A Simplified Model for Multi-Fluid Dual Gradient Drilling Operations

Dual gradient drilling (DGD) is a method for drilling deep water offshore wells safely, efficiently, and to a reduced cost. The dual gradient effect is enforced by using a heavy fluid fitting the drilling window, and a subsea pump to lift the returns up to the rig. Automatic control of this pump is fundamental to ensure a safe and efficient dual gradient drilling operation. In this paper we extend existing work on modelling for DGD in two ways. Firstly, we present a simplified model for operational scenarios where there are multiple fluids in the well, e.g. when changing drilling fluids. Secondly, we make the model more realistic by including a model of the centrifugal subsea pump. The resulting model is then used to simulate a scenario with changing of drilling fluid, and where a PI controller is used to maintain a constant downhole pressure. The simulation confirms that the model is reasonable, and that the PI controller is able to maintain a near constant downhole pressure throughout the operation.