簡介：Roxar ASA公司的多相測量設備不需使用相分離器，對油田產量優化和油藏管理能夠產生明顯影響。
Use of multiphase metering has seen considerable growth and adoption in the oil and gas industry the past several years. As an effective alternative to expensive and labor-intensive well-testing operations — involving test separators, cumbersome mobile well-test units, and maintenance-intensive platform facilities — the potential for growing use of multiphase meters is significant.
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Multiphase meter do not require phase separation. Using a “dual-velocity” method, phase fractions can be calculated based on capacitance and conductivity measurements in combination with a single energy gamma densitometer.
Placed on the wellhead, retrievable choke bridge, jumper, or in the manifold, multiphase meters deliver critical, reliable, and real-time information on oil well capabilities during production — including water saturation, breakthrough, and flow characteristics.
By doing so, multiphase meters support operators with minute-by-minute information used to manage wells, optimize hydrocarbon production, and mitigate water production.
Operators, including Brazilian oil and gas giant Petrobras, have indicated they would like to see multiphase meters on each of their subsea wells and trees.
Moreover, the increasing number of deepwater wet gas fields — defined as being around 95% to 100% gas void fraction (GVF) — has led to the introduction of specialist wet gas meters. In wet gas fields, water detection is critical, as even small amounts of saline water cause large, rapid scaling problems. For example, the desired water detection sensitivity for the Ormen Lange field off Norway, which provides up to 20% of the UK’s gas supply, is 0.005% by volume based on water-volume fraction.
An effective alternative
It is important to understand the benefits of multiphase and wet gas meters compared to other well-testing techniques. Because of poor separation among the three phases, well testing using a separator has limitations in measuring gas and liquid flow rates. Multiphase meters, on the other hand, do not require phase separation, negating the dangers caused by differences in density as well as reducing costs per well tested.
Today’s permanently installed multiphase meters instantly and continuously measure three phase rates — not just at a discrete point in time and not just for one well. Roxar’s multiphase meters, for example, incorporate a “dual-velocity” method, with calculated phase fractions based on capacitance and conductivity measurements in combination with a single energy gamma densitometer.
By detecting changes in multiphase composition at the subsea wellhead,
rising water cut can be immediately detected. And by examining real-time information from the downhole pressure and temperature gauges, the operator can gain an even better understanding of where problems originate.
One main benefit of multiphase meters in subsea wells is limited maintenance requirements. Roxar’s subsea meter, for example, has the option of having a separate, subsea-retrievable canister, which houses the meter’s electronics and flow computing modules.
Finally, as oil and gas reserves become harder to find, well testing will take place in ever more challenging circumstances, such as those found in heavy oil fields.
Separators cannot always acquire the accurate data operators require in such circumstances. In heavy oil environments, for example, fluids may separate poorly due to small differences in density among the phases. Well dynamics and instable wells typical for heavy oils also can cause carry-over and carry-under, leading to inaccuracy of measurement from the test separator.
Most multiphase meters operate in medium-heavy and extra-heavy oil environments, independent from emulsions, and have the ability to handle highly viscous fluids.
Multiphase metering in action
Implementation of multiphase and wet gas metering in the Gulf of Mexico’s (GoM) Independence Hub is one of many good examples of the technology’s current use.
The field’s operator, Anadarko, wanted to conduct well testing without the need to shut the wells down. With each well producing between 25 and 125 MMcf/d, shutdowns would have a highly negative impact on production as well as being a drain on manpower and resources.
One alternative examined, installing a wet gas meter on a flow line that needed to be between 10 and 14 miles (16 and 23 km) long, was expensive and failed to avoid the need to shut down production. Multiswitched manifolds were also considered but wouldn’t have provided enough information from the wells, besides having a high cost impact.
Anadarko opted for a subsea wet gas meter due to its ability to continuously and accurately measure producing wells. The technology also accurately detects saltwater coming into the wet gas stream, allowing Anadarko to distinguish between condensed water and formation water.
A second case example comes from offshore Egypt, where subsea and topside wet gas meters were installed in fields anticipated to produce both condensed water and increasing volumes of formation water over their production lifetime.
For some of these wells, once water production monitoring began, a water breakthrough was almost immediately detected. After reducing choke size for three weeks to reduce the water produced, the operator opened the upper zone of the smart completion. Water production soon began increasing again, leading to the shutting-in of the well for a week.
When production resumed, however, water rate again started increasing dangerously. The operator then went into a period of adjusting the lower inflow control valve position to find optimum gas production at an acceptable water rate. In this manner, by providing early warnings of water produced, the wet gas meters helped the operator save three wells from water breakthrough.
Brownfield well testing
In brownfields, multiphase meters can be the means to bolster well testing in environments characterized by the complex interdependencies at work amongst aging and new technologies.
With many brownfields having limited well-testing capabilities — often lacking test separators and, on platforms, also lacking test lines — installing multiphase meters on wellhead platforms can make well testing easier and more frequent. In doing so, however, as with any technology upgrade, it is important to minimize cost and the disruption of operations.
Roxar installed its multiphase meters for a leading Middle East oil and gas operator in a mature offshore field discovered in the early 1960s. Having significant oil reserves, the well peaked at 50,000 b/d of oil in the early 1970s but has since declined. Only 50% of producing wells were tested each month, with oil being regularly lost due to the shut-in of wells while other wells are being tested.
The multiphase meters were installed on wellhead platforms, each servicing as many as nine wells. The average number of well tests taken per month subsequently increased from 50 to 120, with the amount of lost oil due to well testing reduced from 12,000 bbl to less than 50 bbl per month. Today, the operator has successfully reversed nearly 25 years of steadily declining production.
Where next for multiphase?
Multiphase meters have the potential to significantly impact oilfield production optimization and reservoir management.
As the examples in the GoM and offshore Egypt demonstrate, multiphase and wet gas metering, as an alternative to traditional well testing, already plays a significant role in flow assurance.
When integrated with gauges and other intelligent devices such as pressure and temperature transmitters, the multiphase meter can be a critical component in measuring flow and production rates, thereby supporting better real-time decision-making. Determining choke settings and artificial lift parameters, for example, can be made based on the maximum amount of information.
Additional chapters in the story of multiphase meters remain to be written as their use evolves and their full potential for optimizing reservoir monitoring, flow assurance calculations, production output, and reservoir engineering analysis becomes clear.
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