恒进气量欠平衡钻井方式气侵特征及井口压力控制研究

Characteristics of gas kick in underbalanced drilling with constant air intake flow and of wellhead pressure management

欠平衡钻井打开储层后,随着打开储层的厚度不断增加,储层进入井筒中的气体不断增加,引起井口含气率剧烈变化,从而造成井底压力的失控。计算了欠平衡钻井期间不同井深含气率的变化,分析了不同进气量和井口回压对井筒含气率的影响;在考虑井口回压对井底压力增加值影响的情况下,得到欠平衡钻井打开储层期间恒进气量时井口回压的计算方法。结果表明:井筒中的气体在井口处急剧膨胀,井口回压对井口段气体含气率影响很大,对500 m以下井段含气率影响很小;在恒定进气量下,打开储层厚度越大,井口回压越大,井筒恒进气量越小,需施加的井口回压越大。

In the process of penetrating the reservoir by underbalanced drilling, with the increase of the penetrated reservoir thick- ness, the volume of gas kicking into the wellbore increases, which causes the drastic change of wellhead void fraction, leading to the bottom-hole pressure out of control. The changing of void fractions in different well depths are calculated during underbalanced drilling, and the influence of different air intake flow and wellhead back pressure on wellbore void fractions is analyzed. In consideration of well- head pressure adding value on bottom hole pressure, the computing method of wellhead pressure of constant air intake flow is obtained for penetrating the reservoir by underbalanced drilling. The results show that gas rapidly expands when it reaches the surface, and the wellhead back pressure have a enormous effect on the void fractions at wellhead while have a little influence on the void fraction under 500 meters. Keeping the air intake flow constant, with the increase of the penetrated reservoir thickness, the wellhead pressure increases. The smaller the air intake flow, the bigger the wellhead pressure needs to load.