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Salinity and flow regime independent multiphase flow measurements
Flow Measurement and Instrumentation 21 (2010) 454–461
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Salinity and flow regime independent multiphase flow measurements C. Sætre a,b,∗ , G.A. Johansen a,b , S.A. Tjugum b,c a
Department of Physics and Technology, University of Bergen, 5007 Bergen, Norway
b
The Michelsen Centre for Industrial Measurement Science and Technology, Norway
c
Roxar Flow Measurements, Bergen, Norway
article
info
Article history: Received 23 March 2010 Received in revised form 15 June 2010 Accepted 16 June 2010 Keywords: Gamma-ray densitometry Multibeam Flow regime identification Tomography Gas volume fraction Salinity measurement Scattering measurements Dual modality
abstract For oil production fields, there is a need for downhole measurements of the gas/water/oil multiphase flow. In extreme conditions a relatively simple, robust, and non-intrusive system will be appropriate. A measurement setup that combines multiple gamma beam (MGB) and dual modality densitometry (DMD) measurements, would be able to determine the gas volume fraction (GVF) independently of the flow pattern, and monitor changes in water salinity. MGB measurements of gamma-ray transmission along multiple sections across the oil pipe will provide information on the flow pattern. Whereas the DMD principle will give information on changes in salinity from a combination of transmission and scattering gammaradiation measurements. In this work we present the results from MGB and DMD measurements of a multiphase flow with high-speed gamma-ray tomograph measurements as reference for the flow pattern. The MGB measurements should enable us to distinguish between stratified or wavy/slug and annular or slug flow. Flow patterns with several minor components distributed evenly over the measurement cross section, like bubble flow, will be interpreted as homogeneous flow. The DMD measurements can be used to monitor salinity changes of the water component for intervals where the GVF is low and the water cut of the liquid is high. Combined with other gauges for water cut measurements, the MGB and DMD measurement setup should improve the multiphase flow measurements, and enable increased oil/gas recovery and production water monitoring. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction Measurements on the different components in a gas, oil, and water multiphase pipe flow depend not only on the different densities and volume ratios. They also depend upon the distribution of the components within the pipe if these are not mixed, and also on the salinity of the water component [1]. Gamma radiation is useful for non-intrusive measurements of components with sufficiently different density. Combined with water cut measurements from capacitance [2–4], conductance [5], or more recent measurement principles like single-beam ultrasound [6,7], a multibeam gamma densitometer can be used for gas fraction measurements independent of the flow pattern in a multiphase flow. By measuring at several (in this work three) narrow gamma-ray beams the gas fraction derived from each of these transmission measurements will provide information on the flow regime within the pipe. The total gas fraction of the pipe cross section can then be calculated from the combination of these transmission measurements. If there is no slip between the gas and the
∗ Corresponding address: Department of Physics and Technology, Pbox 7803, 5020 Bergen, Norway. Tel.: +47 55588308. E-mail address: [email protected] (C. Sætre). 0955-5986/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.flowmeasinst.2010.06.002
liquid, that is if all the flow components flow at the same velocity, the measured gas fraction (α ) gives the gas volume fraction (GVF) of the flow. A compact low-energy multibeam gamma-ray densitometer has been built at the University of Bergen in collaboration with Roxar Flow Measurements [8]. The water salinity affects the multiphase measurements when deriving the relative amount of gas from the transmitted gamma radiation. For salinity monitoring of the water component one can combine the measurements of the transmitted and scattered gamma radiation, since these measurements reveal different response of the interaction mechanisms of the radiation and the flowing medium. Whereas the photoelectric effect at a given photon energy is highly dependent on the atomic number of the absorbing material (approximately proportional to Z 4 –Z 5 ) and hence the salinity of the water, Compton scattering is mainly proportional to the density of the scattering material. Using a gamma source Am-241 with characteristic energy of 59.5 keV, there will be both significant photoelectric absorption and Compton scattering for dual modality measurements. By combining the measurements of transmission and scattering radiation the salinity dependency of the gas fraction derived from the transmission measurements can be corrected for. The dual modality measurements of the water salinity should be performed at time segments when firstly the gas volume fraction is low, and secondly that the
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water liquid ratio (WLR) is high. The WLR is equivalent to the water cut at the pressure and temperature prevailing in the flow section [1]. If there is too much gas present, the DMD measurement principle will not work due to low count rates of the scattering measurements. As for the transmission gas fraction measurements, the scattering measurements and thus the salinity independent gas fraction measurements depend on the flow regime. For homogeneously mixed flows there is an empirical relationship between the GVF and the measured scattered radiation intensity, IS , and the transmitted radiation intensity, IT , stating that the GVF is proportional to IS /IT0.55 [9]. For separated flow, like annular or stratified, this relationship does not apply. Tjugum et al. 2002 [8] built a compact low-energy multibeam gamma-ray densitometer with two transmission beams and one detector for Compton scattered radiation. They investigated the flow regime dependency of the GVF in a multiphase flow both theoretically and experimentally. Their conclusion was that a multibeam densitometer provided more accurate GVF measurements compared to a single-beam geometry [10] due to increased resolution, and that the MGB measurement principle yields flowregime information. The salinity dependency of the GVF was also investigated by use of the dual modality principle. Assuming homogeneous flow and correcting the GVF by using the empirical relationship between the GVF and the scattered and transmitted gamma-ray intensity IS /IT0.55 [9], the accuracy of the GVF measurements was improved. The estimate of the salinity had a mean deviation from the true salinity of 1.9%. The feasibility of multiphase flow regime identification was also proven by applying nine gamma-ray beams [11]. By using collimated detectors the build-up caused by scattered radiation could be neglected. A deviation was found between the reference GVF and the measured GVF, possibly related to slip or backflow. Flow measurements with an X-ray based densitometer has been tested by Tjugum and Mihalca [12]. They concluded that the X-ray system can replace traditional gamma-ray densitometer systems, and that the X-ray system showed a higher measurement sensitivity particularly at low density flow. The tests were gas fraction measurements by use of single-beam transmission measurements. In this work a multiphase flow test was run with a compact lowenergy dual modality multibeam gamma-ray measurement setup with narrow collimation of the transmission detectors. A high speed gamma-ray tomograph was used as reference for the actual flow regime [13]. The multibeam and dual modality measurements are also compared to Monte-Carlo simulations. This is the first time the principles of dual modality and multibeam densitometry have been compared to simultaneous tomographic measurements. The main goal of this work is to develop a measurement strategy for flow regime and salinity independent gas volume fraction measurements. 2. Theory For a mixture of gas and liquid the instantaneous gas fraction (α ) of the measurement cross section can be estimated as;
α=
ln(ITflow /ITliq ) ln(ITgas /ITliq )
(1)
where ITflow is the measured transmitted gamma-ray intensity through the flow. ITgas and ITliq are calibration measurements of empty pipe and pipe filled with liquid, respectively. Fig. 1a (top) shows the multibeam and dual modality measurement setup used in this work. With a fan beam collimated source and narrow collimated transmission detectors (T1, T2, and T3) the gas fraction is measured along three beams. The detector labeled T1 in the figure registers the attenuation of the gamma radiation through the center of the pipe. Whereas the detectors labeled T2
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Fig. 1. a: Sketch of the MGB and DMD measurement setup used in the multiphase flow measurements. To the left: Am-241 gamma source with symmetric fan beam collimation. In the middle: PVC pipe cross section, and scattering detector below. To the right: 3 narrowly collimated transmission detectors. b: Monte Carlo simulation model of the measurement system. The beams of the transmission detector measurements are also indicated.
