Determination of Multiphase Flow Meter Reliability and Development of Correction Charts for the Prediction of Oilfield Fluid Flow Rates

 

Samuel S. MOFUNLEWI, Joseph A. AJIENKA and D. APPAH

 

Department of Petroleum & Gas Engineering, University of Port Harcourt, Nigeria

E-mails: Samlewi@engineer.com, jajienka@yahoo.com, dappah@yahoo.co.uk

 

 

Abstract

 

The aim of field testing of Multiphase Flow Meter (MPFM) is to show whether its accuracy compares favourably with that of the Test Separator in accurately measuring the three production phases (oil, gas and water) as well as determining meter reliability in field environment. This study evaluates field test results of the MPFM as compared to reference conventional test separators. Generally, results show that MPFM compares favourably with Test Separator within the specified range of accuracy.

At the moment, there is no legislation for meter proving technique for MPFM. However, this study has developed calibration charts that can be used to correct and improve meter accuracy.

Keywords

Multiphase Flow Meter (MPFM); Test Separator (TS); Correction Charts; Accuracy; Water Liquid Ration (WLR); Oil; Gas.

 

 

Introduction

 

Until recently, large expensive test separators have been used to separate the oil, gas and water which are then measured using conventional technology [1, 2].

Multiphase meter is a device that can be used to measure individual fluid flow rates of oil and gas when more than one fluid is flowing through a pipeline. A multiphase meter provides accurate readings even when different flow regimes are present in the multiphase flow [3]. When using single-phase meters, the fluid mixture (oil and gas) coming from the wellbore must pass through a fluid-separation stage (separator) prior metering. Otherwise, the readings of the single-phase meters will be inaccurate. Separators are not necessary for multiphase metering, and the meters can support different proportions of gas and oil. Multiphase meters provide the advantage of continuous well monitoring, which is not possible using single-phase meters. Additionally, multiphase meters cost less, weigh less and require less space. Multiphase meters are more common in deepwater operations, where well-intervention operations are often prohibitively expensive.

The problem now arises as to whether the accuracy of multiphase meter (MPFM) compare well with that of test separator. How can the MPFM accuracy be improved? This paper proposes solutions to these probes.

 

 

MPFM Test Reference Loop

 

The test reference loop consists of a three-phase separator. Gas and liquid are separated in the test separator. In order to achieve the desired steady water cut concentration, the oil/water volume in the separator and different draw points are adjusted. On separation, the liquid is pumped through a liquid measurement line. In this line, volumetric measurement is performed with PD meters and water cut measurement is performed with the oil/water meter. A vortex meter and rotameters are used to measure gas after compression.

Following the separation is a recombination of gas and liquid phases. On recombination, the combined stream then passes through the multiphase meter and measurement taken accordingly. Figure 1 shows the diagram of the test reference loop [4].

 

 

Test Procedure

 

Below is a list of the procedure for the main testing of MPFM [5]. Test separator is validated as a reference to the multiphase flow meter.

Figure 1. Schematic of test reference loop. Adapted from Tests at Agar Corporation (1999)

 

1.      A purge is time is assigned to each well to be tested.

2.      Data review from test separator to ensure that a steady production condition is attained before starting test.

3.      Data collection from the MPFM and test separator at the same time when test starts.

4.      Initial test result is reviewed.

5.      Enquiry from vendor for any modification for improvement of meter performance was made

6.      Validity of all data collected with a test separator. This involves comparing test results with historical data.

7.      Test is invalidated if major discrepancies are observed.

8.      MPFM inputs were reviewed.

9.      A final list of valid comparison tests was prepared.

10.  Cross-plots of MPFM against test separator were produced.

Process and Performance Conditions

 

Process and performance specifications are given in Tables 1 and 2, respectively.

 

Table 1. Process Conditions

Description

Process Conditions

Watercut

10 90%

GVF

24 85%

Liquid flow rate

1,000 5,000 BPD

Salinity of water

1.5% by weight

Oil viscosity

360cp

Temperature

40oC

 

Table 2. Performance Specification (accuracy)

Description

Specification

Liquid (oil and water)

5%

Crude Oil

5%

Water

5%

Gas

5%

 

The formula below was then used to compute accuracies in each case from the total flow rate and total deviations.

