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What Criteria to Consider in Selecting a Cut Monitor

Cut Monitor Technologies and Factors that Affect Their Performance
Measuring the percentage of water in oil (water cut) is required for both upstream and downstream oil production. Several technologies currently are used to measure water cut across a full range of applications. This article offers a basis of comparison, including advantages and disadvantages of each.

Four basic on-line analytical instrument technologies are used to measure the percentage of water in oil: Capacitance, Microwave, Spectroscopy, and Density. All four rely on electrical and/or mechanical characteristics of the fluid. Because of the differences in each methodology, users should have at least a basic understanding of the strengths and weaknesses of each to select the right instrument for an application.

Capacitance: Capacitance technology has been used successfully to measure water cut for more than 40 years. Its success is due to the significant difference in dielectric constants between oil (k≈2.3) and water (k≈80). Figure 1 shows 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 transmit a radio frequency voltage to the sensing element that measures changes in capacitance. As the amount of water in the flowing oil increases, the net dielectric of the fluid increases causing the capacitance to increase. The instrument’s onboard electronics then computes the relationship between capacitance change and water cut.

Key advantages of capacitive instruments are a stable (and proven) measurement technology, simple design, insensitivity to water conductivity, and an ability to handle a majority of oil patch applications. In addition, capacitive instruments are typically among the lowest cost options relative to other measurement technologies.

Disadvantages include difficulty in handling changing process factors and limitations in measurement range. Capacitive instruments are limited to water cut ranges that are below the inversion point of oil and water. As the fluid becomes water continuous, conductivity dramatically increases, creating an electrical short to ground. The short to ground drives the capacitance to infinity and obscures the dielectric information. This typically occurs at approximately 50% water cut in light oil and at 80% in heavy oil.

Microwave: This technology relies on the different electrical properties of the oil/water mixture to determine the water cut measurement. An oscillator transmits a microwave signal at a precise frequency via an insertion probe that travels through the fluid. As the percentage of water in oil rises, the microwave signal changes in amplitude and frequency. That change in signal is measured electronically, and the relationship between microwave signal change and water cut determined.

Advancements in microwave technology have provided this methodology with several distinct advantages. Two of them are accuracy in the lower cut ranges and the capability to measure the full range of water cut (0-100%). Microwave-based systems also are more robust in handling process factors that can negatively affect other water cut measurement technologies. Disadvantages include its high initial cost relative to other technologies and sensitivity to salinity changes in the higher cut ranges.

Spectroscopy: The basic principle behind spectroscopic measurement of water cut is the response of an oil/water mixture to light. A spectroscopic device emits an infrared beam that ignores the water phase of the mixture. The sole reactant to the selected wavelength is the oil phase. Signal receptors on the device, shown in Figure 2, measure the absorption, reflection, and scatter of the infrared beam and makes a direct correlation to water cut.

Figure 2: Signal receptors for spectroscopic water cut device

Spectroscopy offers several advantages. First is its ability to measure across the full range of water cut. The percentage error actually decreases as the water cut increases. The technology’s accuracy at the high end of the cut ranges separates it from other competitive technologies. Another advantage is the technology is unaffected by changes in density, salinity or entrained gas.

A major disadvantage is the lack the necessary accuracy at the lower cut ranges. That lack of accuracy limits the number of suitable applications. For example, it is not a good 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 recognize that these instruments have a very defined sampling region. The sampling region emits an infrared beam that is reflected, absorbed, and scattered over a potentially small representative region of a very large sample and may or may not provide a true measurement of the entire process flow.

Density: Density measurement is the only methodology that uses a mechanical solution to measure water cut. Usually a coriolis flowmeter performs the measurement. Fluid enters flow tubes that are mechanically driven to vibrate at a certain frequency. Figure 3 provides a typical arrangement. As the fluid’s density changes, the frequency at which the tubes oscillate also changes. The water cut can be determined from those changes.

Figure 3: Coriolis flowmeter

Density measurement does provide the ability to measure the full range of measurement. The technology is cost effective and provides additional information (flow rate, temperature, and density) that can be used as input for process optimization. A drawback occurs when process variables start to change. Introduction of gas and salinity into the process immediately effects the water cut measurement and can significantly impact the accuracy of the device. It is typically confined to light oils due to the limited difference in density between water and heavy oil, and it encounters additional uncertainties when applied to water-flood enhanced oil recovery processes.

Individual Product Capabilities
If choosing the right technology weren’t difficult enough, it is useful to keep in mind additional instrument characteristics.

Range of Accuracy: Some technologies are limited to certain cut ranges. Spectroscopy-based instruments are able to measure the whole range and increase in accuracy at the higher cut ranges. However, that technology is not useful for accurate, low-range cut measurement. On the other hand, capacitance devices offer excellent accuracy and repeatability at the low-cut ranges but are limited by the water/oil inversion point. Microwave measurements offer accuracy throughout the entire range, and their premium price reflects that capability.

Communication Output: While all instruments provide the standard 4-20 mA output, some manufacturers have equipped their devices with additional capabilities. Utilizing digital protocols, embedded relays, multiple 4-20 mA signals and wireless communications are just some of the output options provided.
Sensor Design: Various mounting options are available. Among the most common is the dual flanged spool piece. Figure 4 provides a typical arrangement. A different approach taken by some producers are threaded NPT and slipstream designs that permit a more customizable solution than a spool piece.

Figure 4: Spool-piece cut monitor

Maintenance also should be considered in choosing a sensor design. Users should ask if the probe is susceptible to paraffin buildup. How easy is the device to clean and/or replace? Does the sensing device measure a representative sample of the fluid? Are there any seals, coatings, or fittings that require regular replacement? How well the external electronics stand up to harsh ambient conditions?

An insertion probe that can be installed directly in the process stream offers additional advantages when evaluating sensor designs. The insertion probe used by some capacitance devices allows the sensor to acquire samples over the entire length of the probe, providing a larger representative sample of the mixture and creating a capacitive-averaging effect that allows the electronics to calculate a more accurate measurement.

Net Oil Calculation: Net oil calculations are gaining in popularity with the integration of computing devices, PLCs, flow meters, and water cut instrumentation. A packaged net oil calculation offers end users a dedicated system that eliminates the need for users to piece together individual components to compute net oil calculations.

Start Up and Commissioning: Knowledge and experience are required during installation to get the best performance from a unit. With the increasing complexity of the technologies involved, the level of service and support an OEM provides is of great importance in choosing a device.

Price: Price varies significantly from $2,500 to $30,000 and depends largely on the capabilities of a device. Such extras as digital communications and net oil calculations may add significantly to the base price of the unit.

To ensure the best performance and value from a cut monitor, users need to have comprehensive data on the process parameters and product characteristics that influence performance. Systems should be evaluated on the basis of accuracy, sensing range, process characteristics, mechanical configuration, maintenance requirements, and price prior to a purchase. Armed with right information on the advantages and disadvantages of each technology, users should be able to make the best decision to fit their needs.

Learn more about Drexelbrook water cut monitors here.