Dynamic Simulation of Riser Gas Handling with a Fixed-Choke Constant Outflow

Abstract

It is challenging to optimize operation procedures that safely remove riser gas without causing equipment, personnel, or unnecessary environmental issues. This paper presents results from studies of various scenarios using a fixed-choke constant outflow (FCCO) method that will keep the choke opening constant to measure fluid outflow while circulating the riser gas out through the booster line. A commercial dynamic multiphase flow software program was used to study extreme scenarios of up to 300 bbl of gas influx in the bottom of the riser of a deepwater rig equipped with a riser gas handling system or managed pressure drilling (MPD) system drilling in 12,000 ft of water. The consequences of gas migration and free gas dissolution into both water-based and oil-based mud were studied. The initial fixed-choke opening was selected by modeling a 500-gal/min flow rate through the boost line with a choke sized to initially obtain from 150 to 200 psi surface back pressure. The pump rate was reduced to 200 gal/min or even 0 gal/min as needed to keep choke pressure; hence, the return flow rate is relatively equal to the initial target value. Other scenarios with different influx volumes and choke openings were also studied to check pressure consequences to the riser system. When using the FCCO method to manage the 300 bbl influx with water-based mud, the software calculated a maximum pressure of 580 psi on the riser top. When using oil-based mud, a maximum pressure of 1,112 psi was observed on the riser top for the same scenario. When the same situation was simply shut in (instead of being circulated using the FCCO method), there was a maximum pressure of 2,300 psi on the riser top with water-based mud, but only 1,254 psi when using oil-based mud, ignoring for the moment the benefit provided by pressure relief valves in an actual operation, which could intervene before the riser top pressure reaches 2,300 psi. The analysis of the resulting surface choke pressure and surface gas/liquid flow rate support the concept that circulation of the gas influx while adjusting pump rate to keep the flow rate constant through a choke at the fixed position (even if it means reducing the pump rate to 0 gal/min as flow is driven by gas expansion alone) would be easier to handle than by using other proposed riser gas management procedures. This paper presents the first study of riser gas handling in an oil-based mud scenario considering gas dissolution with an FCCO and provide references for International Association of Drilling Contractors (IADC) riser-gas handling guidelines and deepwater well control guidelines.