Subsea processing and boosting can be key enablers and optimization alternatives for challenging field developments whose benefits increase with water depth, flow rates, and stepouts. This paper summary focuses upon reporting new research-and-development (R&D) initiatives related to subsea processing and boosting offshore Brazil.
Introduction
In the initial exploitation stages of offshore oil and gas fields, the main concern involves the initial investment to construct the production units, drill the wells, and install all equipment necessary for production. Operators usually adopt a conventional solution at this stage, failing to view new technologies as a first alternative. Such technologies are usually considered only when they are vital for a specific field development.
For most fields in the Campos basin offshore Brazil, the main recovery mechanism from the beginning of production is waterflooding. A typical curve of produced fluids starts with the predominance of oil rising to the production plateau until the water breakthrough occurs. At this point, production presents increasing water content in the liquid stream. Therefore, because a massive water injection is necessary to maintain the pressure of the reservoir, after a few years the water cut increases considerably. Additionally, there is a need to overcome the constraints of existing platforms.
There are currently additional motivations for using subsea technology. Revitalization of mature fields can provide opportunities to develop subsea separation technologies for higher flow rates (including higher water cuts) and to develop subsea multiphase pumps for higher differential pressures, among other options. Other possible scenarios include small fields where old floating units can be replaced by subsea systems or fields where subsea boosting or processing systems optimize the production.
For a discussion of past and current proven technologies, concepts, and approaches that have been used as building blocks for new developments in both green and brown fields, please see the complete paper.
Future Vision of Subsea Processing and Boosting: Ongoing R&D Initiatives
There are several new R&D initiatives related to subsea processing and boosting being considered by the operator for application in potential scenarios involving both mature fields and green fields. These initiatives include the following:
- Compact subsea oil/water-separation system
- Compact subsea gas/liquid-separation system
- Subsea gas-compression system
- High-differential-pressure multiphase pump
Compact Subsea Oil/Water-Separation System. One of the main objectives of a subsea oil/water-separation system is to perform the primary processing of fluids on the seabed, providing produced water with a destination other than the topside facilities at the floating production unit. This is an attractive option for most of the fields in their mature phase of production.
Subsea separation is also interesting from the point of view of primary processing, because the sooner the water is removed from the production stream, the easier it is to treat both water and oil streams. All the shearing on the multiphase production stream to the topside installations that takes place when production is routed to a platform is greatly reduced, making treatment at least potentially easier and more effective. This kind of separation system is used mainly for mature fields, and it can be considered for application in a specific area with an existing infrastructure or for revitalization of a mature field in a remote region. To enable production in these mature-field remote regions, in which oil-flow rates are not high enough to justify a dedicated platform, the adoption of a subsea oil/water-separation system can be a solution.
The Troll pilot, installed in 1999, was the first subsea oil/water-separation system installation. In 2007, the Tordis system, designed for higher flow rates, was installed. In 2012, the Marlim three-phase subsea oil/water-separation system, which reinjects water into the same producing reservoir, was installed; it is the world’s first system for deepwater separation of heavy oil and water.
Compact separation technologies will be considered for the project in question, focusing upon a system able to receive the production from a manifold or a single high-capacity producer well and keeping a desirable footprint. Because of the structural requirements (thickness increase) imposed upon conventional separation designs by deepwater and high-flow-rate scenarios, there is a necessity to consider compact separation technologies in order to simplify the deployment, retrieval, and maintenance of the equipment.
Project Development. The first step of this project was the evaluation of possible technological lines to be followed. This assessment considered, as potential alternatives to conventional separators, the use of three core technologies for oil/water separation: tubular separators, hydrocyclones (for high oil contents), and electrocoalescers.
After these evaluations, conceptual studies were initiated with the aim of developing designs of subsea stations with each one of the aforementioned compact separators (or combining the technologies thereof). The main objective of these studies is to map the gaps that must be closed and the challenges that must be overcome for field application. Another objective of the conceptual study is to provide a preliminary qualification plan for the technological gaps identified.
The best choices for closing each gap will depend on the risk the challenge brings to the project as a whole. The gaps associated with high risk would be addressed in a technology qualification program, which corresponds to the last step of the R&D project; those items classified as low-risk can be treated during the equipment-fabrication phase through a common homologation process.
