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SPE Paper Number 120578

Multiphase Meter Production Well Testing Applied to Low GOR Mature Fields

Parviz Mehdizadeh Ph.D., Production Technology Inc., David Farchy, Agar Corporation, and Jairo Suarez, Agar Corporation

Abstract

In many mature fields production well testing is limited by the availability of test separator. Prior industry attempts to utilize the new technology of multiphase metering to fill this gap has been hampered by the cost of multiphase meters. This paper describes the development and field tests of a low cost portable multiphase meter. The meter utilizes the Coriolis flow meter technology combined with a microwave based water cut meter that can measure WC in the 0-100% range. The combination of these techniques provides a light weight metering package that can be mounted on trailer for portable well testing. This multiphase meter can measure oil, water and gas without separation of the production stream at low GOR. These applications can be found in mature fields with low GOR and in heavy oil fields.

The multiphase meter was subjected to field qualification tests prior to deployment in a West Texas field. The paper describes the results from the qualification tests and the performance history during the 18 months of deployment in the field. Well test data from the multiphase meter is compared to conventional test separator and grab samples. Comparison between the test separator and the multiphase meter shows average liquid flow deviation of about ±7%. The water cut was measured within 1-2 points of the water cut from the test separator.

Introduction

Secondary and tertiary recovery methods, starting with water flooding, provide a significant amount of production in many U.S. Mid-Continent areas and throughout the world. These type operations typically handle large volumes of water, small volumes of oil and natural gas. In addition, many formations also produce large amounts of water with small amounts of oil and gas under primary production. Accurate testing of these wells is important to determine reserves, the economics of continued operations and to evaluate the effectiveness of workover projects to improve oil and gas production and to increase well profitability and reserves. There is no substitute for good accurate production data on which to base these decisions and actions.

Production well testing in many of these brown fields is currently performed by centralized separation/metering stations or by portable testers. Centralized systems require extra equipment to be installed and maintained over the field's entire life. This results in increased cost and environmental risks. Portable well systems allow testing at the individual well or at multiple centralized facilities and do not require additional equipment to be installed and maintained. Low cost portable testers using gravity separators are not accurate enough due to separation limitation, sampling frequency and/or gas interference. Higher cost multiphase testing units developed in the past 10 years (1) are out of the economic reach of most stripper, marginal and Brownfield well operators. Also many wells do not have electricity available on site. Thus most Brownfield operators must currently accept poor accuracy or high cost fixed sites.

Current conventional well testing accuracy for determining the flow rates can range from ±5% to ±50%.

In addition, the amount of labor needed to perform well testing, using conventional gravity based test separators or tanks causes the operator to perform well testing infrequently. These two factors combine to produce well rate data uncertainty and inconsistency that results in allocation factors (sum of oil production well tests/oil sales) that vary from 65 to 150% (2).

Multiphase Metering Techniques

The primary information required in the measurement of oil or gas multiphase flow streams is the flow rates of oil, water and gas. The ideal method to obtain this data is to have a multiphase flowmeter that would make direct and independent measurements of these components. Unfortunately, such a device does not exist as yet. Consequently, much of the extensive development in multiphase metering (1-4) has been directed toward inferential techniques that use the instantaneous velocity and cross sectional fraction of each component to make these measurements. Thus the task of any multiphase meter is to estimate the volume fractions and the individual phase velocity in the flow stream. A number of multiphase meters are commercially available today (2-4). These systems use a diverse range of measurement techniques. Prior industry attempts to utilize the new technology of multiphase metering for production well testing in mature fields has been hampered by the high cost of multiphase meters. A number of industry and government supported projects have attempted to develop a "Low Cost" multiphase meters (4, 5). The well streams to be measured by these low cost multiphase meters can vary considerably as shown in Table 1. The challenge to the measurement system is that while these flow streams are considered as low GOR – i.e. low gas producing streams by the operator, they present major difficulties to the measurement devices. From operational perspective the emphasis of measurement would be on the liquid and water cut accuracy. However, the measurement system must deal with flow conditions where the gas phase is occupying as much as 50% of the cross sectional area stream of the flow as noted by the gas volume fraction in Table 1. The impact of gas on liquid and WC measurements must therefore be determined in order to accurately measure the liquid parameters. This paper describes the development and field tests of a "low cost" system called Agar MPFM-50 that is designed to address the low GOR conditions discussed above.

The MPFM-50 multiphase metering system utilizes a Coriolis flow meter and a continuous water cut (OWM) measurement device based on microwave absorption (5, 6). The combination of these techniques provides a light weight metering package that can be mounted on trailer for portable well testing. Figures 1 and 2 show a close up of the meter and the trailer mounted system as it is deployed for well testing. This multiphase meter can measure oil, water and gas in production streams with gas volume fraction of less than 25%. These low gas volume fraction streams can be found at well heads in mature fields as noted in Table 1, the liquid legs of inefficient or under-sized separators(7), and in heavy oil fields(8).

