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

FMI、BHTV/UBI等先进的测井仪器使我们能够非常详细地了解井壁结构。结果表明，在井眼两侧常出现雁列式多裂缝。这种裂缝被称为钻井诱导张性井壁裂缝（Drilling Induced Tensife Wall Fractures，DTF）。已知它们是在钻井时形成在井壁中的，然而，尚未阐明它们是如何形成的。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，以估计在井壁处产生倾斜裂缝所需的临界井眼压力P_，以及<frac>在井壁附近连接它们所需的临界井眼压力P_<link>。该模型考虑了岩石与井内流体温差Dt引起的热应力。模拟结果表明，<frac>P_1和P_2<link>随井眼方位、地应力和Dt的变化而变化。在<link><frac>这种情况下，可以观察到P_ can变得大于P_ DTF。注意，DTF具有沿着钻孔壁的小裂缝长度，这表明与一旦小裂缝连接起来就出现的大的轴向裂缝的开口相比，钻孔壁处的小裂缝开口。如果裂缝开口较小，则可以用钻井泥浆密封开口，并且可以防止显著的循环损失。这可能是在观察到DTF的深度没有任何井漏迹象的原因。为了验证本模型，进行了实验室实验的PMMA立方体试样在双轴压缩。试件是透明的，可以直接看到井壁上的裂缝形成。通过注入液氮冷却井眼，冷却过程中产生热拉应力，在井壁上诱发DTF。实验结果与模型预测吻合较好。

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.