Densitometer
TECHNICAL FIELD OF THE INVENTION 
     This invention relates generally to the measurement of the density of a flowing fluid after material has been added to the fluid and, more particularly, to a method and apparatus for determining the relative density of the flowing fluid based upon the ratio of the fluid&#39;s flow rate before material is added to the flow rate after the material is added. 
     BACKGROUND OF THE INVENTION 
     One of the services provided in the oil field service industry pertains to the stimulation of an oil or gas bearing formation after a well hole has been drilled. Upper and lower plugs or packers are inserted in the well above and below perforations in the well casing, which provide access to the subsurface formation. A fracturing fluid is pumped into the well between the two packers and is forced, under high pressure, through the casing perforations into the formation. It is intended that the fluid will cause the formation to fracture outwardly from the perforations, providing channels for the oil or gas to flow into the well. 
     Once fractures have opened, a proppant is added to the fracturing fluid to be injected into the fractures. The proppant, which can be sand, bauxite, or other like material, props open the fractures to prevent them from closing when pumping of the fracturing fluid ceases and the pressure on the fractures is reduced. It can be appreciated that too little proppant may allow the fractures to close, thereby reducing the flow of oil or gas into the well. It can also be appreciated that too much proppant can clog the fractures, as well as the bore hole, fluid lines, pumps and valves, thereby also adversely affecting production and increasing the required maintenance. 
     To produce proppant-laden fracturing fluid (slurry), a &#34;clean&#34; base fluid is continuously pumped into a blending tank and a proppant is continuously added. Proppant-laden slurry is discharged from the blender and is pumped under high pressure through a pipe or flow line into the well. The amount of proppant added must be carefully monitored and controlled to ensure that production of the well is optimized. 
     One common device used to monitor the amount of proppant which has been added to the base fluid is a nuclear densitometer (or &#34;densometer&#34;) which measures the density of the slurry being discharged from the blender. A radiation source, such as Cesium 137, is positioned against one side of the discharge flow line and a radiation detector is positioned against the opposite side. The radiation is directed through the first side of the flow line, through the discharged fluid, and through the opposite side of the flow line to the detector. The amount of radiation which actually reaches the detector is proportional to the density of the fluid: if the relative amounts of all other components in the slurry remain constant, the greater the density of the slurry (i.e., the more proppant in the slurry), the more radiation will be absorbed in the slurry and the less will be detected. The output signal from the detector can be processed and the density, in units such as pounds of sand added per gallon fluid (PSA), specific gravity units (SGU), or pounds per gallon (PPG) can be displayed for the operators. 
     A nuclear densitometer has many attendant disadvantages, one of which is its reliance upon a radiation source. It is necessary for the operator to have federal and, possibly, state licenses and be subject to extensive regulations. Handling and transportation of the device, while not dangerous, is subject to prescribed procedures. Additionally, as can be appreciated, the radiation source irradiates a portion of the fluid flow line to which the densitometer is attached, causing it to become radioactive. Consequently, that portion of the flow line must be handled and transported with as much care as the nuclear densitometer itself. 
     Other operational disadvantages include the need for separate units for different-size flow lines. Less radiation will be detected through an 8-inch flow line than through a 6-inch flow line, even if the density of the fluid is the same. Consequently, a different densitometer unit is usually used. Additionally, component age, temperature variations, and the decay of the radioactive source tend to cause the device to &#34;drift.&#34; Consequently, various compensation techniques must be employed to prevent such drifting from being interpreted as increased or decreased fluid density. Finally, circuitry associated with a nuclear densitometer may be noisy and slow, resulting in inaccuracies and delays in the monitoring and control of the amount of proppant being added. 
