Gas in riser: On modeling gas influxes in non-aqueous drilling fluids with time-dependent desorption considerations

Abstract

The presence of formation gas in a marine riser above the subsea blowout preventer (SSBOP) can pose great risks to personnel, equipment, and the structural stability of the drilling rig. The offshore drilling industry has been seeking to better understand and simulate gas-in-riser events, to optimize mitigation methods and justify the adoption of new flow control equipment and techniques. This paper presents a modified multiphase model to more realistically describe gas-in-riser events by incorporating time-dependent mass transfer processes occurring in non-aqueous muds (NAMs). A hybrid advection upstream splitting scheme (AUSMV) was used as the numerical scheme. Verification of the model and the numerical scheme were performed using field data from well control events with both NAMs and water-based muds (WBMs). The role of time-dependent mass transfer played during the process, and the proper range of desorption coefficient was evaluated in the case study with NAMs. Furthermore, a comprehensive parametric study was conducted, and results show that the severity of unloading will be significantly underestimated without considering mass transfer in NAMs. Meanwhile, unloading severity can be overestimated if the desorption process is considered as instantaneous instead of time-dependent.