Deep Water Drilling: Full Pressure Profile Control in Open Hole Section Utilizing Model Predictive Control
Authors: Breyholtz Oeyvind , Nygaard Gerhard Haukenes 
Published: May 2016

Abstract
Managed pressure drilling has been introduced to reduce pressure related incidents related to targeting narrow pressure margins prospects or deep water prospects. The full pressure profile method, which will be presented in detail in this paper, classifies as a managed pressure drilling method, and is an extension of the well known constant bottom hole method. A case study on a deep water drilling system is presented to demonstrate how model predictive control can simultaneously control the height of the mud column and the flow rate to control the bottom hole pressure while trying to maintain an optimal casing shoe pressure. A model predictive controller is implemented to fully automate the pressure and flow dynamics. Computer simulations are presented to illustrate the potential of the chosen control strategy. Advantages and limitations associated with the full pressure profile control method is analyzed and discussed.

Introduction
More and more of the world's offshore oil and gas reserves are located in deep waters, and the petroleum industry is demanding new drilling concepts that make exploitation of these resources economically feasible. New deep water drilling concepts have been introduced to overcome the challenges associated with deep water drilling. These concepts have a favourable pressure profile which resembles the natural hydrostatic pressure gradient of water and the pressure exerted by the rock formation.
In the last decade Managed Pressure Drilling (MPD) has been introduced as a powerful tool to precisely control the pressure dynamics in the well (Hannegan et al., 2004). By definition an MPD operation is "an adaptive drilling process used to more precisely control the annular pressure profile throughout the well bore. The objectives are to ascertain the downhole pressure environment limits and to manage the annular hydraulic pressure profile accordingly". MPD operations are often associated with deep water drilling operations, High-Pressure-High-Temperature (HPHT) wells, extended reach drilling, and operations targeting narrow pressure margins prospects. MPD comes in many variants (Hannegan, 2007), such as PMCD (Pressurized Mud Cap Drilling), CBHP (Constant Bottomhole Pressure), RC (Reverse Circulation), and DGD (Dual Gradient Drilling, several methods). To become possible, MPD requires equipment such as drill string non-return valves (NRV), rotating control device, choke manifold, surface separation system, and various pumps. While such equipment offers unprecedented capabilities, it also generates operability challenges that have to be addressed before widespread acceptance of the technology (Rehm et al., 2008). MPD equipment can be used to rapidly counteract pressure disturbances by changing flow rates and/or topside choke opening. Recent experience indicates that in order to fully exploit the opportunities of the MPD equipment, automatic control is needed since optimal manual operation of such equipment is almost impossible. Automatic control of the flow and pressure dynamics in the well can be a challenging task, due to the complex behaviour of multiphase flow, which potentially contains drilling mud, gas oil and cuttings. MPD automation has been limited in the field to date (Santos and Catak, 2007; Fredricks et al., 2008; Bjørkvoll et al., 2008; Godhavn, 2009) but its potential is growing (Thorogood et al., 2009). In (Nygaard et al.,2007) a Nonlinear Model Predictive Controller (NMPC) is introduced as an efficient control scheme for oil drilling systems, and in (Breyholtz et al.,2009a) and (Breyholtz et al.,2009b) the NMPC control has been developed further for optimal multivariable control of different MPD systems. In this work, we focus on one aspect of this automation approach, namely automated control of the pressure profile in the open hole section of the well., by controlling both the bottomhole pressure (BHP) and casing shoe pressure (CSP) in a deep water drilling system. The automation system uses model-predictive control (MPC) as automation technology to coordinate the main mud pump flow rate and subsea pump flow rate.