Measurements matter

When oil and gas is exported, pipelines are used. Due to the density difference between water and oil, and between water and gas, water in oil or water in gas can stratify and accumulate within certain parts of the pipeline, which can lead to corrosion problems. The presence of water in gas during transportation can also potentially lead to the formation of gas hydrate within the pipeline, under certain temperature and pressure conditions. This can lead to a pipeline blockage, causing operational and safety issues.

Knowing the level of water in oil or water moisture in gas can help determine the likelihood of corrosion taking place and/or gas hydrate formation inside the pipeline. It can also help to determine the level of anti-hydrate formation chemicals that may be required to prevent gas hydrate formation or aid in the scheduling of the pipeline inspection and maintenance regime. As these chemicals are often expensive, this can provide an ongoing reduction in OPEX.

With this in mind, accurate measurement of water in oil (crude or condensate oil) is essential for the oil and gas industry. Inaccurate measurement can lead to production process problems, transportation issues, pipeline integrity concerns, and loss of revenue from the sales of oil and/or gas.

Measurement of water in oil can be carried out by sampling and laboratory analysis, using a number of standard methods. It can also be achieved using online water-in-oil measurement devices for which at least four techniques are known to have been commonly used: production, allocation, transportation, and custody transfer.

Custody transfer and fiscal metering are often used interchangeably. They refer to the transactions involving the transportation of oil and gas from one operator (or owner) to another. These include the transferring of oil and gas between tanks and tankers, tankers and ships, and other transactions. Custody transfer in oil and gas measurement is defined as a metering point (location) where oil or gas is being measured for sale from one party to another. For fiscal or custody transfer, water in oil must be measured accurately, and then discounted.

Measurement methods and technologies

Water in oil can be measured using online technologies, as well as laboratory standard methods. Online technologies are increasingly used for fiscal, custody transfer and process operations. These online instruments offer a number of benefits in terms of cost saving, real-time continuous water-in-oil information, and increased productivity. Laboratory standard methods are also important and have traditionally been used to measure water-in-oil concentration. They are also used to calibrate and validate the performance of online water-in-oil measurement devices.

Laboratory standard methods

For fiscal, custody transfer and allocation, laboratory-based water-in-oil concentration measurement standard methods have played an important role – and will continue to do so. They are very well established, and have been practiced by the oil and gas industry for decades. There are a number of standard methods: centrifuge-based, distillation-based, and Karl Fischer titration-based.

All Karl Fischer methods are generally better and more accurate than distillation and centrifuge-based water-in-oil analysis methods. In general, the coulometric Karl Fischer method is better and more accurate than the volumetric Karl Fischer method. However, Karl Fischer titration-based methods are affected by the presence of mercaptan and sulfide (S- or H₂S). In addition to this, Karl Fischer titration methods require the use of expensive, hazardous chemical reagents and delicate glass pieces, and the reagents must be replenished continuously. Further, routine cleaning of the laboratory equipment parts is labour-intensive and time-consuming.

Online water-in-oil measurement

Most of the online water-in-oil measurement instruments that are commercially-available are designed and constructed based on four techniques: capacitance, density, infrared absorption, and microwave.

Capacitance-based technology probably has the longest history, commercially. It is a simple and well-proven technology, and comes at a low cost. As a method, it works well when the oil is in the continuous phase, and it is also relatively insensitive to water salinity. Most of the systems on the market at present can measure low percentages of water in oil accurately.

Density-based water-in-oil measurement technologies may include Coriolis, gamma-ray or x-ray, and are popular for multiphase flow measurement for the oil and gas industry. Whilst they can provide additional information such as flow, viscosity and density, in the case of Coriolis meters, these systems are generally affected by the presence of gas bubbles and solid particles. They are also sensitive to variations in process conditions. Overall, density-based systems have large uncertainty surrounding the measurement of a lower level water-in-oil concentration. Infrared absorption-based technology covers the entire water-in-oil measurement range. Although this approach is also unaffected by changes in density, salinity or entrained gas, it is not particularly accurate at a lower water-in-oil concentration range. Microwave-based technology is more accurate for lower water-in-oil concentration range applications. However, its high initial cost and sensitivity to salinity must be considered, despite its robustness.