Fig. 2. Left: annular flow pattern. Right: stratified flow pattern. Gray is liquid and white is gas. The gas fraction measured along the center transmission gamma beam (T1) is α , and the gas fraction for the entire pipe cross section is αa for annular flow and αs for stratified flow.
and T3 will register the attenuation closer to the pipe wall. The detector labeled S1 measures the scattered radiation. For a homogeneous mixture of gas and liquid the gas fraction derived from Eq. (1) using the intensity measurements from the different transmission detectors in the MGB densitometer, should give the same attenuation coefficient and hence equal gas/liquid fractions. If the flow regime is stratified, with the liquid in the bottom of the pipe across from the radiation source, the gas fraction derived from the transmission detectors at the sides (labeled T2 and T3 in Fig. 1a) should be larger than that derived from the center transmission detector. The geometrical differences in the detector setup are taken into account when the calibration measurements for each of the detectors are used as reference. For annular flow with the gas centered in the middle of the pipe, the gas fraction measured along the center beam will be larger than that of the side transmission detectors. The gas fraction for the total pipe cross section derived from transmission measurements by use of Eq. (1), is valid for homogeneous flow. If annular flow (see Fig. 2, left), the corrected gas fraction αa for the center transmission detector (T1) can be calculated according to Eq. (2), where α is the gas fraction derived from the
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center transmission detector (T1) measurements applied to Eq. (1).
αa = α 2 .
(2)
The corrected gas fraction αs for stratified flow (see Fig. 2, right) for the center transmission detector (T1), can be calculated according to Eq. (3).
αs = 1 −
1 −1 cos (2α − 1) − 2(2α − 1) ∗ α(1 − α) .
π
(3)
For salinity measurements of the water component in the multiphase flow, the DMD principle is applied. Measurements of scattered and transmitted radiation from an Am-241 source, depend on both photoelectric attenuation and Compton scattering cross sections through the flow media. The scattered and transmitted intensities respond differently to the two interaction mechanisms, and it is this difference that is utilized in the DMD principle. The cross section for photoelectric effect is highly dependent on the atomic number of the absorbing material, and hence dependent on the salinity of the water. In a multiphase flow with brine, oil, and gas the salt atoms will give a relatively large contribution to the average atomic value of the mixture since the main components of the flow are low atomic number atoms (hydrogen, oxygen, and carbon). The Compton scattering cross section is roughly proportional to the density of the fluid. By including Compton scattering measurements the salinity of the water component can be monitored during periods of low gas volume fractions and high water liquid ratios. A Monte Carlo simulation model of the MGB and DMD measurement setup has been developed and is used in this study to predict what one can expect to measure with known mixtures and flow regimes. An important advantage for the simulation model is the exact defined reference of flow parameters for both the volume ratios and mixing, and the flow pattern. The simulations performed here were for both two-phase flows (brine and air) and three phase flows (brine with WLR = 50%, diesel, and air). Monte Carlo simulations are helpful in representing measurement systems that utilize ionizing radiation, because they capture the randomness of the radiation transport through a medium. The Monte Carlo model used in this work is based on the Monte Carlo N-Particle (MCNP) code, version 5, developed at the Diagnostics Applications Group, Los Alamos National Laboratory, USA [14]. The entire MGB/DMD measurement setup is reconstructed in the MCNP environment and is benchmarked towards calibration measurements in the flow loop. Fig. 1b shows the geometry of the simulation model. The gas fraction in the simulation geometry is increased stepwise from 0% to 100%. For annular flow the gas is centered in the middle of the pipe in the form of a cylinder with gradually increasing radius. For stratified flow the gas inside the pipe is close to the source, and the plane between gas and liquid is normal to the axis set up by the source and the center transmission detector. The salinity of the water is set equal to that of the measurements, 1.9% and 4.6% NaCl. The densities of the brine solutions are calculated according to Krumgalz et al. 2000 [15]. 3. Experimental setup 3.1. Gamma-ray measurement system The compact MGB and DMD measurement system is intended to be integrated in the pipe wall with no movable parts interfering the oil/water/gas flow. The aim is to have a maintenance-free instrument that is able to sustain the harsh environment below the seabed. The gamma source used is a 100 mCi Am-241 with main characteristic gamma energy of 59.5 keV, and half-life of 432 years. For this photon energy there will be both significant photoelectric effect and Compton scattering. The intention for this measurement system is that it will be integrated in the oil production pipe wall [8]. The diameter of the oil pipes is 2–6 in.,
Fig. 3. Picture of the measurement setup in the former flow rig at Christian Michelsen Research.
typically. The detectors used under testing were Scionix CsI(Na) scintillation detectors. A sketch of the measurement setup as used in the flow loop testing is given in Fig. 1a. The best setup for the instrument is to have the source on top of the pipe, if this is not vertical [8]. During testing the instrument has been clamped on a plastic (PVC) pipe. The attenuation and scattering from the pipe walls are minor. Transmission measurements from empty pipe are used as reference. The integration time was set to 5 s to provide
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Fig. 4. Measured gas fractions (solid legends) derived from two narrow gamma-ray beams plotted against each other, one along the same axis as the center of the source beam (α T1) and one at approximately 25° offset (α T2). The bars display the standard deviation of the averaged measurements. Also plotted are the corresponding results from Monte Carlo simulations of the measurement setup (open legends).
better count rate statistics. The measurement setup is used for exploring measurement principles that can be implemented in a downhole meter. The aim of this work is to develop a measurement strategy. Neither the MGB nor the DMD measurements require energy sensitive detectors, which is an advantage for applications in high temperature environments where thermic noise can be a challenge. Robust detectors without any energy sensitivity can be used, like Geiger Müller tubes. 3.2. Gamma-ray tomograph The high-speed gamma-ray tomograph used in this study was built at the University of Bergen in cooperation with Christian Michelsen Research AS and Norsk Hydro AS [13,16]. Originally it was a dual sensor tomograph for three-component flow consisting of an eight electrode electrical capacitance tomograph and a gamma-ray tomograph. The gamma-ray tomograph consists of a sensor head equipped with five 500 mCi Am-241 fan-beam collimated gamma-ray sources, with characteristic energy of 59.5 keV. Each of these five radiation sources has a corresponding detector module, consisting of 17 collimated CdZnTe detectors. In all the gamma-ray tomograph comprises 85 gamma-ray detector elements. Image reconstruction analysis of the gamma-ray tomograph measurements is described in Maad and Johansen 2008 [17]. For this test the measurement integration times were 1, 10, 50, and 1000 ms, the dead-time of the tomograph electronics was 3 ms. Each measurement series lasted 10 s, not including dead time. The tomographic measurements presented here are based on 10 ms integration time. 3.3. Multiphase flow loop testing The multiphase flow test was run with the MGB and DMD measurement setup together with the high-speed gamma-ray tomograph for flow pattern reference. The distance between the two
measurement sections was approximately 20 cm. The test was run in the former flow loop at Christian Michelsen Research AS, Bergen, Norway. The flow rig had a 3 m3 separator tank, a 15 kW centrifugal pump, and reference instrumentation for phase fractions, flow rates, flow pressure, and temperature. The gamma tomograph was placed approximately 1.5 m after a conversion from the flexible 4 inch pipes of the flow loop to a rigid pipe with inner diameter of 8 cm. This was to ensure a more stable flow pattern. The pipe in the measurement region was a PVC pipe with wall thickness of 0.5 cm. Fig. 3 shows a picture of the gamma tomograph and the MGB/DMD measurement setup in the flow rig. The test matrix applied in the flow loop test is given in Table 1. The tests were done for vertical flow, 45° tilt, and horizontal flow. The flow components were air, regular auto diesel (average chemical formula C12 H23 , 0.04 wt% sulphur), and tap water mixed with NaCl to two different salinities, 1.9% and 4.6%. 4. Measurements 4.1. Flow regime identification from two gamma-ray beams The gas fraction along a narrow gamma-ray beam is derived from transmission measurements applied to Eq. (1). By comparing the calculated gas fraction along two beams, one across the center and one along the side of the pipe, one can obtain information about the distribution of liquid and gas in the measurement cross section. In Fig. 4 MGB measurements are plotted together with simulation results from the Monte Carlo model of the measurement setup. Measurements done for a vertically standing pipe are plotted in red solid legends, for the two salinity mixtures of 1.9% and 4.6% NaCl. For all measurements at vertical flow, except one at the lowest gas fraction value, the gas fractions derived from the center (T1) transmission measurements were larger than the gas fractions from the side (T2) transmission measurements.
