Patent Description:
The present disclosure relates to systems and methods for determining the oil/water content of oil-water mixtures. This is done by using a dielectric sensor or an eddy current sensor.

According to the U. Department of Energy, <NUM> million gallons (<NUM> million liters) of petroleum are spilled into U. waters from vessels and pipelines in a typical year. A major oil spill could easily double that amount. Generally, mechanical surface skimmers remove oil and oil-water mixtures from surface water.

Present systems for measuring oil content in oil-water mixtures encounter difficulties when the oil-water mixture has high electrical conductivity, as can occur with seawater or any highly saline water found in industrial or oil and gas applications. It would be desirable to provide systems and methods that address these difficulties.

Document <CIT> discloses an apparatus for measuring an amount of oil in a flow of fluid, the apparatus comprising: a housing defining an interior passage configured to pass a flow of fluid therethrough; a capacitance sensor configured to respond to a capacitance of the flow of fluid, the capacitance sensor being disposed within the interior passage of the housing and coupled to conductors; and a conductance sensor configured to measure conductance of the flow of fluid, the conductance sensor being disposed on the interior passage of the housing and being further configured to generate a conductance signal.

The present disclosure provides systems and methods for measuring the oil/water content of oil-water mixtures, regardless of the salinity of the water in the mixture. Briefly, two different sensors are used, a dielectric sensor and an eddy current sensor. The dielectric sensor is used if the oil content is above a threshold value, which may be the transition point between oil-in-water and water-in-oil mixtures or emulsions, and the eddy current sensor is used if the oil content is below the threshold value. Computer programs and systems for determining which sensor measurement is more accurate are also described herein.

The subject matter of the invention is defined in the independent claims <NUM> and <NUM>.

The threshold may be an oil content of <NUM> percent. The threshold may alternatively be an oil content corresponding to a resonant frequency that allows for distinction between oil-in-water mixtures and water-in-oil mixtures. In some embodiments, the threshold is set based on the oil content (desired for the particular application). In some implementations, the operating frequency of the second SWR analyzer is selected based on: (i) a radius of a pipe of the sensor cavity, and (ii) a conductivity of the fluid. The second inductor may be a coil wound around a pipe of the sensor cavity. The second inductor is not in direct physical contact with the fluid passing through the sensor cavity.

The one or more processors are further configured to make the determination of whether the oil content is above or below the threshold by using the dielectric sensor. In some implementations, the eddy current sensor is configured to account for water salinity by calibration of the eddy current sensor performed with water having a given salinity. In some implementations, the eddy current sensor is configured to account for water salinity by manual entry of a salinity value. In some implementations, the first inductor, the first capacitor, and the sensor cavity are all connected in parallel.

In accordance with another aspect of the present invention, a method for measuring the oil content of a fluid in a cavity is disclosed. The method includes: using a dielectric sensor, measuring the oil content of the fluid; and using one or more processors to determine whether the oil content is below a threshold; and in response to the determination that the oil content is below the threshold, using an eddy current sensor to measure and report the oil content of the fluid.

The methods may further include: using the one or more processors, determining that the oil content is above the threshold; and in response to the determination that the oil content is above the threshold, using a dielectric sensor to measure and report the oil content of the fluid. In some implementations, the threshold is an oil content of <NUM> percent. In some implementations, the threshold is an oil content that allows for distinction between oil-in-water and water-in-oil mixtures. In some implementations, the eddy current sensor comprises: a resonance circuit formed by a capacitor, an inductor and the cavity, wherein the inductor is configured to expose the fluid of the sensor cavity to a magnetic field; and a standing wave ratio (SWR) analyzer configured to measure a height of a peak of a resonance frequency of the resonance circuit. The inductor can be in the form of a coil wound around a pipe of the sensor cavity.

In some implementations, the determination that the oil content is below the threshold is made based on a measurement from a dielectric sensor. In some implementations, the methods further include accounting for water salinity by calibrating the eddy current sensor using water of a given salinity. In some implementations, the method further includes accounting for water salinity by manually entering a salinity value into the eddy current sensor or the processor(s).

