Annular Pressure Prediction During Well Control

Abstract Annulus pressure rises when circulating out a gas kick because gas expansion reduces the height of mud in the annulus. Kick is the uncontrolled flow of formation fluid and occurs when primary well control is lost (i.e. hydrostatic pressure is less than the formation pressure. The inability to control the kick with the blow out preventer system leads to blowout, the worst disaster while drilling. Existing techniques used in predicting annular pressures during the control process generally assume an ideal situation where the kick enters the wellbore as a continuous slug with no slip velocity relative to the mud as it rises up the annulus. This study presents new analytical models for predicting annular pressures during well control. The models harmonize the cases when the kick is mixed with the mud (most appropriate case) and when the kick comes in as a slug. Analytical solutions and computer programs were developed and validated using published data. Finally, sensitivity studies were conducted to determine the effect of formation permeability, kick volume, and wellbore geometry on annular pressure profiles at any depth of interest. Results show that the "mixed" model demonstrates good prediction of the annular pressures when compared to the "continuous slug" model. Results also show that the annular pressure buildup profiles prior to stabilization are highly sensitive to formation permeability. Also, the "Mixed" model is highly sensitive to kick volume and wellbore geometry while the "Slug" model is slightly sensitive to only kick volume and wellbore geometry.