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Fig. 5. Time series (not length) of tomograms from the gamma-ray tomograph, and on the right examples of the full tomogram for the different flow scenarios. Black is water, white is gas, and the gray tones display the densities between these with darker color for higher density. V1: vertical pipe, Qgas = 25 m3 /h, Qliq = 32 m3 /h. V2: vertical pipe, Qgas = 72 m3 /h, Qliq = 5.5 m3 /h. D1: 45° tilted pipe, Qgas = 26 m3 /h, Qliq = 33 m3 /h. D2: 45° tilted pipe, Qgas = 76 m3 /h, Qliq = 5.5 m3 /h. H1: horizontal pipe, Qgas = 26 m3 /h, Qliq = 33 m3 /h. H2: horizontal pipe, Qgas = 76 m3 /h, Qliq = 5.5 m3 /h. Water liquid ratio for these time series was 100%. The integration time for the tomograph measurements showed here is 10 ms. The time series consist of 1000 tomogram sections, including deadtime of 3 ms a time section of 13 s. Table 1 MGB and DMD measurement setup and gamma tomograph multiphase flow loop test matrix. Reference settings for the volume flow rates (Q , approximate values) for gas and liquid are given in m3 /h. Also given are reference settings for water liquid ratio (WLR) as percent of the liquid component, and gas volume fraction (GVF) in percent of the full volume. Test no.
Qgas
Qliq
WLR
GVF
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
20 40 40 70 20 40 40 70 25 15 35 25 12 20 35 20
5 5 5 5 20 20 20 20 30 40 25 35 40 40 35 40
100 100 0 100 100 100 0 100 100 100 100 0 100 100 100 0
80 89 89 93 50 67 67 78 44 29 58 44 22 36 50 36
Measurements done for horizontal pipe are plotted in blue solid legends, for the same two salinity mixtures as when vertical flow. For all measurements at horizontal pipe, except one at the lowest gas fraction value, the gas fractions derived from the side
(T2) transmission measurements were larger than that derived from the center transmission detector (T1) measurements. The measured gas fractions when the pipe was set at a 45° tilt are plotted as solid black stars. These results align with the results for the horizontal measurements. Monte Carlo simulations of annular flow calculate the expected transmitted radiation to the different detectors. Gas fraction results (Eq. (1)) of the simulations for annular flow are plotted in red open legends in Fig. 4, for the two salinity mixtures. Gas fraction results (Eq. (1)) from the stratified simulations are plotted in blue open legends, for the two salinity mixtures. The solid line in the figure shows where the gas volume fractions derived from the two transmission detectors are equal, which would imply a homogeneous mixture of the gas and the liquid. Measurements for vertical flow give gas fraction ratios between that of homogeneous flow and simulated annular flow. Whereas measurements for 45° tilt and horizontal flow give gas fraction ratios between that of the simulated stratified flow and homogeneous flow. Measurements for the gamma-ray tomograph are shown in Fig. 5. The upper two panels show measurements for vertical pipe (V1 and V2), the middle two are for 45° tilted pipe (D1 and D2), and the lower two are for horizontal pipe (H1 and H2). The volume flow rates for V1, D1, and H1 were at intermediate values, approximately 25 m3 /h for gas and 30 m3 /h for water. Measurements were carried out with diesel mixture, these are not shown here. For V2, D2, and H2 the gas volume flow rate was more than
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Fig. 6. Gas fraction derived from transmission measurements (T1) corrected for the observed flow regime, versus the true GVF from the flow rig reference data. The bars indicate the standard deviation of the averaged measurements.
70 m3 /h, whereas the water volume flow rate was only 5.5 m3 /h. For the vertical measurements (V1 and V2) the gas was centered in the middle of the pipe. When the volume flow rates for gas and liquid were both at intermediate values, about 30 m3 /h, observations showed slug flow which was a mixture of annular and homogeneous flow patterns. This type of slug flow was also observed when the gas volume flow rate was high and the water volume flow rate was low, giving a GVF of more than 80%. The rate of the changing flow pattern was faster for the cases when the gas and the liquid volume flow rates were comparable. Tomographic measurements for the tilted pipe (D1 and D2) showed stratified, annular, and homogeneous flow patterns. The main characteristic for this experiment was stratified slug flow, in the form of relatively large gas bubbles flowing in the upper part of the pipe. The rate of change between the slug flow and the homogeneous flow was, as for the vertical measurements, faster when the gas and liquid volume flow rates were similar. Horizontal tomographic measurements (H1 and H2) showed a stratified slug flow. Also here the changes in the flow pattern were more rapid for similar values of the volume flow rates. Changes in the flow pattern between that of liquid slug flow and annular or stratified flow, could change as fast as 10 s−1 . The lengths in time of the different flow patterns are not consistent, that is the duration of the stratified or annular flow pattern can be interrupted by shorter lasting segments of slug flow. Corrections for annular flow, according to Eq. (2), were performed on the gas fractions for measurements done at vertical flow with 1.9% and 4.6% NaCl brine solutions. The corrected gas fractions for these two solutions are plotted versus true GVF from the flow rig reference data in Fig. 6, legends labeled 90°. Measurements with only diesel and gas are shown in open legends. The corrected gas fraction is lower than the true GVF for all measurement points. The mean deviation of the measured and corrected gas fraction to the reference GVF is 28% for vertical flow. There is no distinction between the water and diesel mixtures. Corrections for stratified flow, according to Eq. (3), were performed on gas fractions for the measurements done at both 45°
Fig. 7. Measured scattered gamma-ray intensity plotted against the liquid ratio. ⟨α⟩ is the mean gas fraction found from transmission measurements for detectors T1 and T2, Eq. (1). The water liquid ratio is 100%. Measurements were done at vertical flow with brine mixtures of 1.9% NaCl (open circles) and 4.6% NaCl (gray colored squares). The standard deviations of the averaged measurements are indicated.
tilted pipe with 1.9% NaCl brine solution, and horizontal pipe with brine solutions of 1.9% and 4.6%. The results are plotted versus the reference GVF in Fig. 6, legends labeled 45° and 0°. The corrected gas fractions are larger or close to equal to the true GVF for GVFs lower than about 0.5. At higher GVFs the corrected gas fractions are lower than the reference. The mean deviations of the measured and corrected gas fractions to the reference GVFs are 15% for horizontal flow and 13% for 45° tilted flow.
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Fig. 8. Simulated scattered gamma-ray intensity plotted against the liquid volume ratio. The water liquid ratio is 100% or 50%. Monte Carlo simulations were performed for annular and stratified flow with brine mixtures of 1.9% NaCl and 4.6% NaCl. The curves are best-fit polynomials to the simulation data points.
4.2. Salinity measurements In Fig. 7 measurements of the scattered gamma-radiation intensity are plotted against the instantaneous mean liquid ratio (1 − ⟨α⟩). The mean gas fraction, ⟨α⟩, is found from Eq. (1) and the measurements at the center and side transmission detectors. Here there are made no corrections according to the flow regime. Due to problems with the detector that measured the scattered radiation, there were only five measurement points with brine mixture of 4.6% NaCl (solid legends in Fig. 7) to compare with measurements of brine mixture with 1.9% NaCl (open legends). The comparisons are done for vertical pipe and water continuous flow (WLR = 100%). When the liquid ratio was less than 0.5 we were not able to distinguish between the two brine mixtures. For measurements done at water filled pipe there is a clear difference between the low salinity (∼750 counts/s) and the higher salinity (∼650 counts/s). Monte Carlo simulations for brine and gas flow with 1.9% and 4.6% NaCl, and water liquid ratios of 100% and 50% are shown in Fig. 8. The lower set of data points show annular flow pattern, and the upper set the stratified flow pattern. Pink and red data points are simulations for the low salinity mixture (1.9% NaCl) with 50% WLR and 100% WLR, respectively. Green and blue data points are simulations for the higher salinity mixture (4.6% NaCl) with 50% WLR and 100% WLR, respectively. The scattered intensity is in unit of counts per simulated photon starting at the source. From the figure one can see that for water continuous flow (100% WLR, blue and red) the liquid volume ratio has to be at least more than 0.7 in order for the simulated DMD principle to register any difference between the two salinities, for both flow regimes. For the three phase flow with 50% WLR the simulated DMD principle has difficulties distinguishing the two salinity mixtures, especially when stratified flow.
