Abstract: Owing to the advantages such as improving ROP，discovering and protecting reservoir，gas drilling is widely used
in drilling industry. However downhole explosion in gas drilling process is high likely to occur when the gas entering the
wellbore in underbalanced condition. This paper introducs the research process in downhole explosion and analyzs the reason
for explosion and presents the influence factors such as mixed combustible gas component，temperature，pressure and inert gas
content in the examples of gas drilling. We also discuss how to avoid explosion by means of restricting the range of oxygen
contents. Nitrogen injection drilling is recommended at the end.

Key words: gas drilling, nitrogen drilling, oxygen content, explosion limit, security