Comparison of Riser Gas Unloading in Water and SBM: Full-ScaleExperiments

Document Type

Conference Proceeding

Publication Date

1-1-2023

Abstract

The potential for a gas-in-riser situation to become uncontrollable by the rapid displacement of mud out ofthe riser is extremely high if the riser-top is left open. The unloading can be catastrophic in synthetic-basedmud (SBM) or oil-based mud (OBM) when the gas remains dissolved and undetected till pressure reductioncauses sudden desorption of dissolved gas closer to the surface. This work demonstrates, investigates,and provides insights into the riser gas unloading phenomena with the help of full-scale gas migrationexperiments. A 5200 ft deep vertical test well (9 5/8" x 2 7/8" casing/drill-pipe) at LSU instrumented with 4 down-holePT gauges was used for the tests. Tests were carried out in water, and SBM. Each test started by injectinga fixed volume of nitrogen gas (5 to 15 bbl) at a low (0.3 bbl/min) or high flow rate (4 bbl/min) from thebottom of the annulus while keeping the annulus open. After the influx, the annulus was either closed atthe surface to study the effects of gas migration under shut-in conditions or left open to study the effectsof gas migration under open-top annulus conditions. The rate of pit-gain reduced during the low-void-fraction gas tests in water, and SBM-filled-annuluswhen gas influx stopped (closing of subsea BOP). However, for the high-void-fraction test in SBM, the pit-gain stopped once influx stopped and remained negative from 6.5 minutes to 35.5 minutes due to a reductionin mud level caused by the dissolution of gas in SBM. The pit gain later resumed and continued to increase.Keeping the annulus open resulted in a rapid exponential increase in pit-gain as the gas-front neared thesurface requiring an immediate shut-in of the annulus to avoid unsafe rapid discharge. The final estimatedoutflow rate based on cumulative pit-gain (Coriolis) was 160gpm for the high-void-fraction test in SBM.Pressure, and differential pressure data from pairs of gauges were used to make real-time decisions duringthe tests and to estimate the location and migration velocities of gas-front and tail. The model developedfor analysis and comparison of test results in water is used here to explain the behavior of gas migrationunder open-top conditions. A thorough investigation with the help of gauge data and pit gain has explicated our understanding ofgas migration behavior and its effect on the dynamics of gas-liquid equilibrium from influx to impendingunloading situation. The interesting results from the tests are extremely useful in explaining the dangers ofusing open-top annulus on rigs.

Publication Source (Journal or Book title)

SPE Latin American and Caribbean Petroleum Engineering Conference Proceedings

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