Vx spectra surface multiphase flowmeter

Literature Reviews

Society of Petroleum Engineers Inc. This paper was prepared for presentation at the SPE International Conference on Health, Safety and Environment in Oil and Gas Exploration and Production held in Kuala Lumpur, Malaysia, 20–22 March 2002. This paper was selected for presentation by an SPE Program Committee following review of information contained in an abstract submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Papers presented at SPE meetings are subject to publication review by Editorial Committees of the Society of Petroleum Engineers. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of where and by whom the paper was presented. Write Librarian, SPE, P.O. Box 833836, Richardson, TX 75083-3836, U.S.A., fax 01-972-952-9435.

Abstract Burning hydrocarbons during cleanup and well-testing operations produces toxic gases, soot, acid rain, unburned hydrocarbons and noise. Not only do these emissions have a damaging impact on the environment, but they also impose an economic impact: the cost of the oil and gas flared and the cost of the equipment used during the flaring operations.

A joint task force between two offshore operating companies in Abu Dhabi, United Arab Emirates (U.A.E.), with the help of the contracted service provider, worked to implement innovative solutions to achieve flaring-emission elimination targets and eliminate environmental risk.

Innovative equipment modifications and design, coupled with new operational procedures, were implemented to neutralize the fluids used to prepare the well for production (allowing well effluents to flow through the test separator during the cleanup phase) and to use a boosting system to pump the oil into the sealine (fully eliminating oil flaring).

The initial stages of the solution neutralized the acid flowed back to surface and eliminated the oil flaring. A multiphase flowmeter was then introduced to the system. Not only did the multiphase flowmeter enhance operation flexibility, confidence in the information acquired, and accuracy of the results, but it also eliminated gas flaring after the cleanup period. The minimal pressure drop across the meter and the high-pressure rating of the meter allow well effluents to flow naturally through the sealine without separating the well-effluent phases. Deployment of the multiphase flowmeter in the system reduced the gas flaring by about 60% during the overall job.

Introduction Conventionally, during cleanup and well-testing operations, both oil and gas are burned away into the atmosphere. The combustion efficiency of fully atmospheric oxidation is insufficient. Oil- and gas-flaring operations create significant amounts of emissions that contain unburned hydrocarbons, carbon monoxide and nitrogen oxides, which produce acid rain, smog, ozone at ground levels, and greenhouse gases in the upper atmosphere.

Acid rain depletes soil, pollutes water, damages forests, endangers animal habitats and food chains, and corrodes human-made structures, such as buildings, statues, automobiles, and other artifacts made of stone or metal. Smog and ozone cause human respiratory ailments, such as asthma, bronchitis and emphysema. Most scientists believe that greenhouse gases are a major cause of global warming. Increased concentrations of water vapor, carbon dioxide, methane and other greenhouse gases trap heat energy in the earth’s atmosphere. A gradual rise in the earth’s surface temperature is expected to melt polar ice caps and glaciers, expanding ocean volume and raising sea level, flooding some coastal regions and even entire islands.

Middle Eastern countries, including the U.A.E. with its low-lying coastal areas, are concerned about rising sea levels and potential flooding caused by global warming. They are concerned about increased radiation of heat and light from global warming, leading to regional desertification. As major producers of oil and gas, they are concerned about the deterioration of air quality from inefficient combustion of a sour gas supply.

Abu Dhabi is blessed with a charming environmental heritage. Nevertheless, the environment in this part of the world is no less fragile than anywhere else.

Recognizing mounting international concerns over environmental issues, a joint task force was formed in June 1997. The Abu Dhabi Marine Operating Company and the Zakum Development Company, with the assistance of Schlumberger, the testing services provider, worked to implement pioneering solutions for optimizing acid neutralization, reducing flaring operations in the short term, and ultimately achieving zero hydrocarbon flaring to eliminate environmental risk during well-testing operations(1-2).

SPE 74106

Environmentally Friendly Well Testing Y. El-Khazindar,SPE, Schlumberger, M. Ramzi Darwish, ADMA-OPCO, and A. Tengirsek, Schlumberger

2 Y. EL-KHAZINDAR, R. DARWISH AND A. TENGIRSEK SPE 74106

Environmentally Friendly Well Testing This paper describes the equipment and processes used to achieve zero hydrocarbon flaring. The First Stages: Acid Neutralization and Oil Reinjection. The separation and reinjection of oil was introduced in June 1998 (see Fig. 1). In this process, single-phase oil-reinjection pumps, specially designed to overcome high sealine pressures of up to 1300 psi, were used to reinject the oil back into the production sealine. They covered the range from 12,000 BOPD at 410 psi to 3500 BOPD at 1300 psi. The use of reinjection pumps, which eliminated the need for oil flaring during postcleanup operations, reduced the total oil flared by 65%.

