<frac> DTF may be observed in such a case. Note that DTF has a small fracture length along the borehole wall suggests a small fracture opening at the borehole wall compared to the opening of large, axial fractures that occur once the small fractures have linked up. If the fracture opening is small, the opening could be sealed with the drilling mud, and significant circulation loss could be prevented. This may be the reason why there was not any indication of lost circulation at the depths where DTF were observed. To verify the present models, laboratory experiments were performed on the PMMA cubical specimens under biaxial compressions. The specimens are transparent, and it allows us to see directly the fracture formation on borehole wall. The borehole was cooled down by injection of liquid nitrogen, and the cooling caused thermal tensile stress to induce DTF on the borehole wall. The experimental results agree well with the model prediction.

先进的测井工具如FMI和BHTV/UBI工具使我们能够非常详细地看到井壁的结构。结果表明，井壁两侧经常出现多条呈阶梯状的裂缝。它们被称为钻井引起的张力墙断裂，简称DTF。众所周知，它们是在钻孔时在井壁中形成的，然而，目前还不清楚它们是如何形成的。在本文中，我们研究了 DTF 形成背后的机制。为此，我们开发了理论模型来估计在钻孔壁处引发倾斜裂缝所需的临界钻孔压力 P_<frac>，以及将它们连接到钻孔壁附近所需的临界钻孔压力 P_<link>。该模型考虑了由岩石和钻孔流体温差 Dt 引起的热应力。我们的模型显示 P_<frac> 和 P_<link> 随钻孔方向、地应力和 Dt 的变化而变化。 P_<link> 可能变得大于 P_<frac> 在这种情况下可以观察到 DTF。请注意，DTF 沿井壁的裂缝长度较小，表明与小裂缝连接后发生的大轴向裂缝的开口相比，井壁处的裂缝开口较小。如果裂缝开口较小，可以用钻井泥浆密封开口，可以防止显着的循环损失。这可能就是在观察到 DTF 的深度没有任何循环漏失迹象的原因。为了验证当前的模型，在双轴压缩下对 PMMA 立方体样品进行了实验室实验。标本是透明的，可以让我们直接看到井壁上的裂缝形成。通过注入液氮来冷却钻孔，冷却引起的热拉应力在钻孔壁上诱发DTF。实验结果与模型预测吻合良好。