5. Discussion High-speed gamma-ray tomograph measurements in the multiphase flow loop showed that for the volume flow rates applied in this test (see Table 1) the flow pattern varied within one second, and could change within one tenth of a second if both the gas volume flow rate and the liquid volume flow rate were at intermediate values, about 30 m3 /h. For horizontal and tilted pipe the flow regime was slug or wavy flow with the gas in the upper part of the pipe. For vertical standing pipe the flow regime was slug flow with gas centered in the middle of the pipe. The MGB measurements had an integration time of 5 s. During this time period there would have been several different flow patterns, not strictly annular or stratified flow. During the integration time of 5 s the count rate of the transmission measurements was high enough to provide good measurement statistics. The MGB data presented here are averaged over several measurements in order to have even better statistics. The high count rate measurements from narrow gamma-ray beams do show the tendency of the flow regime. When horizontal and tilted pipe the MGB measurements showed gas fractions which ratios were between that of simulated stratified flow and homogeneous flow. This corresponds to the layered slug or wavy flow observed by the tomograph. During vertical standing pipe the MGB measurements showed gas fraction ratios that were between that of simulated annular flow and homogeneous flow. This also corresponds to the centered slug flow observed by the gamma-ray tomograph. Flow regime corrections of the gas fraction from the time averaged measurements will not provide a good estimate of the full gas volume fraction. The difference between the corrected gas fraction and the true GVF can be due to slip between the gas and liquid, especially at high GVFs. However, the sampling frequency of the MGB measurements (1/5 s−1 ) is too low compared to the flow pattern variations, and a gas fraction correction of strictly annular or stratified flow in slug or wavy flow will not give the true gas fraction of the pipe cross section.
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For the flow loop test performed here there were two salinity mixtures, one of 1.9% NaCl and one of 4.6% NaCl. DMD scattering measurements for salinity monitoring had an integration time of 5 s. This was the same as for the transmission measurements, and as stated above the sampling frequency was too low relative to the flow pattern variations. Based on the DMD measurements at hand, the instrument was able to distinguish the two salinity mixtures for vertical flow when the liquid volume ratio was more than 0.7 and the water liquid ratio was 100%. From Monte Carlo simulations of the DMD principle setup, one could see that it is difficult for the DMD setup to differentiate the two NaCl solutions when the water liquid ratio is 50%. 6. Measurement strategy In order to estimate the gas fraction for each flow regime of the multiphase flow, the time resolution of the MGB measurements should be sufficiently fast to register all the changes in the flow. For slug flow the required time resolution will depend on length of the liquid slug, length of the stratified or annular gas flow, and the flow velocity. For the measurements performed here, the time resolution should be faster than 0.1 s. With a short integration time one can divide the measurements into time segments according to the flow regime. From this one can obtain running averages of the gas fraction measurements, and perform gas fraction corrections according to the observed flow regime. By including flow rate measurements one can derive an estimate of the true GVF of the flow. Salinity of the water component of the flow is expected to have slow variations compared to the flow pattern variations. With a high sampling frequency of the water-cut and MGB measurements one can select the time sections when the measurements show low gas fraction and high water liquid fraction, and perform DMD for these intervals for salinity monitoring. Also here a running average approach will improve the measurement statistics. Thus, with a high time resolution one can extract the short time intervals where the flow regime is well defined, and perform the corresponding corrections for the gas fraction measurements. The salinity measurements will be iterative and should be performed during flow patterns with low gas fraction (less than 30%) and high water volume ratios (preferably close to 100%). Also the DMD measurement principle will rely on high time resolution measurements. The liquid parameter of the three phase flow can be found based on the water cut measurements and a calculation of ITliq (see Eq. (1)) from the WLR and the calibration measurements of water filled and oil filled pipe. Another approach is an iterative solution process where one uses all the different measurement principles to find the gas fraction and the WLR of the flow. 7. Conclusions In this work a multi gamma beam and dual modality measurement setup has been tested in a multiphase flow loop for flow regime identification and salinity monitoring. For flow pattern
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reference a high-speed gamma-ray tomograph was measuring at a section 20 cm in front of the MGB and DMD measurement setup. Measurements were performed at 0°, 45°, and 90° positioned PVC pipe with inner diameter of 8 cm. The flow components were air, diesel, and tap water mixed with NaCl to a salinity of 1.9% and 4.6%. The integration time of the MGB and DMD measurements was 5 s. According to the high-speed tomograph the flow pattern variations could be one tenth of a second. The sampling frequency of the MGB measurements was thus not sufficiently fast to perform a quantitative analysis of the gas ratio in the multiphase flow. The test did show, however, that the high count rate MGB principle can be used to distinguish the flow regimes of stratified/wavy flow and annular/slug flow. The DMD principle did also show a significant difference between the two salinity mixtures when the water liquid ratio was 100% and the gas volume ratio was less than 30%. If the sampling ratios of the MGB and DMD measurements are faster and complies with Nyquist’s sampling theorem, the compact and rugged MGB and DMD measurement setup should be able to perform flow regime independent gas fraction measurements. References [1] Corneliussen S, Couput JP, Dahl E, Dykesteen E, Frøysa KE, Malde E, Moestue H, Moksnes PO, Scheers L, Tunheim H. Handbook of multiphase flow metering, rev. 2. www.nfogm.no, 2005. [2] Hjertaker BT, Tjugum SA, Hammer EA, Johansen GA. Multimodality tomography for multiphase hydrocarbon flow measurements. IEEE Sensors J 2005;5(2). [3] Kvandal HK, Kjølberg SA, Schüller RB. Water detection in gas/condensate flows by SeCaP technology. Chem Eng J 2009; doi:10.1016/j.cej.2009.11.011. [4] Schüller RB, Gundersen T, Halleraker M, Engebretsen B. Measurement of water concentration in oil/water dispersions with a circular single electrode capacitance probe. IEEE Trans Instrum Meas 2004;53(5):1378–83. [5] Liu X, Hu J, Shan F, Cai B, Su X, Chen Q. Conductance sensor for measurement of the fluid water cut and flow rate in production wells. Chem Eng J 2010;197(2). [6] Toftevåg KR. Ultrasound transit-time flowmeter. Master’s thesis. University of Bergen; 2005 [in Norwegian]. [7] Aandahl BK. Transit-time calculations in turbulence. Master’s thesis. University of Bergen; 2006 [in Norwegian]. [8] Tjugum SA, Frieling J, Johansen GA. A compact low energy multibeam gammaray densitometer for pipe-flow measurements. Nucl Instrum Methods B 2002; 197:301–9. [9] Johansen GA, Jackson P. Salinity independent measurement of gas volume fraction in oil/gas/water pipe flow. Appl Radiat Isot 2000;53:595–601. [10] Gardner RP, Bean RH, Ferrel JK. On the gamma-ray one-shot collimator measurement of two-phase flow void fractions. Nucl Appl Technol 1970;8:88–94. [11] Tjugum SA, Hjertaker BT, Johansen GA. Multiphase flow regime identification by multibeam gamma-ray densitometry. Meas Sci Technol 2002;13:1319–26. [12] Tjugum SA, Mihalca R. X-ray based densitometer for multiphase flow measurements. In: North sea flow measurement workshop 2009 — proceedings. [13] Johansen GA, Frøystein T, Hjertaker BT, Olsen Ø. A dual sensor flow imaging tomographic system. Meas Sci Technol 1996;7:297–307. [14] Brown FB, Barrett RF, Booth TE, Bull JS, Cox LJ, Forster RA, Goorley TJ, Mosteller RD, Post SE, Prael RE, Selcow EC, Sood A, Sweezy J. MCNP Version 5. Trans Am Nucl Soc 2002;87(273). [15] Krumgalz BS, Pogorelskii R, Sokolov A, Pitzer KS. Volumetric ion interaction parameters for single-solute aqueous electrolyte solutions at various temperatures. J Phys Chem Ref Data 2000;29(5). [16] Hjertaker BT, Maad R, Schuster E, Almås OA, Johansen GA. A data acquisition and control system for high-speed gamma-ray tomography. Meas Sci Technol 2008;19(094012). [17] Maad R, Johansen GA. Experimental analysis of high-speed gamma-ray tomography performance. Meas Sci Technol 2008;19(085502).
