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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.

FMI和BHTV/UBI等先进的测井工具使我们能够非常详细地看到井壁结构。结果表明：井壁两侧多裂缝呈呈梯队分布；它们被称为钻致张壁裂缝，简称DTF。已知它们是在钻孔时在井壁中形成的，但它们是如何形成的尚不清楚。在本文中，我们研究了DTF形成的机制。为此，我们建立了理论模型来估计在井壁处启动倾斜裂缝所需的临界井眼压力（P_<frac>）和在井壁附近连接裂缝所需的临界井眼压力（P_<link>）。该模型考虑了由岩石和井内流体温度差异Dt引起的热应力。模拟结果表明，P_<frac>和P_<link>随井眼方位、地应力和Dt的变化而变化。P_<link>可以变得大于P_<frac>在这种情况下可以观察到DTF。注意，DTF沿井壁的裂缝长度较小，这表明与小裂缝连接后出现的大轴向裂缝相比，井壁处的裂缝开口较小。如果裂缝开度较小，则可以用钻井泥浆对裂缝进行密封，防止严重的循环漏失。这可能就是为什么在观察到DTF的深度没有任何循环漏失的迹象的原因。为了验证模型的有效性，对PMMA立方体试件进行了双轴压缩实验。样品是透明的，使我们可以直接看到井壁上的裂缝形成。通过注入液氮对井壁进行冷却，冷却过程产生热拉应力，在井壁上诱发DTF。实验结果与模型预测吻合较好。