Advantageously, the systems and methods described herein have the ability to accurately monitor and report oil-water percentages from <NUM> to <NUM>% regardless of water salinity. By way of comparison, dielectric sensors alone cannot work with high electrical conductivity mixtures, for instance seawater-based mixtures that contain more than ~<NUM>% of sea water. The disclosed approaches work with water of any salinity that is relatively stable (i.e. does not change quickly over time). This includes fresh water, and seawater from different seas, even if very concentrated.

Advantageously, in the approaches described herein, the response is independent of oil-water dispersion, including mixtures that are not homogeneous, homogenous, or an emulsion. In particular, the sensor system can provide accurate results whether the oil-water mixture is an oil-in-water mixture or a water-in-oil mixture.

The sensing system can also operate with an open pipe of any size serving as the sensor cavity. In addition, there is no need to install a flow conditioning device upstream or downstream of the sensor(s). For instance, there is no need for a homogenizer, which generally are not a good option for oil skimming operations as they become quickly clogged during operations and increase pressure drop.

The sensor system measurement can also be carried out at any pressure. The sensor system can also reliably detect the sensor cavity being empty. Metal electrodes do not make physical contact with the oil-water mixture to be tested Advantageously, the sensor uses very minimal power. The measurement can be realized with as little as <NUM> mW of power. The sensor system can be certified as meeting ATEX Level <NUM> criteria (for use in potentially explosive atmospheres).

These and other non-limiting aspects of the present disclosure are described in more detail below.

The following is a brief description of the drawings, which are presented for the purposes of illustrating the exemplary embodiments disclosed herein and not for the purposes of limiting the same.

The present disclosure may be understood more readily by reference to the following detailed description of desired embodiments and the examples included therein. In the following specification and the claims which follow, reference will be made to a number of terms which shall be defined to have the following meanings.

Although specific terms are used in the following description for the sake of clarity, these terms are intended to refer only to the particular structure of the embodiments selected for illustration in the drawings, and are not intended to define or limit the scope of the disclosure. In the drawings and the following description below, it is to be understood that like numeric designations refer to components of like function.

The term "comprising" is used herein as requiring the presence of the named components/steps and allowing the presence of other components/steps. The term "comprising" should be construed to include the term "consisting of", which allows the presence of only the named components/steps.

Numerical values should be understood to include numerical values which are the same when reduced to the same number of significant figures and numerical values which differ from the stated value by less than the experimental error of conventional measurement technique of the type described in the present application to determine the value.

All ranges disclosed herein are inclusive of the recited endpoint and independently combinable (for example, the range of "from <NUM> grams to <NUM> grams" is inclusive of the endpoints, <NUM> grams and <NUM> grams, and all the intermediate values). The endpoints of the ranges and any values disclosed herein are not limited to the precise range or value; they are sufficiently imprecise to include values approximating these ranges and/or values.

The modifier "about" used in connection with a quantity is inclusive of the stated value and has the meaning dictated by the context. When used in the context of a range, the modifier "about" should also be considered as disclosing the range defined by the absolute values of the two endpoints. For example, the range of "from about <NUM> to about <NUM>" also discloses the range "from <NUM> to <NUM>. " The term "about" may refer to plus or minus <NUM>% of the indicated number. For example, "about <NUM>%" may indicate a range of <NUM>% to <NUM>%, and "about <NUM>" may mean from <NUM>-<NUM>.

The present application is related to oil-water mixtures. It should be understood that the oil content plus the water content generally equals <NUM>% (solutes and other materials in the fluid not being considered).

A dielectric sensor may be used to measure water content in oil-water mixtures, gas-water mixtures, and moisture levels in solids. The principle of measurement is related to the large value of water's relative dielectric constant (permittivity), which is about <NUM>. This value is much greater than the dielectric constant of gases (close to <NUM>), organic liquids (like oils and crudes) as well as solids (below about <NUM>). Dielectric sensors are generally constructed as capacitors that contain a cavity that is filled with the oil-water mixture. The cavity can be either a flow through device (e.g. a tube) or a batch device (e.g. a vessel or tank). The dielectric sensor detects changes in electrical capacitance caused by different water content of the fluid in the cavity. Such changes are detected by a direct capacitance measurement, or often by a detection of a frequency shift of a resonance circuit which includes the cavity itself.