Contents lists available at ScienceDirect
Flow Measurement and Instrumentation journal homepage: www.elsevier.com/locate/flowmeasinst
Salinity and flow regime independent multiphase flow measurements C. Sætre a,b,∗ , G.A. Johansen a,b , S.A. Tjugum b,c a
Department of Physics and Technology, University of Bergen, 5007 Bergen, Norway
b
The Michelsen Centre for Industrial Measurement Science and Technology, Norway
c
Roxar Flow Measurements, Bergen, Norway
article
info
Article history: Received 23 March 2010 Received in revised form 15 June 2010 Accepted 16 June 2010 Keywords: Gamma-ray densitometry Multibeam Flow regime identification Tomography Gas volume fraction Salinity measurement Scattering measurements Dual modality
abstract For oil production fields, there is a need for downhole measurements of the gas/water/oil multiphase flow. In extreme conditions a relatively simple, robust, and non-intrusive system will be appropriate. A measurement setup that combines multiple gamma beam (MGB) and dual modality densitometry (DMD) measurements, would be able to determine the gas volume fraction (GVF) independently of the flow pattern, and monitor changes in water salinity. MGB measurements of gamma-ray transmission along multiple sections across the oil pipe will provide information on the flow pattern. Whereas the DMD principle will give information on changes in salinity from a combination of transmission and scattering gammaradiation measurements. In this work we present the results from MGB and DMD measurements of a multiphase flow with high-speed gamma-ray tomograph measurements as reference for the flow pattern. The MGB measurements should enable us to distinguish between stratified or wavy/slug and annular or slug flow. Flow patterns with several minor components distributed evenly over the measurement cross section, like bubble flow, will be interpreted as homogeneous flow. The DMD measurements can be used to monitor salinity changes of the water component for intervals where the GVF is low and the water cut of the liquid is high. Combined with other gauges for water cut measurements, the MGB and DMD measurement setup should improve the multiphase flow measurements, and enable increased oil/gas recovery and production water monitoring. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction Measurements on the different components in a gas, oil, and water multiphase pipe flow depend not only on the different densities and volume ratios. They also depend upon the distribution of the components within the pipe if these are not mixed, and also on the salinity of the water component [1]. Gamma radiation is useful for non-intrusive measurements of components with sufficiently different density. Combined with water cut measurements from capacitance [2–4], conductance [5], or more recent measurement principles like single-beam ultrasound [6,7], a multibeam gamma densitometer can be used for gas fraction measurements independent of the flow pattern in a multiphase flow. By measuring at several (in this work three) narrow gamma-ray beams the gas fraction derived from each of these transmission measurements will provide information on the flow regime within the pipe. The total gas fraction of the pipe cross section can then be calculated from the combination of these transmission measurements. If there is no slip between the gas and the
∗ Corresponding address: Department of Physics and Technology, Pbox 7803, 5020 Bergen, Norway. Tel.: +47 55588308. E-mail address: [email protected] (C. Sætre). 0955-5986/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.flowmeasinst.2010.06.002
liquid, that is if all the flow components flow at the same velocity, the measured gas fraction (α ) gives the gas volume fraction (GVF) of the flow. A compact low-energy multibeam gamma-ray densitometer has been built at the University of Bergen in collaboration with Roxar Flow Measurements [8]. The water salinity affects the multiphase measurements when deriving the relative amount of gas from the transmitted gamma radiation. For salinity monitoring of the water component one can combine the measurements of the transmitted and scattered gamma radiation, since these measurements reveal different response of the interaction mechanisms of the radiation and the flowing medium. Whereas the photoelectric effect at a given photon energy is highly dependent on the atomic number of the absorbing material (approximately proportional to Z 4 –Z 5 ) and hence the salinity of the water, Compton scattering is mainly proportional to the density of the scattering material. Using a gamma source Am-241 with characteristic energy of 59.5 keV, there will be both significant photoelectric absorption and Compton scattering for dual modality measurements. By combining the measurements of transmission and scattering radiation the salinity dependency of the gas fraction derived from the transmission measurements can be corrected for. The dual modality measurements of the water salinity should be performed at time segments when firstly the gas volume fraction is low, and secondly that the
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water liquid ratio (WLR) is high. The WLR is equivalent to the water cut at the pressure and temperature prevailing in the flow section [1]. If there is too much gas present, the DMD measurement principle will not work due to low count rates of the scattering measurements. As for the transmission gas fraction measurements, the scattering measurements and thus the salinity independent gas fraction measurements depend on the flow regime. For homogeneously mixed flows there is an empirical relationship between the GVF and the measured scattered radiation intensity, IS , and the transmitted radiation intensity, IT , stating that the GVF is proportional to IS /IT0.55 [9]. For separated flow, like annular or stratified, this relationship does not apply. Tjugum et al. 2002 [8] built a compact low-energy multibeam gamma-ray densitometer with two transmission beams and one detector for Compton scattered radiation. They investigated the flow regime dependency of the GVF in a multiphase flow both theoretically and experimentally. Their conclusion was that a multibeam densitometer provided more accurate GVF measurements compared to a single-beam geometry [10] due to increased resolution, and that the MGB measurement principle yields flowregime information. The salinity dependency of the GVF was also investigated by use of the dual modality principle. Assuming homogeneous flow and correcting the GVF by using the empirical relationship between the GVF and the scattered and transmitted gamma-ray intensity IS /IT0.55 [9], the accuracy of the GVF measurements was improved. The estimate of the salinity had a mean deviation from the true salinity of 1.9%. The feasibility of multiphase flow regime identification was also proven by applying nine gamma-ray beams [11]. By using collimated detectors the build-up caused by scattered radiation could be neglected. A deviation was found between the reference GVF and the measured GVF, possibly related to slip or backflow. Flow measurements with an X-ray based densitometer has been tested by Tjugum and Mihalca [12]. They concluded that the X-ray system can replace traditional gamma-ray densitometer systems, and that the X-ray system showed a higher measurement sensitivity particularly at low density flow. The tests were gas fraction measurements by use of single-beam transmission measurements. In this work a multiphase flow test was run with a compact lowenergy dual modality multibeam gamma-ray measurement setup with narrow collimation of the transmission detectors. A high speed gamma-ray tomograph was used as reference for the actual flow regime [13]. The multibeam and dual modality measurements are also compared to Monte-Carlo simulations. This is the first time the principles of dual modality and multibeam densitometry have been compared to simultaneous tomographic measurements. The main goal of this work is to develop a measurement strategy for flow regime and salinity independent gas volume fraction measurements. 2. Theory For a mixture of gas and liquid the instantaneous gas fraction (α ) of the measurement cross section can be estimated as;
α=
ln(ITflow /ITliq ) ln(ITgas /ITliq )
(1)
where ITflow is the measured transmitted gamma-ray intensity through the flow. ITgas and ITliq are calibration measurements of empty pipe and pipe filled with liquid, respectively. Fig. 1a (top) shows the multibeam and dual modality measurement setup used in this work. With a fan beam collimated source and narrow collimated transmission detectors (T1, T2, and T3) the gas fraction is measured along three beams. The detector labeled T1 in the figure registers the attenuation of the gamma radiation through the center of the pipe. Whereas the detectors labeled T2
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Fig. 1. a: Sketch of the MGB and DMD measurement setup used in the multiphase flow measurements. To the left: Am-241 gamma source with symmetric fan beam collimation. In the middle: PVC pipe cross section, and scattering detector below. To the right: 3 narrowly collimated transmission detectors. b: Monte Carlo simulation model of the measurement system. The beams of the transmission detector measurements are also indicated.
