Cascaded bottom hole pressure control in managed pressure drilling

Today, in search of oil resources, marginal wells with narrow pressure windows are frequently being drilled. This requires accurate and precise control to balance the bottom hole pressure (BHP) between the pore and fracture pressure of the reservoir. Managed pressure drilling (MPD) is a technique introduced to enable improved pressure control when drilling. This paper presents a cascaded control structure for BHP, choke pressure and choke position in MPD operations. Estimators for the unknown bottom hole flow and uncertain BHP are developed to improve pressure control performance. The presented method is evaluated with field operation data from drilling two 4000 m deep wells. The results show that the pressure is maintained within acceptable margins through a series of operations.