Operational, safety, financial and environmental impact

For fiscal or custody transfer, inaccurate measurement of water in oil will directly impact revenue. For example, oil tankers can typically transport between 500 000 – 4 million bbl of crude oil. For a tanker with a capacity of 1 million bbl of oil, a water-in-oil content of 0.5% means that as much as 5000 bbl of water could be present in the tanker. At US$80/bbl of oil, this means a potential financial exposure of US$400 000.

Around the world, some 90 million bbl of crude oil is produced worldwide, daily, and in pipeline transportation and allocation, water-in-oil content could be much higher than 0.5% in reality. Thus, any inaccurate water-in-oil measurement could have a significant financial impact on all of the parties involved. This includes production partners, oil and gas commodity sellers, and buyers.

For oil and gas transportation using pipelines, pipeline integrity is paramount for operators. Corrosion-related pipeline incidents can lead to catastrophic consequences. By measuring water in oil (or condensate) and moisture in gas accurately, and making sure that the concentration is within the specification set by the pipeline operators, the risk of water-led corrosion is reduced, and in turn corrosion-induced pipeline leaking or worse – pipeline rupturing.

Any pipeline leakage of oil and gas resulting from pipeline corrosion is obviously bad in terms of safety, environmental pollution, and loss of revenue. Inaccurate water-in-oil measurement could also affect the performance of produced water treatment systems or refinery wastewater treatment systems, which then impact the quality of treated water for discharge. The discharge of produced water or refinery wastewater is strictly regulated.

Trends and needs in water-in-oil measurements

Traditionally, water-in-oil determination in custody transfer and allocation has been achieved by sampling and laboratory analysis. This process is laborious and time consuming. Online continuous water-in-oil measurement provides real time determination of water in a flowing hydrocarbon stream. This approach offers many advantages, and can potentially improve system efficiency and operational safety, and streamline system operations – if it works both reliably and accurately.

Online continuous water-in-oil measurement devices have been available on the market for a long time. However, few collaborated studies have been conducted in which these online water-cut measurement devices are tested and evaluated independently and in a collective manner. The only known tests were conducted back in the 1990s as part of a joint industry project (JIP), in which commercially-available devices were tested using a specially designed and developed flow loop. Since then, measurement technologies have advanced and been improved, and new instruments have also been developed. Therefore, there is a need to test and evaluate such devices again.

There also seems to be a technology gap in online measurement of very low water content in gas condensate or crude oil applications in the parts per million (ppm) range. For gas and gas condensate production, separated gas and condensate streams are often recombined and exported. Thus, water content in condensate oil will affect the overall water moisture level in the gas/condensate export line.

The future of accurate water measurement

Measurement of water in oil is crucially important for the oil and gas industry in relation to fiscal or custody transfer, production and production allocation, and pipeline transportation. Inaccurate water-in-oil measurement can directly impact operators’ finance, production operations, pipeline integrity and safety, and the environment.

For custody transfer and allocation, measurement is often carried out by sampling and laboratory analysis, for which standard measurement methods are available, based on using centrifuge, distillation, or the Karl Fischer method. However, there is an increasing demand for online measurement devices, for which the oil and gas industry continues to work together to come up with standard practices and guidelines.

For oil and gas production operations, online water-in-oil devices are already widely used as part of multi-phase flow measurement. The vast majority of water-in-oil devices use one of four measurement technologies: capacitance, density, infrared absorption or microwave. For online water-in-oil measurement, there is a need to independently test and evaluate instruments that are available on the market.

Written by Dr Ming Yang, TÜV SÜD National Engineering Laboratory.

Read the article online at: https://www.hydrocarbonengineering.com/special-reports/27072023/measurements-matter/