To increase the oil recovery in the Albacora field, significant water injection is required that was not considered in the initial project-development phases. Technical and economic constraints do not allow the use of conventional seawater-injection plants because current production units have no area available to implement a conventional water-injection system. Application of subsea raw-water-injection (SRWI) systems in the field involved challenges that required a detailed and systematic analysis to evaluate the technical feasibility and establish requirements for implementation. This alternative enabled seawater to be injected into the reservoir with minimum treatment by use of equipment installed on the seabed.
Introduction
In some offshore fields, mainly mature fields, the addition of conventional technologies to topside facilities can constrain the use of seawater injection. Conventional systems require the installation of too many pieces of equipment at the production units, which requires large areas that sometimes are not available. Other restrictions include swivel constraints on floating production, storage, and offloading vessels (FPSOs) and load limitations.
One alternative is SRWI technology in which most of the system is installed at the seabed and seawater is injected with minimum treatment. This system has minimal effect on the topside-facilities footprint. The SRWI alternative involves technical challenges that include seawater compatibility with the reservoir rock and fluids, microbiological control, corrosion, water properties, reliability of the subsea equipment, and power, which limit the use of this technology in some scenarios.
Petrobras established a project to evaluate and develop this technology, involving technical-feasibility studies and evaluation of various scenarios and preliminary specifications. During this project, Albacora-field reservoir studies indicated the need for large amounts of water injection, and SRWI became the only economically feasible alternative identified to address this demand.
Albacora Field
Albacora is a giant field in the Campos basin, approximately 100 km offshore Brazil, covering an area of approximately 235 km2, with water depths of 250 to 1100 m. The field was discovered in September 1984 and has an estimated oil-in-place volume of 4.4 billion bbl. Pilot-phase production started in 1987 with the FPSO P.P. Moraes. In 1993, a second development phase started with the semisubmersible P-24, which was replaced later by the definitive production system with two production units: the semisubmersible P-25 and the FPSO P-31, installed in 1996 and 1998, respectively. Field oil production peaked in 1999.
During the production phase, it was determined that the aquifer was less active in some reservoirs than expected. Thus, higher water-injection rates are required. Because of space restrictions, a conventional water-injection system could not be installed on P-25. P-31 has sufficient space for this installation, but there are constraints in the FPSO turret that would require too much work to be performed with the platform in operation. Also, new oil accumulations were discovered in the Albacora field and there were opportunities to perform infill drilling of developed reservoirs.
SRWI Feasibility. Traditionally, offshore seawater injection is performed by injecting deaerated seawater, filtering the water to make it compatible with the reservoir rock, and adding biocides. The injection of nondeaerated seawater with minimum treatment would require adjustments of the injection procedures and was not feasible in some reservoirs. The main effects of untreated water were potential loss of injectivity from a higher number of solid particles, corrosion of materials in contact with the aerated seawater, and bacteria growth in the injection system. Adjustments to the water-injection process were necessary to deal with potential effects in injection flow rate and equipment integrity. The feasibility and effects of SRWI implementation depend on seawater properties and reservoir characteristics.
Campos Basin Seawater Characteristics
The quality and characteristics of the raw seawater vary with the location, the depth, the season, and, in shallow waters, the meteorological conditions. The size distribution and volume of the particles, the quantity and nature of microorganisms, and the amount of dissolved oxygen are highly variable. To assess the technical feasibility and specify an SRWI system, a survey was made of seawater physicochemical and biological parameters. The main parameters sampled at the Albacora field were solid-particle count and size, dissolved oxygen, and bacteria quantities. There is a tendency for more solid particles to exist near sea level and near the seabed. These occurrences are the result of the amount of marine life near the sea surface and the suspension of particles generated by deep ocean currents. Therefore, it was proposed to position the SRWI water intake 100 m above the seabed. At this depth, seawater quality is good and, in some cases, close to the quality of conventionally treated seawater in terms of solid-particle content.
