Subsea separation and produced water reinjection (PWRI) or discharge comprise an integral part of the subsea processing strategy that can bring many benefits, including economic, operational, and environmental, for the oil and gas industry. The importance of subsea separation and PWRI to economics has been demonstrated through work done by Statoil, which installed the world’s first full-scale subsea separation system at its Tordis field in the North Sea.
Statoil estimated that the system’s installation would enable the company to achieve an additional total field oil recovery of 6%, which is equivalent to an extra 26 million bbl of oil. At a price of USD 50/bbl, this would lead to more than USD 1 billion of extra revenue.
Continuous Measurement
The most economic means of implementing subsea separation and PWRI or discharge operations is to use continuous online subsea water-quality measurement devices.
The alternative of using a remotely operated vehicle (ROV) to extract and deliver produced water samples to the surface for offline analysis has been estimated by the Research Partnership to Secure Energy for America (RPSEA) to cost as much as USD 250,000 per day. It is also time consuming, and the processing conditions could change before the results are obtained.
However, few subsea online continuous monitoring devices exist for measuring produced water quality. But there are moves within the industry to address this problem, given the enabling status that the technology holds for deepwater/ultradeepwater and marginal field development.
A number of joint industry projects (JIPs) and other initiatives have been launched globally and have made progress in developing technical specifications, identifying possible technologies, and assessing the performance of potential sensors under laboratory and field (at surface) conditions.
Technical Specifications
The most obvious consideration is that subsea separation and PWRI or discharge require online monitors to operate reliably and accurately at a water depth of up to 9,850 ft, which brings the additional challenge of operation at high temperatures and pressures.
Depending upon the type of operations, there are also differences in the technical requirements. For subsea discharges, the focus will be on the measurement of oil-in-water, while for reinjection, the emphasis will be on the measurement of solids and oil for concentration and particle size.
Table 1 summarizes the technical specifications for some of the key parameters of a subsea produced water quality measurement sensor. The table includes measurement applications for produced water discharge, reinjection, and separation operations. The specifications related to discharge applications have been obtained from the RPSEA project, while the specifications for reinjection and separation applications are from JIPs in which NEL is involved.
Technology Development
There are a number of technologies available on the market that could be developed for subsea applications, including:
- Laser-induced fluorescence (LIF)
- Light scattering
- Microscopy image analysis
- Ultrasonic acoustic
- Combination of some of the above
While these all offer the functionality to measure oil-in-water concentration subsea, only the microscopy image analysis and ultrasonic acoustically based technologies are also suitable for measuring solid concentration and particle size. All of the technologies listed above have been applied in measuring produced water quality on the surface, primarily for process optimization.
However, most of the listed technologies are optically based. Thus, one of the main issues to be addressed is fouling of the optical windows. For surface applications, the industry normally relies on automatic cleaning technology and regular maintenance. However it becomes difficult and very expensive to perform maintenance in a subsea environment, especially in deep water. In subsea settings, maintenance will need to be virtually zero.
A number of cleaning technologies have been trialled and incorporated into the oil-in-water measurement devices for surface and subsea operations. These technologies include:
- Jetting spray
- Ultrasonic
- Hydrodynamic, in which produced water fluid is passed through a venturi device fitted with optical sensors and detectors at the throat, where the fluid velocity is greatest.
Research so far has shown that an ultrasonically based cleaning mechanism, typically effective in surface, low-pressure environments, does not work well in high-pressure subsea settings. Jetting spray and hydrodynamics are believed to offer more potential in these environments.
There is also research to develop coating materials that could prevent fouling of the optical windows.
What Research Is Revealing
A significant level of research and development has been progressing for a number of years, sponsored by oil and gas operators, equipment manufacturers, independent research organizations such as NEL, and government-backed programs such as RPSEA.
Among the operators, ExxonMobil, Petrobras, and Statoil have been most active. ExxonMobil has developed two produced water quality monitoring prototypes based on the use of equipment manufacturer JM Canty’s microscopy imaging technology. A jetting spray cleaning mechanism was incorporated into the prototypes, and they have been flow-loop tested. The published results (SPE 174808) have confirmed the prototypes’ capability.
Petrobras focused on developing and qualifying an oil-in-water monitoring system, based on light-scattering technology, for its Marlim subsea separation system. A hydrodynamic mechanism was incorporated to prevent fouling. The monitor was installed in 2012, but few details have been published on its operation and performance.
Statoil views the development of a subsea oil-in-water monitor as an important part of its stated goal to develop a “subsea factory” by 2020. Through an internal testing and evaluation program, the company selected three technologies—microscopy, LIF, and ultrasonic acoustic—for a surface field trial in the North Sea, which was recently completed. Following the trial, one of the technologies is expected to be chosen for marinization and further development.
Manufacturers, including Advanced Sensors, JM Canty, Jorin, ProAnalysis, and Clearview Subsea, have been working with operators and independent organizations such as NEL to advance their technologies.
NEL has initiated three JIPs in the past 6 years to accelerate the development of these devices. A number of potentially suitable technologies have been tested in flow loops with simulated produced water and evaluated, including LIF, microscopy, and ultrasonically based systems. The results have pointed toward the use of a microscopy-based technique with jetting as a potential fouling mitigation mechanism for subsea applications.
From 2014 to 2016, RPSEA conducted the Subsea Produced Water Sensor Development project, which was led by Clearview Subsea and assisted by NEL. The goal was to develop a subsea produced water quality measurement sensor with an emphasis on subsea separation and produced water discharge applications.
Phase 1 of the project focused on the development of sensor technical requirements, gap analysis of existing technologies potentially suited for subsea applications, and proof of concept of a new sensor that uses a confocal laser fluorescence microscopy (CLFM) technology. Phase 2 of the project involved the design, construction, and bench-scale testing of sensors selected from the first phase.
The RPSEA project was completed in October 2016, following the bench-scale testing of four sensors using LIF, light scattering, microscopy image analysis, and CLFM technologies, respectively. The project results are being disseminated through RPSEA, SPE, and NEL events.
Summary
The wide deployment of subsea separation and PWRI or discharge systems to help unlock oil resources through subsea production depends on closing the technology gaps in subsea water-quality-measurement. A significant amount of industry research and development work has been done to develop such instruments. However, this will still require more collective effort, particularly in marinization, system integration, environmental testing, and subsea field trials. JIPs are a cost-effective and relatively low-risk forum for conducting this work. Real progress can be anticipated in the next few years.
Reference
SPE 174808 Flow-Loop Testing of Produced Water Quality Monitoring Sensor Prototypes by X. Yin, P. Moore, and K. Gul et al.