空气钻井携岩及防燃爆最小注气量计算方法

The minimum air rate required for cuttings-carrying and downhole explosion prevention during air drilling

空气钻井是一种特殊的欠平衡钻井方式,当钻遇气层时,天然气就会侵入井内与空气混合,在井下高温高压的环境下极易发生燃爆。注入的空气量既要能有效地携带岩屑,又要能防止井下燃爆。针对空气钻井中确定最小注气量的难题,根据修正的Angel模型,应用最小动能法和最小速度法,建立了携带岩屑的最小注气量方程;根据气侵速度和甲烷在井下的燃爆极限,通过控制甲烷的体积浓度,得出了防止燃爆最小注气量的计算方法;通过计算机编程,对川中某气田空气钻井的最小注气量进行了计算,其结果与实际作业中的最小注气量数据非常吻合。

Air drilling is a kind of special under-balanced drilling technology. When drilling into gas zones, the gas will enter the wellbore and mix with the air, which will probably cause fire due to the HPHT conditions downhole. Determination of the minimum air rate is critical for air drilling, which should not only ensure efficient cuttings-carrying, but also will not cause downhole fires. The minimum air rate equation for cuttings carrying is established by minimum kinetic energy model and minimum speed model based on the modified Angel model. And the minimum air rate for downhole fire prevention can be determined by regulating the volumetric concentration of methane according to gas influx rate and methane downhole burning limits. A program is then compiled by those equations. The calculated results of this program match with the actual air rate when applied to an air drilling case in central Sichuan basin.