气体钻井低温破岩机理分析

气体钻井有着较高的机械钻速在很大程度上归因于钻头水眼处的焦耳–汤姆森低温效应。这种效应对井底岩石产生了热冲击应力,使得井底岩石的强度降低,进而促进了机械破岩的作用。首先建立了非对称冷却条件下井底岩石的温度场的分布模型,并以此建立了井底岩石三维动态热应力分布模型,对气体钻井井底热冲击应力进行了深入的剖析。其次,通过莫尔–库仑准则,对岩石的黏聚力变化进行了分析,得出随着冷却时间的加长,岩石强度迅速降低,有利于岩石的破坏。最后,为验证理论模型,对砂岩岩样进行液氮冷却试验,并对其进行声波实时测量,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。

Mechanism of cryogenic rock failure in gas drilling

The factors contributing to high penetration rate of gas drilling are complex. The isentropic flow is generated when gas passes through bit nozzle during gas drilling. This phenomenon will lead to cryogenic effects, and then the resulted thermal shock stress at bottom hole rock will reduce the rock strength, contributing to the role of the rock failure. First, a model for the temperature distribution of bottom hole rock under asymmetric cooling is established. The three-dimensional dynamic thermal shock stress distribution model is established based on the temperature field. Then, the change of the rock cohesion is analyzed by using the Mohr-Coulomb criterion. The results demonstrate that as the temperature decreases, the strength of rock is greatly reduced, resulting in increased ROP. Finally the liquid nitrogen cooling tests and real-time measurements of acoustic waves are conducted to verify the above theory. The first wave amplitude has a dramatic delay, which illustrates that the cooling has an important impact on the internal structure of rock. The mechanism of rock failure under dynamic low temperature in gas drilling is clearly depicted.