Journal of Process Control 69 (2018) 58–69 Contents lists available at ScienceDirect Journal of Process Control j our na l ho me pa g e: www.elsevier.com/locate/jprocont Oil production increase in unstable gas lift systems through nonlinear model predictive control Fabio C. Diehl a,b,∗ , Cristina S. Almeida a , Thiago K. Anzai a , Giovani Gerevini b , Saul S. Neto a , Oscar F. Von Meien a , Mario C.M.M. Campos a , Marcelo Farenzena b , Jorge O. Trierweiler b a Research and Development Center, Petrobras, 21941915, Rio de Janeiro, Brazil b Chemical Engineering Department, Federal University of Rio Grande do Sul, 90040040, Porto Alegre, Brazil a r t i c l e i n f o Article history: Received 1 May 2017 Received in revised form 7 July 2018 Accepted 11 July 2018 Keywords: NMPC FOWM Severe slug flow Deepwater Ultra-deepwater Offshore crude production a b s t r a c t Oil production employing gas lift techniques enable the production of no natural flow wells and supply the energy lost in the reservoir caused by the field depletion, keeping the production in brown fields feasible. The multiphase flow conditions and the long pipes used to transport the fluids from the reservoir to the surface facilities, especially in deep and ultra-deepwater cases, may create unstable flow situations. Several publications in process control have discussed this problem since the 1980s, but the potential multivariable actions on the choke valve and gas lift flow have not been explored so far. In this paper the operating oil production system is treated through a nonlinear predictive control strategy. The strategy evaluation in a rigorous model (OLGA) shows the association between predictive capability and the integrated actuation in the manipulated variables results in an oil production increase and a partial or entire suppression of the instabilities in the multiphase flow. Furthermore, the rate of acting required on the valves is lower in the multivariable approach, allowing the use of slow choke valves as a final control element. © 2018 Elsevier Ltd. All rights reserved. 1. Introduction The onshore oil industry was responsible for supplying 90% of the world’s crude oil in the 1970s. This number has dropped to around 70% these days, driven mainly by new discoveries in the offshore environment. The evolution of technologies in seismic has made it possible to improve exploration in saline basins and deeper waters, which reinforces the perspective of increasing the partici- pation of the offshore industry in the world’s oil supply in the next years. Notwithstanding, producing hydrocarbons in offshore con- ditions is more complex than in the onshore environment, which makes exploration and production more dependent on technolog- ical capacity building. In recent years, the most relevant discoveries of new offshore carbon sources were reported in deep or ultra-deepwaters. The Brazilian pre-salt is an example of a new exploratory frontier at high depths of water. Wells installed in this area may require more ∗ Corresponding author at: Research and Development Center, Petrobras, 21941915, Rio de Janeiro, Brazil. E-mail address: fabio.diehl@petrobras.com.br (F.C. Diehl). than 10 km of piping to transport the reservoir fluids to the surface facilities. In deep and ultra-deepwater, pipelines typically carry the multiphase mixture containing oil, gas, water, and sediments across a series of obstacles including rocks, seabed, and ocean, which impose conditions of horizontal, vertical, and inclined flow to the fluids. One of the implications of this configuration is the appear- ance of instabilities in the transport flow of the multiphase mixture. Depending on the characteristics of the fluids (mass fractions of the phases, viscosity, etc.) and the flow conditions (phase velocity, flow directions, etc.), it is possible to form regions of liquid accu- mulation with the effect of blocking the incoming gas upstream of the liquid accumulation. This situation forces the pressure in the gas side to increase until this pressure is high enough to push the entire mass of liquid in front of it. This kind of instability is known as terrain slugging and can occur in production columns when the production column presents a horizontal part, or in the subsea flowline where it is most common due to the irregular seabed. When this phenomenon occurs in the connection between the flowline and the riser, also called low point, the instability is known as severe slugging (riser-induced slugging) due to the sig- nificant pressure amplitude resulting in the flow. The slugging is a cyclic phenomenon that results in permanent oscillations in the https://doi.org/10.1016/j.jprocont.2018.07.009 0959-1524/© 2018 Elsevier Ltd. All rights reserved.