However, the dielectric measurement is not reliable when the mixture being measured is highly electrically conductive, for instance seawater or any highly saline water found in industrial or oil and gas applications. The high conductivity of the mixture can be represented in the dielectric sensor as a low resistance connected in parallel with the cavity's capacitance which effectively shortens the cavity. This effect cannot be solved by modifying the shape or size of the cavity in which the fluid / mixture is measured, since the relative contribution of the cavity's capacitance and resistance is geometry-independent. Theoretically, the relative resistance contribution can be reduced by increasing the frequency of the dielectric measurement, since the capacitance contribution is increased at higher frequencies, while the resistive contribution remains constant. However, a frequency increase causes reduction of skin depth, which determines the extent of penetration of electromagnetic waves into the tested mixture. This decrease in skin depth penetration makes a sensor sensitive only to areas close to its electrodes, which may be a small fraction of overall sensor volume, especially for larger sensors. Due to the high salinity of typical seawater, the conductivity effect is severe enough to prevent successful development of dielectric sensors that by themselves are effective in marine environments. In the case of oil-water sensors operating with seawater, dielectric sensors are capable of measuring oil content only for low conductivity, water-in-oil mixtures / emulsions that contain a minimum of <NUM>-<NUM>% oil.

The systems and methods described herein relate to sensor configurations capable of measuring the oil or water content of oil-water mixtures. The measurements are accurate even in cases where the water contains large amounts of salt, for example seawater. Very generally, measurement are made using: (<NUM>) a dielectric measurement, and (<NUM>) an eddy current loss measurement. The dielectric measurement may be used for high oil content mixtures/emulsions that are of the water-in-oil type, and therefore have relatively low electrical conductivity. For this type of mixture/emulsion, the dielectric sensor provides a reliable measurement for oil content. On the other hand, the eddy current measurement is used for low oil content mixtures/emulsions that are of the oil-in-water type and have high electrical conductivity. For this type of mixture/emulsion, the eddy current measurement provides a reliable measurement for oil content. The measurements can be compared to a reference table that accounts for oil content and salinity, etc. Using both sensors in the implementations as described herein allows for unambiguous and accurate measurement of oil or water content for a broad range of oil-water mixtures including mixtures of crude oils and/or mixtures with saline water.

The systems and methods described herein provide a reliable oil content measurement for a broad range of mixtures ranging from including pure saline water to pure oil. In one aspect, the device works at different water salinities and for different oil and crude types, and is insensitive to oil-water dispersion state. The sensors can be used in any application where oil-water mixtures need to be evaluated for oil content or water cut. One application of the sensor systems described herein is for offshore applications, for instance for evaluation of efficiency of oil recovery during spill clean-up operations.

The sensors can be operated at relatively high operating frequencies, from <NUM> up to <NUM>, which minimizes concerns related to electrode polarization. The proposed sensors also provide a relatively uniform sensitivity across a pipe cross section, which allows for use of open pipes with a broad diameter range, without any mixing or homogenizing devices while still obtaining accurate measurements.

<FIG> presents a design of a dielectric sensor <NUM>, which is used in some embodiments of the present disclosure. The sensor is installed on a cavity <NUM> that contains the fluid whose oil/water content will be measured. The cavity itself can be considered part of the sensor system. The cavity is illustrated here as a pipe or tube through which fluid can flow. The cavity can be made of any suitable material, for example acrylic or PVC. The cavity may have any desired diameter. In experiments reported further herein, the cavity has a diameter of <NUM> inches. A pair of electrodes <NUM> is attached to the cavity <NUM>. The electrodes are made of a suitable metal. The electrodes are attached to the exterior of the cavity <NUM>. Together, the cavity and electrodes act as a capacitor, whose capacitance will change depending on the fluid in the cavity that is being measured.