To stimulate wells after drilling or workover operations, 10 to 15 gal/ft of 15% HCl are pumped downhole. The spent acid flowing back to surface typically has a pH of 2 to 3. In February 2000, an acid neutralization system was introduced (see Fig. 2). In this process, neutralizing agents are used to keep the pH of the effluents flowing back to surface after an acid job between 5.5 and 6. Na2CO3 was chosen because it is soluble in water, cost effective and generally available on well sites.

The neutralization system keeps the pH of water dumped overboard the rig between 5.5 and 6. Furthermore, it allows oil separation from the start of flowback operations so that the reinjection pumps can inject 100% of the oil back into the production sealine.

The neutralization system involves pumping demulsifying agents upstream of the choke manifold and pumping the neutralizing agent, Na2CO3, downstream of the choke manifold. The neutralizing agent then flows into a test separator, a 100-bbl dual-compartment skimmer, where chemical treatment reduces the oil-in-water content to 100 ppm before the water is dumped overboard.

Introducing the neutralization system and the reinjection pumps eliminated the need for oil-flaring operations. Even though initial stages considerably reduced emissions to atmosphere, improving the system to eliminate gas flaring during cleanup and well-testing operations remained a challenge. The PhaseTester multiphase flowmeter. In May 2001, the PhaseTester* portable multiphase periodic well testing equipment(3-5) was introduced to the setup (see Fig. 3). The PhaseTester’s high working pressure of 5000 psi allows placing the tool upstream of the choke manifold (see Fig. 4). This tool was selected to

• reduce the amount of gas flared by approximately 60%

• eliminate unnecessary shut-ins and flow disturbance during testing operations (for example, during a helicopter landing)

• provide a reliable, stand-alone flow rate measurement.

* Mark of Schlumberger

The PhaseTester multiphase flowmeter simultaneously measures the total mass flow rate of the stream passing through, with a venturi meter, and the individual phase fractions, with a dual-energy gamma fraction meter. Working on the multiple-energy principle, the detector picks up the total count of photons hitting the sensor, as well as the energy of each hit. The high-speed detector produces a signature count rate, over the two peak energy bands, which is a function of the measured medium. Hits at two different energy levels are filtered out and used for the fraction of oil, water and gas calculations. The sampling frequency of 45 Hz is sufficient to define the continuous variations in the process flow.

The measuring section includes the venturi meter and the dual-energy gamma fraction meter. Both measurements— performed at the same time and place—eliminate anomalies or inaccuracies associated with meters that have a series of sensors implemented along a pipe and that do not detect the same flow instantaneously.

The PhaseTester multiphase flowmeter uses a blind tee in the flowline upstream of the measuring section to impose a predictable flow shape onto the flowstream. Effectively removing flow anomalies imposed by downhole conditions and surface piping, this tee eliminates high-frequency flow instabilities in the measuring section.

With an American Petroleum Institute (API) 6A rating, the PhaseTester multiphaseflowmeter does not require additional shutdown and pressure-relief systems. The PhaseTester Vx* multiphase well testing technology can be used to accept fluids directly from the flowline and, after measurement, flow them back naturally to the production sealine without any boosting services. Pressure loss across the system is typically 3 to 30 psi, much lower than for a conventional test-separation system. Testing. Between May 15 and October 1, 2001, 23 wells were cleaned up and tested using the reinjection, neutralization system and PhaseTester multiphase flowmeter combination. The operation involved 36 postcleanup flow tests. Flowing conditions during the flow periods were

• wellhead flowing pressure range: 90–1300 psi • oil flow rate range: 400–5500 BOPD • gas flow rate range: 0.3–4.0 MMscf/D • gas volume fraction (GVF) range: 60–98% • CO2 range: 1.5–8.0% • H2S range: 0–4% • basic sediment and water (BS&W) range: 0–34%

Figs. 5 through 11 plot the distribution of these parameters against the number of flow periods.

The 36 flow tests were compared to a traditional test separator(6-7). Various authors have mentioned the difficulty of making comparisons with test separators (for example, Ting8 or Amdal et al.9). Separators are limited to performing a reasonable liquid-to-liquid separation. In most tests the water cut is calculated from in-line sampling from the choke manifold. The PhaseTester tool obtains reliable water measurements with the dual-energy spectral gamma-ray

SPE 74106 ENVIRONMENTALLY FRIENDLY WELL TESTING 3

composition meter because this nuclear determination is not sensitive to the distribution of the phases.

