Selecting a Cut Monitor - Criteria to Consider

Cut Monitor Technologies and Factors that Affect Their Performance

Measuring the percentage of water in oil (water cut) is essential for both downstream and upstream oil production. Presently, numerous technologies are used to measure water cut across a whole range of applications. This article suggests a basis of comparison, including advantages and disadvantages of each.

Technologies

To measure the percentage of water in oil, four basic on-line analytical instrument technologies are used—capacitance, microwave, spectroscopy, and density. All four depend on mechanical and/or electrical features of the fluid. Due to the variations in each methodology, users should have at least a standard understanding of the advantages and disadvantages of each to choose the correct instrument for an application.

Capacitance

Capacitance technology has been used successfully to measure water cut for over four decades. Its success is attributed to the significant variance in dielectric constants between oil (k≈2.3) and water (k≈80). Figure 1 illustrates the sensing element, with diameter ‘a’, and the pipe wall, diameter ‘b’, that form the two plates of a cylindrical capacitor.

Figure 1. Standard cylindrical capacitor

The system’s electronics convey a radio frequency voltage to the sensing element that measures variations in capacitance. As the quantity of water in the flowing oil increases, the net dielectric of the fluid increases resulting in the capacitance to increase. The instrument’s onboard electronics then computes the connection between capacitance change and water cut.

The main benefits of capacitive instruments are a simple design, stable (and proven) measurement technology, insensitivity to water conductivity, and an ability to handle a bulk of oil patch applications. Furthermore, capacitive instruments are usually among the lowest cost options compared with other measurement technologies.

Disadvantages include trouble in handling changing process factors and restrictions in the measurement range. Capacitive instruments are restricted to water cut ranges that are lower than the inversion point of water and oil. As the fluid turns into water continuous, conductivity dramatically increases, forming an electrical short to ground. The short to ground pushes the capacitance to infinity and conceals the dielectric information. This usually happens at approximately 80% water cut in heavy oil and at 50% in light oil.

Microwave

This technology depends on the various electrical properties of the oil/water mixture to establish the water cut measurement. An oscillator conveys a microwave signal at a precise frequency via an insertion probe that moves through the fluid. As the percentage of water in oil increases, the microwave signal varies in amplitude and frequency. That variation in signal is measured electronically, and the relationship between microwave signal change and water cut established.

Progress in microwave technology has given this methodology several unique advantages. Two of them are accuracy in the lower cut ranges and the capability to calculate the complete range of water cut (0-100%). Microwave-based systems also are more powerful in managing process factors that can negatively impact other water cut measurement technologies. Drawbacks include its high initial cost in comparison to other technologies and sensitivity to salinity changes in the higher cut ranges.

Spectroscopy

The standard principle behind spectroscopic measurement of water cut is the reaction of an oil/water mixture to light. A spectroscopic device discharges an infrared beam that overlooks the water phase of the mixture. The only reactant to the selected wavelength is the oil phase. Signal receptors on the device, illustrated in Figure 2, measure the absorption, reflection, and scatter of the infrared beam and makes a direct association to water cut.

Figure 2. Signal receptors for spectroscopic water cut device

Spectroscopy offers a number of benefits. First is its ability to measure across the whole range of water cut. The percentage error truly decreases as the water cut increases. The technology’s accuracy at the high end of the cut ranges sets it apart from other competitive technologies. Another benefit is that the technology is not impacted by variations in salinity, density, or entrained gas.

A major drawback is the lack of the required accuracy at the lower cut ranges. That absence of accuracy restricts the number of suitable applications. For instance, it is not an ideal choice for Lease and Automatic Custody Transfer (L.A.C.T.) sites that have cut ranges of 0-3% water in oil per API Specification 11N. Users of infrared devices also must realize that these instruments have a well-defined sampling region. The sampling region produces an infrared beam that is reflected, absorbed, and scattered over a potentially small representative area of a huge sample and may or may not provide a correct measurement of the total process flow.

Density

Density measurement is the only approach that uses a mechanical solution to calculate water cut. Typically, a Coriolis Flowmeter carries out the measurement. Fluid enters flow tubes that are mechanically motivated to vibrate at a definite frequency. A typical arrangement is shown in Figure 3. As the fluid’s density varies, the frequency at which the tubes oscillate also varies. The water cut can be established from those variations.

Figure 3. Coriolis flowmeter

Density measurement does offer the ability to measure the complete range of measurement. The technology is economical and provides more information (temperature, flow rate, and density) that can be used as input for process enhancement. A disadvantage occurs when process variables begin to vary. Introduction of gas and salinity into the process directly affects the water cut measurement and can greatly influence the accuracy of the device. It is usually restricted to light oils because of the limited difference in density between heavy oil and water, and it encounters more uncertainties when applied to water-flood enhanced oil recovery processes.

Individual Product Capabilities

If picking the correct technology weren’t hard enough, it is beneficial to bear in mind extra instrument features.

Range of Accuracy

A few of the technologies are restricted to certain cut ranges. Spectroscopy-based instruments can measure the entire range and increase in accuracy at the higher cut ranges. However, that technology is not beneficial for accurate, low-range cut measurement. Alternatively, capacitance devices offer superior accuracy and repeatability at the low-cut ranges but are restricted by the water/oil inversion point. Microwave measurements provide accuracy across the whole range, and their premium price echoes that capability.

Communication Output

While all instruments offer the basic 4-20 mA output, certain manufacturers have fitted their devices with extra features. Utilizing embedded relays, digital protocols, multiple 4-20 mA signals, and wireless communications are a few of the output options provided.

Sensor Design

A number of mounting options are available. Among the most typical is the dual-flanged spool piece. Figure 4 illustrates a characteristic arrangement. A different method taken by certain manufacturers are threaded NPT and slipstream designs that allow a more tailor-made solution than a spool piece.

Figure 4. Spool-piece cut monitor

Maintenance also should be considered in selecting a sensor design. Users should ask if the probe is vulnerable to paraffin accumulation. Is the device easy to clean and/or replace? Does the sensing device measure a characteristic sample of the fluid? Are there any coatings, seals, or fittings that require systematic replacement? How well the external electronics resist harsh ambient conditions?

An insertion probe that can be fitted directly into the process stream offers more advantages when assessing sensor designs. The insertion probe used by certain capacitance devices allows the sensor to obtain samples over the whole length of the probe, providing a larger representative sample of the mixture and forming a capacitive-averaging effect that enables the electronics to compute a more accurate measurement.

Net Oil Calculation

Net oil calculations are increasing in popularity with the incorporation of computing devices, flow meters, PLCs, and water cut instrumentation. A packaged net oil calculation offers end users an exclusive system that removes the need for users to put together individual components to measure net oil calculations.

Start Up and Commissioning

Knowledge and experience are essential during installation to obtain the best performance from a unit. With the growing complexity of the technologies involved, the level of service and support an OEM offers is of great significance in selecting a device.

Price

Price differs greatly from $2,500 to $30,000 and relies mostly on a device’s capabilities. Such add-ons as net oil calculations and digital communications may add considerably to the base price of the unit.

Conclusion

To ensure the finest performance and value from a cut monitor, users need to possess complete data on the process parameters and product features that impact performance. Systems should be assessed on the basis of mechanical configuration, accuracy, process characteristics, sensing range, maintenance requirements, and price before a purchase. Equipped with the right information on the benefits and drawbacks of each technology, users should be able to make the ideal decision to suit their requirements.

This information has been sourced, reviewed and adapted from materials provided by Ametek STC.

For more information on this source, please visit Ametek STC.