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Rock failure by thermal stress in gas drilling

The factors that contributing high penetration rate of gas drilling is complex. Isentropic flow was generated when gas pass through bit nozzle during gas drilling. This phenomenon would lead to cryogenic effects. The thermal stress on the bottom hole rock was formed by the effect, which can reduces the strength of the rock and promotes the mechanical rock breaking. First, the temperature distribution model of bottom hole rock under the condition of asymmetric cooling was built in the paper. The three-dimensional dynamic thermal shock stress distribution model was established based on the temperature model. Then, the change of the rock cohesion was analysed by using the Mohr Coulomb criterion. The results demonstrate that with the temperature decreases, the strength of rock is greatly reduced, can improve ROP. Last, the liquid nitrogen cooling test and real time measurement of acoustic wave were conducted to verify the above analyse. The First wave amplitude has a dramatic delay, which illustrate the cooling have an important impact on rock internal structure. The paper clearly depicts the mechanism of rock failure under the condition of dynamic low temperature in gas drilling. [Received: September 7, 2017; Accepted: June 20, 2018]