Abstract:
A microwave-gamma ray water-in-crude monitor measures the percent quantity of water in crude oil flowing in a pipe line by causing the crude oil to flow through a measuring cell. A microwave transmitter and a gamma ray source are arranged with the measuring cell and transmits microwave energy and gamma rays through the measuring cell. A microwave receiver and a gamma ray detector receive the energies transmitted through the measuring cell and provides signals in accordance with the received energies. Apparatus connected to the microwave receiver and to the gamma ray detector provide a display of the water content of the crude oil in accordance with the signals from the microwave receiver and the gamma ray detector.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to monitors in general and, more particularly, to a water-in-crude monitor. 
     SUMMARY OF THE INVENTION 
     A microwave-gamma ray water-in-crude monitoring system measures the percent quantity of fresh water or salt water in crude oil flowing in a pipe line. The system includes a measuring cell arranged with the pipe line so that the crude oil flows through the measuring cell. A microwave transmitter subsystem and a gamma ray source are arranged with the measuring cell so that microwave energy and gamma rays are transmitted through the measuring cell. A microwave receiving subsystem and a gamma ray detector provide signals corresponding to received microwave energy and to the received gamma rays, respectively. Apparatus connected to the microwave receiver and to the gamma ray detector provides an indication of the percentage of water in the crude oil. 
     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 which follow, wherein one embodiment of the invention is 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 
     FIG. 1 is a plot of relative loss factor versus temperature at a particular wavelength. 
     FIG. 2 is a simplified block diagram of a water-in-crude monitor constructed in accordance with the present invention. 
     FIG. 3 is a detailed block diagram of the monitor shown in Fig. 2. 
    
    
     DESCRIPTION OF THE INVENTION 
     A method of determining the fresh water or salt water content of flowing streams of gas free crude oil under the conditions encountered in well flow lines utilizes the measurement of microwave attenuation caused by water present in the crude stream. Dielectric relaxation of the water molecules at microwave frequencies causes severe attenuation of electromagnetic waves of centimeter wavelength. Attenuation of centimeter waves has been used to measure moisture content in many materials such as concrete and core slabs, see &#34;Microwave Attenuation--A New Tool for Monitoring Saturations in Laboratory Flooding Experiments,&#34; by R. W. Parsons, Marathon Oil Company, Society of Petroleum Engineers Journal, August 1975, pp. 302-310. 
     The propagation of a plane-parallel electromagnetic wave can be respresented by the following equations: 
     
         E=Eoe.sup.-αx ·e.sup.j2π(vt-βx/2π) (1) 
    
     
         H=H.sub.o e.sup.-αx ·e.sup.j2π(vt-βx/2π) (2) 
    
     where E and H are the electric and magnetic field vectors, x and t the direction of propagation and propagation time in space and v the frequency of the wave. Obviously the wave has a time period T=l/v and a space period λ=2π/β (wavelength). Also the wave is attenuated by the factor e - αx as it proceeds along the x-direction. The attenuation factor α is a function of dielectric and magnetic characteristics of the propagating material at the frequency of the wave and is of interest only to the water-in-crude determination. Assuming no magnetic losses in the propagating materials which is certainly true for crude streams, the attenuation factor is given by the following equation: 
     
         α=(2π/λ.sub.o){(K&#39;/2)[(1+(K&#34;/K&#39;).sup.2).sup.1/2 -1]}.sup.1/2(3) 
    
     where 
     λ o  =wavelength in empty space (air), 
     k&#39;=relative dielectric constant, and 
     k&#34;=relative loss factor. 
     The parameters k&#39; and k&#34; are dependent on the frequency of the wave, the temperature and the material composition of the propagating material. The hydrocarbons in crude oils have low values of k&#39; and k&#34; in comparison of those of water and aqueous solutions of sodium chloride. As an example k&#34;, the relative loss factor, for pure water and for a 253 kppm aqueous NaCl solution is plotted versus temperature for a wavelength of 1.267 cm (23.68 GHz) in FIG. 1. Data for the plot was obtained from &#34;The Dielectric Properties of Water in Solutions&#34; by J. B. Hasted, S. M. M. El Sabeh, Transaction of the Faraday Society, V. 49, 1953, London. Note that the loss factor has a maximum at about 28° C. This maximum shifts to lower temperatures at lower frequencies and to higher temperatures at higher frequencies. Also it should be noted that the values for the almost saturated aqueous NaCl solution is only slightly lower than that for the pure water. Values of k&#34; for the hydrocarbons in most crude oils are less than about 0.05 at the above wavelength, as can be determined from &#34;Tables of Dielectric Dispersion Data for Pure Liquids and Dilute Solutions&#34; by Floyd Buckley et al, Nov. 1958, NTIS PB-188296. 
     The attenuation coefficients, α for the pure water and the 253 kppm NaCl solution were computed for the temperature range 0° to 60° C. (32° to 140° F.) at a wavelength of 1.267 cm. The attenuation coefficient for the liquid hydrocarbons in crude oils computes to less than 0.0013 cm -1  over the same temperature range. 
     The foregoing assumes that the crude oil does not contain free gas. When the crude oil contains free gas the free gas must be accounted for to determine a correct water in crude content. 
     Let L be the total microwave beam path, then microwave attenuation of initial value I o  to the value I can be expressed by the following equation: 
     
