July 2017, Vol. 244, No. 7


Taking Measure of Flow Meters in Phase-Contaminated Oil Flow

By Dennis van Puten, DNV GL-Oil & Gas

Accurate allocation measurement results are a key financial and commercial driver for E&P companies. Yet despite limited evidence of the performance of Coriolis and ultrasonic flow meter technology, they are being increasingly deployed for oil flows contaminated with water or natural gas.

This article updates efforts to increase knowledge and understanding of Coriolis and ultrasonic meters at a time when oil and gas operators are facing significant measurement challenges in optimizing production and generating more from reservoirs.

Short-Term Agility, Long-Term Resilience, DNV GL’s seventh annual benchmark study on the outlook for the oil and gas industry, surveyed over 700 senior sector players and revealed that nearly half of respondents (47%) believed their organization would look to increase the efficiency of the assets already in operation. A further 39% expected their business to focus on extending the lifespan of assets over the next 12 months, while 11% of global respondents planned investment in new technologies to target enhanced oil recovery (EOR). There is little doubt that production optimization is of high importance for the oil and gas sector.

One main challenge, however, is how to deal with the measurement of multiphase flows. The application of multiphase flow meters (MPFMs) has increased significantly over the last several decades to overcome this challenge. In principle, these MPFMs have the capability to cover the entire multiphase flow regime, but might not be the best choice when considering costs and accuracy.

Many oil fields are producing small levels of phase contamination (i.e., small volume fractions of water and gas). This may be caused by using enhanced oil recovery (EOR) techniques or due to the production toward the end-of-life of a field. These phase contaminations are typically small compared to the main flow, and, therefore, single-phase measurements with appropriate compensation methods might be used.

In many situations, an MPFM may not be fit-for-purpose and the single-phase flow meter might outperform the MPFM in terms of accuracy and costs. For these applications, an uncertainty of 5-10% is typically allowed, which aids in the successful application of single-phase flow meters.

Single-phase flow meters are often installed in situations where normally a pure oil stream is expected, for example, downstream of a separator, at custody transfer locations, or at bunkering stations. The transported fluids are often near their bubble point and may evaporate due to changes in process conditions, either planned or caused by process upsets. The phase equilibrium by itself is extremely sensitive and calculations require accurate input data, like upstream process conditions and composition of the fluids, to predict the onset of degassing.

The effect on the introduction of a second phase should be well understood as it can lead to relative high biases due to the non-linear behavior of a multiphase flow. These systematic errors enter the allocation process and can lead to large financial risk. For phase-contaminated oil flows, both over-reading and under-reading of a single-phase flow meter is possible, as the type of contamination and its physical behavior determine the interaction with the continuous phase.

Recent preliminary tests at DNV GL show liquid mass flow under-readings of a Coriolis flow meter can be as high as 40% for relative small gas volume fractions of 5% (Figure 1).

A large dependence on the flow regime was observed, which adds to the complexity and uncertainty of the measurement. At large mass flow rates, fully homogeneous gas-liquid mixtures were produced, which reduced the systematic mass flow bias to about 8% at 5% gas entrainment. Similar deviations on the mass flow rate were found in other studies, but no physical explanation has been devised. Also, large differences in physical response to gas entrainment between different manufacturers were observed and were inexplicable.

figure 1 DNV Flow Meter

Figure 1: Preliminary results of liquid mass flow error for a vertically installed Coriolis meter as function of gas volume fraction.

Correction methods for phase-contaminated flows for Coriolis meters have been developed in recent years. It is generally accepted that the density measurement of a Coriolis flow meter can be used for accurate measurement of the water liquid ratio in absence of gas. Many publications of meter manufacturers also state that accurate compensation for the gas entrainment is possible. Based on these observations, the technology seems ready for large-scale field application of contaminated oil streams.

However, the performance tests of these meters are executed at different conditions and often performed on in-house, small-scale, low-pressure testing facilities. Differences between published data by manufacturers and third-party testing facilities have been observed. Since the intellectual property of the dedicated correction methods is protected, the performance of the flow meter, including the correction methods, can only be evaluated by means of a performance test. It is essential that these tests are performed at realistic field conditions and that an equal playing field is provided to all manufacturers. As there is currently no general accepted methodology for testing these flow meters, there is no fair comparison possible between the different manufacturers.

The current ultrasonic meter technology is less advanced when considering gas entrainment. It is a general conception that bubbles block and disperse the ultrasonic signal, and therefore, no valid measurement can be performed. However, ultrasonic technology has advanced in recent years and the performance and its systematic bias is expected to depend on the gas void fraction. It is necessary to explore the applicability of these ultrasonic meters and understand the sensitivity of these meters for water-oil mixtures and gas entrainment such that the over- or under-reading of the meter can be predicted.

The in-house developed correction methods are often based on laboratory-scale experiments. The scalability of these results to field conditions is hampered by the multi-dimensional parameter space of a multiphase flow. The complexity of a multiphase flow and the size of the parameter space are greatly reduced by using dimensional analysis in which the behavior is expressed in terms of dimensionless numbers, such as the Froude number or Reynolds number.

This approach has already been successfully applied by DNV GL in wet gas environments for ultrasonic meters and explored by several manufacturers in phase-contaminated liquid flows. To cover a large envelope within the dimensionless parameter space in terms of the density ratio and liquid Reynolds number, for example, large-scale experiments at high pressure and variable liquid viscosity are mandatory.

Increasing knowledge and understanding of Coriolis and the behavior of ultrasonic meters in phase-contaminated oil flow allocation will support the application in a wider range of flow conditions. To achieve this, DNV GL is executing a joint testing project (JTP) that aims to set up testing guidelines, evaluate the performance, and develop correction algorithms for Coriolis and ultrasonic meters used in phase-contaminated oil flow allocation, endorsed by key industry stakeholders.

The core deliverable of this JTP is to develop a systematic test program close to real field applications (natural gas/water/oil mixtures) based on the fundamentals of multiphase flow – flow regime and physical properties. Secondly, to develop a test plan for an unbiased and level playing field performance evaluation of the meters, including corrected and uncorrected data from the manufacturer. And finally, to develop from the test program a correction algorithm for Coriolis and ultrasonic meters in phase-contaminated oil flow. The first test results are expected to be presented at the North Sea Flow Measurement Workshop later this year.

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