Fixed Choke Constant Outflow Circulation Method for Riser Gas Handling: Full-Scale Tests in Water- and Synthetic-Based Mud with Gauges and Distributed Fiber-Optic Sensors

Abstract

Summary Conventional methods of managing gas-in-riser events are not optimal when the drilling riser is isolated from the formation by a closed subsea blowout preventer (BOP) on rigs equipped with mud gas separator (MGS), managed pressure drilling (MPD), or riser gas-handling equipment. The industry is concerned about exceeding the pressure limit of the riser and the flow capacity of the MGS and hence has not been able to reach a consensus on a circulation method for riser gas. This work is an analysis of the first-ever demonstration of the fixed-choke constant-outflow (FCCO) circulation method in synthetic-based mud (SBM) carried out in June 2022. The first-ever demonstration of the FCCO circulation method in water was performed in March 2021. The results from the water tests were discussed in IADC Gas-in-Riser/Riser Gas-Handling Subcommittee meetings, and the new fixed-choke method was named FCCO in November of 2021. The reason for using the acronym FCCO for the new method is that it allows the use of a fixed-choke opening percentage throughout the circulation period by managing the outflow and backpressure by varying only the pump rate. This work includes the comparison of the actual test results from the March 2021 FCCO tests in water with results estimated using a new model. This is followed by a discussion of the results from the June 2022 FCCO test in SBM. Nitrogen gas was injected into the bottom of an annulus 5,200-ft deep, vertical test well (9 5/8×2 7/8 in. casing/tubing) initially filled with water and instrumented with four downhole pressue and temperature gauges, and distributed fiber-optic sensors [distributed temperature sensing (DTS) and distributed acoustic sensing (DAS)] for water tests, and later filled with SBM. We started direct circulation to produce flow out of the annulus through a choke kept at a fixed open position (%) required for a preplanned applied surface backpressure (ASBP). We reduced pump rate if/as necessary to maintain this ASBP to ensure outflow rate within MGS flow capacity. We performed tests at different fixed-choke positions, different average ASBPs, and initial pump rates. We tested constant bottomhole pressure (CBHP) circulation and fixed pump rate methods also for comparison with the FCCO circulation method. The results from the FCCO tests demonstrated better control of outflow compared with the other methods. There is no need to use high ASBP. The use of a high ASBP suppressed the value of peak pressure. Installation of more than one gauge inside the riser enhances safety by allowing real-time influx detection capability, estimation of gas position, gas velocity, and gas void fraction. The FCCO method can be preplanned and easily substituted as the preferred circulation method for staying within the handling capacity of the existing MGS on the rig during gas-in-riser situations.