Fig. 2. Left: annular flow pattern. Right: stratified flow pattern. Gray is liquid and white is gas. The gas fraction measured along the center transmission gamma beam (T1) is α , and the gas fraction for the entire pipe cross section is αa for annular flow and αs for stratified flow.
and T3 will register the attenuation closer to the pipe wall. The detector labeled S1 measures the scattered radiation. For a homogeneous mixture of gas and liquid the gas fraction derived from Eq. (1) using the intensity measurements from the different transmission detectors in the MGB densitometer, should give the same attenuation coefficient and hence equal gas/liquid fractions. If the flow regime is stratified, with the liquid in the bottom of the pipe across from the radiation source, the gas fraction derived from the transmission detectors at the sides (labeled T2 and T3 in Fig. 1a) should be larger than that derived from the center transmission detector. The geometrical differences in the detector setup are taken into account when the calibration measurements for each of the detectors are used as reference. For annular flow with the gas centered in the middle of the pipe, the gas fraction measured along the center beam will be larger than that of the side transmission detectors. The gas fraction for the total pipe cross section derived from transmission measurements by use of Eq. (1), is valid for homogeneous flow. If annular flow (see Fig. 2, left), the corrected gas fraction αa for the center transmission detector (T1) can be calculated according to Eq. (2), where α is the gas fraction derived from the
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center transmission detector (T1) measurements applied to Eq. (1).
αa = α 2 .
(2)
The corrected gas fraction αs for stratified flow (see Fig. 2, right) for the center transmission detector (T1), can be calculated according to Eq. (3).
αs = 1 −
1 −1 cos (2α − 1) − 2(2α − 1) ∗ α(1 − α) .
π
(3)
For salinity measurements of the water component in the multiphase flow, the DMD principle is applied. Measurements of scattered and transmitted radiation from an Am-241 source, depend on both photoelectric attenuation and Compton scattering cross sections through the flow media. The scattered and transmitted intensities respond differently to the two interaction mechanisms, and it is this difference that is utilized in the DMD principle. The cross section for photoelectric effect is highly dependent on the atomic number of the absorbing material, and hence dependent on the salinity of the water. In a multiphase flow with brine, oil, and gas the salt atoms will give a relatively large contribution to the average atomic value of the mixture since the main components of the flow are low atomic number atoms (hydrogen, oxygen, and carbon). The Compton scattering cross section is roughly proportional to the density of the fluid. By including Compton scattering measurements the salinity of the water component can be monitored during periods of low gas volume fractions and high water liquid ratios. A Monte Carlo simulation model of the MGB and DMD measurement setup has been developed and is used in this study to predict what one can expect to measure with known mixtures and flow regimes. An important advantage for the simulation model is the exact defined reference of flow parameters for both the volume ratios and mixing, and the flow pattern. The simulations performed here were for both two-phase flows (brine and air) and three phase flows (brine with WLR = 50%, diesel, and air). Monte Carlo simulations are helpful in representing measurement systems that utilize ionizing radiation, because they capture the randomness of the radiation transport through a medium. The Monte Carlo model used in this work is based on the Monte Carlo N-Particle (MCNP) code, version 5, developed at the Diagnostics Applications Group, Los Alamos National Laboratory, USA [14]. The entire MGB/DMD measurement setup is reconstructed in the MCNP environment and is benchmarked towards calibration measurements in the flow loop. Fig. 1b shows the geometry of the simulation model. The gas fraction in the simulation geometry is increased stepwise from 0% to 100%. For annular flow the gas is centered in the middle of the pipe in the form of a cylinder with gradually increasing radius. For stratified flow the gas inside the pipe is close to the source, and the plane between gas and liquid is normal to the axis set up by the source and the center transmission detector. The salinity of the water is set equal to that of the measurements, 1.9% and 4.6% NaCl. The densities of the brine solutions are calculated according to Krumgalz et al. 2000 [15]. 3. Experimental setup 3.1. Gamma-ray measurement system The compact MGB and DMD measurement system is intended to be integrated in the pipe wall with no movable parts interfering the oil/water/gas flow. The aim is to have a maintenance-free instrument that is able to sustain the harsh environment below the seabed. The gamma source used is a 100 mCi Am-241 with main characteristic gamma energy of 59.5 keV, and half-life of 432 years. For this photon energy there will be both significant photoelectric effect and Compton scattering. The intention for this measurement system is that it will be integrated in the oil production pipe wall [8]. The diameter of the oil pipes is 2–6 in.,
Fig. 3. Picture of the measurement setup in the former flow rig at Christian Michelsen Research.
typically. The detectors used under testing were Scionix CsI(Na) scintillation detectors. A sketch of the measurement setup as used in the flow loop testing is given in Fig. 1a. The best setup for the instrument is to have the source on top of the pipe, if this is not vertical [8]. During testing the instrument has been clamped on a plastic (PVC) pipe. The attenuation and scattering from the pipe walls are minor. Transmission measurements from empty pipe are used as reference. The integration time was set to 5 s to provide
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Fig. 4. Measured gas fractions (solid legends) derived from two narrow gamma-ray beams plotted against each other, one along the same axis as the center of the source beam (α T1) and one at approximately 25° offset (α T2). The bars display the standard deviation of the averaged measurements. Also plotted are the corresponding results from Monte Carlo simulations of the measurement setup (open legends).
better count rate statistics. The measurement setup is used for exploring measurement principles that can be implemented in a downhole meter. The aim of this work is to develop a measurement strategy. Neither the MGB nor the DMD measurements require energy sensitive detectors, which is an advantage for applications in high temperature environments where thermic noise can be a challenge. Robust detectors without any energy sensitivity can be used, like Geiger Müller tubes. 3.2. Gamma-ray tomograph The high-speed gamma-ray tomograph used in this study was built at the University of Bergen in cooperation with Christian Michelsen Research AS and Norsk Hydro AS [13,16]. Originally it was a dual sensor tomograph for three-component flow consisting of an eight electrode electrical capacitance tomograph and a gamma-ray tomograph. The gamma-ray tomograph consists of a sensor head equipped with five 500 mCi Am-241 fan-beam collimated gamma-ray sources, with characteristic energy of 59.5 keV. Each of these five radiation sources has a corresponding detector module, consisting of 17 collimated CdZnTe detectors. In all the gamma-ray tomograph comprises 85 gamma-ray detector elements. Image reconstruction analysis of the gamma-ray tomograph measurements is described in Maad and Johansen 2008 [17]. For this test the measurement integration times were 1, 10, 50, and 1000 ms, the dead-time of the tomograph electronics was 3 ms. Each measurement series lasted 10 s, not including dead time. The tomographic measurements presented here are based on 10 ms integration time. 3.3. Multiphase flow loop testing The multiphase flow test was run with the MGB and DMD measurement setup together with the high-speed gamma-ray tomograph for flow pattern reference. The distance between the two
measurement sections was approximately 20 cm. The test was run in the former flow loop at Christian Michelsen Research AS, Bergen, Norway. The flow rig had a 3 m3 separator tank, a 15 kW centrifugal pump, and reference instrumentation for phase fractions, flow rates, flow pressure, and temperature. The gamma tomograph was placed approximately 1.5 m after a conversion from the flexible 4 inch pipes of the flow loop to a rigid pipe with inner diameter of 8 cm. This was to ensure a more stable flow pattern. The pipe in the measurement region was a PVC pipe with wall thickness of 0.5 cm. Fig. 3 shows a picture of the gamma tomograph and the MGB/DMD measurement setup in the flow rig. The test matrix applied in the flow loop test is given in Table 1. The tests were done for vertical flow, 45° tilt, and horizontal flow. The flow components were air, regular auto diesel (average chemical formula C12 H23 , 0.04 wt% sulphur), and tap water mixed with NaCl to two different salinities, 1.9% and 4.6%. 4. Measurements 4.1. Flow regime identification from two gamma-ray beams The gas fraction along a narrow gamma-ray beam is derived from transmission measurements applied to Eq. (1). By comparing the calculated gas fraction along two beams, one across the center and one along the side of the pipe, one can obtain information about the distribution of liquid and gas in the measurement cross section. In Fig. 4 MGB measurements are plotted together with simulation results from the Monte Carlo model of the measurement setup. Measurements done for a vertically standing pipe are plotted in red solid legends, for the two salinity mixtures of 1.9% and 4.6% NaCl. For all measurements at vertical flow, except one at the lowest gas fraction value, the gas fractions derived from the center (T1) transmission measurements were larger than the gas fractions from the side (T2) transmission measurements.
