1 Introduction & Background

Initial developments for multiphase flow measurement started in the early eighties as a response to the needs of the oil and gas production industry. These developments were collaborative efforts by research organizations and meter manufacturers with oil and gas production companies. Out of the early prototypes developed based on dissimilar design and function, some were abandoned while others became commercially available.

As a first step towards its field applications, multiphase flow meters (MPFM) replaced conventional test separators installed on gathering line headers for combined flow measurement. Availability of low cost and compact multiphase flow meters further boosted interest in their use, and MPFM installations have become a mandatory part of every new wellhead configuration. Today, many associated fields have installed multiphase flow meters on a one-per-well basis in subsea, topsides, and onshore assets. The number of total installed MPFMs is close to 10,000 units as of 2021.

The development of MPFM is based on the principle of measurement of multiphase fluid flow by determining individual phase fractions, phase velocities and phase densities of gas, water and oil. The MPFM is set up to measure component velocities and two component fractions since the fractional sum has to be equal to 1. Commonly, for density measurement, dual gamma-ray densitometers with different energy levels are used to calculate the gas fraction and water cut of the multiphase mixture.

Today the scope of multiphase measurement covers three types of applications with different technologies commercially available after thorough testing and site validation:

  1. In-line (installed directly on the production line) MPFMs for measurements of oil, water and gas volumes flowing in the fluid passing through them
  2. Partial and Full separation MPFMs
  3. Wet gas meters (a subset of multiphase flows)

2 Advantages of Using Multiphase Flow Meters

Over time, multiphase flow meters have been recognized as beneficial for continuous metering (or monitoring) of unprocessed well fluids coming out of a single well, thereby evaluating well productivity and reservoir performance. Inline multiphase meters can improve operators’ bottom line as they provide invaluable information for better reservoir management continuously. Conventional test separators can provide improved measurement certainty but at a higher cost and have limited applications. Real-time production monitoring, especially in complex subsea installations, is only possible with multiphase flow meters.

Starting with an initial role of production measurement and allocation, today MPFMs are increasingly being used for other beneficial applications, like:

  1. Exploration and Appraisal Well Testing
  2. Pre-production Well Monitoring
  3. Reservoir Management through Production Well Surveillance
  4. Control and Optimization of Well Production

3 Understanding the Uncertainties of Multiphase Flow Measurement

MPFMs measure complex fluid mixtures are coming out of a well that is yet unprocessed. As such, these meters are subjected to extreme conditions of high temperatures, high pressures, erosion, highly corrosive H2S/CO2 and deposition of solids, waxes and hydrates. All these variables change during the production life of a reservoir. This reality contributes to measurement uncertainties more significant for multiphase flow meters than measurement uncertainties specified for single-phase meters. Another factor that may significantly affect the uncertainty of measurement of the multiphase flow is the quality of PVT sampling data obtained as a representative sample of the well fluid.

Generally, MPFMs (essentially in subsea installations) are expected to perform continuously and permanently once installed and are not subjected to routine calibrations as for single-phase meters. Thus in-situ validations of multiphase flow measurements are traditionally done through comparison with test-separator results. Various organizations like TUV SUD National Engineering Laboratory, Norwegian Society for Oil and Gas Measurement (NFOGM) and American Petroleum Institute have been actively involved in developing guidelines and standards for improved performance MPFMs.

  • Norwegian Society for Oil and Gas Measurement (NFOGM) initially provided guidelines for inline multiphase fluid measurement systems through a handbook in 1995, which was updated in 2005.
  • API Upstream Allocation Task Group developed API Recommended Practice RP 86-2005 – Measurement of Multiphase Flow. With the Issuance of API MPMS Chapter 20.3 in 2013, API RP 86 stands withdrawn.
  • Guidance Notes for Petroleum Measurement (earlier published by UK Department of Trade and Industry (DTI) in 2003) was updated by the UK’s Oil and Gas Authority in 2015.

With the establishment of the Advanced Multiphase Facility (AMF) by TUV SUD in 2019 for multiphase meter testing, fiscal applications are envisaged by MPFMs. Dr Brian Millington, Managing Director of TÜV SÜD National Engineering Laboratory, said:[1]

“The AMF’s world-leading research facilities will support the global oil & gas industry with both current and future measurement challenges, from well optimization to fiscal accounting. While significant production opportunities exist in extreme environments, higher operating pressures and temperatures can impact the performance of multiphase flow measurement devices. The AMF will increase the viability of well exploitation by helping operators to measure multiphase flow more accurately and better understand the performance of production operations in these challenging but potentially profitable environments.”

The use of multiphase flow meters in fiscal applications is now being established and accepted as essential for correct estimation and production recovery from the reservoir. MPFMs are now available, with improvements in meter performance having uncertainties typically application-dependent and not always quantifiable. However, measurement uncertainty can be minimized by adopting best Practice in meter selection, maintenance, operation and verification. Fiscal multiphase measurements are appropriate in production allocation applications where hydrocarbons from more than one field are blended in a shared production facility and where cost-benefit considerations indicate that single-phase measurement of each field is not economically justified.

In subsea applications, MPFMs are typically used in well testing, allocation measurement, fiscal measurement, well management, and/or flow assurance applications. The categorization of MPFM application is vital since it can determine the required level of factory testing, independent verification, field maintenance, and ongoing verification required during operation.

4 MPFM Knowledge Base under API

It is recommended that engineers who are involved in multiphase flow measurement should consult the following, in addition to API MPMS 20.3, to acquire adequate knowledge:

API RP 17S (2015) – Recommended Practice for the Design, Testing, and Operation of Subsea Multiphase Flow Meters

Provides recommendations for the sizing, specification, system integration, and testing of subsea flow meters to measure full stream, multiphase flow. This document includes wet gas flow meters as a subset of MPFMs. In-line MPFMs are typically used in subsea applications and are the focus of this document.

