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.
Several researchers have run field tests to study rapid unloading of gas in deepwater 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.
Mitigating Gas in Riser Rapid Unloading for Deepwater Dual-Gradient Well Control
19 December 2016