Multiphase flow meter may force paradigm shift

Cheaper, lighter, more reliable

William Furlow
Technology Editor

New developments in the size, price, and reliability of multiphase flow meters could make them standard issue for offshore platforms, according to Dennis Perry, Manager of Applications for PetroTraces.

Perry is working with Conoco to develop a reliable multiphase meter that could replace the large, heavy, cumbersome separators that are currently the industry standard. In addition to technological hurdles, Perry said the project is also facing ideological barriers.

Conventional three-stage separators process large volumes of gas, oil, and water, separating the three phases so each can be measured accurately. The raw production from several wells can feed into these vessels which use a variety of methods to expedite the natural separation process.

Separators are originally calibrated based on the production from the fields in which they operate. When originally installed they offer a high degree of accuracy. To perpetuate the precision of this process requires an expensive maintenance program. Since this well testing is a measurement function and doesn't directly contribute to the bottom line, it is important to the operator that it be performed at a low cost.

Over time, wells become depleted and the composition of their production changes. This means that over time, the accuracy of the well testing equipment that was calibrated for the original production rates and composition begins to fall off. In addition, the testing equipment only gives average flow rate readings, so the information is not delivered in real-time.

That means on average, any problem detected in production rates will be more or less half of the interval between tests. If there are a number of problems during a year, then the lost production between when the problem occurs and when it is detected and corrected can quickly add up. In addition, these vessels are large and very heavy taking up valuable space on the platform and adding to the finite weight capacity of the installation.

Multiphase solution

A multiphase meter that can accurately detect anomalies in production in real time offers a number of advantages. This technology could tell operators right away when there is something wrong with their production and, depending on where the meter is located, it could help track the problem to an individual well.

In addition, the multiphase meters are dramatically smaller than the traditional three phase separator and require far less maintenance. Perry said the savings in maintenance costs alone could easily justify switching over from a conventional system in some cases. The production received by these systems is not always pure. There is oil, gas, and water, but there also are impurities, such as sand, that can cause damage and wear to the internal parts of the separator. This accounts for much of the maintenance costs.

Perception problems

The question about multiphase meters is one of reliability and perceived reliability. Because the three-phase separators have been around for so long, and they have performed reliably there is an understandable resistance to the new technology.

In test loops, the multi-flow meters perform well, but these test situations only simulate the production a meter would encounter in the field. To determine the accuracy of the multi-flow meter in a test loop, the composition of the fluids it is testing must be known.

To predetermine the composition, the test operators blend premeasured amounts of oil, gas, and water. For the purpose of safety, nitrogen is used in place of natural gas. Also, the oil used in such a test has been refined and the water has been filtered. As one would suspect these elements in the flow loop, while accurately measured my the equipment being tested, perform differently than unrefined production in the field.

Baseline readings

A field test would seem to be the natural alternative; use this equipment side-by-side with the separator and compare the readings. But the question then becomes, how accurate is the separator to begin with?

If the composition and rate of the flow have changed since the unit was installed and adjusted, and the separator is batch processing the production it may give readings that are less accurate than the real-time computerized readings from a flow meter. It is a question of baseline. Should the goal of the multiphase flow meter be to mimic the results the operator is getting from the separator, or is there a more accurate standard?

Perry is quick to point out that the separator systems have not been through the rigorous testing that this new technology is receiving. The amount of water cut and gas volume fraction (GVF)could account for discrepancies between the separator's readings and those of the multiphase meter. The question is which system gives the more accurate reading?

The multiphase meter gives real-time readings, meaning changes in water cut and GVF would show up in altered readings. The separator batch processes this production in large volumes. The average of the many readings from the multiphase meter would presumable be more accurate, all things being equal, because it is based on more data points.

In the flow loop tests, the meter performed better at low GVF. As the amount of GVF was increased, the accuracy of the readings from the meter decreased. Under these conditions, Perry said the meter tended to overstate the liquid as the GVF increased. Still, he pointed out that the overall volume was accurate, the difficulty was in measuring the proportions of gas, and liquids.

Field test

Perry took the multiphase meter onto an offshore platform in the Gulf of Mexico to field test it against a three-phase separator. For six months, the meter was reading flows from 11 wells. These readings were graphed so they could be evaluated. The variance in gas readings was much broader than that of the liquids, indicating there was a problem with the gas lift system used on the field. In the area of water cut, the field test correlated with the flow loop results.

The gas readings were less definite. Still, Perry points out the accuracy of these field readings was measured against data from the separator. If there were inaccuracies in these measurements they would cause abnormalities in the accuracy of the multiphase results. Perry also said the multiphase meter was operating at its lower operating limit and might require higher volumes of flow to give accurate readings.

At the present, multiphase meters are no real threat to the three-phase separator market. They are more of a specialty application, being too expensive to justify replacing the units already in the field.

Also, the meters currently on the market require a high flow rate to be accurate. Perry said further development is required to come up with a multiphase meter that will be inexpensive and reliable enough to be retrofitted into existing platforms and part of the initial design on new installations.