A multiphase flow meter enhances data quality and quantity obtained from mobile and permanent well testing equipment, improving well performance assessment and reservoir understanding.

A novel multiphase flow meter has proven successful in a range of well testing applications worldwide, providing an efficient and cost-effective means to obtain flow rate data at every stage of a well's life. When testing wells, the Schlumberger multiphase meter - which incorporates Vx technology - displayed greater operational flexibility and more accurate and extensive data acquisition capabilities compared to conventional testing methods. Well tests performed with the multiphase meter generated accurate, continuous and instantaneous flow rate data representative of the well and surrounding reservoir, thus enabling more accurate assessment of well performance for better reservoir understanding.
Conventional well tests
Testing wells to determine reservoir parameters, evaluate recovery potential and optimize production rates is a necessary task during all stages of field life, from exploration and appraisal to development and production. Conventional well tests typically are performed using single-phase flow meters installed on each of the gas, oil and water outlets of standard separators and tanks. Problems such as poor fluid separation and meter calibration can prevent accurate measurement of gas, oil and water flow rates.1 Fluid level instability in large separators can result in flow measurements having insufficient resolution to identify smaller flow events or subtle transient behavior. High flow rates and heavy oils can cause emulsions and foams to form. If these are not broken using standard methods such as heat treatment, chemical injection or gravity separation, flow rate data may be inaccurate. Therefore, conventional well testing methods make it difficult, if not impossible, to obtain accurate flow measurements that are representative of the well and the surrounding reservoir.
Multiphase tests
The multiphase flow meter continuously measures each phase - gas, oil and water - without the need for prior separation. This eliminates the need for large separators with their inherent measurement problems. Multiphase meter flow measurements can be more accurate, extensive and precise throughout a range of transient conditions and producing stages.
The Schlumberger multiphase flow meter uses Vx technology that compactly combines two metering technologies into a simple device with no moving parts. A dual-energy composition meter is situated at the throat of a Venturi, which measures the flow rates of three produced phases regardless of the inlet flow regime (Figure 1). The two main elements of its measurement section are:
• a Venturi that measures the total mass flow rate using differential pressure sensors; and
• a dual-energy spectral gamma-ray detector combined with a radioactive chemical source that measures the gas, oil and water fractions.
Incorporated into the detector design is advanced logging and space technology that enables high-frequency measurements, making the meter insensitive to flow regime. Being an absolute measurement technique, the metering technology combination does not require onsite flow calibration as do other well testing methods.
Two multiphase meter designs have been developed. PhaseTester is a portable meter used in mobile well testing operations during the exploration and appraisal phases. During the subsequent development and production phases, either a portable meter can be used for periodic well tests or the permanent version, PhaseWatcher, can be installed for continuous well testing. Whether installed for permanent or mobile testing operations, the multiphase metering equipment weighs less and has a smaller footprint than conventional separator-based testing equipment.
For mobile well testing, rapid and efficient equipment deployment, rig-up and set-up are key to a successful operation.2 These three tasks can be accomplished less than 90 minutes after the equipment arrives at the wellsite during routine operations. Improved efficiency and shortened turnaround time for installation and operation are particularly important in high-cost environments such as deep water. Depending on testing requirements, test duration ranges from less than an hour to several days. For shorter tests, up to three wells in different locations can be tested in a given day.
As with any multiphase meter, knowledge of fluid properties is required to identify individual phases. The densities and attenuation of the gas, oil and water are its basic inputs. The phase densities typically are obtained from onsite fluid samples. The attenuation of each phase is determined from either in-situ measurements or elemental analysis computations.
The meter generates a full record of flow periods throughout the well test, thus providing flow rate history required for transient analysis. Flow rate data can be acquired at surface pressures below and far above the operating limit of the separator, which expands the operating envelope for flow rate data acquisition.
Multiphase vs. conventional testing
Multiphase flow meters provide a well testing solution that is better than conventional test methods. Benefits include:
• reduced risk and increased safety;
• improved data quantity and quality;
• enhanced data availability and continuity; and
• streamlined logistics and shortened test duration.
Multiphase flow rate metering reduces well condition uncertainty and provides accurate well test data for prospect assessment, completion design, field development and subsequent reservoir and production management. Operational control and safety are improved by reducing maintenance requirements and eliminating the need for pressure safety valves and relief lines.