Principle of Operation

The MPFM-50 consists of the following components: a Coriolis mass flow meter; a watercut device, one pressure transmitter, and one differential pressure transmitter. The temperature is measured with a RTD sensor. The meter's transducers continuously measure pressure, mass flow, density, temperature, and dielectric properties of the flow stream. The signals from the sensors are connected to the analog inputs of the Data Acquisition System (DAS).The DAS computer determines the gas, water, and oil flow rates from the raw data. The result is a real-time measurement of flow rates. The principle of operation is described in the following section.

To calculate the flow rates of liquid and gas, the meter first uses the water fraction measured by the water cut meter to estimate actual liquid density based on the known oil and water densities provide to the system. Then using the Coriolis measured mass flow and fluid density in combination with the estimated liquid density, the gas volume fraction of the flow stream is calculated. Volumetric flow rate of oil water and gas is then determined according to the following equations:

The effects of changes in fluid salinity, viscosity, temperature and density as well as the PVT correction are accounted in the calculation modeL. Figure 3 provides a simplified view of the meter calculation modeL.

Data Collection and Reporting

Raw data coming from the system instrumentations is collected by the data acquisition system (OAS) at 1 cycle per second frequency. Flow calculations are performed for each measurement cycle. In most flow regimes, a single set of transmitted flow data does not represent the true flow rates. Ouring slugging, the contents may be 100% liquid one second and i 00% gas the next. For this reason, data is averaged for a period of time to provide a more accurate representation of the flow. Figure 4 shows graphs of liquid and gas flow rates as well as water cut values during the well tests for a number of wells.

The reported flow rate is a continuous average of all the individual cycles. Flow data and volumetric totals can be reported by the OAS as:

• Printed logs
• RS-232 output
• The computer's main storage media (hard disk or compact flash card)
• Analog outputs
• SCADA system data through a Modbus protocol

The operational perfom1ance envelop for the meter is shown in Figure 5. As with all the multiphase flow measurement systems, the operational envelop of the MPFM-50 meter is constrained by the gas volume fraction of the flow stream. The 2-inch meter that was used in the current field tests was tested by vendor in a multiphase test loop prior to the field deployment. The test loop results established the following vendor claimed accuracy for oil, water and gas measurements:

• Oil and Water Rates = :f2% of full scale:f 5% of readings
• Gas Rates = :f5% of full scale :f 5% of the reading.

Field Qualification Trials

An 18 month field testing campaign has been conducted in a New Mexico field to assess the performance of the MPFM-50 metering system. The wells are e02 gas lifted. The campaign started in the October of 2007. During the initial qualification phase the performance of the multiphase metering trailer was indexed against a conventional gravity based 3-phase test separator. The test separator was equipped with turbine meters that measured the liquid and gas phases. For these qualification tests temporarily flexible hose connections were made at the well test manifold in order to allow each well stream to be selectively diverted to the multiphase meter before reaching the test separator. Gross and net volumes of oil, gas, and water discharges from test separator during each test period were used to estimate daily net flow rates as well as water cut. Table 2 shows the results from these qualification tests conducted in October of 2007 during the initial field deployment of the multiphase metering trailer. A second and similar qualification trial was conducted during May 2008. These tests involved the same 5 wells that were tested previously plus additional wells that were not included in the first triaL. The objective was to check the repeatability as well as performance sustainability of the multi phase system. The results of these tests are shown in Table 3. In between these qualification tests the multiphase metering trailer has been used by the operator to conduct production well testing in various locations in this field where a conventional test separator was not available. These operational well tests have been continuing since October 2007. During these production tests the operator has collected and analyzed fluid samples from selected wells. The we data obtained from these samples has been utilized to check the performance of the multiphase meter in locations where a conventional test separator was not available. The quality of the tests performed by the multiphase meter as assessed by these grab sample analysis has been determined to be "good" and the multiphase meter tests have been accepted for normal production measurements.

Tables 2 and 3 show the comparison of the flow rates and water cut data between the multi phase and the conventional test separator. For this comparison, the well testing procedure and corresponding separator measured values have been assumed to have absolute accuracy and were used as reference. In reality the test separator measurements have some uncertainties. But since test separator data is the acceptable yard stick for well testing by the operator, all multi phase meter measurements were "indexed" to the test separator values. The comparison columns in Tables 2 and 3 were therefore developed with this indexing methodology.

One of the features claimed for of multi phase metering technology is that the measurements performed by these meters are more consistent than measurements made by the conventional test separators (9). Table 4 shows the results from a recent 24 hour test for a number of wells compared with historical data for the same wells. These tests were conducted to establish the consistency of data produced by the multiphase meter over a period of time.