The dielectric sensor <NUM> also includes a first capacitor <NUM> and a first inductor <NUM>. The first capacitor may have a capacitance of about <NUM> picofarads (pF) to about <NUM> pF. The first inductor may have an inductance of about <NUM> microhenrys (µH) to about <NUM>µH. The electrodes <NUM>, the first capacitor <NUM>, and the first inductor <NUM> are connected to each other in parallel, i.e. in a parallel circuit. Together, these components form a first resonance circuit <NUM>.

A secondary pickup coil <NUM> is installed proximate the first inductor <NUM>, and connected to a first radio frequency Standing Wave Ratio (SWR) analyzer <NUM>. The first SWR analyzer is used to identify the peak resonance frequency of the first resonance circuit. Examples of suitable SWR analyzers include the AA-<NUM> and AA-<NUM>, both produced by Rig Expert. If desired, a different device or method can be employed to measure the resonance peak. For instance, a frequency counter can be used to determine the resonance frequency. The distance between the first inductor and the secondary pickup coil can be adjusted to optimize the sharpness of the resonance peak. The SWR analyzer will scan an operating frequency range, desirably near the expected resonance frequency. The resonance frequency, defined by a maximum peak, is recorded for the oil-water mixture.

The sensor systems of the present disclosure also include an eddy current sensor. An eddy current measurement is effective for measuring the oil/water content in oil-in-water mixtures, while free of the electrical conductivity measurement drawbacks present in the dielectric sensor. The eddy current effect is observed in all conductive materials that are exposed to changing magnetic fields. Eddy currents are electrical currents that cause two effects: (<NUM>) they have an orientation and intensity that tends to cancel the external magnetic field that generates them, and (<NUM>) they cause energy losses due to heat generation in the conductive media. In fact, the eddy currents are responsible for the finite skin depth penetration of electromagnetic waves in conductive media. Energy losses due to eddy currents allow for measurement of this effect by means of a resonance circuit. If the inductor generating the eddy current effect is a part of the resonance circuit, eddy current energy losses in the tested material will cause losses in the resonance circuit. These losses will cause broadening and height reduction of a resonance peak, both easily measurable effects.

<FIG> illustrates an eddy current sensor <NUM>. A second inductor <NUM> is attached to the cavity <NUM> in a manner that exposes the oil-water mixture within the cavity to a magnetic field. Desirably, the second inductor <NUM> is in the form of a coil wound around the cavity <NUM>, as this ensures the most uniform sensitivity across the entire cross section of the cavity. The coils of the second inductor are on the exterior of the cavity, and are not in direct electrical contact with the tested fluid within the cavity, since this would introduce direct conductivity effects and result in additional measuring problems. The coils of the inductors described in the present disclosure can be made from any suitable conductive metal, copper being the most suitable.

The second inductor <NUM> is connected to a second capacitor <NUM> and forms a second resonance circuit <NUM>. The second inductor and the second capacitor are arranged in series with each other. The second capacitor <NUM> may have a capacitance of about <NUM> to about <NUM> pF, including from about <NUM> pF to about <NUM> pF. The second inductor may have an inductance of about <NUM> microhenrys (µH) to about <NUM>µH.

A secondary pickup coil <NUM> is installed proximate the second inductor <NUM>, and connected to a second radio frequency Standing Wave Ratio (SWR) analyzer <NUM>. This configuration for the eddy current sensor has a resonance frequency of about <NUM> megahertz (MHz). The second SWR analyzer is used to identify the magnitude of the resonance peak, which correlates with the oil/water content of the fluid within the cavity. Again, examples of suitable SWR analyzers include the AA-<NUM> and AA-<NUM>, both produced by Rig Expert. The distance between the second inductor and the secondary pickup coil can be adjusted to optimize their coupling and the sharpness of the resonance peak.

It has been found that the measurement of the dielectric sensor is most accurate for oil-water mixtures having an oil content above a threshold value, while the measurement of the eddy current sensor is most accurate for oil-water mixtures having an oil content below the threshold value. In one embodiment, the threshold value can be an oil content of <NUM> percent. In other embodiments, the threshold value can be an oil content corresponding to a resonance frequency that allows for distinction between oil-in-water mixtures and water-in-oil mixtures (this resonance frequency may vary depending on the values of the capacitor and inductor used in the dielectric sensor). Thus, a system using both types of sensors is expected to be most accurate over the entire range of possible oil/water values. The measurements made by the respective sensor can be compared to reference tables to determine the oil content.

