Acquisition Study of Downhole Explosion Limit in Air Drilling

Abstract

Summary Downhole explosions adversely affect air drilling; however, the explosion limit can facilitate the adjustment of the flow rate to prevent downhole explosions. The downhole explosion limit, comprising lower and upper limits, can be determined through diverse methodologies. In this study, the interrelation between absolute enthalpy and adiabatic flame temperature (AFT) enabled the deduction of the lower explosion limit (LEL) prediction method, and an equilibrium between heat generation and heat transfer formed the basis for the upper explosion limit (UEL) prediction method. Additionally, a variable-volume apparatus was established to measure the explosion limit and adjust the pressure precisely without changing the amount of gas. The explosion limit of methane in the air was determined using both predicted and experimental methods, mirroring the scenarios of natural gas entering the wellbore during air drilling. The theoretical model exhibited the same reliability as the experimental results, and the prediction method for the explosion limit proved more efficient. Moreover, the effects of initial temperature and pressure on the explosion limit are discussed. The LEL decreased by an average of 15.5% when the initial pressure increased from 0.1 MPa to 2 MPa, while the UEL experienced a significant increase by an average of 152%. The UEL exhibited a logarithmic dependence on the initial pressure. The effect of the initial temperature on the explosion limit was noticeably less pronounced than that of the initial pressure. This study provides the essential theoretical basis and experimental results for avoiding downhole explosions during air drilling.