The data in Table 4 indicate very good measurements consistency between the latest test and the historical data.

Conclusions

The development and field tests of a low cost portable multi phase meter suitable for low GOR production applications has been discussed in this paper. The meter utilizes the eoriolis flow meter technology combined with a microwave based water cut meter that can measure we in the 0-100% range. The combination of these techniques provides a light weight compact metering package shown in Figure 1 that can be mounted on trailer for portable well testing as shown in Figure 2. This multi phase meter can measure oil, water and gas without separation for the type of production streams expected in mature field as shown in Table I. The principle of operation for the multi phase meter is based on mass and density measurements as schematically shown in Figure 3. The capacity as well as the performance of the meter is influenced by the volume fraction of the gas as noted in Figure 4. eurrently this meter is limited to operation in streams with gas volume fraction of less than 25%.

An 18 month field testing campaign has been conducted in a West Texas field to assess the performance of the trailer mounted multi phase metering system. During this campaign a number of field qualifications were conducted to index the performance of the multiphase meter against the field test separator. The results of these field qualification tests are shoW11 in Tables 2 and 3. Tables 2 and 3 show that the MPFM-50 multiphase meter can measure the we values within the 1-2 (:f 1%) points of the values obtained by the test separator. The liquid rates from the multiphase meter are within :f 7% of the rates measured by the test separator. With these we and liquid measurement accuracies, the oil rate can be measured to within:f 20 BPD accuracy as noted in Table 3. The low gas rates at test locations caused high uncertainty in the gas measurements by the test separator. The gas measured by the multiphase meter is within :f 10 mscfd of the test separator. These accuracy results are in line with the "vendor claimed" accuracy specifications discussed in the previous section.

In addition to the qualification tests the multi phase metering trailer has been used by the operator to conduct production well testing in various locations in this field where a conventional test separator was not available. During these production tests the operator has collected and analyzed fluid samples from selected wells. The we data obtained from these samples has been utilized to check the performance of the multiphase meter in locations where a conventional test separator was not available. The quality of the tests performed by the multi phase meter as assessed by these grab sample analysis has been determined to be "acceptable" and the multiphase meter tests have been accepted for normal production measurements.

Table 4 shows the results from a recent 24 hour test for a number of wells compared with historical data for the same wells. These tests were conducted to establish the consistency of data produced by the multiphase meter over a period of time. The data in Table 4 indicates very good measurement consistency between the latest test results and the historical production data.

The results of the work described in this paper indicate that the compact multi phase metering system tested in this project has suitable measurement accuracy for production measurements and monitoring in low GOR fields. The measurement accuracy, light weight, and reasonable cost of the system should afford the operator serious consideration for deploying the system in locations where conventional gravity based test separators are not available or are impractical to instalL.

Acknowledgement

The authors wish to thank the assistance and support provided by the Agar eorporation during this project.

References

1. Pub. 2566, State of Art, Multiphase Flow Metering, API Committee on Petroleum Measurements, May 2004.

2. Mehdizadeh, P.: "Multi phase Flow Measurements", paper 81 10, 79th Intemational School of Hydrocarbon Measurements, 18-20 May 2004, Oklahoma City, OK.

3. Mehdizadeh, P., Marrelli, 1., and Ting, V.C.: "Wet Gas Metering: Trends in Application and Technical Development", SPE 77351, SPE Annual Technical Conference and Exhibition, 29 September-2 October 2002, San Antonio, TX.

4. Oglesby, K., Mehdizadeh, P., and Rodgers, G.: "Portable Multiphase Production Tester for High Water -Cut Wells", SPE 103087, 2006 SPE Annual Technical Conference and Exhibition held in San Antonio, Texas, U.S.A., 24-27 September 2006.

5. Lansangan, R., Dutton, R., Tombs, M., Manus, H., and Duta M.: "Three-Phase flow MeasuremntTechnique using a Coriolis Flow Meter and a Water Cut Probe", 4th International SEA Hydorcarbon Flow Measurement workshop, 9-11 March, 2005.

6. Mehdizadeh P.: "Test Verifies Water-Cut Meter Accuracy in Steamflood", Oil & Gas J., 2 October 2000.

7. Means, S.R., and Mehdizadeh, P.: "New Technology Improves Well Testing Units", Oil & Gas 1., 30 October 2000.

8. Bertolin, L., Mehdizadeh, P., and Stobie G.: "Petrozuata - An Application of Multiphase Metering Technology" SPE JPT, June 2005.

9. Theuvney B.C., Mehdizadeh P."Multiphase Flowmeter Application for Well and Fiscal Allocation", SPE paper 76766 to be presented at the PE Westem Regional lAAPG Pacific Section Joint Meeting, Anchorage, Alaska, USA, 20-22 May 2002.