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Fig. 5. Time series (not length) of tomograms from the gamma-ray tomograph, and on the right examples of the full tomogram for the different flow scenarios. Black is water, white is gas, and the gray tones display the densities between these with darker color for higher density. V1: vertical pipe, Qgas = 25 m3 /h, Qliq = 32 m3 /h. V2: vertical pipe, Qgas = 72 m3 /h, Qliq = 5.5 m3 /h. D1: 45° tilted pipe, Qgas = 26 m3 /h, Qliq = 33 m3 /h. D2: 45° tilted pipe, Qgas = 76 m3 /h, Qliq = 5.5 m3 /h. H1: horizontal pipe, Qgas = 26 m3 /h, Qliq = 33 m3 /h. H2: horizontal pipe, Qgas = 76 m3 /h, Qliq = 5.5 m3 /h. Water liquid ratio for these time series was 100%. The integration time for the tomograph measurements showed here is 10 ms. The time series consist of 1000 tomogram sections, including deadtime of 3 ms a time section of 13 s. Table 1 MGB and DMD measurement setup and gamma tomograph multiphase flow loop test matrix. Reference settings for the volume flow rates (Q , approximate values) for gas and liquid are given in m3 /h. Also given are reference settings for water liquid ratio (WLR) as percent of the liquid component, and gas volume fraction (GVF) in percent of the full volume. Test no.
Qgas
Qliq
WLR
GVF
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
20 40 40 70 20 40 40 70 25 15 35 25 12 20 35 20
5 5 5 5 20 20 20 20 30 40 25 35 40 40 35 40
100 100 0 100 100 100 0 100 100 100 100 0 100 100 100 0
80 89 89 93 50 67 67 78 44 29 58 44 22 36 50 36
Measurements done for horizontal pipe are plotted in blue solid legends, for the same two salinity mixtures as when vertical flow. For all measurements at horizontal pipe, except one at the lowest gas fraction value, the gas fractions derived from the side
(T2) transmission measurements were larger than that derived from the center transmission detector (T1) measurements. The measured gas fractions when the pipe was set at a 45° tilt are plotted as solid black stars. These results align with the results for the horizontal measurements. Monte Carlo simulations of annular flow calculate the expected transmitted radiation to the different detectors. Gas fraction results (Eq. (1)) of the simulations for annular flow are plotted in red open legends in Fig. 4, for the two salinity mixtures. Gas fraction results (Eq. (1)) from the stratified simulations are plotted in blue open legends, for the two salinity mixtures. The solid line in the figure shows where the gas volume fractions derived from the two transmission detectors are equal, which would imply a homogeneous mixture of the gas and the liquid. Measurements for vertical flow give gas fraction ratios between that of homogeneous flow and simulated annular flow. Whereas measurements for 45° tilt and horizontal flow give gas fraction ratios between that of the simulated stratified flow and homogeneous flow. Measurements for the gamma-ray tomograph are shown in Fig. 5. The upper two panels show measurements for vertical pipe (V1 and V2), the middle two are for 45° tilted pipe (D1 and D2), and the lower two are for horizontal pipe (H1 and H2). The volume flow rates for V1, D1, and H1 were at intermediate values, approximately 25 m3 /h for gas and 30 m3 /h for water. Measurements were carried out with diesel mixture, these are not shown here. For V2, D2, and H2 the gas volume flow rate was more than
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Fig. 6. Gas fraction derived from transmission measurements (T1) corrected for the observed flow regime, versus the true GVF from the flow rig reference data. The bars indicate the standard deviation of the averaged measurements.
70 m3 /h, whereas the water volume flow rate was only 5.5 m3 /h. For the vertical measurements (V1 and V2) the gas was centered in the middle of the pipe. When the volume flow rates for gas and liquid were both at intermediate values, about 30 m3 /h, observations showed slug flow which was a mixture of annular and homogeneous flow patterns. This type of slug flow was also observed when the gas volume flow rate was high and the water volume flow rate was low, giving a GVF of more than 80%. The rate of the changing flow pattern was faster for the cases when the gas and the liquid volume flow rates were comparable. Tomographic measurements for the tilted pipe (D1 and D2) showed stratified, annular, and homogeneous flow patterns. The main characteristic for this experiment was stratified slug flow, in the form of relatively large gas bubbles flowing in the upper part of the pipe. The rate of change between the slug flow and the homogeneous flow was, as for the vertical measurements, faster when the gas and liquid volume flow rates were similar. Horizontal tomographic measurements (H1 and H2) showed a stratified slug flow. Also here the changes in the flow pattern were more rapid for similar values of the volume flow rates. Changes in the flow pattern between that of liquid slug flow and annular or stratified flow, could change as fast as 10 s−1 . The lengths in time of the different flow patterns are not consistent, that is the duration of the stratified or annular flow pattern can be interrupted by shorter lasting segments of slug flow. Corrections for annular flow, according to Eq. (2), were performed on the gas fractions for measurements done at vertical flow with 1.9% and 4.6% NaCl brine solutions. The corrected gas fractions for these two solutions are plotted versus true GVF from the flow rig reference data in Fig. 6, legends labeled 90°. Measurements with only diesel and gas are shown in open legends. The corrected gas fraction is lower than the true GVF for all measurement points. The mean deviation of the measured and corrected gas fraction to the reference GVF is 28% for vertical flow. There is no distinction between the water and diesel mixtures. Corrections for stratified flow, according to Eq. (3), were performed on gas fractions for the measurements done at both 45°
Fig. 7. Measured scattered gamma-ray intensity plotted against the liquid ratio. ⟨α⟩ is the mean gas fraction found from transmission measurements for detectors T1 and T2, Eq. (1). The water liquid ratio is 100%. Measurements were done at vertical flow with brine mixtures of 1.9% NaCl (open circles) and 4.6% NaCl (gray colored squares). The standard deviations of the averaged measurements are indicated.
tilted pipe with 1.9% NaCl brine solution, and horizontal pipe with brine solutions of 1.9% and 4.6%. The results are plotted versus the reference GVF in Fig. 6, legends labeled 45° and 0°. The corrected gas fractions are larger or close to equal to the true GVF for GVFs lower than about 0.5. At higher GVFs the corrected gas fractions are lower than the reference. The mean deviations of the measured and corrected gas fractions to the reference GVFs are 15% for horizontal flow and 13% for 45° tilted flow.
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Fig. 8. Simulated scattered gamma-ray intensity plotted against the liquid volume ratio. The water liquid ratio is 100% or 50%. Monte Carlo simulations were performed for annular and stratified flow with brine mixtures of 1.9% NaCl and 4.6% NaCl. The curves are best-fit polynomials to the simulation data points.
4.2. Salinity measurements In Fig. 7 measurements of the scattered gamma-radiation intensity are plotted against the instantaneous mean liquid ratio (1 − ⟨α⟩). The mean gas fraction, ⟨α⟩, is found from Eq. (1) and the measurements at the center and side transmission detectors. Here there are made no corrections according to the flow regime. Due to problems with the detector that measured the scattered radiation, there were only five measurement points with brine mixture of 4.6% NaCl (solid legends in Fig. 7) to compare with measurements of brine mixture with 1.9% NaCl (open legends). The comparisons are done for vertical pipe and water continuous flow (WLR = 100%). When the liquid ratio was less than 0.5 we were not able to distinguish between the two brine mixtures. For measurements done at water filled pipe there is a clear difference between the low salinity (∼750 counts/s) and the higher salinity (∼650 counts/s). Monte Carlo simulations for brine and gas flow with 1.9% and 4.6% NaCl, and water liquid ratios of 100% and 50% are shown in Fig. 8. The lower set of data points show annular flow pattern, and the upper set the stratified flow pattern. Pink and red data points are simulations for the low salinity mixture (1.9% NaCl) with 50% WLR and 100% WLR, respectively. Green and blue data points are simulations for the higher salinity mixture (4.6% NaCl) with 50% WLR and 100% WLR, respectively. The scattered intensity is in unit of counts per simulated photon starting at the source. From the figure one can see that for water continuous flow (100% WLR, blue and red) the liquid volume ratio has to be at least more than 0.7 in order for the simulated DMD principle to register any difference between the two salinities, for both flow regimes. For the three phase flow with 50% WLR the simulated DMD principle has difficulties distinguishing the two salinity mixtures, especially when stratified flow.
