Abstract:
A crude oil production stream analyzer includes a device which measures the dielectric constant of the crude oil production stream and provides a signal corresponding thereto. A densitometer provides a density signal representative of the density of the crude oil production stream. At least one constituent of the crude oil production stream is determined in accordance with the dielectric constant signal and the density signal and the temperature of the crude oil production stream which is sensed by a temperature sensor.

Description:
BACKGROUND OF THE INVENTION 
     FIELD OF THE INVENTION 
     The present invention relates to oil industry monitors in general and, more particularly, to water-in-crude monitors. 
     SUMMARY OF THE INVENTION 
     A crude oil production stream analyzer includes devices, apparatus and a sensor which measures the dielectric constant, the density and the temperature of the crude oil production stream, respectively. The quantity of at least one constituent of the crude oil production stream is determined in accordance with the measured dielectric constant, density and temperature. 
     The objects and advantages of the invention will appear more fully hereinafter from a consideration of the detailed description which follows, taken together with the accompanying drawings wherein two embodiments of the invention are illustrated by way of example. It is to be expressly understood, however, that the drawings are for illustration purposes only and are not to be construed as defining the limits of the invention. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIGS. 1, 2, 3 and 4 are plots of &#34;relative dielectric constant&#34; versus &#34;Δθ phase difference,&#34; &#34;dielectric constant&#34; versus &#34;V g  &#34; fractional volume of gas, &#34;phase difference&#34; versus &#34;V g  &#34; and &#34;εw&#34; dielectric constant of water versus &#34;temperature,&#34; respectively. 
     FIG. 5 is a simplified block diagram of a water-in-crude monitor constructed in accordance with the present invention. 
     FIG. 6 is a detailed block diagram of the monitor means shown in FIG. 5. 
    
    
     DESCRIPTION OF THE INVENTION 
     THEORY 
     Production streams in the recovery of crude oil from an earth formation generally are not 100% crude oil but contain in various degrees water (or brine) or gas. The present invention has discovered a relationship among characteristics of a crude oil production stream that allows the determination of the quantities of those production stream constituents. 
     Theoretical calculations using plane wave theory indicate that the dielectric constant of a material may be measured by measuring the phase difference between the signals received at two separated points from a common source. This phase shift is representative of the wave travel time between the two points. Mathematically, ##EQU1## where d is the distance between the points, λ air  is the wavelength in air and ε is the dielectric constant of the material. Rearranging, 
     
         ε=[(Δθ)(λ.sub.air)/((d)(360))].sup.2. (2) 
    
     The phase difference measurement for a given dielectric constant is shown in FIG. 1. This chart is constructed based on plane wave theory, for a frequency of 200 MHz and a spacing of 4 inches between coils in a device of the present invention as hereinafter described. 
     The influence of gas and water on the dielectric constant is shown in FIG. 2. This again is based on theoretical plane wave theory. Also, the assumption has been generally made that the oil is in the continuous phase. 
     Inserting the data of FIG. 2 into the system measurement characteristics of FIG. 1 yields the curve shown in FIG. 3. FIG. 4 shows the effect of temperature on the dielectric constant of water. 
     FIG. 1 is a plot of the relative dielectric constant vs. phase difference in degrees while FIG. 2 is a plot of the following equation 
     
         ln ε.sub.m =V.sub.o ln ε.sub.o +V.sub.w ln ε.sub.w (3) 
    
     where V o  is the volume fraction of oil, V w  is the volume fraction of water, ε m  is the measured dielectric constant of the production stream, ε o  is the relative dielectric constant of oil and ε w  is the relative dielectric constant of water. 
     Relating the dielectric constant to the volume fraction of gas, FIG. 3 is a plot of phase difference in degrees vs. volume fraction of gas. 
     The determination of V g  involves the following relationship 
     
         V.sub.g =1-V.sub.L.                                        (4) 
    
     The density, which is related to the constituent volumes of the production stream as hereinafter described, is determined with a gamma ray density gauge which irradiates the production stream with gamma radiation. The gamma radiation, after passing through the production stream, is detected and a count rate C is developed. The count rate C is related to the measured density ρ m  in the following equation 
     
         ρ.sub.m =K ln (C.sub.0 /C)                             (5) 
    
     where K and C 0  are calibration constants which are predetermined. Density ρ m  is related to the densities of the different constituents of the production stream as shown in the following equation 
     
         ρ.sub.m =V.sub.g ρ.sub.g +V.sub.o ρ.sub.o +V.sub.w ρ.sub.w (6) 
    
     where V g , V o , V w , ρ g , ρ o  and ρ w  are the volume fractions and densities of the gas, oil and water phases of the production stream, respectively. A volume V L , the liquid phase fraction of the fluid, is given by the following equation 
     
         V.sub.L =V.sub.o +V.sub.w, ρ.sub.L =(V.sub.o ρ.sub.o +V.sub.w ρ.sub.w)/V.sub.L                                      (7) 
    
     so that equation (6) may be rewritten as 
     
         ρ.sub.m =V.sub.g ρ.sub.g +V.sub.L ρ.sub.L.     (8) 
    
     It should be noted that ρ g  &lt;&lt;ρ L  and that equation (8) may be written as an approximation 
     
         ρ.sub.m ≅V.sub.L ρ.sub.L                 (9) 
    
     and hence 
     
         V.sub.L =(ρ.sub.m /ρ.sub.L).                       (10) 
    
