Evaluation of control methods for drilling operations with unexpected gas influx

This paper presents an evaluation of various control methods to be used during drilling operations where an unexpected gas influx occurs. In the event of an unexpected gas influx the current industry procedure is to control the pressure in the well manually. The drilling industry term for this manual procedure is well control. The focus of the paper is threefold. Firstly, to design an automatic sequence which is similar to the existing manual procedure. Secondly, to evaluate three different control algorithms for pressure control during an unexpected gas influx, and thirdly, to evaluate control parameter tuning needed when implementing different control algorithms.The control methods have been evaluated on various drilling scenarios with unexpected gas influx, referred to as a kick. After a kick of reservoir gas has entered the well, automatic control of the well control choke and rig pump is applied to compensate for pressure fluctuations while circulating out the gas. A PI controller is designed to stabilize the well pressure by controlling the well control choke, an internal model controller (IMC) controls the pressure by manipulating the choke and the rig pump flowrate, and a model predictive controller (MPC) uses coordinated control of the choke and the pump flowrate to stabilize the well pressure. The model based controllers use a simple first order model of the well. Simulations are performed using a detailed flow model of the well to test the controller performance and robustness. Several cases with different amounts of gas influx are investigated.The simulations show that it is feasible to control the pressure using automatic control of the choke valve and pump during an unexpected gas influx by use of all the presented control methods. The control methods are robust against changes in process conditions and disturbances, as they are able to handle several pressure levels and gas volumes without requiring re-tuning. However, since the pressure dynamics in the well are influenced when gas is entering the well, the model based controllers could probably be further improved if the models were updated after the gas influx occurred.The results indicate that adaption of the automatic sequence to the current manual procedure is applicable. However, to avoid a reduction in downhole pressure when stopping the pump and shutting in the well, the automatic sequence may be further improved beyond what is feasible with manual operation.