Zhu Zhongxi, Wang Chaofei, Guan Zhigang, Lei Wanneng
National Engineering Laboratory of Petroleum Drilling Technology, Leak Resistance & Sealing Technology Research Department, Yangtze University, Daxue Road 111, Caidian, Wuhan 430100, China.
Engineering Technology Department of PetroChina Xinjiang Oilfield Company, Kalamay 834000, China.
ACS Omega. 2021 Jul 14;6(29):19026-19037. doi: 10.1021/acsomega.1c02311. eCollection 2021 Jul 27.
Due to high temperature of formation and low temperature of the drilling fluid while drilling in the hot dry rock, thermal stress around the borehole is easy to appear. The thermal stress causes cracks in the borehole wall. In severe cases, the cracks caused borehole instability and lost-circulation. Based on the theory of transient heat transfer in formation, the method of Bessel infinite series is adopted to analyze the distribution of temperature around the borehole. The results showed that the specific heat of rock and the coefficient of heat conduction changing with temperature fiercely influences the transient heat conduction in the rock and had no significant effect on the heat conduction velocity in the rock. The crack propagation is related to the stress intensity factor at the crack tip and the crack opening width. Compared with the thermal conductivity coefficient and elastic modulus, the linear expansion coefficient has a stronger effect on the peak value of thermal stress. The deeper the crack around the borehole, the weaker the influence of the drilling fluid temperature on it. The temperature gradient of the borehole wall changes gently in space. The stress intensity factor of the crack tip around the borehole is far higher than the fracture toughness of the rock, and the crack opening width near the crack tip is more than 1 mm. Therefore, the rock around the borehole while drilling in the hot dry rock would almost inevitably crack and expand, influenced by the thermal stress.
在干热岩中钻进时，由于地层温度高而钻井液温度低，钻孔周围容易出现热应力。热应力会导致钻孔壁出现裂缝。在严重情况下，裂缝会导致钻孔失稳和漏失。基于地层瞬态传热理论，采用贝塞尔无穷级数法分析钻孔周围温度分布。结果表明，岩石的比热容和导热系数随温度的剧烈变化对岩石中的瞬态热传导有很大影响，而对岩石中的热传导速度影响不显著。裂纹扩展与裂纹尖端的应力强度因子和裂纹张开宽度有关。与导热系数和弹性模量相比，线性膨胀系数对热应力峰值的影响更强。钻孔周围裂纹越深，钻井液温度对其影响越弱。钻孔壁温度梯度在空间上变化平缓。钻孔周围裂纹尖端的应力强度因子远高于岩石的断裂韧性，裂纹尖端附近的裂纹张开宽度超过1mm。因此，在干热岩中钻进时，钻孔周围的岩石几乎不可避免地会在热应力作用下产生裂缝并扩展。
