Wellbore pressure management of offshore wells during casing running in narrow safety pressure window formations

Abstract

Casing running in subsea wells has long been considered a daunting task and repeatedly proved unsuccessful, largely due to the narrow safety pressure window formations. Mud losses to formations, formation fluid influx and well wall collapsing have been the three most common causes in such casing running failures. The problem can be solved with the managed wellbore pressure casing running technique, which was developed based on managed pressure cementing and implemented by controlling mud level. Accurate calculation of surge-swab pressure is the key to successful managed pressure casing running. In this study, we show a dynamic surge-swab pressure model for the managed pressure casing running method. By analyzing the factors influencing surge-swab pressure, it is found that the surge-swab pressure of wellbore rises with the increase in initial position of the bottom of the casing, casing running speed, drilling mud density and viscosity. By contrast, changing mud level has little impact on surge-swab pressure. The surge-swab pressure during the deceleration stage of the casing has a negative value, namely the swab pressure. Therefore, we propose a managed pressure casing running method based on the accurate calculation dynamic surge-swab pressure. This method controls the wellbore pressure by adjusting the mud level according to the surge-swab pressure. Managed wellbore pressure casing running can reduce not only the risk of lost circulation and fluid influx during casing running, but also the time for casing running operation.