Study On Vibrating Properties Of The Drill String In Air And Gas Drilling Well

With the advantage of high drilling speed and the ability of protecting pay formation, Air and Gas drilling technology has been used in large scale. But during the process of drilling, a problem has directly raised the drilling cost and threaten the safety of drilling, which is that the drill string breaks frequently and seriously. The gas adopted in Air and Gas drilling has a low density and damping action, which led the drill string bear more complicated stress under well. Based on theory study, finite element and mathematical model was built, detailed numerical simulation on properties of the drill string in Air and Gas drilling well was done in this paper with the ANSYS software. The results show that the natural frequency value of torsional vibration and axial vibration is big, the resonance range is narrow. But the natural frequencies value of lateral vibration is small and between various steps natural frequency is separated slightly and the resonance range becomes wider. The property of torsional vibration, axial vibration and lateral vibration change little when string dimension and wall thickness change, but change a lot when the length of drill collar or whole drill string change. The natural frequency value of axial vibration and lateral vibration is much bigger than the value of common drilling with mud. The results of harmonic response analysis show that the low frequency resonance becomes stronger and the high frequency resonance becomes weaker because of the hydrodynamic effects. Besides, drilling mud has a positive effect on the safety of drill string. The drill string vibrate more strenuously and collide more seriously with the borehole wall when it resonate because the low damping of gas in Air and Gas drilling well, which causes transverse stress in drill string exceed it's fatigue resistance and the drill string fatigue break eventually. During the process of drilling with air hammaer, the rotate speed adopted can avert the critical speed of a same BHA so that the probability of drill string resonance is reduced. The recommend speed changed with well depth is given by calculating with the built model. The results are helpful for the optimization of drilling parameters and rotate speeds in a Air and Gas drilling well.