A system using both types of measurements can be built with the cavity in the form of a single pipe or tube through which the oil-water mixture flows, with the two sensors being mounted on the single pipe and spaced apart from each other. The two measurements (by the dielectric sensor and the eddy current sensor) can be performed simultaneously or in short succession. It is expected that two simultaneous and continuous measurements will be possible with proper selection of operating frequencies that do not overlap, including harmonics overlap. Both measurements require very small power; the SWR analyzer used in both sensors has an output power of -<NUM> dBm, which is equivalent to <NUM> mW.

<FIG> schematically illustrates an algorithm that can be used to report an accurate measurement of the oil/water content. The first part <NUM> of the algorithm identifies what type of mixture or emulsion is filling the sensor cavity. This can be determined by the resonance frequency obtained by the dielectric sensor. For instance, an oil-in-water emulsion is identified if the resonance frequency is less than a given value (which is affected by the capacitor and inductor values), and a water-in-oil emulsion is identified if the resonance frequency is above this value. This determines whether the reported oil/water content is based on the measurement from the dielectric sensor or the eddy current sensor. If the oil-water mixture is a water-in-oil emulsion, the measurement from the dielectric sensor <NUM> is reported. If the oil-water mixture is an oil-in-water emulsion, the measurement from the eddy current sensor <NUM> is reported.

<FIG> is a block diagram illustrating an exemplary embodiment of a sensor system of the present disclosure. The sensor system <NUM> includes components that are proximate the sensor cavity <NUM>, electronics <NUM> for processing the measurements made by the sensors, and external controls <NUM>. The components proximate the sensor cavity <NUM> include the dielectric sensor <NUM>, the eddy current detector <NUM>, and a temperature sensor <NUM> that is used to account for changes in water conductivity due to temperature. As described above, the components of the dielectric sensor <NUM> and the eddy current detector <NUM> are external to the sensor cavity, and do not need to contact the oil-water mixture that is present within the sensor cavity. The electronics <NUM> include the first SWR analyzer <NUM> for the dielectric sensor <NUM> and the second SWR analyzer <NUM> for the eddy current detector <NUM>. A single board computer <NUM> is used to control the SWR analyzers, carry out algorithm calculations, and handle input and output operations. For example, the single board computer can compare the sensor measurements to reference tables or databases that identify the oil / water content based on the measurements made by the dielectric sensor and/or the eddy current sensor.

The SWR analyzer(s) can scan a preselected frequency range anywhere from a fraction of a megahertz up to tens, or for some units, hundreds of megahertz. SWR analyzers are available as handheld instruments with a simple keyboard and a display. Selected SWR analyzers are intended to be imbedded in larger instruments, and are constructed as a single electronic board without peripherals. These SWR analyzers use serial or USB ports for external communication and control. The output power of SWR analyzers is very small, on the order of milliwatts, which is sufficient for the present application.

In some embodiments, two RigExpert AA-<NUM> ZERO single-board analyzers are used for the sensors. A BeagleBone Black open-source single-board computer can be used to control the SWR analyzers, carry out algorithm calculations, and handle input and output operations. The BeagleBone Black provides direct and simultaneous control of two AA-<NUM> analyzers plus one temperature sensor, on-board storage of oil fraction data, and hosting of a website used for user interface.

For the dielectric sensor, operating frequencies below <NUM> should be avoided because of electrode and oil-water interface polarization effects. Generally, any operating frequency in the range of about <NUM> to about <NUM> can be used. Other operating frequency ranges include about <NUM> to about <NUM>, and about <NUM> to about <NUM>. Skin depth consideration does not significantly affect the dielectric measurement, due to very low conductivities of water-in-oil mixtures.