         I=I.sub.o ·exp[-α·lw]              (4) 
    
     where lw=length of beam in water. However, we have 
     
         lw=l.sub.L ·η                                 (5) 
    
     where 
     l L  =length of beam in liquid (crude and water) 
     η=water/liquid fraction, 
     and 
     
         l.sub.L =L·θ                                (6) 
    
     where θ=fraction of length L which is filled with liquid. 
     We can now write equation (4) using equations (5) and (6) as follows: 
     
         I=I.sub.o ·exp[-α·L·θ·η], (7) 
    
     or 
     
         θ·η=(1/αL) ln (I.sub.o /I),       (7a) 
    
     θis also the average void fraction in the measuring path. This void fraction can be measured e.g. with a gamma ray density gauge as follows. 
     A gamma ray beam traversing the measuring section of length L is attenuated by the material (gas, water, oil) within this section. This attenuation can be described by equation (5). 
     ti I y  =I oy  ·exp{-[μ g  ·l g  +μ w  ·l w  +μ oil  ·l oil  ]}, (8) 
     where μ g , μ w , μ oil  are the gamma ray attenuation coefficients per unit length for gas, water and oil, respectively; l g , l w , l oil  are the thicknesses of gas, water and oil, respectively, within the path length; I oy  is the initial gamma ray intensity, I y  the attenuated gamma ray intensity. 
     The following relationships between the above parameters  i  (i.e. g, w, oil), L, η and θ exist. 
     
         L=l.sub.g +l.sub.w +l.sub.oil                              (9) 
    
     
         l.sub.L =l.sub.w +l.sub.oil =L·θ            (10) 
    
     
         l.sub.w =l.sub.L ·η                           (11) 
    
     
         l.sub.g =L(1-θ).                                     (12) 
    
     Using the relationships of (9) through (12) to substitute for l g , l w  and l oil  in equation (8) the following is obtained 
     
         I.sub.y =I.sub.oy exp{-L[μ.sub.g +θ(μ.sub.oil -μ.sub.g +η(μ.sub.w -μ.sub.oil))]},                      (13) 
    
     and 
     
         (1/L)[ ln (I.sub.oy /I.sub.y)]=μ.sub.g +θ[(μ.sub.oil -μ.sub.g)+η(μ.sub.w -μ.sub.oil)],            (14) 
    
     or 
     
         θ={(1/L)[ ln (I.sub.oy /I.sub.y)]-μ.sub.g }/[(μ.sub.oil -μ.sub.g)+η(μ.sub.w -μ.sub.oil)].            (15) 
    
     Substituting equation (15) into equation (7a) to eliminate θ and rearranging terms yields equation (16) for determining η, the water/liquid ratio, from measured and known parameters 
     
         η=[(1/αL) ln (I.sub.o /I)·(μ.sub.oil -μ.sub.g)]/[(1/L) ln (I.sub.oy /I.sub.y)-μ.sub.g -(1/αL)(μ.sub.w -μ.sub.oil)·ln (I.sub.o /I)]. (16) 
    
     In many cases μ w  =μ oil  =μ and μ g  &lt;&lt;μ may be utilized without making large errors, equation (16) then reduces to 
     
         η=[(1/αL)·μ·ln (I.sub.o /I)]/[(1/L) ln (I.sub.oy /I.sub.y)-μ.sub.g ].                         (17) 
    
     Using again relationships of (9) through (12) the fractional water oil ratio can be calculated from the water/liquid ratio by equation (18) 
     
         water/oil fraction=η/(1-η).                        (18) 
    
