Riser gas unloading events in deepwater subsea well construction operations are very hazardous and difficult to control. Gas kicks may go unnoticed when gas dissolves in non-aqueous fluids (NAF), with dissolved gas volumes not showing significant pit gains on surface when conventional well control indicators are used. Once dissolved gas is circulated above subsea blow-out preventers (BOP) and comes out of solution at pressure and temperature conditions below the bubble point, it can quickly displace a large mud volume in the riser at relatively shallow depths, leaving little time to react for diverting the gas at surface.
In this paper, we present a variant of the constant bottom-hole pressure (CBHP) approach to managed pressure drilling (MPD) that offers a comprehensive way to manage and control riser gas unloading behavior. The technique enables early kick detection, thereby allowing more sensitive diagnosis of gas kicks going into solution, and preventing them from creating a hazardous situation in the first place. It also allows for the application of back-pressure on the dissolved gas-NAF fluid mixture in the riser, in order to delay, minimize or even prevent gas breaking out close to surface (in case dissolved gas is allowed above the subsea BOPs). Complicating factors include the pressure integrity limits of the marine riser, and the fact that the nature of any kick (whether it is gas, fluid or a combination, its detailed composition, volume, etc.) is generally unknown. It is argued here that such challenges are best met in the field by using automated MPD control methodology.
The designed controller (triple-mode constant bottom-hole pressure (TCBHP)) combines three modes of operation: pressure control, flow control and solubility control, each with their advantages and limitations. The controller can automatically switch between control modes based on observed kick behavior, thereby gaining the ability to compensate for the limitations of the individual control modes and, more importantly, deal with kicks agnostically, i.e. independent of their nature. We describe the architecture and underlying details of the new controller, and show its functionality by simulating several riser gas unloading scenarios. Two distinct cases that are given special consideration are: (1) the case where the subsea BOPs remain open after the kick has passed them and the controller regulates pressure using maximum allowable surface pressure (MASP) / downhole fracture gradient information as an upper limit; (2) the case where the subsea BOPs are closed after the kick has passed them and the controller regulates pressure within riser pressure limits when the kick is circulated to surface using the riser booster pump. Simulation results show that the new TCBHP controller can quickly and robustly control riser gas unloading situations with complicated transient conditions, without fracturing downhole formations and/or jeopardizing the pressure integrity of the riser. Its development aims to help mitigate some of the fears and concerns around riser gas unloading, and making automated subsea well control using CBHP- MPD technology a reality in the near-future.