For the eddy current sensor, operating frequencies below <NUM> should also be avoided because of electrode and oil-water interface polarization effects. The operating frequency should be selected to provide effective dynamic range of the eddy current measurement. Two frequency selection methods can be used. First, the operating frequency is selected to be the maximum frequency, which allows for reliable eddy current height measurement for pure water with maximum salinity expected for a given sensor and electronic implementation. Second, the operating frequency is selected to provide a pure water skin depth penetration close to a radius of the sensor cavity. This can be determined according to the following Equation (<NUM>): <MAT> where R is the radius of the sensor cavity in cm, and σ is the conductivity of the fluid in S/m. For instance, for seawater with conductivity of <NUM>/m and a pipe that is <NUM> inches in diameter, the operating frequency of the order of <NUM> may be appropriate. In some particular embodiments, the operating frequency for the eddy current sensor is from about <NUM> to about <NUM>.

Overlap between the operating frequencies of the dielectric sensor and the eddy current sensor, including their harmonics, should be avoided to prevent signal interference.

The physics of the eddy current effect can be affected by the salinity of the water. Thus, various embodiments described herein account for water salinity. This can be realized, for example, by calibration of the sensor system performed with pure water having a given salinity, or by directly entering the salinity value into the sensor. This permits the appropriate reference tables / databases to be used to identify the oil / water content based on the sensor measurements.

If there are no significant interactions between the oil and water, the electrical conductivity of the oil-water mixture (CMixture) can be expressed as the product of the pure saline water electrical conductivity (CWater) and a geometrical factor (g), which accounts for presence of oil in the water, as shown in Equation (<NUM>) below.

The pure saline water conductivity (CWater) is a function of water salinity and temperature, and its value can be calculated using published correlations. The geometrical factor (g) is a function of oil volume fraction, and oil-water dispersion. Experiments carried out with different oil types, various oil-in-water mixtures and emulsions, and frequencies in megahertz range (<NUM>-<NUM>) indicate that equation <NUM> above can be approximately solved by the following Equation (<NUM>). <MAT> where A is a constant, and is between <NUM>% and <NUM>%. The mixture conductivity (CMixture), measured by the eddy current sensor, is a monotonic function of the eddy peak height, which can be determined by suitable sensor calibration procedure. Importantly, the oil fraction measurement based on Equation (<NUM>) is not sensitive to the homogeneity of the oil-water dispersion, and can be used with both coarse mixtures and stable emulsions.

In some embodiments, the sensor systems described herein are specifically designed to meet the needs of the oil recovery industry. Currently, there is no way for ships of opportunity to know the ratio of oil to water in skimming operations. The disclosed sensor systems of this present disclosure will solve that problem. The approaches described herein save operators money by reducing storage of seawater and increasing space for recovered oil. Furthermore, the approaches described herein will allow for fewer trips between recovery vessels and storage barges, and will further result in reduced cost for waste processing and filtering of seawater. These sensor systems will help improve efficiency and reduce the amount of seawater that must be processed, thereby reducing operating costs.

The systems and methods described herein may also be used in oil processing and as an oil cut sensor or water cut sensor to detect when wells are producing an unexpected amount of ground water.

The systems described herein have the ability to monitor oil-water percentages from <NUM>% to <NUM>% regardless of water salinity. In addition, the sensitivity of the sensor system is very uniform across the sensor cavity, making the sensor system independent of the oil-water dispersion, including mixtures that are not homogeneous, homogenous, or in an emulsion state.

The techniques described herein are suitably implemented in the form of one or more electronic processors executing instructions read from a non-transitory storage medium such as a hard drive or other magnetic storage medium, an optical disk or other optical storage medium, a cloud-based storage medium such as a RAID disk array, flash memory or other non-volatile electronic storage medium, or so forth. Some embodiments also include computers connected via an electronic network (e.g. WiFi, Ethernet, Internet, various combinations thereof, or so forth) to form a parallel computing resource, ad hoc cloud computing resource, or so forth.

The following examples are provided to illustrate the systems and methods of the present disclosure. The examples are merely illustrative and are not intended to limit the disclosure to the materials, conditions, or process parameters set forth therein.

