Supersonic Flow Conditions During Gas-In-Riser Unloading and Implications

Abstract A transient flow model based on the drift-flux approach has been used to investigate if supersonic flow conditions can occur at the outlet when gas unloads the riser. The transition to supersonic condition will take place when the fluid velocity exceeds the sonic velocity in the two-phase mixture. The sound speed was estimated by an expression found in literature. During supersonic flow, the pressure at the outlet of the riser will increase above atmospheric pressure and the numerical scheme was modified to estimate what the pressure build-up could be become. A simulation study was performed where riser depths, mud weights and initial gas volumes at bottom of riser condition were varied. Only water-based mud was considered. The gas was circulated slowly towards the surface until the riser was unloaded. It was seen that for riser depths between 2000 and 3000 meter, a pressure build-up would take place for initial gas volumes above 10 m3. The pressure build-up became more severe for deeper risers, higher mud weights and larger initial gas volumes. The pressure build-up demonstrated can increase the risk for overboard circulation. It can also possibly pose a threat to the integrity of the equipment. Further studies are recommended to quantify the magnitude of the pressure build-ups and evaluate equipment selection for readiness when dealing with gas in riser.