Method and Apparatus for Net Oil Measurement

The present invention relates to a method for determining the amount of oil in an oil/water mixture.

The accuracy of net oil measurement is extremely important to buyers and sellers of oil. If the oil contains water, the buyer does not want to pay for the gross amount of liquid shipped to him. Rather, he is to pay only for the net amount of oil present in the total volume delivered. Net oil measurement is also required in oil fields for royalty payments and in enhanced oil recovery fields for pumping rate control.

There are in the prior art a number of instruments which have been used to measure water content in an oil/water mixture. Most of such instruments in the prior art rely on the difference between the dielectric constant of water and the dielectric constant of oil. As such, the main problem with these devices is their inability to deal with mixtures where the water constituent of the mixture is in the continuous phase rather than the oil. By definition, the dielectric constant is the ratio of the capacitance of a capacitor field with a given dielectric to that of the same capacitor having only a vacuum as the dielectric. Therefore, in using the devices for oil/water measurement, when water is the continuous phase the instrument will peg at full scale because the electric path between the two parallel plates of the capacitor will be shorted by the water in continuous phase. This is so even though oil may still comprise some 40 to 50 percent or more of the overall mixture.

A relatively typical capacitance probe for use in determining oil/water ratios is found in U.S. Pat. No. 3,200,312 to Callahan. Callahan relies on the measurement of the mixture's dielectric constant. As such, the probe must be non-functional when water is in the continuous phase.

Yet another capacitance-type probe is taught in U.S. Pat. No. 3,025,464 to Bond. The Bond probe is designed specifically for pipeline use where there is typically low water content and oil is in the continuous phase. For that purpose, the Bond probe will function adequately. However, because the Bond probe is a capacitance probe, it will not function in mixtures where water becomes the continuous phase.

Still another prior art capacitance probe is shown in U.S. Pat. No. 3,523,245 to Love et al. It has the same shortcomings as the prior art references mentioned above. In fact, FIG. 2 of the Love et al patent depicts a graph for water fraction versus probe capacitance. It is noted that the water fraction portion of the graph does not go above 0.5. In fact, the Love et al reference in discussing FIG. 2 specifically states that when the water fractions get above 0.5, the water tends to separate out and the capacitance quickly approaches the value at free water.

U.S. Pat. No. 3,368,147 to Graham teaches a capacitance measuring circuit to determine the sediment and water content of oil well production. Because Graham relies on capacitance, such reference is also insufficient to determine oil/water ratios where water is in the continuous phase.

U.S. Pat. No. 3,550,019 to Thomas seems to teach a linearizing circuit for net oil analyzers. However, Thomas does not teach the use of a digital linearizer or any means of overcoming the jump in the electrical characteristic of the mixture as the oil/water mixture moves from oil being in the continuous phase to water being in the continuous phase.

Yet another net oil computer is described in U.S. Pat. No. 3,385,108 to Rosso. Rosso relies on a capacitance probe and does not teach the use of a digital linearizer or any means of overcoming the jump in the electrical characteristic of the mixture when the oil/water mixture goes from oil being in the continuous phase to water being in the continuous phase.

Another typical capacitance probe having the same inadequacies as those probes mentioned above is found in U.S. Pat. No. 3,006,189 to Warren et al.

It is therefore an object of the present invention to provide an apparatus and method for determining the percentage of water present in a mixture where either oil or water is in the continuous phase.

Yet another object of the present invention is to provide an apparatus and method for determining the percentage of water in an oil/water mixture where the amount of water present is in the range from 0 to 100 percent.

Still a further object of the present invention is to provide a method and apparatus for determining the water content in oil/water mixtures by measurement of the mixtures' electrical properties wherein the output signal is linearized.

Still another object of the present invention is to teach a method and apparatus for determining whether oil or water is in the continuous phase by measuring an electrical property of the mixture.

Yet a further object of the present invention is to provide a method and apparatus for selecting one of two curves which represent the possible conditions of the media.

It is a further object of the present invention to provide an apparatus and method for determining the percentages of oil and water in an oil/water mixture by measuring its electrical properties wherein it is determined whether the oil or the water is in the continuous phase.

Yet another object of the present invention is to provide two empirically or theoretically derived curves which plot percentage of water versus current which allows determination of whether the oil or the water is in the continuous phase.

