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Advanced subsea systems unlock opportunities as the E&P frontier pushes to more challenging deepwater and ultradeepwater developments. Meanwhile, as economics change and the drive to reduce risk continues, operators now prioritize an accurate understanding of well behavior at the seabed.
By and large, operators rely heavily on computer modeling that can be enhanced with real-time field data. Those independent technologies that deliver reliable real-time data not only validate model accuracy, they improve critical operational decisions and ultimately drive higher lifetime productivity and returns for subsea field investments.
Meeting the technical challenge
Real-time subsea data delivery is never a simple or easy task. In addition to water-cut data, a Weatherford client required hydrate-inhibitor-to-liquid-fraction measurement in nearly dry gas conditions where the gas-volume fraction (GVF) is close to 99.9%.
The goal for the data is flow assurance or ensuring that oil and gas is transported from the reservoir to the end user reliably and economically. In the production phase, a well’s flowing volume is directly tied to the operator revenue, so assuring constant flow of hydrocarbons is critical. The risk of hydrates blocking the production lines and compromising the integrity of systems, pipelines and risers increases in the cold and pressurized deep subsea environment, further compounded by long pipelines like tiebacks.
The challenge was especially stark in Egypt given that the project included the longest tieback in the world at 136.7 miles (220 km).
Controlling solids formation is based largely on predictive analysis and hence requires a constant balancing act with thermodynamic, kinetic and manual controls. Common preventative measures include injecting monoethylene glycol (MEG) or methanol at the subsea tree and downhole to control hydrate formation. The dosage rates are determined by hydrate stability curve models using PVT data and salinity estimates. Moreover, inhibition options are limited in high water-cut wells so a constant water-to-liquid ratio and hydrate-inhibitor-to-liquid ratio are necessary to determine dosage.
These chemicals are expensive and need continuous recycling for constant use. However, due to value of a subsea installation and the cost of remediating hydrate formations in the lines, most operators opt for over-injecting chemicals to reduce risk of downtime and costly interventions.
Driving value beyond capex
A quarter of any subsea field development capex is spent on equipment, which makes that line item a key focus area for cost optimization and the reason why equipment reliability is paramount. This heavy focus on just capex, however, often overlooks recurring costs from opex, RISEX and RAMEX that can add substantially to the life-of-asset operating costs. (RISEX = risk costs associated with loss of well control (blowouts) during installation, normal production operations and during recompletions. RAMEX = the reliability, availability and maintainability costs associated with subsea component failures.)
Real-time multiphase measurement remains a complex arena where no single metering technology has yet been a clear winner across all applications. While one technology might adapt well to a certain application profile, it might not to another. However, for flow-measurement equipment, nuclear-based multiphase flowmeters (MPFM) are the industry norm. Their less-than-stellar reliability track record and million-dollar costs, however, have made them ripe for optimization. In addition to a high-upfront investment, opex considerations are equally important for subsea MPFMs because of their need for maintenance and services with exclusive OEM offshore access requirements.
Reducing costs in Egypt
Weatherford's first priority in Egypt was to safely and reliably meet operational specification, followed by a secondary priority for reducing both project capex and opex. Given the 99.9% GVF and customer desire to optimize flow assurance and MEG injection, the Weatherford team proposed Red Eye subsea water-cut meters to complement the planned subsea MPFM to deliver accurate water and hydrate fraction measurements.
These meters provide real-time measurements across the full range of water cut from 0% to 100%. Using patented optical-sensor technology, Red Eye subsea meters ignore changes in salinity and distinguish between gas, water, methanol and condensate suspended in flowlines. The system is a simple and economical way to detect water breakthrough, measure hydrate-inhibitor concentration and trend water behavior in the reservoir. They operate independent of flow rate, PVT or topside hardware needs, and they are integrated and commissioned by the subsea integrator on their subsea architecture.
In addition to providing water cut, Red Eye also provides an accurate measurement of methanol- (or glycol) to-liquid ratio in real time. These capabilities reduce the risk of underdosing and eliminate the unnecessary cost of overdosing while serving as a key enabler in protecting the integrity of the riser systems during transient conditions.
The Red Eye subsea meters enabled the operator to reduce opex by optimizing the hydrate stability curve and the water-to-MEG ratio in real time. Using the continuous data from the Red Eye subsea meters, the operator was able to reduce the MEG injection rate by a ratio of 15:1, which translates to a 90% reduction in MEG recycling and transportation costs and saves more than $2 million per year.
In short, reliable and accurate measurements ensured operator can confidently manage its hydrate-inhibitor strategy and very quickly adapt to water breakthrough regardless of its severity.
Virtual metering and digitalization
As mentioned above, real-time multiphase measurement is complex and there is no correct single answer. A promising frontier is digitalization, which has thus far delivered subsea digital twins and underwater autonomous inspection drones. On the metering front, virtual flow metering (VFM) is a promising solution.
VFMs are software models that derive real-time multiphase flow measurement values using the data from standard instrumentation installed on subsea production systems. In the subsea world, they are mostly used as backups to subsea MPFMs, but they have been useful in cases where the physical meters were either temporarily unavailable or deemed inaccurate due to issues like drifting. They might, one day, serve as a suitable alternative to subsea MPFMs apart from a missing ingredient: accurate water-cut measurement, which cannot be attained by pressure and temperature data alone.
As VFM technology continues to evolve, a hybrid solution has been adapted by a North Sea operator. The solution design has multiple wells sharing a common subsea test line with a physical MPFM, while each tree is instrumented with a Red Eye water-cut meter. In addition to input from other process instruments and valves, water-cut data enable the operator to enhance the VFM model, which are optimized with well test data from the subsea MPFM. With these three technologies complementing each other, the operator established an accurate, robust and economical flow metering solution that expends less opex and capex.
As the industry unlocks opportunities in ultradeepwater developments, accurate flow measurement data reduce operational risks and costs by delivering a better understanding of well behavior at the seabed. The future of this discipline will likely combine field-proven technology, including the Red Eye subsea water-cut meter, and new techniques that include VFMs. Together, these systems can improve operational decisions and ultimately drive higher returns for subsea fields.
A personal note from the author:
"On a shuttle from the Cairo airport in the summer of 2019, I struck up a conversation with Mr. Ahmed, a local taxi driver. It was my first trip to Egypt, so I was interested in some local insight. As Mr. Ahmed inquired about the purpose of my visit, I tried to explain in my broken Arabic that I was here to support the startup of some critical wells at an offshore gas field. He knew exactly what I meant. If I were to summarize his reaction and the context, this project meant a great deal to the Egyptian people as it would significantly contribute to their domestic energy needs. While I had an idea about the magnitude of the project and the enabling role we were to play as a technology provider, it was only at that point I realized the importance and value that we, and our industry as a whole, drive all over the world.
In the two years since meeting Mr. Ahmed, I’m proud to say that I played a part in starting those wells in the Mediterranean Sea and contributed to bringing more affordable energy to people of Egypt. Right now, a series of Red Eye subsea water-cut meters sit on the tree of each well, helping the operator accurately trend their water cut in real time from the ultradeep seabed."
—Sameer Siddiqui, Technical Manager of Testing, Weatherford
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