     Referring now to FIG. 2, oil flowing from a well head enters a measuring cell 1 by way of a pipe (not shown) and leaves cell 1 by way of a pipe (not shown). Cell 1 includes ceramic windows 7, 9 and steel windows 10, 11. A conventional type klystron 12 provides microwave frequency radiation through wave guides 14 to an isolator 16. Isolator 16 stops reflected microwaves from entering klystron 12; a tuner 24 provides a mechanical type tuning for matching the transmission subsystem when isolator 16 provides microwaves to attenuator 28. 
     Attenuator 28 provides attenuated microwaves to a conventional horn antenna 33 which propagates the microwaves through window 7 and through window 9 to a second horn antenna 38. Horn antenna 38 provides the received microwaves to a microwave detector 42 which in turn provides an electrical signal E1 to an amplifier 46. The amplified signal from amplifier 46 is provided to an integrator 50 receiving pulse E 2  from a clock 54 and provides an integrated signal E 3  to a monitor 60. 
     A gamma ray source 65, located in a shield and collimator 67, provides gamma ray beam through window 10 across measuring cell 1 and through window 11. Shield and collimator 67 may be made of lead or tungsten. 
     A radiation detector 69, which may be a conventional type sodium iodide (thallium activated) crystal detector, detects the gamma radiation passing through window 11 and provides light pulses, corresponding in number and amplitude to the detected gamma radiation, to a photo multiplier tube 73. Detector 69 and photomultiplier tube 73 are surrounded by a shield 75 which prevents the gamma radiation passing through window 11 from escaping from the area. Photomultiplier tube 73 provides electrical pulses on a one-for-one basis with the light pulses from detector 69 to an amplifier 80 where they are amplified and provided to a gain stabilizer 83. Gain stabilizer 83 may be a type manufactured by Hawshaw Chemical Co. as their part No. NA-22AGC Amplifier. Gain stabilizer 83 provides corresponding pulses to a conventional type discriminator and count rate meter 88. Discriminator and count rate meter 88 provides a signal E5, corresponding to the detected radiation, to monitor 60. 
     Referring now to FIG. 3, signal E3 corresponding to the term I in equation 16 is applied to a divider 100 in monitor 60 where it is divided into a DC voltage corresponding to a value of I o . Divider 100 provides a signal to a natural log function generator 104 which in turn provides a signal corresponding to the term ln (I o  /I) in equation 16. 
     A divider 108 divides a DC voltage corresponding to a value for L, the distance from source 65 to detector 69, into a DC voltage corresponding to a value of 1. Divider 108 divides a signal to another divider 110 where it has a DC voltage corresponding to the attenuation constant α into it to provide a signal. The attenuation constant α may be determined prior to operation by determining I o  with measuring cell 1 empty, then filling measuring cell 1 with water and determining I and solving equation 4 for α knowing the length of the microwave beam in the water. A multiplier 114 multiplies the signals from natural log function generator 104 and divider 110 to provide another signal to a multiplier 116. 
     Subtracting means 120 subtracts a direct current voltage corresponding to μ g  from a direct current voltage corresponding to μ oil  to provide a difference signal to multiplier 116 where it is multiplied with the signal from multiplier 114 to provide a product signal. The constants μ g , μ oil  are determined in the same manner as the dielectric constant by utilizing equation 4, using I y  for I and I oy  for I o , and filling the measuring cell 1 with gas of the type encountered in the producing well for the constant μ g  and then filling measuring cell 1 with oil of the type provided by the well to obtain the constant μ oil . 
     Signal E5 is applied to a divider 125 where it is divided into a DC voltage corresponding to the term I oy  in equation 16. The value of I oy  is determined prior to operation by detecting the gamma radiation from source 65 while measuring cell 1 is empty. An output signal from divider 125 is applied to a natural log function generator 128 which provides a signal corresponding to the term ln (I oy  /I y ) in equation 16. A multiplier 130 multiplies the signal provided by divider 108 with the signal from natural log function generator 128 to provide a corresponding signal. Subtracting means 131 subtracts the voltage corresponding to μ g  from the signal provided by multiplier 130 to provide a signal. 
     Subtracting means 134 subtracts the voltage corresponding to μ oil  from the voltage corresponding to μ w  to provide a difference signal. A multiplier 137 multiplies the signals from multiplier 114 and subtracting means 134 to provide a product signal which is subtracted from the signal provided by subtracting means 131 by subtracting means 140. A divider 144 divides the signal provided by subtracting means 140 into the signal from multiplier 116 to provide a signal corresponding to the term η to a divider 148 and to subtracting means 149. 
     Subtracting means 149 subtracts the signal provided by divider 144 from the voltage corresponding to the value of 1 to provide a signal which is divided into the signal provided by divider 144 by a divider 148. Divider 148 provides a signal, corresponding to the water to oil fraction, to a recorder 150 and to an analog to digital converter 160. Converter 160 converts the analog signal from divider 148 to digital signals which are provided to readout means 164. 
     The present invention as hereinbefore described is a water-in-crude monitor that determines the water to oil fraction utilizing microwaves and gamma rays so that water-in-crude oil that contains gas may be monitored.