中文摘要：

欠平衡钻井过程中气层逐渐被打开时，气体将以小气泡的形式从地层进入环空流体中，气泡进入环空后将膨胀运移，变成大气泡。因此，气泡生成半径对环空流体的流动规律有重要影响。建立不同气侵量下气泡生成直径的计算模型，并给出气侵量高低的判别标准。分析得出：井底欠压值和地层孔隙半径是影响气泡生成直径的重要因素。在欠压值恒定时，气泡直径随着表面张力和孔隙半径的增加而增大，而随钻井液密度的增加而减小；随着气体侵入量的增加，欠压值增大，钻井液密度和表面张力对气泡直径的影响减弱。

英文摘要：

During underbalance drilling gas reservoir is opened gradually, gas is in the shape of spherical from formation into annular flow and bubbles will be inflation migration, so radius of bubbles formation is important for the annular fluid . The study establish bubbles generated invasion of the diameter calculation model under different of gas kick, and given the rule of flow pattern. Analysis derived that underbalanced pressure drop and pore radius is an important factor of bubble diameter, when the pressure drop is constant, the bubble diameter will increasing as surface tension and pore radius increasing;While gas kick volume increases further, pressure drop becomes larger, the affect of fluid density and surface tension is little.