Briefly stated, the foregoing and numerous other features, objects and advantages of the present invention will become readily apparent upon reading the detailed description, claims and drawings set forth hereinafter. These features, objects and advantages are accomplished by utilizing a microprocessor or comparator circuit which is able to distinguish between oil being in the continuous phase and water being in the continuous phase based on the electrical properties of the mixture. A few techniques are available to measure the electrical properties of the mixture. For example, the conductivity of the media may be measured at a high frequency. While these techniques avoid the saturation effect which is typical of measuring capacitance, they produce two distinct, non-linear curves of output signal typically plotting current versus the percentage of water in the mixture. These curves may be empirically or theoretically derived. The first of these curves is for the case where the water is in the continuous phase, while the second curve occurs where oil is the continuous phase. It should be understood that the change in phase does not occur at a predetermined oil/water ratio. Other variables are involved, including droplet size, surface tension and emulsifying chemicals present. Typically however, the change occurs when the amount of water present in the mixture is in the range of 35 to 75 percent of the total. Thus, just measuring the energy absorption properties of the mixture is not the complete solution. Because there are two distinct curves or equations it is necessary to determine which curve or equation is to be used in calculating the percentage of water present.

As noted earlier, the step jump occurs in the data when the mixture changes over from oil being in the continuous phase to water being in the continuous phase. It is very desirable to eliminate this step jump from the data and to linearize the two distinct curves.

It should be apparent that the step change represents a rapid jump in the mixture's conductivity. This change in conductivity is measured by a conductivity meter or energy absorption detector usually in units of milliamps of output. This information is fed to a comparator to select one of two memories, those being where water is in the continuous phase and where oil is in the continuous phase. Generally, it has been determined that if the oil/water monitor in a particular configuraiton measures a current of say, less than 5 milliamps, then oil is in the continuous phase and if a current greater than 5 milliamps is measured then water is in the continuous phase. The linearized output from the selected memory is fed to an output stage, display or multiplier. The multiplier is used to determine the net water by multiplying the gross flow rate by the percentage of water present. The difference between the gross flow rate and the net water equals the net oil present.

FIG. 1 is an elevational view of a probe and oil/water monitor as used in conjunction with the present invention.

FIG. 2 is a schematic diagram of the flow of data through the apparatus of the present invention.

FIG. 3 is a graph of the two empirically derived curves plotting current absorbed or admittance versus percentage of water.

FIG. 4 is a schematic of a circuit for an oil/water monitor.

Turning first to FIG. 1 and there is shown a probe 10 mounted within a conduit 12. Energy is transmitted into the medium from oil/water monitor 14 through probe 10. In such manner, oil/water monitor 14 can measure the electrical properties of the media flowing through conduit 12. Typically, this could be performed by measuring the conductivity, energy absorption, capacitance, admittance and/or impedance of the media with oil/water monitor 14. As used herein the term "electrical properties" includes all of such terms singly or in combination.

One such oil/water monitor 14, which can be used with the present invention, is the Agar OW-101 water in oil monitor.

The Agar OW-101 measures the energy absorption properties of the oil/water mixture, rather than just the capacitance. It is used in conjunction with an empirically generated curve plotting milliamps versus percentage of water. The curve contains a pronounced step jump as the mixture goes from oil being in the continuous phase to water being in the continuous phase. Because the location of the step is affected by a number of variables, it can be difficult to determine precisely what pecentage of water is present.

Another device which may be used for oil/water monitor 14 is the Invalco Model No. CX-745-200GP.

U.S. Pat. No. 4,503,383 to Agar et al teaches another device which can be used in the capacity of oil/water monitor 14.

Still another device which can be used as oil/water monitor 14 is shown schematically in FIG. 4. It includes an alternating current generator 15, a capacitor 17 and an ammeter 19. The capacitor 17 should be in the form of a probe inserted into the fluid. The ammeter 19 measures current [I]so that when water is in the continuous phase the circuit can be defined by the equation:

which is Ohm's Law, where I is the current through the ammeter 19, V is the voltage of the generator 15, and R is the effective resistance of the media.

When oil is in the continuous phase the circuit can be defined by the equation:

where "j" is the square root of -1, "w" represents the radial frequency and "c" represents capacitance of the probe with the mixture inside it. Thus there can be theoretically derived two distinct curves or equations representing some electrical property plotted against the percentage of water present. It is known that the effective capacitance of a parallel plates capacitor is given by the equation:

where "C" is effective capacitance, "K" is a dimensional constant, "E" is the dielectric constant of the media between the plates, "A" is the area of the plates and "D" is the distance between the plates. It is further known that the effective resistance of a media contained between two plates is given by the equation:

where "R" is the effective resistance, "D" is the distance between the plates, "A" is the area of the plates and "G" is the conductivity of the media. Because both the dielectric constant and the conductivity of the media are proportional to the percentage of water present in media, the derivation of two distinct equations is possible. However, the dielectric constant and conductivity of the media depend not only on the percentages of water and oil present, but also on which constituent is in the continuous phase. As mentioned earlier, the constituent which is in the continuous phase is affected by a number of other variables. Therefore, it is probably simpler to use the empirically generated curves shown in FIG. 3.

