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.

诸如FMI和BHTV/UBI工具之类的高级记录工具使我们能够非常详细地看到钻孔墙的结构。结果表明，在钻孔壁的相对侧面，经常出现多个骨折。它们被称为钻探诱导的伸长式壁断裂，简称DTF。众所周知，它们是在钻孔时在钻孔墙中形成的，但是尚未澄清它们的形成方式。在本文中，我们研究了形成或DTF背后的机制。为此，我们开发了Theoreticai模型，以估计在钻孔壁上引发倾斜裂缝所需的临界钻孔压力，P_ <FRAC>，并且将它们连接到Borheole壁附近的borheole壁，P_ <link>。这些模型被认为是由岩石和钻孔流体温度差异引起的热应力。我们的建模表明，P_ <frac>和P_ <link>通过钻孔定向，原位应力和DT更改。在这种情况下，可以观察到P_ <link>可以比P_ <frac> DTF大。请注意，DTF沿着钻孔壁的断裂长度很小，这表明钻孔壁的断裂开口很小，与一旦小裂缝连接起来，发生的大型轴向裂缝的开口就会发生。如果裂缝开口很小，则可以用钻泥密封开口，并可以防止大量循环损失。这可能就是为什么在观察到DTF的深处没有任何迹象表明循环失落的原因。为了验证当前模型，在双轴压缩下对PMMA立方标本进行了实验室实验。样品是透明的，它使我们可以直接看到钻孔壁上的断裂形成。通过注射液氮来冷却钻孔，冷却导致热拉伸应力在井眼壁上诱导DTF。实验结果与模型预测非常吻合。