气体钻井井底低温对岩石破碎的影响机制

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,通过引入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。

Influence of low temperature induced by Joule-Thomson effect on rock breaking during gas drilling

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