Dual frequency microwave water cut monitoring means and method

Abstract: The percentage water content by volume in a petroleum stream is determined. A source (3) supplies microwave energy at two frequencies. An antenna (12) transmits a selected supply into the stream (17) and receives test energy either by reflection or transmission. A detector (32) provides a test energy intensity signal, difference means (9,34,40,44) provides a signal representative of the phase difference between the supplied and the test energy, and a sensor (52) provides a signal representative of the temperature of the stream. An indicator (50) determines the water percentage in accordance with the intensity, phase difference and temperature signals. The use of two frequencies for measurement in turn provides two phase difference signals to permit resolution of any ambiguity for high dielectric streams or for large diameter pipelines where the difference may be the correct phase shift plus a multiple of 360°.

Description: The present invention relates to apparatus and methods for determining the water cut, i.e. the percentage water content by volume, in a petroleum stream. EP-A-0384593 describes apparatus for determining the percentage water content by volume in a petroleum stream, comprising: a source for supplying microwave energy; antenna means for transmitting microwave energy supplied by said source into the stream and for receiving test microwave energy that has either been reflected back from the stream or has transmitted through the stream; a detector responsive to the test microwave energy to provide an intensity signal representative of the intensity of the test microwave energy; phase difference means responsive to the supplied microwave energy and to the test microwave energy, said phase difference means comprising a voltage controlled phase shifter for shifting the phase of the supplied microwave energy in accordance with a phase shift signal to provide reference microwave energy, and difference means responsive to the test microwave energy and the reference microwave energy to provide said phase shift signal as the phase difference between said test and reference energies until there is substantially a 90° said phase difference, whereupon said phase shifter provides an enabling signal; a temperature sensor to provide a temperature signal representative of the temperature of the petroleum stream; and indicator means connected to the detector, the phase shifter, the phase difference means and the temperature sensor to provide an indication of the percentage water content in accordance with said intensity signal, said phase shift signal and said temperature signal. The present invention is characterized in that said source is adapted to supply microwave energy at first and second different frequencies; and said indicator means includes a stored table of phase shifts at each of said first and second frequencies whereby to permit resolution of ambiguity where the phase difference signal may represent a correct phase shift plus an integral multiple of 360°. US-A-4812739 describes the use of first and second microwave beams differing in frequency, reception location or both, with the particular aim of detecting the passage of large slugs of water. The antenna means may include an antenna adapted to transmit a selected said supply of microwave energy into the stream and to receive test microwave energy that has been reflected back from the stream, and there is also provided a circulator connected to said source and to said antenna to direct energy from the source to the antenna and to direct energy from the antenna to the detector. In another embodiment the antenna means includes an antenna adapted to receive test microwave energy that has propagated through the stream and to pass the test microwave energy to the detector. An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawing which is a simplified block diagram of a microwave water cut monitor according to the invention. The water cut monitor shown in the drawing includes a microwave source 3 for supplying microwave energy at two microwave frequencies. Preferred frequencies of 10.119 GHz and 10.369 GHz are used, although the true criteria is that there be a substantial difference between the two frequencies. Source 3 is low powered and may use a microwave gun source. Source 3 provides the microwave energies to switch means 4 via microwave conductors 5 and 6. Switch means 4 is controlled by a signal E1 to pass the microwave energy from either conductor 5 or 6 and provide it to a directional coupler 7. Directional coupler 7 provides the selected microwave energy to a circulator 8 and to a conventional voltage controlled phase shifter 9. All conductance or carrying of microwave energy is accomplished by using conventional waveguides. Circulator 8 provides microwave energy to an antenna 12. Antenna 12 provides the microwave energy through a window 14, which may be made of a conductive ceramic or Teflon, to a petroleum stream comprising at least oil and water, passing thro a pipe 17. Pipe 17 may be a portion of a pipeline having windows 14 or it may be made of the "window" material. The microwave energy provided by antenna 12 passes through the petroleum stream and another window 14 and is received by an antenna 20. Antenna 20 provides the received microwave energy to a switch means 24 which in turn provides the received microwave as test microwave energy to a directional coupler 28, as hereinafter explained. Directional coupler 28 provides the test microwave energy to a detector 32 and to a mixer 34. Detector 32 provides a signal E2 corresponding to the intensity of the microwave energy received by antenna 20. The petroleum stream also reflects some of the microwave energy back to antenna 12 which passes back through antenna 12 to circulator 8. Circulator 8 blocks the reflected microwave energy from feeding back to source 3 and provides the reflected microwave energy to switch means 24. Reflected microwave energy becomes more important as the distance between antennas 12 and 20 increases. This is especially true where a large pipeline carrying the petroleum stream is being monitored. A positive direct current voltage +V is provided to a switch 36 which is connected to switch means 24. With switch 36 open, switch means 24 provides microwave energy from antenna 20 as test microwave energy. When switch 36 is closed, the reflected microwave energy from circulator 8 is provided by switch means 24 as the test microwave energy. The microwave energy from voltage controlled phase shifter 9, hereinafter called the reference microwave energy, and the test microwave energy from directional coupler 28, are provided to mixer 34 which mixes them to provide two electrical signals E3, E4, representative of the phases of the reference microwave energy and the test microwave energy, respectively. A differential amplifier 40 provides an output signal E0 in accordance with the difference between signals E3 and E4. Signal E0 is a function of the phase difference between the reference microwave energy and the test microwave energy and is provided to a feedback network 44. Feedback network 44 provides a signal C to voltage control phase shifter 9, controlling the phase of the reference microwave energy, and to a mini-computer means 50. Signal E0, and hence the signal C, decreases in amplitude until there is substantially 90° phase difference between the reference microwave energy and the test microwave energy. Voltage control phase shifter 9 indicates the amount of phase shift required to eliminate the phase difference. Signal E2 from detector 32 is also provided to computer means 50 which contains within it memory means having data related to temperature and phase and amplitude for various percentages of water cuts that could be encountered in the production stream. It has been discovered that the phase difference for measurements in a fluid stream may exceed 360 degrees under certain circumstances. These circumstances include cases where the dielectric of the stream is large, for example when the percentage of water in the petroleum is large, and when the emulsion is water continuous and in cases where the distance between antennas is large as in the case of using larger pipe 17 of Figure 1. In those cases the true phase shift may be the measured phase shift plus some integer multiple of 360 degrees. The present invention resolves this ambiguity by monitoring the petroleum at two substantially different frequencies, the main frequency f 1 , and a secondary frequency, f 2 , and using the difference in measured phase shift, (phase 1 - phase 2) at the two frequencies to determine the correct integer multiplier to use when computing the true phase shift. The correct integer is chosen from a table created from knowledge of the frequencies involved. The maximum possible size of the integer that may be resolved is limited by the separation of frequencies f 1 and f 2 . In the present case integer size of up to 40 can be resolved. Reduction of frequency separation would increase the maximum integer size further limited only by resolution of the phase shift measurement. A temperature sensor 52 sensing the temperature of the petroleum stream in pipe 17 and provides a signal T to computer means 50 representative of the sensed temperature. Phase Shifter 9 also provides an enable signal to computer means 50 allowing computer means 50 to utilize signals T, C and E2. Computer means 50 also provides signal E1 to switch means 4 so that computer means 50 can correlate signal E2 to a particular frequency. Internally computer means 50 uses the phase shift signal C, signal T and the two amplitude levels of signal E2 to address computer means 50 memory means to select the proper water cut values. Computer means 50 provides signals, corresponding to the selected water cut value, to readout means 54 which may be either digital display means or record means or a combination of the two.