(1)

 

 

Test Result Summary

 

The test result summary is presented in Table 3 for clarity.

 

Table 3. Test results summary

No.

Reference Measurements (BPD)

Test Measurement (BPD)

Deviations

 

Oil

Water

Gas

(ACFD)

Liquid

WLR

Oil

Water

Gas

(ACFD)

Liquid

WLR

Oil

Water

Gas

Liquid

WLR

1

1054

129

40229

1183

0.109

1128

84

39954

1212

0.069

74

-45

-275

29

-0.04

2

2701

363

20080

3064

0.118

2819

305

19026

3124

0.098

118

-58

-1054

60

-0.02

3

3276

433

6835

3709

0.117

3382

363

5427

3745

0.097

106

-70

-1408

36

-0.02

4

786

701

47241

1487

0.471

730

729

47376

1459

0.5

-56

28

135

-28

0.029

5

2373

2575

30098

4948

0.521

2480

2488

28839

4968

0.501

107

-87

-1259

20

-0.019

6

180

1319

48773

1498

0.881

218

1204

50196

1423

0.846

38

-115

1423

-75

-0.035

7

487

4504

29236

4991

0.902

661

4493

27048

5154

0.872

174

-11

-2188

168

-0.03

Total

10857

10024

222492

20880

3.119

11418

9666

217866

21805

2.983

561

-358

-4626

205

-0.135

 

The following results in Table 4 were obtained using equation 1 above.

It can be inferred from the results in Table 4 that the MPFM compare well with test separator. The percent accuracy falls within the specifications in Table 2. This means that the overall performance of the meter was excellent.

 

Table 4. Percent accuracy of Oil, Water, Gas, Liquid and Watercut

Description

Oil

Water

Gas

Liquid

Watercut

Accuracy

5.17%

3.57%

2.08%

0.98%

4.34%

 

 

Development of Correction Charts and Descriptive Statistics

 

The correction charts below are developed from the test result summary (Table 4). They are to be used for improving meter accuracy. These charts are developed by selecting the best trend line for oil, gas water, liquid and water liquid ration (WLR) separately. They are presented in Figures 2 6. Descriptive statistics are also presented.

 

Correction Charts for Oil Rate

Figure 2. Cross plot for oil rate for test separator versus oil rate for MPFM

 

Descriptive Statistics for Oil Rate

 

Table 5a. Descriptive Statistics

Regression Statistics

R Square

0.9969

Standard Errors

73.90

Observations

7

Table 5b. Descriptive Statistics

 

Coefficients

Standard Error

t Stat

P-Value

Lower 95%

Upper 95%

Intercept

-43.39

48.58

-0.89

0.41

-163.26

81.48

X Variable

0.98

0.02

40.12

1.81E-07

0.91

1.04

 

 

Correction Charts for Water Rate

Figure 3. Cross plot for water rate for test separator versus water rate for MPFM

 

Descriptive Statistics for Water Rate

 

Table 6a. Descriptive Statistics

Regression Statistics

R Square

0.9996

Standard Errors

51.8

Observations

7

 

Table 6b. Descriptive Statistics

 

Coefficients

Standard Error

t Stat

P-Value

Lower 95%

Upper 95%

Intercept

56.90

26.81

2.12

0.087

-12.02

125.82

X Variable

1.00

0.013

75.06

7.95E-09

0.96

1.02

 

Correction Charts for Liquid Rate

Figure 4. Cross plot for liquid rate for test separator versus liquid rate for MPFM

 
Descriptive Statistics for Liquid Rate

 

Table 7a. Descriptive Statistics

Regression Statistics

R Square

0.999

Standard Errors

57.6

Observations

7

 

Table 7b. Descriptive Statistics

 

Coefficients

Standard Error

t Stat

P-Value

Lower 95%

Upper 95%

Intercept

64.17

47.26

1.36

0.23

-57.31

185.65

X Variable

0.97

0.014

69.6

1.16E-08

0.93

1.00

 

Correction Charts for Gas Rate

Figure 5. Cross plot for gas rate for test separator versus gas rate for MPFM

 

