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A proposed methodology for improved multiphase metering

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Oilfield Technology,

Dennis van Putten, and Bert Tinge DNV GL explain how improved multiphase metering could optimise well production

Multiphase flow is the passage of more than one liquid, gas or chemical substance through a pipeline, which is an increasing concern on subsea (and topsides) production piping systems. While measurement of single-phase fluid flow in the oil and gas industry has now reached maturity, a new challenge is the simultaneous measurement of commingled gas, oil and water streams. This has driven the development of multiphase flow meters over the last two decades. The main issue in the application of these meters in the field is the uncertainty of the measurement due to the immaturity of the technology.

As flow rates increase and E&P goes deeper and into more harsh environments, the development of products and equipment are becoming more complex to improve production performance – though new innovations in subsea processing and separation are on the horizon. Finding the balance between design, simulation and monitoring is crucial to maintain integrity of equipment and push ahead to recover more reserves from the reservoir.

Multiphase meters are used to optimise the production from a well and deviations in flow rate data may lead to smaller production amounts from oil and gas reserves. Erroneous measurements in the allocation process can lead to enormous loss of profit for E&P companies. Multiphase meters are, besides allocation purposes, also used for field management, i.e. monitoring the production from a well real time. Increased water production could for example indicate the depletion of the field.

For protection of assets, multiphase meters are also used to identify water breakthrough in oil and gas production. The introduction of water in combination with an acidic gas can lead to large corrosion risks in the downstream segment of the production pipeline.

Current application

An increased number of small and remote fields have been produced over the last number of years. Since it is not thought to be economically viable to build process facilities on each of these fields, an increasing number of production pipelines are shared among different E&P companies with the need to allocate the total production to the various suppliers to that line.

Moreover, multiphase flow meters provide an economically attractive solution for subsea field development. Cost-effectiveness and efficiency is the driving force in developing this technology further.

Separators are the common solution, but these are costly in both Capex and Opex (i.e. acquiring them and maintaining them). For some fields, for example, small and stranded fields, this cost cannot be overcome by the revenue. In these cases, multiphase flow meters are a solution, despite the fact that at this stage, they are generally considered less accurate than separators. Improving the accuracy and the range of applications are key topics for the collaborative R&D efforts in the oil and gas industry.

Accurate multiphase flow meters allow for a fair division of the fluids produced. Contracts between E&Ps (and governments) require allocation measurements with less than 2% uncertainty, which pushes the improvement of existing technologies and the emergence of new measurement technologies further.

Proper calibration of multiphase flow meters is essential for accurate allocation processes and efficient well production. Multiphase flow facilities with the capability of performing tests at realistic field conditions reduce the uncertainty of these processes.

In downstream applications, single phase flow is expected and uncertainties of less than 0.5% are typically attained. In recent years, E&P’s have explored the boundaries of their upstream process equipment and as a result moderate multiphase flows are more frequently observed in downstream pipelines. The behaviour of flow meters on small amounts of phase contamination is very non-linear. As an example, for single phase gas flow meters, the introduction of 1% of liquid volume fraction can lead to more than 5% systematic bias in the gas volume flow reading.

Assessing performance

An impartial performance test is necessary to prove the capabilities of these multiphase flow meters. Currently, performance tests are executed at several test facilities in the world. None of these facilities are capable of covering the complete set of parameters of flow conditions and fluid properties. As future field development is expected to lead to even more extreme field conditions, the industry needs to anticipate these changes and develop guidelines to manage them with the available test facility resources.

Accurate measurement of the production of oilfields is an important means of reducing the financial risks that E&P companies face in the allocation processes. Inaccurate results related to the oil flow can present both operational and financial risks by negatively influencing decision-making and understanding of operational efficiency.

DNV GL’s Multiphase Flow Laboratory in Groningen, the Netherlands, allows equipment manufacturers and oil and gas companies to test, validate and calibrate multiphase technologies, such as separators and multiphase flow meters used in oil and gas production, well management and hydrocarbon allocation.

The facility can recreate conditions experienced in the field. These include a full range of multiphase fluid compositions at realistic temperatures, pressures and flow rates. Experts assist the development of standard industry protocols for equipment and testing.

Transfer of field conditions to laboratory scale

The use of true-to-nature fluids (fluids that are encountered in the field: natural gas, oil(s) and (salt) water), in contrast to testing with water and air under atmospheric conditions in the test facility, is a necessity for recreating the actual physical conditions that the multiphase flow meter will face in the field. The basic strategy of testing a multiphase meter for a certain application is to closely replicate the conditions in the field, which means supplying the fluids from the well and selecting a test facility that is capable of attaining the demanded flow rates and thermodynamic conditions. Due to large variety in the field this is an insurmountable task.

A potential strategy is to reduce the parameter space by using dimensionless numbers. This means that instead of defining fluid densities, viscosities, surface tension and pipe size for example, a series of dimensionless numbers are defined. This approach has been very successful in many physical problems ranging from single phase flow meter calibration to scaled experiments in aerospace science.

For multiphase flow, the dimensional analysis is much more complex and derivation from the fundamental multiphase flow equations is necessary to demonstrate which dimensionless parameters are needed for a general multiphase flow. The analysis from the fundamental equations ensures that for same dimensionless numbers the physical behaviour, and therefore the topology of the multiphase flow, is identical.

By definition, the number of independent variables resulting from dimensional analysis is smaller than the set of dimension-full parameters, reducing the required test time. This analysis leads to a better, more fundamental understanding of multiphase flow and the way multiphase metering should be undertaken. This will improve the accuracy of multiphase metering and increase the range of applications of multiphase meters.

The approach is aided by the distinction of many sub-regimes in the multiphase domain, of which the wet gas flow regime is an example. These sub-regimes are distinguished by certain industry specified boundaries often based on dimensionless numbers. In these sub-regimes, assumptions can be made on the expected dependency of the multiphase flow topology on certain dimensionless numbers and therefore reduce the parameter space even more. Typically, wet gas flow regimes are stationary, gas continuous and often the combined liquid flow of oil and water is well mixed.

Current status and outlook

Looking solely to the multiphase flow regimes is not sufficient to verify the multiphase meter performance and in-depth knowledge on the measurement principle is required. This involves analysing very different measurement principles which are used in multiphase flow meters, such as nuclear magnetic resonance, gamma ray absorption and capacitance tomography. Fundamental understanding of the physics behind all these principles is a pre-requisite to properly evaluate the performance of these meters.

Specific meter designs exhibit different sensitivities to various physical properties. As an example, fraction measurements based on conductance behave differently on changes in water salinity than gamma source based methods. Although presenting no change in multiphase topology, these sensitivities need to be taken into account.

Studies are initiated to facilitate the inter-comparison between test laboratories and the representativeness of these test conditions for metering technology in the field. These studies will aid in the understanding of the differences between facilities but do not judge the representativeness of the conditions compared to actual field conditions.

There has been a growth in the amount of experimental data on flow regimes, especially in wet gas, in recent years. The tests demonstrate that the flow regimes of natural gas and inert heavy gases like argon can be matched by taking the appropriate pressure. Also, the complex process of atomisation of stratified flow can be predicted for different liquids by means of dimensionless numbers.

The proposed methodology will be applied to other sub-regimes of the multiphase flow domain in the coming years, which is expected to provide more evidence on the approach.

Liquid injection section in Groningen Multiphase Flow Laboratory.

Multiphase Bornemann pump

Adapted from an article by DNV GL Louise Mulhall

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