Uncertainties are not limited to those of the individual oil and gas meters installed on a test separator. Other factors that can affect the separator uncertainties are

• meter factor drift caused by gas entrainment in the liquid leg of the test separator

• slugging of the wells, which affects test-separator performance

• an operator-dependent process • poor liquid-to-liquid separation • overall accuracy affected by the whole system (back

pressures, lines, controls, etc.) • frequency of data gathering. For a calibrated separator operated by a dedicated crew,

the uncertainties are expected to be • gas uncertainty in the range of 5 to 8% • total liquid uncertainty in the range of 5 to 8%. Comprehensive preparation of the reference separator was

performed in advance, ensuring a proper conclusion. A comparison of flow rate results between the PhaseTester tool and the reference separator indicated that the PhaseTester tool performed within its operating specifications, yielding the PhaseTester multiphase flowmeter uncertainty figures +/- the separator uncertainties. The flow rate uncertainties(10) for the PhaseTester flowmeter are as follows:

Liquid: The larger of 2.5% of the reading, or 300 bbl/d, for GVF between 0 and 98%

Gas: The larger of 1% of the reading, or 140 scf/d, for GVF between 0% and 30% The larger of 3% of the reading, or 420 scf/d, for GVF between 30% and 60% The larger of 10% of the reading, or 1410 scf/d, for GVF between 60% and 90% The larger of 15% of the reading, or 2120 scf/d, for GVF between 90% and 95%

Water-Liquid Ratio (WLR): ± 3% absolute for GVF between 0% and 70% ± 4% absolute for GVF between 70% and 80% ± 5% absolute for GVF between 80% and 90% ± 8% absolute for GVF between 90% and 95%.

Toward Zero Gas Flaring. The ultimate goal is to achieve a flaring-free operation. As a step toward this goal, the PhaseTester multiphase flowmeter was deployed to reduce gas flaring to a minimum. The PhaseTester flowmeter was expected to decrease gas flaring by approximately 60% during the overall job, as well as eliminate unnecessary shut-ins and flow disturbance during the testing operations.

After the flowback and acid neutralization, once the wellhead pressure increased to overcome the production sealine pressure, well effluents bypassed the conventional separator system and flowed naturally through the PhaseTester

tool into the production sealine. Separating, boosting and flaring operations were not required. The extremely low pressure drop across the PhaseTester tool helped to achieve gas-flaring reduction.

Confidence in the PhaseTester multiphase flowmeter’s measurement reliability and flexibility was developed during its evaluation over 36 flow tests. The PhaseTester multiphase flowmeter was then deployed on three tests to reduce gas flaring. Table 1 shows the reduction in gas flaring when the PhaseTester multiphase flowmeter was integrated into the system. (Gas is typically flared during the initial cleanup phase.)

TABLE 1—[Summary of gas flaring reduction results] Well no. 1 Well no. 2 Well no. 3 Total job duration, hours 75 80 41 Total gas produced, MMscf 13.6 8.3 3.6 Gas flared, MMscf 3.20 0.54 0.77 Gas flared, % 24 7 21 Gas injected, MMscf 10.4 7.7 2.9 Gas Injected, % 76 93 79 Future Plans. Various studies have been carried out to define practical and economical means of fully eliminating the gas flaring. A multiphase pump is currently being considered as a solution. The challenges to designing a multiphase pump for this application include the pump capability to cover a wide range of flow conditions, as well as size and power requirements. The joint task force is currently studying technical details and performance of a multiphase pump specifically designed to suit a wide range of flow conditions.

The current water treatment skimmer system is capable of achieving an oil-in-water level of approximately 100 ppm. Different options were considered to achieve an oil-in-water level of less than 15 ppm, the environmental emissions guidelines limit specified by Abu Dhabi National Oil Company’s Health, Safety and Environment policy. The joint task force proposed an integrated water deoiling package. Full feasibility and Hazardous and Operability studies were performed. The water deoiling unit, currently being manufactured, is planned for utilization in January 2002.

The water deoiling unit was designed to meet the following criteria:

• a compact integrated solution including a degasser, hydrocyclone, reject oil tank, pumping units and control panel, all on one skid that can be accommodated easily on the rig/barge without inflicting space limitations

• the ability to recycle the liquids within the system to ensure an optimum oil-in-water level

• the ability to cover a wide range of flow rates and cope with changing flowing conditions. The system is capable of handling up to 6,000 B/D of water with oil-in-water levels of up to 1% at the inlet of the system.

4 Y. EL-KHAZINDAR, R. DARWISH AND A. TENGIRSEK SPE 74106

Figure 12 is a schematic for the proposed future setup.

Conclusions During the initial stages of the project, utilizing acid neutralization and oil-reinjection systems achieved zero oil flaring. After integrating the PhaseTester multiphase flowmeter into these systems, the following benefits were observed:

• When the wellhead pressure is high enough to overcome the production sealine pressure, the well effluents flow naturally through the PhaseTester multiphase flowmeter (bypassing the separator) into the production sealine. Gas flaring is eliminated (because of the minimal pressure drop across the PhaseTester flowmeter), reducing the amount of gas flared during the whole job by more than the targeted 60%.