Descriptive Statistics for Gas Rate

 

Table 8a. Descriptive Statistics

Regression Statistics

R Square

0.997

Standard Errors

839.36

Observations

7

 

Table 8b. Descriptive Statistics

 

Coefficients

Standard Error

t Stat

P-Value

Lower 95%

Upper 95%

Intercept

2438.41

738.53

3.30

0.021

539.96

4336.86

X Variable

0.94

0.021

44.00

1.14E-07

0.89

1.00

 

Correction Charts for WLR Rate

Figure 6. Cross plot of WLR for test separator versus WLR rate for MPFM

 
Descriptive Statistics for Gas Rate

 

Table 9a. Descriptive Statistics

Regression Statistics

R Square

0.996

Standard Errors

0.025

Observations

7

 

Table 9b. Descriptive Statistics

 

Coefficients

Standard Error

t Stat

P-Value

Lower 95%

Upper 95%

Intercept

0.019

0.016

1.24

0.27

-0.021

0.06

X Variable

1.00

0.029

34.18

4.03E-07

0.92

1.07

 

Interpretation of Charts

 

The plots in Figures 2 6 can be used to predict what the rate (oil, gas, water, liquid or WLR) of a MPFM will be if that of test separator is known.

For example, if the oil rate for test separator is 2000BPD, then the predicted value of MPFM will 2200BPD. Also, if the gas rate for test separator is 1000ACFD, the predicted MPFM rate will be 800ACFD. The closer the value of R2 is to unity (1), the better. For rates that fall outside those presented in the charts above, the corresponding correlations can be used to determine the predicted values. That is if the value of test separator is know, make substitution into the appropriate equation to get the corresponding value of MPFM. For example, if the test separator rate for liquid is 10,000BPD, it will be better to substitue into the liquid rate equation to obtain the value for MPFM. Doing this, we will get 10254BPD.

The equations, R2 and P values are summarised below:

 

Table 10. Equations and R2 values for different rates

S/No.

Description

Equation

R2 Value

P-Value

1

Oil rate

y = 0.9775x 43.388

0.9969

0.412

2

Water rate

y = 0.9958x + 56.899

0.9991

0.087

3

Liquid rate

y = 0.969x + 64.169

0.999

0.233

4

Gas rate

y = 0.9429x + 2438.4

0.9974

0.0214

5

WLR rate

y = x + 0.0194

0.9957

0.269

 

 

Conclusions

 

This study has been able to show that the MPFM accuracy compare favourable with that of test separators. Hence, due to the economic benefits and the dependability of its accuracy, it is important to spread the expertise in MPFM through the oil industry. Both field and laboratory testing should be conducted to determine meter accuracy for added confidence.

Also, the correction charts developed in this study are useful tools for predicting the values of MPFM fluid flow rates when the flow rates of test separators are known. However, the charts are limited to the ranges shown on them. For fluid flow rates outside those obtainable on the charts, the equations developed are recommended for use.

 

References

 

1.      Mofunlewi S. S., Ajienka J. A., Economic Evaluation of Multphase Meters, Leonardo J. Sci, 2007, 11, p. 2.

2.      Mofunlewi S. S., Evaluating the Efficiency of Multiphase Meters, M. Eng. Thesis, Department of Petroleum & Gas Engineering, University of Port Harcourt, Port Harcourt, Nigeria, 2003, p. 2.

3.      Schlumberger Oilfield Glossary, Multiphase Meter, http://www.glossary.oilfield.slb.com/Display.cfm?Term=multiphase%20meter, 2008, ID 11235.

4.      Sonatrach, Anardarko, Lasmo, Quobba, Multiphase Meter and Watercut Meter Test, Tests at Agar Corporation, Houston, Texas, 1999, p. 1-3.

5.      Al-Taweel A. B., Barlow S. G., Field-Testing of Multiphase Meters, Saudi Aramco Journal of Technology, http://www.saudiaramco.com/irj/go/km/docs// SaudiAramcoPublic/Publications/EN/Journal%20of%20Technology/Spring2000/p50to59.pdf, Spring, 2000, p. 50-59.