Abstract: Gas drilling is of great use in the development of low pressure oil-gas fields. Gas drilling can increase drilling rate and protect production formation effectively. However, it is found in practice that drill string vibration and drill tools failure are severe problems in gas drilling. In this paper, the drill string vibration and failure problems found in gas drilling and the ways to solve them are introduced, mathematical models of longitudinal vibration of drill string induced by bit displacement and mechanical analysis software presented, influence of drilling fluid viscosity on drill string longitudinal vibration studied. Calculation indicates that drilling fluid viscosity is of great influence on drill string longitudinal vibration. Drill string longitudinal vibration increases as drilling fluid viscosity decreases. The influence of drilling fluid viscosity on resonant frequency is little. Although gas drilling increases penetration rate, it needs more advanced technology to prevent drill string from failure.

Key words: drill string, longitudinal vibration, mathematical model, gas drilling, drilling fluid, viscosity, resonance vibration, anti-vibration