The present invention relates to a method and apparatus for measuring the fractions and water salinity of a multiphase mixture based on a measurement of the complex dielectric constant and density of the multiphase mixture. The complex dielectric constant is determined by measuring the fre¬ quency location of a phase shift between two receiving antennas and the spread of the phase shift divided by the frequency location of the phase shift. The method is par¬ ticularly suitable for high precision measurement of the volume fraction and salinity of small amounts of water dis¬ persed in a hydrocarbon continuous mixture such as measure¬ ment of water fraction and water salinity of a hydrocarbon wet gas stream (typical water fraction for a wet gas stream is in the order of 0,001 - 0,5 -% of pipeline volume) . The method is not useable for measurement of the complex dielectric constant of a highly lossy medium such as a mul¬ tiphase mixture where hydrocarbons are dispersed in water and the salinity of the water is above approximately 0,5% NaCl by weight.

A flowing fluid mixture of oil water and gas or condensate water and gas is a common occurrence in the oil industry being a product of an unprocessed well stream. Such a well stream is often referred as a multiphase flow mixture where oil, water and gas are referred to as individual phases or fractions.

When the gas fraction of the well stream is above 90% gas, the well is commonly referred to as a wetgas well. Wetgas wells may also be referred to as a gas/condensate well where the condensate is condensed liquid from the gas. The composition of a gas/condensate well could typical be 98% gas, 1,9% condensate and 0,1% liquid fresh water which have been condensed from the gas. The formation water in the hydrocarbon reservoir is typical saline water and its salinity is usually known to the operator. Under normal situations, the well should not produce any formation water. In fact, formation water in the pipeline can cause hydrate and scale formation and could also cause severe pipeline corrosion. If the amount of formation and fresh water (also referred as total water fraction) in a well is known to the field operator, chemical inhibitors can be injected into the well stream in order to limit the un¬ wanted effects due to the water or the production rate from the well can be changed in order to minimize the formation water production. It is of particular interest to measure the formation and fresh water content of remotely operated subsea wells since the cost of the pipelines in such an in¬ stallation is severe. By measuring the total water (forma- tion water plus fresh (condensed) water) fraction and water salinity, the fresh water and formation water fraction of the well can be determined since the salinity of the forma¬ tion water is known to the operator. In order to fulfill the requirements of the field operator, an instrument for measuring the total water fraction and water salinity of the wells would need to be able to perform reliable and repeatable measurements of the water fraction and water salinity with formation water fractions as small as a few per thousand of the total area in the pipeline.

Microwaves are widely used for measurement of composition and water salinity of a multiphase mixture. US 4,458,524 (1984) discloses a multiphase flow meter that measures the dielectric constant (permittivity) , density, temperature and pressure. Such device uses phase shift between two re- ceiving antennas to determine the dielectric constant. Other techniques are further known being based on resonance frequency measurement. Examples of such techniques are dis¬ closed in W03/034051 and US 6,466,035. US 5,103,181 describe a method based on measurement of constructive and destructive interference patterns in the pipe.

However, none of the above described methods are able to measure the complex dielectric constant such that the water salinity of the multiphase mixture can be determined.

It is well known that the complex dielectric constant of a media can be measured by measuring the phase shift and attenuation of an electromagnetic wave through the media. US "4,902,961 describe a method for measuring complex dielectric constant based on measurement of phase shift and power attenuation. The measurement is performed at two dif¬ ferent (fixed) frequencies, one in the X-band and the other in the S-and. Since this method rely on power measurements for measuring the complex dielectric constant, it would not have the required sensitivity and stability in order to perform accurate measurement of water fraction and water salinity of a hydrocarbon wetgas stream. NO 200 10 616 dis- closes a method for measurement of the water conductivity of the continuous phase of a multiphase mixture based on a power and phase measurement at microwave frequencies. However, this method also relies on a power measurement and the method is not able to perform accurate measurement of the complex dielectric constant of hydrocarbon continuous mixture with a dispersed water phase.

It is the purpose of this invention to provide accurate measurement of the complex dielectric constant. It is the purpose of this invention to perform accurate measurements of the dispersed water fraction and water salinity of a hydrocarbon continuous medium. It is the purpose of this invention to perform accurate measurements of the oil, gas and water fraction and water salinity of a multiphase mixture containing small amounts of water and large amounts of gas which is typical for hydrocarbon wet gas streams.