5. Discussion High-speed gamma-ray tomograph measurements in the multiphase flow loop showed that for the volume flow rates applied in this test (see Table 1) the flow pattern varied within one second, and could change within one tenth of a second if both the gas volume flow rate and the liquid volume flow rate were at intermediate values, about 30 m3 /h. For horizontal and tilted pipe the flow regime was slug or wavy flow with the gas in the upper part of the pipe. For vertical standing pipe the flow regime was slug flow with gas centered in the middle of the pipe. The MGB measurements had an integration time of 5 s. During this time period there would have been several different flow patterns, not strictly annular or stratified flow. During the integration time of 5 s the count rate of the transmission measurements was high enough to provide good measurement statistics. The MGB data presented here are averaged over several measurements in order to have even better statistics. The high count rate measurements from narrow gamma-ray beams do show the tendency of the flow regime. When horizontal and tilted pipe the MGB measurements showed gas fractions which ratios were between that of simulated stratified flow and homogeneous flow. This corresponds to the layered slug or wavy flow observed by the tomograph. During vertical standing pipe the MGB measurements showed gas fraction ratios that were between that of simulated annular flow and homogeneous flow. This also corresponds to the centered slug flow observed by the gamma-ray tomograph. Flow regime corrections of the gas fraction from the time averaged measurements will not provide a good estimate of the full gas volume fraction. The difference between the corrected gas fraction and the true GVF can be due to slip between the gas and liquid, especially at high GVFs. However, the sampling frequency of the MGB measurements (1/5 s−1 ) is too low compared to the flow pattern variations, and a gas fraction correction of strictly annular or stratified flow in slug or wavy flow will not give the true gas fraction of the pipe cross section.
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For the flow loop test performed here there were two salinity mixtures, one of 1.9% NaCl and one of 4.6% NaCl. DMD scattering measurements for salinity monitoring had an integration time of 5 s. This was the same as for the transmission measurements, and as stated above the sampling frequency was too low relative to the flow pattern variations. Based on the DMD measurements at hand, the instrument was able to distinguish the two salinity mixtures for vertical flow when the liquid volume ratio was more than 0.7 and the water liquid ratio was 100%. From Monte Carlo simulations of the DMD principle setup, one could see that it is difficult for the DMD setup to differentiate the two NaCl solutions when the water liquid ratio is 50%. 6. Measurement strategy In order to estimate the gas fraction for each flow regime of the multiphase flow, the time resolution of the MGB measurements should be sufficiently fast to register all the changes in the flow. For slug flow the required time resolution will depend on length of the liquid slug, length of the stratified or annular gas flow, and the flow velocity. For the measurements performed here, the time resolution should be faster than 0.1 s. With a short integration time one can divide the measurements into time segments according to the flow regime. From this one can obtain running averages of the gas fraction measurements, and perform gas fraction corrections according to the observed flow regime. By including flow rate measurements one can derive an estimate of the true GVF of the flow. Salinity of the water component of the flow is expected to have slow variations compared to the flow pattern variations. With a high sampling frequency of the water-cut and MGB measurements one can select the time sections when the measurements show low gas fraction and high water liquid fraction, and perform DMD for these intervals for salinity monitoring. Also here a running average approach will improve the measurement statistics. Thus, with a high time resolution one can extract the short time intervals where the flow regime is well defined, and perform the corresponding corrections for the gas fraction measurements. The salinity measurements will be iterative and should be performed during flow patterns with low gas fraction (less than 30%) and high water volume ratios (preferably close to 100%). Also the DMD measurement principle will rely on high time resolution measurements. The liquid parameter of the three phase flow can be found based on the water cut measurements and a calculation of ITliq (see Eq. (1)) from the WLR and the calibration measurements of water filled and oil filled pipe. Another approach is an iterative solution process where one uses all the different measurement principles to find the gas fraction and the WLR of the flow. 7. Conclusions In this work a multi gamma beam and dual modality measurement setup has been tested in a multiphase flow loop for flow regime identification and salinity monitoring. For flow pattern
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reference a high-speed gamma-ray tomograph was measuring at a section 20 cm in front of the MGB and DMD measurement setup. Measurements were performed at 0°, 45°, and 90° positioned PVC pipe with inner diameter of 8 cm. The flow components were air, diesel, and tap water mixed with NaCl to a salinity of 1.9% and 4.6%. The integration time of the MGB and DMD measurements was 5 s. According to the high-speed tomograph the flow pattern variations could be one tenth of a second. The sampling frequency of the MGB measurements was thus not sufficiently fast to perform a quantitative analysis of the gas ratio in the multiphase flow. The test did show, however, that the high count rate MGB principle can be used to distinguish the flow regimes of stratified/wavy flow and annular/slug flow. The DMD principle did also show a significant difference between the two salinity mixtures when the water liquid ratio was 100% and the gas volume ratio was less than 30%. If the sampling ratios of the MGB and DMD measurements are faster and complies with Nyquist’s sampling theorem, the compact and rugged MGB and DMD measurement setup should be able to perform flow regime independent gas fraction measurements. References [1] Corneliussen S, Couput JP, Dahl E, Dykesteen E, Frøysa KE, Malde E, Moestue H, Moksnes PO, Scheers L, Tunheim H. Handbook of multiphase flow metering, rev. 2. www.nfogm.no, 2005. [2] Hjertaker BT, Tjugum SA, Hammer EA, Johansen GA. Multimodality tomography for multiphase hydrocarbon flow measurements. IEEE Sensors J 2005;5(2). [3] Kvandal HK, Kjølberg SA, Schüller RB. Water detection in gas/condensate flows by SeCaP technology. Chem Eng J 2009; doi:10.1016/j.cej.2009.11.011. [4] Schüller RB, Gundersen T, Halleraker M, Engebretsen B. Measurement of water concentration in oil/water dispersions with a circular single electrode capacitance probe. IEEE Trans Instrum Meas 2004;53(5):1378–83. [5] Liu X, Hu J, Shan F, Cai B, Su X, Chen Q. Conductance sensor for measurement of the fluid water cut and flow rate in production wells. Chem Eng J 2010;197(2). [6] Toftevåg KR. Ultrasound transit-time flowmeter. Master’s thesis. University of Bergen; 2005 [in Norwegian]. [7] Aandahl BK. Transit-time calculations in turbulence. Master’s thesis. University of Bergen; 2006 [in Norwegian]. [8] Tjugum SA, Frieling J, Johansen GA. A compact low energy multibeam gammaray densitometer for pipe-flow measurements. Nucl Instrum Methods B 2002; 197:301–9. [9] Johansen GA, Jackson P. Salinity independent measurement of gas volume fraction in oil/gas/water pipe flow. Appl Radiat Isot 2000;53:595–601. [10] Gardner RP, Bean RH, Ferrel JK. On the gamma-ray one-shot collimator measurement of two-phase flow void fractions. Nucl Appl Technol 1970;8:88–94. [11] Tjugum SA, Hjertaker BT, Johansen GA. Multiphase flow regime identification by multibeam gamma-ray densitometry. Meas Sci Technol 2002;13:1319–26. [12] Tjugum SA, Mihalca R. X-ray based densitometer for multiphase flow measurements. In: North sea flow measurement workshop 2009 — proceedings. [13] Johansen GA, Frøystein T, Hjertaker BT, Olsen Ø. A dual sensor flow imaging tomographic system. Meas Sci Technol 1996;7:297–307. [14] Brown FB, Barrett RF, Booth TE, Bull JS, Cox LJ, Forster RA, Goorley TJ, Mosteller RD, Post SE, Prael RE, Selcow EC, Sood A, Sweezy J. MCNP Version 5. Trans Am Nucl Soc 2002;87(273). [15] Krumgalz BS, Pogorelskii R, Sokolov A, Pitzer KS. Volumetric ion interaction parameters for single-solute aqueous electrolyte solutions at various temperatures. J Phys Chem Ref Data 2000;29(5). [16] Hjertaker BT, Maad R, Schuster E, Almås OA, Johansen GA. A data acquisition and control system for high-speed gamma-ray tomography. Meas Sci Technol 2008;19(094012). [17] Maad R, Johansen GA. Experimental analysis of high-speed gamma-ray tomography performance. Meas Sci Technol 2008;19(085502).