• The PhaseTester multiphase flowmeter results, compared to a reference test separator, showed that it performed within its operating specifications and provided reliable, stand-alone flow rate measurements.

• Shut-ins to accommodate helicopter landings were eliminated because fluids flow directly through the PhaseTester multiphase flowmeter into the production sealine. Furthermore, helicopter flights are no longer dependent of the flow programs.

Eliminating the risk to the environment posed by flaring hydrocarbons during well testing operations is a challenge. Keys to achieve environmentally friendly well testing operations:

- understanding the flow conditions and operational constraints.

- setting reasonable targets to achieve flaring free operations.

- examining and revising the existing operational procedures.

- looking for areas in which a significant value could be achieved by implementing innovative ideas and solutions.

The deployment of an acid neutralization system and oil reinjection pumps to completely eliminate oil flaring as well as the utilization of the PhaseTester multiphase flow meter to reduce the gas flaring by more than 60% are examples of solutions that can be implemented to achieve environmentally friendly well testing operations.

Acknowledgments The authors would like to thank the management of ADMA- OPCO and Schlumberger for permission to publish this paper.

References

1. Messiri A., Al Attas, M.O., and Mohamed, N.: “Towards Zero Flaring Emission,” paper ADIPEC 0964, presented at the 2000 Abu Dhabi International Petroleum Exhibition and Conference, Abu Dhabi, UAE, October 15–18.

2. Hassan, M.M., Fadaq, A.S. and Beadie, G.: “Reduction of Well Emission During Clean Up and Testing Operations by Rig,” paper SPE 68151, presented at the 2001 SPE Middle East Oil Show and Conference, Bahrain, March 17–20.

3. Kontha, I.N.H, Weimer, B, Retnanto, A., Azim A and Martinon, D.: “Monitoring Well Performance Using Multiphase Flow Meters,” paper SPE 68718, presented at the 2001 SPE Asia Pacific Oil and Gas Conference and Exhibition, Jakarta, Java, Indonesia, April 17–19.

4. “Multiphase Flow Meters: A New Way to test Wells in Production,” Hart First Look (September 1999).

5. Mus, E. A, Toskey, E.D and Bascoul, S.J.F: “Added Value of a Multiphase Flow Meter in Exploration Well Testing,” paper OTC 13146, presented at the 2001 Offshore Technology Conference, Houston, Texas, USA, April 30– May 3.

6. Atkinson, D.I, Berard, M and Segeral, G.: “Qualification of a Nonintrusive Multiphase Flow Meter in Viscous Flows,” paper SPE 63118, presented at the 2000 SPE Annual Technical Conference and Exhibition, Dallas, Texas, USA, October 1–4.

7. Theuveny, B.C., Segeral, G. and Pinguet, B.: “Multiphase Flowmeters in Well Testing Applications,” paper SPE 71475, presented at the 2001 SPE Annual Technical Conference and Exhibition, New Orleans, Louisiana, USA, September 30–October 3.

8. Ting, V.C.: “Effects of Non-Standard Operating Conditions on the Accuracy of Orifice Meters,” SPE Production & Facilities (February 1993) 58.

9. Amdal, J., Danielsen, Dykesteen, E, Flølo, D., Grendstad, J., Hide, H.O., Moestue, H., and Torkildsen, B.H.: Handbook of Multiphase Metering, Norwegian Society for Oil and Gas Measurement, (1997).

10. Retnanto, A.: “Production Optimization Using Multiphase Well Testing: A Case Study from East Kalimantan, Indonesia,” paper SPE 71556, presented at the 2001 SPE Annual Technical Conference and Exhibition, New Orleans, Louisiana, USA, September 30–October 3.

Metric Conversion Factors cp x 1.0* E–03 = Pa·s bar x 1.013 25* E+05 = Pa psi x 6.894 757 E+00 = kPa bbl x 1.589 873 E–01 = m3 B/D x 6.624 471 E–03 = m3/h ft3 x 2.831 685 E–02 = m3 ft3/D x 1.179 869 E–03 = m3/h lb/ft3 x 1.601 846 E+01 = kg/m3

*Conversion factor is exact.

SPE 74106 ENVIRONMENTALLY FRIENDLY WELL TESTING 5

Fig. 1—Schematic of the surface well testing setup and reinjection pump

Fig. 2—Schematic of the surface well testing setup, reinjection pump and neutralization package

Fig. 3—Schematic of the surface well testing setup, reinjection pump, neutralization package and PhaseTester multiphase flowmeter

Fig. 4—The PhaseTester multiphase flowmeter

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6 Y. EL-KHAZINDAR, R. DARWISH AND A. TENGIRSEK SPE 74106

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Fig. 12—Schematic of the reinjection pump, neutralization package, PhaseTester multiphase flowmeter, water deoiling unit and multiphase pump