The current or electrical signal generated in oil/water monitor 14 is transmitted to a zero to span adjuster 16 which allows the device of the present invention to be calibrated. From the zero to span adjuster 16 the data is transmitted to an analog to digital converter 18 and to a comparator 20. The comparator 20 uses this information to select one of two memories. There is a continuous water phase memory 22 and a continuous oil phase memory 24.

The continuous water phase memory 22 and the continuous oil phase memory 24 represent curves 23 and 25 respectively as shown in FIG. 3. These curves can be arrived at empirically, or theoretically. Curves 23 and 25 plot some electrical signal versus percentage of water present in the mixture. The electrical signal may be in the form of a measurement of current, voltage, frequency, energy, conductivity, capacitance, admittance, impedance or the like. It should be recognized that curves 23 and 25 represent two separate and distinct equations. Note that curves 23 and 25 have been projected past the points where they intersect the step jump 27 as shown by the dashed lines. The comparator 20 is, in actuality, a microprocessor or other computing device which compares the measured electrical signal shown in FIG. 3 as current with a preset value, say 5 milliamps. If the measured current is greater than the preset value, then water is in the continuous phase and the comparator 20 selects water continuous phase memory 22 with data plotted as the upper milliamperage curve 23. If the measured current is less than the preset value, then the oil is in the continuous phase and the comparator 20 selects oil continuous phase memory 24 containing data plotted as the lower milliamperage curve 25.

The data transmitted from the oil/water monitor 14 provides the comparator 20 with the amount of current measured so that the comparator 20 can compare that value to the preset value.

Depending on which continuous memory 22 or 24 is selected, the data is transmitted from analog to digital converter 18 to that particular phase memory 22 or 24 where the amount of current is used to determine the percentage of water present by way of curve 23 or curve 25. The digitized data representing the percentage of water present is then transmitted to multiplier 26 and simultaneously, to a digital to analog converter 28. The data from the digital to analog converter 28 is then transmitted to a meter 30 where the percent of water can be directly read.

The flow rate of the oil/water mixture flowing through conduit 12 is measured by flow meter 32. Flow meter 32 is preferably a positive displacement type flow meter or of some other type of high accuracy type flow meter. Output from flow meter 32 is transmitted simultaneously to multiplier 26, subtractor 34 and gross flow totalizer 36. Gross flow totalizer 36 keeps a running tabulation of the total volume pumped through conduit 12. The gross flow data transmitted from flow meter 32 to multiplier 26 is multiplied by the percentage of water data transmitted to multiplier 26 from memories 22 and 24. The data is then transmitted from multiplier 26 simultaneously to net water totalizer 38 and to subtractor 34. Net water totalizer 38 keeps a running tabulation of the total amount of water which has been pumped through conduit 12. Within subtractor 34, the total water volume is subtracted from the gross flow with the result being transmitted to the net oil totalizer 40. Net oil totalizer 40 keeps a running tabulation of the total volume of oil which has been pumped through conduit 12.

FIG. 3 is a graph depicting a somewhat typical step jump 27 between the two non-linear curves 23 and 25 generated when oil/water ratios are determined by measuring the electrical properties of the mixture. It is highly desirable to eliminate the step jump from the data. It is also highly desirable to linearize the data. The present invention accomplishes these goals through the use of comparator 20, and memories 22 and 24, and analog to digital converter 18. Further, by relying on other electrical properties ties of the media such as energy absorption, rather than the dielectric constant alone, the present invention allows measurement of the ratio of oil to water regardless of which component is in the continuous phase up to and including the situation where there is no true mixture and 100 percent of the volume is water.

For purposes of clarification, the component in the continuous phase can be defined as that liquid which contains and surrounds the droplets of the second liquid such that the second liquid is present within the first liquid in the form of individual, discreet units.

It should be understood that, the comparator circuit of the present invention is useful in evaluating data in any two curve situation where automatic identification of the proper curve is necessary. An example would be a conventional turbine flow meter. The comparator circuit, taught herein, could be used to distinguish between laminar and turbulent flow, and correct the "meter factor" accordingly.

From the foregoing, it will be seen that this invention is one well adapted to obtain all of the ends and objects hereinabove set forth, together with other advantages which are apparent and which are inherent to the apparatus.

It will be understood that certain features and subcombinations are of utility and may be employed with reference to other features and subcombinations. This is contemplated by and is within the scope of the claims.

As many possible embodiments may be made of the invention without departing from the scope thereof, it is to be understood that all matter herein set forth and shown in the accompanying drawings is to be interpreted as illustrative and not in a limiting sense.