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

先进的测井仪器，如FMI和BHTV/UBI仪器，使我们能够非常详细地了解井壁结构。结果表明，井壁两侧经常出现梯形多条裂缝。它们被称为钻井诱发的廷尼菲墙体裂缝，简称DTF。它们是在钻井过程中在井壁中形成的，但它们是如何形成的，目前还不清楚。在这篇文章中，我们研究了形成DTF的机制。为此，我们建立了理论模型，以估计在井壁P_t；裂缝处引发倾斜裂缝所需的临界井眼压力，以及在井壁附近将它们连接起来所必需的临界井眼压力。该模型考虑了岩石和井筒流体温差dT引起的热应力。我们的模拟表明，P&lt；Frc&Gt；和P&lt；link&gt；随着井眼方位、地应力和DT；而发生变化。在这种情况下，P_&lt；link&>可能会变得大于P_&lt；Frc&>DTF。请注意，DTF沿井壁的裂缝长度较小，这意味着井壁上的裂缝开口较小，而一旦小裂缝连接起来，就会出现大的轴向裂缝。如果裂缝开度较小，则可用钻井液将其封堵，可防止严重的井漏。这可能是为什么在观察到DTF的深处没有任何迹象表明漏出的原因。为了验证所提出的模型，对PMMA立方体试件进行了双向压缩的室内试验。样品是透明的，它使我们能够直接看到井壁上的裂缝形成。通过注入液氮对井筒进行降温，冷却过程中产生的热拉应力在井壁上产生DTF。实验结果与模型预测结果吻合较好。

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.