     When in equation 6 and 8, the term V g  ρ g  is not negligible the gas density ρ g  can be determined from temperature and pressure and pressure-volume-temperature (PVT) data for the particular oil-gas system according to well known techniques. 
     With the densities ρ o , ρ w  and ρ g  and dielectric constants of ε o  and ε w  known there are three equations with three unknown variables V g , V o , and V w  as follows: 
     
         ln ε.sub.m =V.sub.o ln ε.sub.o +V.sub.w ln ε.sub.w (11) 
    
     
         1=V.sub.o +V.sub.w +V.sub.g                                (12) 
    
     
         ρ.sub.m =V.sub.g ρ.sub.g +V.sub.w ρ.sub.w +V.sub.o ρ.sub.o (13) 
    
     The variables can be determined from equations 11-13, ε m  and ρ m  are measured. 
     Thus, knowing the dielectric constants and densities for oil and water and measuring the dielectric constant, temperature and density of the production stream, the water volume fraction can be determined. Further if the water phase is brine, the salinity of the brine may be measured separately and the salt content may then be determined from V w  and the salinity if so desired. When the gas pressure is not negligible, the water volume fraction can still be determined by measuring the pressure of the production stream and the water volume fraction is determined accordingly. 
     WATER-IN-CRUDE MONITOR 
     The foregoing theory shows the interrelationship of the different constituents of the crude oil production stream. Those equations are all well known in the art and are part of the public domain. The present invention utilizes the novel relationship between those equations and does so without actually using the equations. With reference to FIG. 5, the production stream, consisting of various phases of crude oil, water and gas, flows through a pipe 1 a portion of which is non-conductive and non-magnetic. A Cs 137  gamma ray source 4 is mounted within a lead collimator 6 and irradiates the production stream in pipe 1 with gamma radiation. 
     Monitor means 8 receives the gamma radiation after passage through the production stream. Monitor means 8 also provides an electrical VHF or UHF signal E1, preferably at a frequency of 200 MHz, to a transmitting coil 12 encircling the non-conductive, non-magnetic portion of pipe 1 so as to develop electromagnetic energy within the production stream. A receiver coil 16, spaced a predetermined distance from coil 12 and also encircling non-conductive, non-magnetic portion of pipe 1, provides a signal E2, at substantially the same frequency as signal E1, to monitor means 8 in accordance with received electromagnetic energy from the production stream. A temperature sensor 18 senses the temperature of the production stream and provides a corresponding signal T to monitor means 8. Monitor means 8 provides an indication of the water content of the production stream in accordance with the received signals E2 and T and the received gamma radiation as hereinafter explained. 
     Referring now to FIG. 6, a geiger counter 20 provides pulses in accordance with the gamma radiation that has passed through the production stream. A count rate meter 23 receives the pulses from geiger counter 20 and provides a signal corresponding to the count rate of the pulses from geiger counter 23 and hence to the detected gamma radiation. The count rate signal from the meter 23 is converted to digital signals by an analog-to-digital converter 26 which provides the digital signals to address signal means 30. 
     An oscillator 35 provides signal E1, which preferably is at a frequency of 200 MHz, to transmitter coil 12 and also to a mixer 38. Signal E2 from coil 16 is provided to another mixer 40. Mixers 38 and 40 receive a signal E3 from a local oscillator 44. Mixers 38 and 40 provide signals to a flip-flop 47 in accordance with the transmitted signals E1, the received signals E2 and the local oscillator frequency signal E3 with a frequency slightly different from that of oscillator 35 and preferably at a frequency of 199.998 MH z . The output from flip-flop 47 is provided to an integrating circuit 50 which provides an analog, signal corresponding to the phase difference and hence the phase shift due to the dielectric constant of the production stream. The signal from integrating circuit 50 is provided to another analog-to-digital converter 52 which provides corresponding digital signals to address signal means 30. 
     Signal T from temperature sensor 18 is converted to digital signals by an analog-to-digital converter 55 which provides the digital signals to address signal means 30. 
     Address signal means 30 provides memory address signals to memory means 60 to select information stored in memory means 60. Different volumes, in digital form, of constituents of the production stream are stored in memory means 60. In the present example, for each combination of temperature, measured phase shift and detected gamma radiation there is stored a corresponding value for V w , as determined by prior calibration. The address signals cause memory means 60 to select the appropriate water fractional volume of the production stream and provide signals to display means 70 for display. Of course, it would be obvious to one skilled in the art that the gas and crude oil fractional volumes may also be stored. If the water is brine then the salt content may be stored in lieu of the fractional water volume value if so desired. 
     In another embodiment, which is so minor in description as not to warrant a separate figure, a second receiver coil providing a signal E2a, corresponding to received electromagnetic energy from the production stream, may be used in the determination of the phase shift. With reference to FIGS. 5 and 6, the two receiver coils are spaced a predetermined distance apart. Signal E1 is not provided to mixer 38, but mixer 38 now receives signal E2a indicated by the dashed line. The monitor then operates as previously described in the first embodiment of the present invention. 
     In yet another embodiment, when the gas density is not negligible, the present invention may be modified by using a pressure sensor 73 to sense the pressure of the production stream. Sensor 73 would then provide a corresponding analog pressure signal p to monitor means 8. In monitor means 8, signal p would be converted to digital signals by an analog-to-digital converter 75 and applied to address signal means 30. Of course, memory means 60 would have to have the gas density data stored within it and controlled to select the proper ρ g . However, due to the complexity now encountered, it may be more desirable to use a computer to solve operations (11) through (13) to determine the water volume fraction. 
     The crude oil production stream analyzer as hereinbefore described determines the water cut of a production stream in accordance with the density, the dielectric constant and the temperature of the production stream.