The development of gas drilling is restricted seriously because of frequent failure of drilling tools. To study the fatigue failure cycle of drill string, the whole drill string was taken in deep-vertical drilling as research subject. Based on Forman model in drill string fatigue life prediction, and taking consideration of the dynamic characteristics of drill string and the effect of local thermal factors on drill string fatigue life in gas drilling,the calculation equation of drill string fatigue life in gas drilling was established. Taking a real borehole in the west of Sichuan province as an example,the fatigue life of drill string in gas drilling was calculated with the established equation, then compared with that in mud drilling. Consequently, the relationship model of the fatigue life in gas and mud drilling was obtained. The results showed that the fatigue life of drilling tools was very sensitive to the initial crack. The initial crack develops larger,the fatigue life drops sharper. The fatigue life of drill string in each interval in gas drilling was obviously lower than that in mud drilling.%钻柱的频繁失效严重制约着气体钻井的发展.为研究钻柱的疲劳失效周期,以深直井全井钻柱为研究对象,基于疲劳寿命预测的Forman模型,综合考虑深直井钻柱的动力学特性和气体钻井过程中局部热因素对钻柱疲劳寿命的影响,建立了气体钻井钻柱疲劳寿命计算方程.以川西某实钻井为例,利用建立的气体钻井钻柱疲劳寿命计算方程,对气体钻井条件下的钻柱疲劳寿命进行了计算,并与该层段钻井液钻井参数下的钻柱疲劳寿命进行了对比,初步获得了气体钻井与钻井液钻井的钻柱疲劳寿命关系模型.分析结果表明,气体钻井的钻柱疲劳寿命对初始裂纹非常敏感,初始裂纹越大钻柱寿命下降越明显;气体钻井各井段钻柱的疲劳寿命均明显低于钻井液钻井.
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