The method according to the present invention compromises the following steps:

a. electromagnetic phase measurements are performed between two receiving antennas located at a dif¬ ferent distance from a sending antenna,

b. based on an empirically determined constant (s) and the above measurements, the effective and imaginary dielectric constants are determined,

c. the mixture density is determined,

d. the temperature and pressure are determined

e. based on the knowledge of densities, effective dielectric constants and imaginary dielectric constants of the components of the fluid mixture and the result from the above steps a-d, the vol¬ ume fractions of the gas and liquid or liquids of the fluid mixture and salinity of water are cal¬ culated.

The apparatus according to the invention is further charac¬ terized by the features as defined in the independent claim

Dependent claims 2 - 7 and 9 - 12 define preferred embodi¬ ments of the invention.

The invention will be further described in the following with reference to the figures, where:

Fig. 1 shows a schematic longitudinal sectional view of an exemplified embodiment of a meter for measuring the compo¬ sition and water salinity according to the invention,

Fig. 2 shows a schematic cross sectional view along the line III-III in figure 1,

Fig. 3 shows a graph of the real and imaginary part of the dielectric constant for water, Fig. 4 shows a graph of the imaginary part of the dielec¬ tric constant for water normalized to the values for fresh water,

Fig. 5 shows a graph of the phase measurements in fresh and saline water,

Fig. 6 shows a graph of the measured and theoretical effec¬ tive and imaginary dielectric constant for water,

Fig. 7 shows a graph of the phase measurements of a typical wet gas stream,

Fig. 8 shows a graph of the measured water fraction of a typical wet gas stream,

Fig. 9 shows a graph of the measured s-factor vs. the imaginary dielectric constant for water,

Fig. 10 shows a graph of the measured conductivity vs. the theoretical conductivity of the water fraction, and

Fig. 11 shows a graph of the measured salinity vs. the theoretical salinity of the water fraction.

The exemplifying composition meter according to the inven¬ tion shown in figure 1 includes three main elements as follows:

1) The effective dielectric constant, as defined be¬ low, of the multiphase mixture is derived by per¬ forming phase measurements over a wide frequency band. A RF signal is transmitted from a sending antenna 3 (probe) and received at two receiving antenna 4 located in a pipe 1 wherein a mixture of oil, water and gas is flowing. The pipe and antenna arrangement of figure 1 may also be referred to as a sensor. The frequency of the RF signal is typically varied from 10 Mhz to 3.500 Mhz. For pipes or venturi throats with an inner diameter below 50 mm, the upper frequency may be as high as 10.000 Mhz. By recording the frequency at several predetermined phase differences and using a calibration constant for the system, the effective dielectric constant within the pipe can be measured.

2) The imaginary part of the dielectric constant of the multiphase mixture is derived by measuring a parameter called the S-factor (shape factor) of the phase difference vs. frequency. The S-factor is here defined as the measured frequency differ¬ ence at a upper and lower predetermined phase dif- ference divided by the measured frequency at a predetermined phase difference in between the upper and lower phase difference. By using a cali¬ bration constant for the system, the imaginary part of the dielectric constant of the multiphase mixture can be derived.

3) Based on a determination of temperature, pressure and density and knowledge of the effective and imaginary dielectric constant and density of oil, gas and water, the fraction of oil, gas and water and the salinity of the water can be derived.

The effective dielectric constant is defined as:

%=V(*')2+(*")2~

Where:

ε1 : Real part of dielectric constant ε ' ' : Imaginary part of dielectric constant

Instruments for measuring temperature, pressure and density can be included in the composition meter for determining the temperature, pressure and density. Alternatively, the temperature, pressure and density can obtained from other sources such as temperature and pressure transmitters located upstream or downstream the composition meter and transferred electronically to the composition meter. The density of the flowing fluid through the composition meter can also be determined using PVT (Pressure, Volume and Tem¬ perature) algorithms. Based on the composition of the hydrocarbons and the measured water fraction from the com¬ position meter, the multiphase mixture density can then be calculated in an iterative process based on equations of state for hydrocarbons as a function of temperature and pressure and the measured water fraction.

Disadvantages with the existing solutions.

