Modelling two-phase flow for control design in oil well drilling

Today, marginal oil wells are being drilled and the operating margins for the bottom-hole well pressures during drilling are becoming narrower. This requires an improved control of the pressure balance between the reservoir pore pressure and the well bottom-hole pressure. In oil well drilling applications, the pressure is typically controlled manually by adjusting the choke valve. This paper proposes a simple feedback PI-control scheme with feed-forward compensation of the known disturbances. A low-dimensional dynamic state model for two-phase flow has been developed to be able to tune the control parameters. The proposed method is presented and evaluated using a detailed oil well drilling simulator. The results show that the proposed control design keeps the bottom-hole pressure within the operating margins