Claims: Apparatus for determining the percentage water content by volume in a petroleum stream, comprising: a source (3) for supplying microwave energy; antenna means (12,20) for transmitting microwave energy supplied by said source into the stream and for receiving test microwave energy that has either been reflected back from the stream or has transmitted through the stream; a detector (32) responsive to the test microwave energy to provide an intensity signal (E2) representative of the intensity of the test microwave energy; phase difference means (9,34,40,44) responsive to the supplied microwave energy and to the test microwave energy, said phase difference means (9,34,40,44) comprising a voltage controlled phase shifter (9) for shifting the phase of the supplied microwave energy in accordance with a phase shift signal (C) to provide reference microwave energy, and difference means (34,40,44) responsive to the test microwave energy and the reference microwave energy to provide said phase shift signal as the phase difference between said test and reference energies until there is substantially a 90° said phase difference, whereupon said phase shifter provides an enabling signal; a temperature sensor (52) to provide a temperature signal (T) representative of the temperature of the petroleum stream; and indicator means (50) connected to the detector (32), the phase shifter (9), the phase difference means (9,34,40,44) and the temperature sensor (52) to provide an indication of the percentage water content in accordance with said intensity signal (E2), said phase shift signal (C) and said temperature signal (T); characterized in that: said source (3) is adapted to supply microwave energy at first and second different frequencies; and said indicator means (50) includes a stored table of phase shifts at each of said first and second frequencies whereby to permit resolution of ambiguity where the phase difference signal may represent a correct phase shift plus an integral multiple of 360°. Apparatus according to Claim 1 characterized in that said difference means (34,40,44) comprises a mixer (34) responsive to the test and reference energies to provide electrical signals (E3,E4) representative of the phases of the test and reference energies respectively, a differential amplifier (40) connected to the mixer to provide an output signal representative of the difference between said two electrical signals, and a feedback network (44) connected to the differential amplifier to provide said phase difference signal in accordance with said output signal. Apparatus according to Claim 1 or Claim 2 characterized in that said antenna means includes an antenna (12) adapted to transmit a selected said supply of microwave energy into the stream and to receive test microwave energy that has been reflected back from the stream, and a circulator (8) connected to said source (3) and to said antenna (12) to direct energy from the source to the antenna (12) and to direct energy from the antenna to the detector (32). Apparatus according to any one of Claims 1 to 3 characterized in that said antenna means includes an antenna (20) adapted to receive test microwave energy that has propagated through the stream and to pass the test microwave energy to the detector (32). Apparatus according to Claim 4 characterized by switch means (24) operable to select test microwave energy from said circulator (8) or from said antenna (20) for passing to said detector (32). Apparatus according to any one of Claims 1 to 5 characterized by a switch (4) operable to select a supply of microwave energy at said first or said second frequency for supply to said antenna means (12,20).

Inventors: DURRETT MICHAEL GREGORY, HATTON GREGORY JOHN, HELMS DAVID ALBERT, DURRETT, MICHAEL GREGORY, HATTON, GREGORY JOHN, HELMS, DAVID ALBERT
Applicants: TEXACO DEVELOPMENT CORP, TEXACO DEVELOPMENT CORPORATION
Application year: 1991
Main classification: G01N22
IPC codes: E21B47, G01N22