It is well known that the complex dielectric constant of a media can be measured by measuring the phase shift and attenuation of an electromagnetic wave through the media. The main disadvantage of such solutions for measurement of complex dielectric constant is that they have limited accu¬ racy and are unable to sense small variations since they rely on an accurate power or loss measurement. Accurate power and loss measurements at microwave frequencies are difficult to perform partly due to impedance mismatch, which is very common for any microwave based industrial device for measuring dielectric constant, and partly due to limitations of the electronics itself. Consequently, the limitations of the measurement electronics and standing waves due to impedance mismatches makes it difficult to obtain the required accuracy, repeatability and sensitivity for accurate composition and water salinity measurement for wetgas wells and crude oil wells with high gas frac¬ tions where it is critical to perform accurate measurements of small fluctuations in the water fraction and salinity.

Uniqueness of the present invention.

The uniqueness of the invention is the ability to provide accurate, repeatable measurements of the complex dielectric constant and its ability to sense small variations in the complex dielectric constant without the need to perform any power and/or loss measurements. Instead the complex dielec¬ tric constant is measured based on a differential measure¬ ment of phase and measurement of frequency where highly accurate measurements can be performed since the phase is far less affected by impedance miss matches compared to power/loss measurements. Also, any discrepancies in the sensor, cable and electronic measurement paths can easily be removed by using the same physical length of both paths. Hence, the present invention is far less affected by meas¬ urement distortions related to power variations in the fre¬ quency spectrum and standing waves (rippel) in the measure¬ ment electronics sensor arrangement compared to techniques based on measurement of electrical power and/or electrical loss. -

Detailed description of the innovation

The lowest cut-off frequency of a circular wave guide is TEi1 at:

Equation 1:

where:

fc Cut-off frequency

r Radius of pipe

ε Effective Dielectric constant (permittivity) inside the wave guide (pipe)

μ Permeability inside the wave guide (pipe) Below the cut-off frequency, the electric field will propa¬ gate according to plane wave theory. When the field in the pipe 1 changes from plane wave propagation into TEi1, a step occurs in the phase difference of the receiving probes 4 of figure 1. By applying a frequency sweep on the trans¬ mitter 3 and measuring the frequency location of the phase shift 25, the cutoff frequency fc Equation 1 can be rear¬ ranged as:

Equation 2

where:

0.293 k2 = Nt*

Frequency of electromagnetic wave (cut-off frequency of TEn)

Effective dielectric constant in¬ side the pipe

hence Y.2 can be determined by measuring the frequency fc with a known dielectric constant inside the pipe.

It is well known that .the real and imaginary parts of the dielectric constant can be described as:

Equation 3:

ε=ε'-jε"

where:

Complex dielectric constant

Real part of the complex dielectric constant

Imaginary part of the complex dielectric constant

For air, gas, oil and condensate, the imaginary part of the dielectric constant is for all practical purposes zero. For water, the complex dielectric constant can be described by a single Debye relaxation law where:

Equation 4 :

εs - ε∞ water S water ~ ^∞ + ' a> ^- -J' i+O— \ ωεn )\-η co,.

where :

εwater = Complex dielectric constant of water

ε∞ = Dielectric constant at infinite frequencies

εs = Static dielectric constant

ω = Frequency

ωr = Debye relaxation frequency

η = Cole-Cole spread factor

o~water = Conductivity of water

ε0 = Boltzmann' s constant

According to J. B. Hasted, Aqueous Dielectrics (1973) , page 29, the dielectric constant of water may also be described as: Equation 5 :

p — p — i( pU -L Water \ ° water as J \ dielectric ' ωεn J

where:

εwater = Complex dielectric constant of water

εs = Static dielectric constant

ε"dieiectrio = Imaginary part of dielectric constant for fresh water

σwater = Conductivity of water

ω = Frequency

ε0 = Boltzmann' s constant

Measurements and equations of the static dielectric con¬ stant of water and the imaginary part of the dielectric constant for fresh water are well described in the litera- ture. Some examples are Malmberg and Maryott (1956) , Res. Nat. Bur Standards, 56, 1 (static dielectric constant), Akelδf and Oshry(1950) Am. Chem Soc, 12 284 (static dielectric constant), Barthel (1991), Phys. Chem, 95, p853 (imaginary dielectric constant for fresh water) and Meissner and Wentz, Report from Boeing/AER inverstigation for CMIS (imaginary dielectric constant for fresh water) .

