Study of Sour Gas Kicks Taken During Managed Pressure Drilling Operations

Abstract

Abstract The well control safety is always an important issue in the drilling engineering, especially for the sour gas reservoir in deep carbonatite, in which the formation gas invade into the wellbore quite easily and that is difficult to be detected due to narrow mud window and high solubility of sour gas respectively. However managed pressure drilling (MPD) well control is an effective new method to deal with gas kick problems, which allows circulating a certain influxes out of the wellbore without shut-in procedure while keeping the bottomhole pressure relatively constant. Therefore it might be an appropriate way to be used in sour gas kicks with high H2S content. A transient multiphase flow model for complete kick circulations of MPD well control operations is proposed. The presented model takes gas solubility in Water Based Mud (SBM) into account, which is often ignored in most hydraulics calculations, by using PR equations of state (EOS) to predict the PVT behavior of the drilling fluid with dissolved gas. The validity of the model including liquid volume fraction and pressure drop have been both verified with large-scale flow loop experimental date at The University of Tulsa. There are three main variables (maximum pit gain/casing pressure/casing shoe pressure) during complete circulation of a kick that are calculated and analysed. As the content of H2S increases, the maximum pit gain/casing pressure both decrease while the maximum casing shoe pressure remains almost the same. The peak value of casing pressure appears obviously later than the moment that gas front migrates to the wellhead. Moreover, the effects of degrees of choke opening on the annular pressure during the MPD well control operations are studied. As a result, this study has an important guiding significance to improve MPD well control theory for sour gas reservoir development with lower non-productive time (NPT).