As multiphase meters gain wider acceptance, their potential to add value in new ways is being recognized.

The promise multiphase meters hold for reducing hardware requirements for offshore production facilities, particularly subsea well tie-backs to deepwater projects, has been one of the most important drivers for the rapid development of this technology during the past decade. But other benefits are becoming apparent as the number of installations increases. One of these is the potential for multiphase meters to significantly improve our ability to properly allocate production to individual wells.

Over the past 6 years, the number of multiphase meter installations worldwide has grown from roughly 50 to more than 1,000. According to Parviz Mehdizadeh, an independent consultant who has followed the numbers, "Both offshore and onshore installations have increased, but the more rapid recent growth has been in the number of meters onshore." According to Mehdizadeh, multiphase metering technology has firmly established itself and the prospect for rapid growth is evident, particularly with the advent of mobile multiphase test systems that can be used for exploration well tests. "This capability opens up the possibility of true real-time well test analysis and as such, is an enabling technology."

Gerald Smith, Schlumberger's business development manager for multiphase testing, agrees. "While multiphase testing has not yet reached a capability where it can be all things for all people, there have been steady improvements in measurement quality as the technology has evolved, particularly over the past 2 years." Schlumberger introduced its multiphase testing technology about 3 years ago and has quickly moved to apply it as a mobile testing service. Its PhaseTester service incorporates a skid-mounted, portable, multiphase meter system. The PhaseWatcher service is a permanent well monitoring service that utilizes the same technology, in this case, mass measurement using a venturi, coupled with a dual-energy gamma ray measurement of phase holdup (cross-sectional fraction). Schlumberger considered other approaches when researching multiphase meters during the mid 1990s, but settled on this combination as the most robust for well testing. The pressure drop across the venturi, the attenuation of the gamma ray beam and the impedance of the mixture are measured. A relationship between these measurements and the flow rates of the respective oil, gas and water phases, established by calibration, is used to establish the three flow rates.

A second basic approach involves measuring the basic parameters of phase velocities and hold-ups, sometimes after homogenizing the mixture upstream of the meter. A number of different approaches have been taken to arrive at the required measurements. (Note: A good summary of these approaches can be found in an SPE paper by G. Falcone, et al., SPE 74689, "Multiphase Flow Metering: Current Trends and Future Developments.")

The smaller size and lighter weight of multiphase meters have made them ideal for offshore installations, particularly where subsea satellite wells developing smaller accumulations are being tied back to deepwater hub platforms. One example is the tie-back of Macaroni, Serrano and Oregano development wells to Shell's Auger platform in the Gulf of Mexico. In these situations, the relatively small size of the multiphase meter equipment allows it to be added to the platform to provide well testing capability for the satellite wells without the need to further burden existing test separators. This approach can allow the development of what might otherwise be marginal accumulations. Multiphase meters also help to avoid the shutdown cycles associated with well tests that can lead to lost production and in some cases, well damage.

But as more and more multiphase meters are installed, one of the less obvious benefits is becoming apparent. "The use of test separators to routinely test wells has not allowed us to recognize the transient nature of the flowing conditions of many wells," says Mehdizadeh. "With continuous test data from multiphase meters we are able to see what individual wells are actually doing, and to begin to quantify the real impact of inaccurate allocation of production based on well tests. The greatest beneficiaries of multiphase metering may actually be the reservoir engineers, who will finally get an accurate idea of how much oil, gas and water has been produced from a given well bore."

This problem is particularly bad for wells with unstable flow, on artificial lift, with irregular or high water cuts, stable emulsions and improperly sized test separators. The impact of such factors on the accuracy of back allocation of production could be much more significant than has been recognized. While multiphase meters cannot yet guarantee metering performance within the ±0.5% range on all phases needed for "fiscal" allocation, they can be used to perform important diagnostics of existing systems. One approach recommended by Theuveny and Mehdizadeh (SPE paper 76766, "Multiphase Flowmeter Application for Well and Fiscal Allocation," May 2002), is to deploy mobile multiphase meters to learn the actual producing behavior of wells, with the testing duration proportional to the volumes to be back allocated. Rather than using the existing test separators as a reference to "train" the multiphase meter, the multiphase meter should be used to examine the quality of the test separator data. "Operators are beginning to recognize this application, and that is one of the reasons for the increase in onshore multiphase meter installations," says Mehdizadeh.

In spite of the growth in installations and applications, no industry-wide standards currently exist for multiphase meters. An effort is under way by API to begin the process of developing such standards. Operators could benefit from a set of objective guidelines that describe how these meters work, what they can and cannot do, and what sort of questions need to be asked when considering them for a production facility.

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