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.