关键词: 钻具失效, 冲蚀, 环空返速, 有限元分析, 螺纹应力

Abstract: Because of Gas drilling’s distinguishing feature, drilling tool is wornouted seriously by high velocity liquid’s erosive action and drilling tool substitute thread is cracked easily by air hammaer upward’s shock wave, all of which impact drilling speed and increase drilling cost. Based on the theory of rock mechanics and gas/solid two⁃phase flow, drilling tool erosived by liquid in gas drilling is analysed, return velocity model of mixture in annular is established, return velocity in annular impacted by back pressure is calculated with illustration. In order to avert erosiving by high velocity, return velocity of mixture in annular has to be between minimal carrier velocity and interface erosive velocity. Increasing well head back pressure could decreased the drill tool’s erosiving. Based on mechanical theory，the deformation harmonization equation of drilling tool substitute thread under the influence of pretightening load is fetched. The stress of drilling tool thread is analysed by finite element. The screw thread load distribution regular pattern of drilling tool substitute thread is found out. The thread stress centralized area is obtained, which could provide measure to prevent drilling tool failure and to enhance thread strength.

Key words: Drilling tool failure, Erosive, Annular return velocity, Finite element analysis, Thread stress