Two sections of <NUM>-inch schedule <NUM> acrylic pipes were connected to form a closed loop fluid recirculation system able to pump mixtures of oil and saline water, and served as the sensor cavity. One of the pipe sections was used as the sensing cavity. A propeller powered by a variable speed motor was inserted into the second pipe section and used to force flow through the sensor cavity. Two metal electrodes were attached to the outside of the sensory cavity pipe section on opposite sides of the pipe. These electrodes were connected with a capacitor (e.g., <NUM>-<NUM> pF) and an inductor (e.g., several turns, <NUM>-inch diameter) to form a dielectric sensor as illustrated in <FIG>.

<FIG> shows a typical response of the dielectric sensor for several mixtures of HYDROCAL <NUM> refined oil and <NUM> wt% solutions of Red Sea salt (mostly NaCl with some minerals characteristic of a marine environment), where oil+water equaled <NUM>%. Each oil content mixture was tested at four values of mixer speed from <NUM> to <NUM>,<NUM> revolutions per minute (rpm). The minimum <NUM> rpm speed was chosen because it provided a uniform oil-water mixing without formation of small droplets or emulsion. The <NUM>,<NUM>-rpm speed generated stable emulsions especially for oil content over <NUM>%. The operating frequency was between <NUM> and <NUM>.

As it is clearly visible in <FIG>, the resonance frequency versus oil content relation has two distinctive regimes. For mixtures above <NUM>% oil content, the frequency is almost a linear function of oil content, as expected from the dielectric sensor principles. For these mixtures, the relation can be easily inversed thus allowing for calculation of oil content based on a measured resonance frequency. Importantly, for this part of the plot, the resonance frequency does not strongly depend on the mixer speed indicating low sensitivity to oil-water dispersion.

In contrast, mixtures with oil content below <NUM>% generate very similar resonance frequency for all values of oil content with the exemption of data obtained at the lowest mixer speed of <NUM> rpm. This part of the frequency versus oil content cannot be reversed, meaning the oil content cannot be measured by the dielectric method for mixtures of this type. The response of the dielectric measurement shown in <FIG> does not appreciably improve with frequency. This was demonstrated in equivalent tests carried out at <NUM> and <NUM> that produced very similar results.

The two distinctive parts of data presented in <FIG> correlated with emulsion types observed during experiments. Oil-in-water mixtures or emulsions have much lower viscosity as compared to water-in-oil mixtures or emulsion. This difference of viscosity can be easily determined visually during experiments. All mixtures with oil content below <NUM>% were low viscosity, therefore oil-in-water type. In contrast, all mixtures with oil content above <NUM>% were viscous and of the water-in-oil type. Mixtures with <NUM>% oil content can be of both types depending on how this oil content was achieved. Different oils behave differently when dispersed in water and the transition point between the two types of emulsions may be different, although it will generally occur between <NUM>-<NUM>% oil content. Other factors like temperature, presence of dispersing additives or impurities, mixing conditions, and mixture history may also affect the transition point between the two mixture types. Finally, the transition is known to have significant hysteresis effect and not be very reproducible.

The eddy current measurement was tested for the same range of oil-water mixtures as used in the dielectric measurements. The main quantity recorded during the eddy currents test was the height of a resonance peak. The resonance frequency was also recorded, however, it remained constant (<NUM>) for all mixtures as well as for an empty pipe measurement. <FIG> shows the response of the eddy current sensor to oil mixtures with different oil content and mixer speeds.

The response of the eddy currents sensor was measured as the height of the resonance peak. The peak height increased monotonically with oil content up to <NUM>% and was independent of the mixer speed. The peak height dependence on oil content was quite strong, considering that the decibel scale is logarithmic. Together, <FIG> and <FIG> indicate that the dielectric and the eddy current measurements are complementary and, if used in conjunction in one sensor system, can provide a reliable oil content measurement for the entire range of oil-water mixtures.

It should be noted that the resonance frequency of the dielectric sensor (<FIG>) changed from about <NUM> for the oil-in-water mixtures to about <NUM> for the water-in-oil mixtures. Thus, for example, resonance frequency values between <NUM> and <NUM> could be used by the sensor system to determine whether to report the dielectric sensor measurement or the eddy current sensor measurement (see <FIG>).