For these conceptual studies, a typical mature-field scenario is being considered, with a water depth of approximately 900 m. According to this scenario, the compact subsea oil/water-separation system will receive the production from four wells (a single flowline, coming from a four-hub manifold). Separated gas and oil (with residual water) will be sent separately topside, while the water will be injected in three existing wells, or new wells, in a piggy-back configuration.
In order to be applicable in deep water, and for increased production, the use of compact technologies is mandatory, instead of the use of large vessels. Tubular, cyclonic, and electrostatic technologies, all compact, are considered.
Compact Subsea Gas/Liquid-Separation System. The gas/liquid technology is applied subsea with the main goals of increasing the oil-flow rate and improving the recovery factor of a field. In specific scenarios, the gas/liquid separation can be an enabler technology, allowing production of fields or areas that would not be economical if development considered conventional technologies. Finally, this kind of separation can be applied in association with subsea compressors or pumping systems.
The operator has operational experience with subsea gas/liquid-separation systems. One important characteristic of the gas/liquid-separation technology is that it can be applied to both brownfields and green fields. Therefore, its development is very attractive for oil companies because the qualification of a robust solution can bring benefits for a broad range of scenarios. For a detailed discussion of the project-development process for this system, please see the complete paper.
Subsea Gas-Compression System. The objective of subsea gas-compression technology is to increase the pressure of the produced-gas wellstream so that potential developments may address the following applications and conditions:
- Increase reservoir recovery.
- Maintain the production plateau as long as possible.
- Accelerate production.
- Reduce the acid-gas emissions or increase oilfield recovery by means of a gas-reinjection system.
- Apply in long-step-out wells/fields.
- Apply in high-gas/oil-ratio (GOR) fields (coupled with gas/liquid-separation system) and gas fields.
- Reduce field-development costs.
On the basis of the technologies developed and qualified in the current subsea-compression systems, the operator intends to analyze the implementation of subsea gas-boosting technology in potential scenarios such as transport in long tiebacks and reinjection of gas. Some of the primary benefits of these possible scenarios could be evident in gas reservoirs or in oil reservoirs with high GOR, where the subsea layout can be designed with a subsea gas/liquid-separation system that removes gas from the wellstream, a boosting station for the liquid phase, and a subsea compressor to transport the separated gas or to inject it into the reservoir in order to increase oilfield recovery or to prevent acid-gas emissions. Nevertheless, for a specific scenario, these evaluations will be compared with possible topside conventional alternatives, as usual.
The operator is engaged in a specific R&D project on subsea compression. This project aims to carry out a technical-feasibility study and gap analysis of a subsea gas-compression station, identifying the technical status of each essential component and describing how far the current technologies are from each potential application scenario.
High-Differential-Pressure Multiphase Pump (MPP). Subsea multiphase pumping is the most basic type of subsea processing, with a substantial number of applications. It consists essentially of adding hydraulic energy to the multiphase flow without separation. Much effort has gone toward developing subsea-MPP technology, and relevant technologies are available for use. Each multiphase-pumping technology has its application niche, operating envelope, and technical limitations. The choice of the best alternative for a given application depends on several factors such as required differential pressure, flow rate, gas volume fraction, suction pressure, and viscosity.
There are more than 15 subsea multiphase-pumping systems already installed worldwide, but the maximum pressure differential of these systems is less than 45 bar. However, efforts are currently under way to support the development and installation of pilot systems of twin-screw and helicoaxial pumps with differential pressures of 60 bar. There are some remaining technological gaps, though, related to important needs. Scenario studies indicate that the development of high differential pressures and high-flow-rate MPPs could be attractive because of water-depth factors, longer tiebacks, and high flow rates. However, the dissemination of MPP installations would require overcoming constraints in many applications; these constraints include the effect of topside equipment on production-platform operation, umbilical costs, and subsea-installation optimization. Consequently, an R&D project has been established to evaluate and qualify subsea high-differential-pressure MPPs and to support the development of technologies that optimize or enable MPP use.
This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 24498, “Subsea Processing and Boosting in Brazil: Status and Future Vision” by F.A. Albuquerque, M.G. Morais, M.L. Euphemio, C. Kuchpil, D.G. Duarte, and R.T. Orlowski, Petrobras, prepared for the 2013 Offshore Technology Conference Brasil, Rio de Janeiro, 29–31 October. The paper has not been peer reviewed. Copyright 2013 Offshore Technology Conference. Reproduced by permission.