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

Abstract

Abstract The potential for a gas-in-riser situation to become uncontrollable by the rapid displacement of mud out of the riser is extremely high if the riser-top is left open. The unloading can be catastrophic in synthetic-based mud (SBM) or oil-based mud (OBM) when the gas remains dissolved and undetected till pressure reduction causes 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 migration experiments. A 5200 ft deep vertical test well (9 5/8" x 2 7/8" casing/drill-pipe) at LSU instrumented with 4 down-hole PT gauges was used for the tests. Tests were carried out in water, and SBM. Each test started by injecting a fixed volume of nitrogen gas (5 to 15 bbl) at a low (0.3 bbl/min) or high flow rate (4 bbl/min) from the bottom of the annulus while keeping the annulus open. After the influx, the annulus was either closed at the surface to study the effects of gas migration under shut-in conditions or left open to study the effects of 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-annulus when 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 reduction in 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 the surface requiring an immediate shut-in of the annulus to avoid unsafe rapid discharge. The final estimated outflow 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 during the tests and to estimate the location and migration velocities of gas-front and tail. The model developed for analysis and comparison of test results in water is used here to explain the behavior of gas migration under open-top conditions. A thorough investigation with the help of gauge data and pit gain has explicated our understanding of gas migration behavior and its effect on the dynamics of gas-liquid equilibrium from influx to impending unloading situation. The interesting results from the tests are extremely useful in explaining the dangers of using open-top annulus on rigs.