The present invention relates to a method and apparatus for measuring the water conductivity and water volume fraction of a multiphase mixture in a pipe. The invention is based on a measurement of the complex dielectric constant of the multiphase mixture. The complex dielectric constant is determined by measuring the wave phase constant β of a plane electromagnetic wave propagating near the inside wall of the pipe. The measurement of β is based on a measurement of the phase difference between two receiving antennas inside the pipe located at different distances from a third transmitting antenna. The phase measurement is performed at least two frequencies in the range between 10 Mhz and 10 Ghz.
The method and apparatus is particularly suitable for high precision measurement of the water conductivity and water volume fraction of a flowing fluid of hydrocarbons dispersed in a continuous water phase. E.g, the invention can be used to measure the volume fraction and water conductivity of a two phase oil water mixture flowing in a pipe where the oil is dispersed in a continuous water phase. Another example is to determine the water conductivity of the water phase of a multiphase mixture containing oil, gas and water, where the oil and gas is dispersed in a continuous water phase.
A flowing 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 to as a multiphase mixture where oil, water and gas are referred to as individual phases or fractions. The conductivity of the water phase, often referred to as the salinity of the water, can affect the measurement accuracy of measurement systems used to measure characteristics of tie mixture, such as a multiphase flow meter for measurement of oil, water and gas volume and/or mass flow rates. It is therefore common for such instruments to use the conductivity or salinity of the water phase as a calibration/configuration parameter. In many cases the water conductivity is not known. E.g. the water in the flow line may be based on the commingled production of many wells with different water salinities or a well with multiple production zones where each zone has a different water salinity. In other cases sea-water, fresh water or steam may be injected into the reservoir in order to boost the production of the wells., which also has the effect of altering the salt content of the water in the reservoir and wells over time. Under such circumstances, it is often difficult or almost impossible to obtain a reliable value of the water conductivity over time. For subsea (seabed) installations it is difficult and costly to obtain a sample from the relevant location in order to analyze the conductivity of the water phase.
A multiphase flow meter utilizing the present invention is capable of measuring the conductivity and salinity of the water fraction and hence providing reliable measurements of the oil, water and gas flow rates even for wells or commingled flow lines with changing water salinity over time.
The present invention can also be used to determine the water volume fraction and water conductivity or salinity of a two phase oil water mixture where water is the continuous phase. This is a flow condition commonly occurring in hydrocarbon process plants and oil transportation pipe lines. An apparatus for measurement of the water volume fraction of an oil/water mixture is frequently referred to as a water analyzer or watercut meter. Many such instruments rely on the water conductivity as a calibration constant. In many applications the water conductivity is not known since the water in the flow line may be based on the commingled production of many wells having different water salinity. As a consequence, the water volume fraction measurement may be incorrect. A water analyzer based on the present invention will provide reliable measurements of the water volume fraction irrespectively of any change in the water conductivity.
Microwaves are widely used for measurement of composition and water salinity of a multiphase mixture. U.S. Pat. No. 4,458,524 (1984) discloses a multiphase flow meter that measures the dielectric constant (dielectric constant), density, temperature and pressure. Such device uses phase shift between two receiving antennas to determine the dielectric constant. Other techniques are further known being based on resonance frequency measurement. Examples of such techniques are disclosed in WO3/034051 and U.S. Pat. No. 6,466,035. U.S. Pat. No. 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. U.S. Pat. No. 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 different (fixed) frequencies, one in the X-band and the other in the S-band. Other examples can be found in NO 200 10 616 which discloses 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, U.S. Pat. No. 5,341,100 describing a method and apparatus for measurement of fluid conductivity and hydrocarbon volume based on a measurement of phase shift and attenuation (power) of an electromagnetic wave and U.S. Pat. No. 5,107,219 describing a method and apparatus for measurement of the conductance of a fluid based on measurement of microwave energy (power/loss) and phase difference.
The main disadvantages of the above methods and apparatuses are limited accuracy and ability 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 make it difficult to obtain the required accuracy, repeatability and sensitivity for accurate fraction and/or water salinity/conductivity measurements.
NO 20043470 describes a method an apparatus for determining water salinity based on phase measurement(s) only. However, this method can not be used for a flowing fluid of hydrocarbons dispersed in a continuous phase of saline water.
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