Figure 3 shows a graph the real 6 and imaginary 7 part of the dielectric constant of fresh water and the imaginary part of saline water 8, 9, 10, 11, 12 at 25 0C with a sa- unity of 0,5%, 1.0%, 1,5% and 2,0% NaCl by weight respec¬ tively. Figure 4 shows the same data for the imaginary part of saline water 13, 14, 15, 16, 17 with a salinity of 0,5%, 1,0%, 1,5% and 2,0 % NaCl by weight respectively, normalized to the value of the imaginary part of the dielectric constant for fresh water. Figure 5 shows the phase difference 18, 19, 20, 21, 22 when the sensor of figure 1 is filled with 100% water with a sa¬ linity of 0,0%, 0,05%, 0,10%, 0,15% and 0,20% NaCl by weight respectively. Figure 5 shows the corresponding meas- urement of the effective dielectric constant 23 and imagi¬ nary part 24 of the dielectric constant vs. the theoretical values.

Figure 7 shows the phase measurement at a typical wetgas situation. Initial, the sensor is empty with a response as shown in 25. When 0.09% fresh water is injected into the sensor, the phase difference curve 26 shits approximately 4,5 Mhz to the left. Furthermore, as the salinity of the water is increased to 0,5%, 1,0%, 1,5% and 2,0% NaCl by weight respectively, the slope of the phase curve 27, 28, 29, 30 is reduced as seen in figure 7.

In order to measure the composition of oil, water and gas (%oil, %water & %gas) , the following equations can be used based on a measurement of the mixture dielectric constant £mix and the mixture density ρmix:

Equation 6:

Φoil + Φwater+ Φgas = l

where:

ΦOii = Cross sectional volume fraction of oil (or condensate) Φwater = Cross sectional volume fraction of water

Φgas = Cross sectional volume fraction of gas Equation 7 :

Φoil X Poil + Φwater X Pwater +Φgas X Pgas = Pmix

where:

Poii = Density of oil (or condensate)

Pwater = Density of water

Pgas = Density of gas

Pmix = Measured density

A temperature and pressure measurement is also required in order to compensate the above density parameters for tem- perature and pressure variations but, for simplicity, these will be ignored for the following discussions of the meas¬ urement principle.

The Bruggeman-Hanai mixing equation relates the dielectric constant of a two component mixture to the volume fractions of the components. If the two component mixture is droplets as an inner phase dispersed in a continuous media of an outer phase, the equation become:

Equation 8 :

_ 1 Yn J Φ inner + Ψo

where:

Sinner = Dielectric constant of the inner phase (dispersed phase)

εouter = Dielectric constant of the outer phase (continuous phase)

εmix = Measured dielectric constant of the mixture

Φinner = Volume fraction of inner phase (dis¬ persed phase)

Φouter = Volume fraction of outer phase (con- tinuous phase)

A temperature and pressure measurement is also required in order to compensate the above dielectric constant parame¬ ters for temperature and pressure variations but, for sim¬ plicity, these will be ignored for the following discus- sions of the measurement principle.

The equation above can also be used for a three-phase mix¬ ture such as oil, water and gas in which the inner phase is a well mixed combination of two of the phases dispersed in an outer phase. E.g., an inner oil/water mixture may be dispersed in an outer continuous media of gas and simi¬ larly, gas bubbles may be dispersed in an outer continuous media of an oil/water mixture.

Figure 8 shows the measured water fraction of the raw meas¬ urements of figure 7. The dielectric constant is calculated based on equation 1 and 2 and the volume fraction of the water fraction is calculated based on equation 8.

The imaginary part of the dielectric constant of the multi¬ phase mixture is derived by measuring a parameter called the S-factor (shape factor) of the phase difference vs. frequency. The S-factor is here defined as the measured frequency difference at a upper and lower predetermined phase difference divided by the measured frequency at a predetermined phase difference in between the upper and lower phase difference.

Figure 9 shows the measured normalized S-factor of the data of figure 6 vs. the normalized imaginary part of the dielectric constant for water. The data is normalized to the corresponding values for fresh water. By applying an experimental derived correction factor to the normalized S- factor measurement, the conductivity, and thereby salinity, of the water fraction can be measured. One way to experi- mentally obtain the correction factor above is to circulate gas or air with a known water content and known water sa¬ linity through the sensor and record the corresponding S- factor measurements. By adjusting the amount of gas, water and water salinity of the fluid mixture circulating through the sensor, the correction factor (s) can be derived for a wide operational range of the composition meter.

Figure 10 shows the measured water conductivity of the water fraction vs. the reference conductivity and figure 11 shows the measured water salinity vs. the reference salin- ity of the raw data measurement of figure 7.

Since the salinity of the formation water is known to the field operator, the formation water fraction and fresh water fraction can be calculated based on the above meas¬ urements of the total water fraction and water fraction salinity.