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

Key words:  gas drilling  thermal stress at low temperature  stress intensity factor  rock cracking  acoustic test

Abstract:

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.