API MPMS Chapter 1

Vocabulary Provides definitions and terms used throughout the API Manual of Petroleum Measurement Standards (MPMS).

API RP 85

Use of Subsea Wet-Gas Flowmeters in Allocation Measurement Systems

(includes Addendum 1 dated January 2013, whereby some sections were over-ridden by API MPMS 20.3)

This document presents a recommended allocation methodology that best fits the application and that equitably accommodates variances in the uncertainty level between meters in the system. Through this document intent of API is to advise the user on various aspects of the use of subsea wet-gas flowmeters in allocation measurement systems. Marinization, operation, abnormal operation, and meter testing are essential topics included in this RP. Most importantly, this document proposes techniques to be used in the allocation of total production to individual contributing streams.

5 How to Make your MPFM Perform Optimally

Having stated all the measurement and well fluid flow uncertainties inherent in multiphase flow measurements, it is now time to understand how vital considerations during selection and use of a multiphase flow meter can ensure better measurement results.

Intended Purpose: Describing specific intent of multiphase measurement and closely matching well production profile predictions with the Operating Envelope of the selected meter. Operating Envelop describes performance expectations for a multiphase flow meter regarding liquid and gas flow rates, gas volume fraction and water-liquid ratio.

Accessibility & Location: Describing meter accessibility in terms of remote/unmanned land-based operations, offshore unmanned operations, topside / onshore manned operations, and subsea applications are generally crucial for the smooth functioning of multiphase meter throughout its operational life. Similarly, the location of the meter relative to other wellhead components like chokes, meter isolation fittings, other diameter changes and bends needs to be specified as they affect meter readings. For subsea applications, additional specifications like insulation, maintenance, replacement, retrieval and placement of meter in subsea pipework – jumper, pipeline end termination (PLET), manifold, tree, others.

Production Profile Prediction: Predicting flow regimes that are likely to be encountered and identification and quantification of flowing fluid properties are vital to selecting meter selection. Identification of various multiphase flow regimes that may be experienced during the life of producing field helps define meter response during each regime and during the transition from one regime to another.

Fluid Properties and Variations: Standards make it mandatory that all fluid properties required for configuration of the multiphase meter shall be obtained from the PVT studies performed on reservoir fluids. All properties like gas composition (including corrosive contaminants), gas density, oil density, water salinity, liquid viscosity, chemical additives, sand, etc., should be quantified at actual line conditions. A description of meter sensitivity to changes in fluid properties during the life of producing field helps in addressing possibilities of measurement bias of multiphase meter. This is also important for the selection of material for meter body and internal wetted parts.

Sampling: Identifying the frequency of sampling and type of fluid sampling requirements for maintaining the specified accuracy of the multiphase flow meter is crucial. Miniaturized PVT sampling and multi-trace techniques may be used with the correct location of the sampling point depending on flow composition. A 12 o’clock sample point position on horizontal pipe may be difficult to sample oil at high gas volume fractions (GVF), while a 6 o’clock sample point position may make gas sampling impossible at low GVF. The use of injectable sampling probes coupled with double-block-and-bleed (DBB) valve assembly is recommended to minimize sampling errors.

Pressure and Temperature Measurement and Variability: Multiphase flow meter is sensitive to large pressure and temperature variations due to multiple variations in fluid composition, operating envelop (OE) and computations. A multiphase flow meter should not operate beyond the OE to ensure specified metering performance and long-term reliability.

Right Communication and Computation Requirements: To get the best use of multiphase meters, the data communication requirements should be specified, including parameters related to production intended for routine collection and those related to meter diagnostics to fulfil operational needs. All types of communications between meter, flow computer, and control room shall be specified for data rates, frequency, and redundancy levels. All raw and processed data should be trended, ensuring that the computation of results is sufficiently fast and robust for predicted operating conditions.

Calibration and Field Verification: To achieve optimal performance of MPFM, all sensors of a multiphase meter should be calibrated as per detailed procedures specified by OEM / vendor. In some cases where calibration is not possible, the expected drift of the meter sensor should be documented with possible effects on overall meter performance. The multiphase meter user needs to pay attention to models and methodologies used to convert sensor readings into useable information and the source of each input for flow rate calculations. Understanding of inter-dependency of key sensor parameters by the user helps make a judgement about meter performance and diagnosis of malfunction.

Latest MPFMs are equipped with reference multiphase flow loop testing certificates, which means that at least one MPFM representative of those used in a field application has been flow loop tested and witnessed by a third party. Multiphase flow loop tests are carried out for defined well production profile and meter OE and conclude with a meter performance capability acceptance test called Blind Tests. Once the meter configuration has been completed, the flow loop tests proceed to a conclusion without further intervention.

Field verification of multiphase meters is carried out under a plan for continual verification of MPFM in field applications. This includes meter diagnostics for verification and field comparison of MPFM results with other field measurement systems part of system balance.

NOTE: Traditionally, for MPFM field verification, a Test Separator is the best practical option. Verification is done by flowing comparable multiphase flow through the test separator / MPFM and comparing MPFM results with (metered) volumes (of gas, oil and water) obtained downstream of the test separator.

6 HSE requirements related to MPFM

In addition to local regulations, international standards apply for handling MPFMs installed with a radioactive source. Following standards should be consulted to safeguard against radiation exposure and safe transportation of MPFMs:

  • IAEA DS 379, Safety Standards for Protection Against Ionizing Radiation
  • IAEA DS 387, Safe Transport of Radioactive Material

[1] https://www.tuvsud.com/en/press-and-media/2019/october/tuv-sud-launches-16-million-pound-facility

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