A Switching MPD Controller for Mitigating Riser Gas Unloading Events in Offshore Drilling
Authors: Gu Qifan , Fallah AmirHossein , Ambrus Adrian , Ma Zheren , Chen Dongemi , Ashok Pradeep , van Oort Eric 
Published: January 2020

Summary
Riser gas unloading events in subsea well construction are hazardous and difficult to control. When a gas influx enters the wellbore and dissolves in nonaqueous fluids (NAFs), it may go unnoticed because the pit gain on the surface may be minimal and remain below the detection threshold when using conventional well control indicators. Once the dissolved gas is circulated up and comes out of the solution at pressure and temperature conditions below the bubblepoint, it can quickly displace a large volume of mud in the riser or the chokeline. When this breaking out of gas occurs at a shallow depth, it leaves little time for the rig crew to react.
In this paper, we present a novel managed pressure drilling (MPD) approach for riser gas unloading control that makes use of a pressurized riser drilling (PRD) controller. The PRD method employs the constant bottomhole pressure (CBHP) controller for normal operations (e.g., drilling and circulation of kick), whereas it offers a more comprehensive way to manage and control a riser gas unloading behavior. The PRD choke controller dynamically applies a backpressure on the dissolved gas–NAF mixture in the riser to delay, minimize, or even prevent the gas breaking out at locations closer to the surface. The control algorithm considers the pressure limits of the riser and of the openhole formations and can adjust for kick uncertainties (e.g., whether the kick is a gas, liquid, or both, its volume and distribution).
The proposed PRD controller consists of three operation modes: pressure control mode, flow control mode, and solubility control mode, with each mode applicable to its corresponding operating condition. The controller can automatically switch among different modes on the basis of the observed kick behavior, thereby gaining the ability to compensate for the limitations of the individual control modes and, more importantly, to deal with kicks agnostically, i.e., independent of their nature. The proposed controller is evaluated by simulating different riser gas unloading scenarios. Here, two distinct cases are given special consideration: (1) the case in which the subsea blowout preventers (BOPs) remain open after the kick passes them and the controller regulates the pressure using the maximum allowable surface pressure (MASP) or downhole fracture gradient as an upper limit and (2) the case in which the subsea BOPs are closed after the kick passes them and the controller regulates the pressure within the riser pressure limits when the kick is circulated to the surface using the riser booster pump.
Simulation results show that the proposed controller can quickly and robustly control the riser gas unloading situations with complicated transient conditions, without fracturing downhole formations or jeopardizing the pressure integrity of the riser. The developed PRD controller aims to help mitigate some of the concerns about riser gas unloading when the dissolved gas is allowed to pass the subsea BOPs and enter the riser and to facilitate the implementation of more automated subsea well control using MPD technology in the foreseeable future.