Lars Anders Ruden
Kenneth Olsvik
Roxar
Many operators see multiphase metering as an important factor in increasing production rates. Industry analysts Douglas-Westwood Ltd and OTM Consulting, for example, predict that more than 1,000 additional multiphase meters will be deployed by 2015. According to Rystad Energy Global, 12% of global oil & gas production today, for instance, is facilitated by Roxar multiphase meters.
With the increased market penetration of multiphase meters come challenges – particularly in environments such as deepwater operations where scaling is prevalent, and other environments such as heavy oil and sour gas fields.
The heavy oil challenge
According to the United States Geological Survey, heavy oil is known to occur in 127 basins throughout the world with 3,424 Bbbl of “in place” heavy oil.
A major difficulty with heavy oil is its high viscosity and associated weak flow, which makes it difficult to extract. Technologies that work with light and medium oil grades often fail with heavy oil due to the different process conditions such as low gas rates, low density contrast among liquids, unpredictable emulsion properties, viscous fluids, and the presence of wax.
The fact that there is no need to separate phases in multiphase meters, as opposed to standard test separators, benefits multiphase meters. Fluids may separate poorly in heavy oil fields due to the small differences in densities among the phases. Well dynamics in heavy oil wells also can cause carry over and carry under, leading to inaccurate test separator measurements.
Multiphase meters must, however, deal with large variations in oil densities and viscosities in heavy oil fields. This can be counteracted with direct-phase slip measurements. In the case of Roxar’s meters, for example, for phase velocity determination, cross correlation and dual cross correlation is used below 90% gas void fraction (GVF). The cross correlation algorithms are not dependent on viscosity, and dual cross correlation enables direct-phase slip measurements.
Venturi mass flow measurement also is used in multiphase meters, with such venturi models able to cater for large variations in oil densities and viscosities. The three velocity measurement method ensures that the system has a built-in redundancy and self-verification.
Highly viscous fluids often have wax, raising concerns about clogging the process impulse tubing (sense lines). To counteract this threat, multiphase metering should include self-draining impulse tubing. Such impulse tubing is more sensitive and guarantees a higher turndown, leading to higher sensitivity and accuracy of the meter/measurements, compared to alternative solutions such as remote seals.
Sour service environments
Sour service environments also pose challenges to the reliability of multiphase equipment.
There is the challenge of measuring flow rates of oil, water, and gas reliably and accurately under the presence of high and fluctuating H2S concentrations, and there are the HSE implications for a multiphase meter of producing a potentially hazardous gas.
A robust measurement principle for multiphase flow is essential in sour service. Meters which apply fractional measurements using electrical impedance measurements, in combination with either non-gamma software or single, high-energy gamma for density measurements, are robust against H2S concentration variations.
The electrical impendence measurements calculate the mixed conductivity and permittivity to determine the phase fractions and it is highly unlikely that the oil permittivity and water conductivity will change significantly in the presence of H2S gas and sulphur atoms.
When dealing with wellstreams containing hazardous sour gases, it is important to consider the safety and environmental implications that the cleaning and interruption of the well flow have.
Limited maintenance
In this case, remote and limited maintenance requirements are important. The normal maintenance schedule of the meter will be a yearly empty-pipe calibration of the gamma system and a check-up of the meter’s electronics and transmitters to ensure there is no drift. All maintenance actions can be done remotely from the service console.
Remote monitoring also allows operators and service engineers to control the multiphase meters from a safe, remote location -- of great importance when the meters are to be installed at unmanned locations. One example is the Kasaghan project in the Caspian Sea (23% mol H2S), an area of over 5,500 sq km (2,124 sq mi) and where there is a potential for 50 to 200 multiphase meters to be installed on unmanned platforms.
Roxar has supplied a number of projects with multiphase meters around the world where process conditions indicate high concentrations of H2S. In 2003, Roxar supplied multiphase meters to a field in Qatar with v/v 2% H2S. The meters monitor the well rates and input data to update the production model. In 2004, Roxar delivered 19 meters to a major operator in Kazakhstan. This field is known for extreme high H2S levels, up to 16 mol%. These meters have shown consistently good measurements.
The problem of scaling
Scaling – the term used to describe a deposit inside a pipeline, borehole, or reservoir which forms after a chemical reaction – represents one of the most significant production and well integrity challenges in oil and gas production today.
Scaling can plug both production and injection wells as well lines, pumps, and valves. It can lead to inaccurate multiphase measurements, if the scale forms a layer on the inside a meter’s sensors.
The materials that make up the surface that forms the inside of the capacitance/inductive sensor is important. A PEEK (Polyetheretherketone) surface, for example, is more resistant to scale build-up than steel or metallic components.
Another preventative measure in applications with known scaling potential is to inject scale inhibitor upstream of the meters – at the trees, for example. This should be done as an early preventative measure before scale problems occur.
In cases where scale build-up does have the potential to influence meter readings, it is important for the multiphase meters to detect and solve such scaling problems through both preventative and corrective actions.
There are other remedial measures -- the use of a scale inhibitor to prevent the formation of scale and to increase oil and gas flow, for example, and the direct removal of the scale with the use of a manual brush when access to the meter inner wall is possible.
