Mitigating Gas in Riser Rapid Unloading for Deepwater Dual-Gradient Well Control
Authors: Carpenter Chris 
Published: February 2017

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 27242, “Mitigating Gas in Riser Rapid Unloading for Deepwater Dual-Gradient Well Control,” Zhaoguang Yuan and Dan Morrell, Schlumberger; Paul Sonnemann, Safekick; and Colin Leach, Mulberry Well Systems, prepared for the 2016 Offshore Technology Conference, Houston, 2–5 May. The paper has not been peer reviewed. Copyright 2016 Offshore Technology Conference. Reproduced by permission.
In deepwater dual-gradient wells, if gas comes into the riser, a rapid unloading event may occur if removal of the gas is not carried out properly. Oil-based muds (OBMs) present an even greater challenge when compared with water-based muds (WBMs) to avoid gas in riser events. A study using a dynamic multiphase-flow software simulated a rapid-unloading event and determined the gas fraction in the riser annulus and the effect on riser fluid levels.
Case Description and Model Verification
Several researchers have run field tests to study rapid unloading of gas in deep-water risers. These tests used WBMs; there are no comparable data from tests using OBMs.
Previous papers described the results from field tests of procedures for handling various well-control scenarios in a dual-gradient drilling application. The test well is a vertical well located in 1,214 ft of water. The dual-gradient drilling system being tested is a controlled-mud-level-type system, so the level of mud in the riser is used to achieve the dual-gradient effect. The riser mud level was lowered 328 ft below the Kelly bushing. Mud used during the test was 8.91-lbm/gal WBM, with a plastic viscosity of 9.0 cp and a yield point of 21 lbf/100 ft2. The drillstring mud-pump rate was 660 gal/min, and a top-fill pump flowing at 158 gal/min injected mud at the top of the riser.
Several different procedures were tested; however, Test 5 produced a rapid unloading of the riser. 550 lbm of nitrogen gas was injected through the drillstring and circulated into the riser. This gas bubble was then pumped up the riser with an open blowout preventer (BOP). This study uses a dynamic multiphase-flow software to reproduce the standpipe pressure and surface liquid rate in this test. The simulated standpipe-pressure trend matches the recorded data very well. The first field observation was of a “small spray of mud,” which most likely came from the top-fill pump as the air in the riser was pushed out by the mud and gas injected into the riser. The next field observation noted a fountain of mud with a height of approximately 33 ft. Calculations show a maximum fluid velocity of 46 ft/sec, while the simulation results show a 49-ft/sec maximum mud velocity. At the moment of rapid unloading, there are some fractions of free gas coming out with the drilling fluid.
The field test shows that gas migration and rapid unloading occur at 35 minutes, while the simulation shows the event to be over by 48 minutes. The software was not able to fully match the exact timing of the event. However, the simulation was able to match all the trends in pressure and velocity involving gas migration and rapid unloading.