System Description
It was decided to install three SRWI systems in the Albacora field, injecting a total of 16 500 m3/d of seawater into seven wells in the Marlim and Albacora reservoirs to increase the reservoir pressure. All the systems would be connected to semisubmersible P-25. One system injects water into three wells in a piggyback layout, and the other two systems inject into two wells each, also in piggyback layout. The water, from the sea approximately 100 m above the seabed, goes through a filter before reaching the pump inlet. From this point, the water moves through flowlines to the first subsea tree where part of the water is injected, controlled by a choke. The remainder of the water flows to the other well(s). Fig. 1 shows an Albacora SRWI system on the left and the overall subsea layout on the right.
The main Albacora SRWI components include the following.
- Electrical-power cable
- Umbilical
- Water-intake system
- Subsea filter
- Pump module
- Subsea control and monitoring system
- Barrier-fluid hydraulic-power unit
- Topside pump and control system
Pump Module. The pump module includes the raw-water-injection pump, seawater filter, and valves for operation, injection of chemicals, and water sampling. Fig. 2 shows the pump module and its components. Although each location has specific flow-rate and pressure requirements, all the pumps are identical to keep spare-part and total-system cost as low as possible. The differences are the speed and power level at which they run. Each pump is driven by its own variable-speed drive (VSD), and the operating points are maintained close to the best efficiency for each pump. Therefore, all the pumps are interchangeable between the locations, with power limited by the respective VSDs.
Because the initial reservoir-pressure depletion was low, the injection rate could be achieved without running the pumps. Therefore, a bypass line allows water injection by use of hydrostatic pressure alone, which saves power. Later, pressure in the regions near the injector wells is expected to increase and injectivity will decline, requiring the use of the pumps to maintain the specified injection rate.
Subsea Filter. Each pump module is equipped with one filter unit, positioned on the suction side of the water-injection pump. The filter consists of 46 cylindrical rods in two circular patterns. Raw seawater is conducted into the rods from both ends and flows out through small slots that make up the filter area. Particles larger than 50 µm are caught on the filter walls. When required, embedded particles will be removed from rod walls by backflushing.
A backflush sequence consists of two concurrent operations: continuously rotating backflush arms to isolate two filter rods at a time with the flush arms and opening the filter-backflush valve to backflush the two isolated filter rods. The sequence progresses through the entire set of filter rods. The backflushing sequence could be started by time (typically once/week), by differential-pressure reading across the filter unit, or manually. During this operation, water from the clean side of the filter is pulled back through the filter rods by an ejector, which is driven by the discharge pressure from the water-injection pump. Full flow rate is maintained through the filter, and flushed water is dumped to sea.
Water-Intake System. The water-intake system consists of a 6-in.-internal-diameter flexible riser 200 m long, connected to a buoy, approximately 100 m above the seabed. The position of the collecting point was determined after several samplings performed around P-25 to minimize the possibility of clogging the filters. There are two strainers connected to the upper end of the riser, for a rough filtration, avoiding big particles or fish that could enter the system. The strainers are retrievable by use of remotely operated vehicles and are coated with antifouling paint to prevent marine growth. At the lower end, there is a vertical-connection module, which is connected to the pump flow base.
Installation and Operation
One complete SRWI system has been installed. The installation of the flow base and pump module was performed by cable with a subsea-equipment support vessel. No relevant problems were faced during this installation phase. Despite having fewer pieces of equipment topside with the SRWI system, the effect of installing the equipment was high because of the interference with the regular activities of the platform. The development of alternatives that affect topside facilities less would be helpful to future projects (e.g., subsea VSD and pumps without barrier fluid). Water injection started 1 October 2012. The injection was by hydrostatic pressure because pump operation is not necessary during the initial phase of reservoir pressurization. Injectivity decline had not been detected at the time this paper was written.
This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper OTC 24167, “Albacora Subsea Raw-Water-Injection Systems,” by L. Buk Jr., C.A. Andrade, J.B. Azevedo, E.J.J. Coelho, O.C. Costa, C. Kuchpil, A.G. Siqueira, and A.L.S. Souza, Petrobras, prepared for the 2013 Offshore Technology Conference, Houston, 6–9 May. The paper has not been peer reviewed. Copyright 2013 Offshore Technology Conference. Reproduced by permission.