One important feature of the eddy current measurement is its dependence on water salinity. This is demonstrated in <FIG>, which shows the eddy current signals obtained at <NUM> and <NUM> wt% water salinities.

The effectiveness of combining a dielectric sensor with an eddy current sensor was tested for several types of oil, both refined and crude, and for salinity levels <NUM>%, <NUM>%, and <NUM>%. <FIG> shows the comparison between true oil fraction and the oil fraction obtained from the combined sensor system. The agreement was excellent across the entire range of oil-water mixtures. Average error of measurement was below <NUM>%, and the maximum error was below <NUM>% (both measured in units of oil fraction).

In addition, some embodiments, which are based on a combination of one dielectric sensor and one eddy current sensor, provide an accurate oil fraction information if the sensor cavity is fully filled with liquid oil-water mixture with no or only a minimum amount of air. However, if the tested stream contains a significant volume of air, or other gas or vapor, the sensor will interpret the gas volume as oil and effectively overestimate the oil fraction. This is particularly limiting for many types of oil recovery operations that produce streams containing a significant fraction of air and result in a "partially empty" condition in pipes and hoses used in these operations. A typical practice of oil recovery operations involves long and horizontal hoses or pipes and flow velocities that are too small to prevent flow stratification due to gravity. These conditions result in a stratified flow with oil-water mixture occupying the bottom part of the hose and air present on top.

An effective method to mitigate this adverse effect for sensors with streams containing air is to use a U-trap arrangement <NUM>, as shown in <FIG>. In this arrangement, the sensors <NUM>, <NUM> are installed below the inlet <NUM> and outlet <NUM> of the U-trap arrangement <NUM>, ensuring its cavity remains predominantly filled with liquids. In such an arrangement, passing air pockets are quickly displaced by buoyancy and spend relatively shorter time inside of the sensors <NUM>, <NUM>. This ensures that the effects of air are reduced, and the measured oil fraction is closer to its true value.

Embodiments relating to the U-trap method can be further improved if a second connection <NUM> (e.g. an air passage) is installed at the top of the main U-trap <NUM> to allow for air passage. This connection should be of smaller cross section since air has much lower viscosity as compared to oil or water. A smaller diameter connection will ensure relatively free flow of air but will restrict flow of liquids. For instance, for a <NUM>- or <NUM>-inch diameter sensor, and a ½ inch diameter air transfer connection may be used.

The example shown in <FIG> shows the U-trap design with the sensor mounted horizontally. However, this is not required. Any sensor orientation, including vertical, can be used. Still, the sensor mounted horizontally offers some benefits including: the overall height of the U-trap is minimized; the U-trap is less likely to trap solid impurities that may be present in the oil-water stream; and oil-water separation due to buoyancy is minimized.

Claim 1:
A method for measuring oil content of a fluid in a sensor cavity (<NUM>, <NUM>), comprising:
using a dielectric sensor (<NUM>, <NUM>) comprising (i) a first resonance circuit (<NUM>) formed by a first capacitor (<NUM>), a first inductor (<NUM>), and a pair of electrodes (<NUM>) attached to the sensor cavity (<NUM>, <NUM>), and (ii) a device (<NUM>, <NUM>) configured to measure a resonance frequency of the first resonance circuit, measuring the oil content of the fluid in the sensor cavity;
using one or more processors (<NUM>), determining whether the oil content is above or below a threshold; and
in response to the determination that the oil content is below the threshold, using an eddy current sensor (<NUM>, <NUM>), comprising (i) a second resonance circuit (<NUM>) formed by a second capacitor (<NUM>), and a second inductor (<NUM>) configured to produce a magnetic field within the sensor cavity (<NUM>, <NUM>), and (ii) a device (<NUM>, <NUM>) configured to measure a resonance frequency of the second resonance circuit, measuring the oil content of the fluid;
wherein the components of the dielectric sensor (<NUM>, <NUM>) and the eddy current sensor (<NUM>, <NUM>) are external to the sensor cavity (<NUM>, <NUM>).