Multiphase flow meters can measure three phases without having to separate them, which increases accuracy and reduces the sources of error that jeopardize separator measurements. Emulsions and foams do not affect the multiphase meter's ability to provide accurate flow rate measurements. Moreover, data quality can be controlled by changing the back pressure on the multiphase meter, adding further confidence to the acquired flow rate measurements.
Data acquired from separators or tanks during conventional well testing operations typically take the form of discrete, tabulated flow rate and pressure data measurements. Continuous flow rate measurements better reflect well and reservoir performance, as do continuous pressure readings. The complete, continuous flow rate data set generated by multiphase metering enables analysts to interpret the data and well test events better. Additionally, the measurements generated by the meter are instantaneous, providing real-time information. Instantaneous and continuous flow rate measurements facilitate decisions and shorten flow periods and well test duration, saving time and money.
Broader testing applications
Multiphase flow meters can be applied to well testing during a field's exploration, appraisal, development and production stages. Numerous field tests and flow loops have proven these capabilities over a range of conditions during a 3-year period.
The multiphase flow meter not only improves data quantity and quality, it also improves the execution of the test. For example, the cleanup period prior to shut-in can be included in the test. Monitoring an exploration well's flow rate history during cleanup is important for accurate pressure transient analysis. By measuring flow rates throughout the test periods, the data and the resulting calculations truly represent the reservoir and its behavior.
In the conventional well test using separation equipment, produced fluids often are not measured during cleanup. Therefore, assumptions regarding the flow rate history must be made when performing the subsequent analysis. Flawed well test procedure and flow assumptions can generate inaccurate analyses, and as a result, incorrect conclusions regarding well performance or reservoir potential can be drawn. However, the information typically missing from conventional well testing methods can be provided via multiphase metering during critical flow periods, adding value to exploration well testing.
The multiphase flow meter also can be used during the producing stage to optimize a well or field's production. Its ability to continuously measure the individual produced phases permits quick performance trend analysis and well diagnostics. Rapid, accurate diagnosis enables timely decisions regarding production rate changes or intervention planning.
The production optimization process can be described as a loop that begins and ends with a well test. First, existing conditions are measured; then production enhancement actions such as reperforation, stimulation or gas-lift valve change-out are carried out; and then resulting improvements are assessed. Continuous monitoring and efficient testing help optimize individual well and field production and maximize reservoir recovery and producing life.
One particular field application illustrates the benefits of multiphase flow meter vs. conventional separator well testing. Wells on an unmanned offshore platform were each tested for 12 hours. One well on gas lift had 3-hour cyclic variations in its producing water and gas cuts (Figure 2). Though this well could not be tested using a separator, a multiphase meter generated quality flow rate data. After careful analysis, the gas lift parameters were changed. As a result, oil production was optimized, increasing producing rates by 650 b/d.
The multiphase flow meter also enables well testing to be conducted under more challenging conditions. At flowing pressures too low for separator operations, its use has enabled testing to continue that otherwise might have been aborted using conventional testing schemes. It performs well with difficult fluids and phases, including emulsions, foams, waxes, viscous oil and significant H2S concentrations. In colder environments, flow rates can be measured without heat exchangers, which streamlines logistics, enhances safety and saves operating costs.
While challenging flow conditions and difficult fluids might impede the safe and practical acquisition of accurate test data in separators and tanks, they do not negatively affect multiphase flow meter measurements. Numerous field application successes have elevated multiphase flow meters using Vx technology as the preferred data acquisition method because of the comprehensive, continuous and instantaneous nature of the flow rate data generated. Moreover, its successful application has been demonstrated throughout the stages of this and other fields' lives over a range of conditions around the world.
References
1. Mus, E.A., Toskey, E.D., Bascoul, S.J.F., and Norris, R.J.: "Added Value of a Multiphase Flowmeter in Exploration Well Testing," paper OTC 13146, presented at the Offshore Technology Conference, Houston, Texas, April 30-May 3, 2001.
2. Theuveny, B.C., Segeral, G., and Pinguet, B.: "Multiphase Flowmeters in Well Testing Applications," paper SPE 71475, presented at the SPE Annual Technical Conference and Exhibition, New Orleans, La., Sept. 30-Oct. 3, 2001.

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