Marlim 3-Phase Subsea Separation System (Petrobras): Introduction to the Involved Reservoir Background

Abstract

Abstract This paper presents an overview of the reservoir requirements regarding thedevelopment of the Marlim 3-Phase Subsea Separation System - SSAO (for theterms in Portuguese). Produced water re-injection (PWRI) in the reservoir insuch scenario is a challenging task, regarding the well injectivity maintenance(SSAO-treated produced water suspended solids and oil content, sub-sea andsub-surface facilities materials compatible with the produced watercorrosiveness), geomechanical effects, and biogenic reservoir souringcontrol. Lessons learned from previous PWRI tests and the implications of somealternatives to the subsea separation and re-injection systems are reported. This work comprehends also the definition of chemicals applyed to preventundesirable effects: a bio-dispersant (to avoid biocorrosion and biofouling inthe re-injection system) and a nitrate salt (to oppose biogenic reservoirsouring potential). Relevant operational aspects are mentioned as well, fromchemical injection to the monitoring of both solid particles and oil dropletscontents in the re-injection water stream. Introduction Marlim field area is 149.4 km2 and the water depth range is 650 - 1050 m. Themain reservoirs of Marlim field are turbidites of Oligocene/Miocene age. All ofthe sandstone facies are poorly-consolidated. Reservoir-rock porosity andpermeability are relatively homogeneous, typically ranging 27 - 30 %, and 1,000- 6,500 mD (4,000 mD average), respectively. The oil API varies from 17° to 24° in the main field area and oil saturationpressure is 265 kgf/cm2, which is 22 kgf/cm2 below the reservoir originalpressure. The Marlim reservoir mechanism is solution gas drive. Water injectionhas been selected as the most feasible reservoir pressure maintenance method, thanks to its simplicity, besides factors like deepwater location, reservoircontinuity, relative scarcity of gas, and favorable characteristics of therelative permeability curves. Initial oil production from the Marlim field was in March 1991 and the peak ofproduction was approximately 630,000 bpd of oil in April, 2002. Seawaterinjection started in 1994. Injection pumps were designed to matrix injectionconditions, but considerable injectivity decline was observed in some of theearlier injection wells. Marlim field was developed with subsea wells, so anyworkover operation to restore the injectivity requires a floating rig to beperformed, which makes them very expensive. Therefore, injectivity preservationis essential to assure that the sustainment of the required seawater injectionvolumes. A comprehensive program was established to control the injected waterquality, imposing strict limits to its suspended solids concentration andparticle size, oil and grease content, as well as corrosion-related parameters(oxygen, CO2 and sulfide content, bacteria counts). Injection rate, injectivityindex, frequency of tubing or flowline substitution, corrosion rates andfilterability are also permanently monitored (Pinto et al., 2001). Some of the Marlim field water treatment facilities are nearly reaching theircapacity. An alternative to reduce the topsides water load is to remove water,as much as possible, on the seafloor. So far, it is not considered feasible, onthe subsea system, to achieve the required water quality to discharge it on thesea. The only alternative is to dispose the separated water in subsurface. InMarlim field, it was decided to re-inject the separated water in the reservoiritself, because there are no proper adjacent geological formations to takeit.