中文摘要：

首先分析井底钻头喷嘴的Joule-Thomson效应,其次,建立急剧冷却条件下井底岩石的温度场分布模型,并以此建立井底岩石动态热应力分布模型,对气体钻井井底热冲击应力进行深入剖析.此外,通过引人井底岩石裂纹尖端应力强度因子,对钻头载荷和热应力共同作用下的井底岩石裂纹扩展失稳进行分析,对砂岩岩样进行液氮冷却试验,并对其进行声波实时测量.结果表明：在热应力的作用下岩石裂纹的抗扩展阻力存在极小值,气体钻井岩屑粒度较小;随着温度的降低,波速在岩石中呈线性衰减,声波的首波波幅也有明显的延迟,说明冷却处理对岩心内部结构产生了很大影响.

英文摘要：

Low temperature at the outlet of drilling bit nozzles can be induced due to Joule-Thomson effect during well drillingusing gases as drilling fluid. In this study,a numerical simulation model for analyzing the temperature distribution around thebottom of the hole was established based on the Joule-Thomson cooling effect, and then the induced thermal shock stress wascalculated according to the temperature distribution. The expansion and instability of rock cracks were analyzed in terms of astress intensity factor at the cracking tips of rock. Experiments were conducted using liquid nitrogen for cooling of rock samplesin order to verify the ther^nal cracking models, and a real-time acoustic technique was used to measure the crackingeffect. The results show that, under the thermal stress, a minimal cracking resistance of anti-propagation exists, and cuttingswith small sizes were obtained during gas drilling. It appears that the wave velocity decreases at low temperatures, and thefirst wave amplitude has a dramatic delay, which illustrate that the cooling can have an important impact on the internal structureof rocks.