Analysis of Riser Gas Pressure from Full-Scale Gas-in-Riser Experiments with Instrumentation

Summary Because the use of adaptive drilling processes, such as managed pressure drilling (MPD), facilitate drilling of otherwise nondrillable wells with faster corrective action, the drilling industry should include the effect of gas dispersion, bubble suspension, fluid compressibility, and riser ballooning to avoid the overestimation of riser pressure and to produce more efficient well control methods. The IADC Deepwater Well Control Guideline recommends always addressing riser gas first, before proceeding to control the well in a well control situation. The intent is to remove the risk of gas reaching the surface and the rig floor, putting personnel and assets at risk. However, with the availability of equipment on the rig dedicated to handling riser gas and the fact that the riser is isolated from the wellbore, the atmosphere reduces the level of risk of gas in the riser, whereas the well below the subsea blowout preventer (SSBOP) poses a greater risk. In this paper, we discuss the results from full-scale experiments recently conducted in an extensively instrumented test well (LSU Well-2) and demonstrate that the riser pressures resulting from upward transport or aggregation of riser gas are much lower than the values estimated using the single-bubble model and industry thumb rules. We explain the danger of using an open-top riser top during the monitoring of gas-in-riser and how the situation can get out of control due to the potential dynamic unloading situation. Our research also demonstrates the minimal fluid bleedoff volumes required to reduce pressure buildup consequences of free gas migration in a fully closed riser containing low-compressibility liquid. A differential pressure methodology used in this paper for analysis was also used for detecting the presence, position, void fraction, and lead and tail velocity of the gas column in real time during each of the tests to make decisions during the tests. The results from a successful application of the fixed choke constant outflow (FCCO) method as a new method for circulating out gas from the riser by staying within the gas-handling capacity of the existing mud gas separator (MGS) on the rig are presented. This is the industry’s first test of the FCCO method.