     Consequently, a need has arisen for an apparatus and method for determining the amount of material added to a flowing fluid which substantially reduce or avoid the foregoing disadvantages. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, an apparatus and method are provided for determining the relative density of a flowing fluid. The apparatus comprises an input stage for generating two flow signals representative of volumetric flow rates of a fluid flowing through two sections of a flow line, a calculating stage for generating a density signal representative of the relative density of the fluid flowing through the second stage of the flow line, and an output stage for displaying the density signal or otherwise making it available to an operator. The method comprises the steps of generating two signals representative of the flow rates of the fluid flowing through the two sections of flow line, deriving a ratio of the two signals, comparing the ratio to a first predetermined constant, deriving a second ratio of a second predetermined constant to the first ratio, and multiplying the result of the comparing step times the second ratio to generate an output density signal representative of the relative density of the fluid flowing through the second section of flow line (i.e., after material, such as proppant, has been added to the fluid). 
     In a specific embodiment, the input stage of the apparatus includes two inputs for receiving signals from suction and discharge flow rate sensors, each input signal having a frequency representative of the respective flow rate, and two outputs for providing two voltage signals proportional to the two frequencies. The calculating stage includes a first dividing means for providing the first ratio signal representative of the one voltage signal divided by the other, a comparing means for providing a difference signal representative of the difference between the first ratio signal and a first reference signal, a second dividing means for providing a second ratio signal representative of a second reference signal divided by the first ratio signal, and a multiplying means for providing a density signal substantially equal to the product of the difference signal times the second ratio signal and representing the amount of proppant in the fracturing fluid. 
     The present invention advantageously uses signals from flow rate sensors already in place and is sufficiently flexible to provide direct read-out of the relative density in any of several commonly-used units. Because the present invention relies upon a ratio of two input signals, different turbine sizes can be used without having to recalibrate the apparatus or use an entirely different unit. The method can be employed in conjunction with either an analog or a digital apparatus and can be incorporated into a feedback loop to monitor and quickly control the amount of proppant being added to the base fluid. The present invention does not employ a nuclear radiation source, and thus it does not have the attendant disadvantages thereof. Furthermore, the present invention is relatively fast, has reduced susceptibility to noise, and retains its accuracy despite temperature fluctuations and component aging. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 schematically illustrates one embodiment of the present invention in use during a fracturing job; 
     FIG. 2 illustrates a flow chart of an embodiment of the present invention; 
     FIG. 3 illustrates a block diagram of an embodiment of the present invention in which the density signal is generated through analog circuitry; and 
     FIG. 4 schematically illustrates an embodiment of the present invention in which the density signal is generated by a digital computer. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 schematically illustrates an embodiment of the present invention 10 in use during a fracturing job. For clarity, unrelated pipes, valves and other equipment are not included in FIG. 1. A well 12 has been bored into a subsurface oil- or gas-bearing formation 14 and a well casing 16 with perforations 18 has been set in concrete 20 within the well bore 22. Perforations 18 through the casing 16 and concrete 18 provide access to the formation 14. Upper and lower packers 24 and 26, respectively, are positioned within the casing 16 above and below the perforations 18 to seal off the intermediate area. A tubing string 28 from the surface penetrates the upper packer 24 into the isolated area between the packers 24 and 26. 
     Base fluid from a storage device or tank 30 flows through a suction flow line 32 into a blender 34. Proppant from a dry proppant supply (not shown) flows through a supply line 36 into the blender 34. Other materials, such as gel setting agents and fluid loss control agents, can also be added to the fluid to form the fracturing fluid or slurry. Blended fracturing fluid then flows from the blender 34 through a discharge flow line 38 and is pumped by a high-pressure pump 40 through the tubing string 28 into the portion of the well hole isolated by the upper and lower packers 24 and 26. The total input to the blender 34 is equal to the total output so that the fluid level in the blender 34 remains constant. Pressure from the fracturing fluid causes fractures 42 to form in the subsurface formation 14 and radiate outwardly from the perforations 18. Proppant which has been added to the slurry props open the fractures 42 to maintain channels within the subsurface formation 14 leading to the well 12. 
     The &#34;clean&#34; base fluid from the tank 30 passes by a suction flow-rate sensor 44 before entering the blender 34. The suction flow-rate sensor 44 is typically a turbine-like device in which the rotation of the turbine, caused by the fluid flow, is detected by a magnetic sensor which generates a series of pulses to be transmitted by a wire 46, processed and converted into a display, such as a digital suction flow-rate display 48. Similarly, the &#34;dirty&#34; proppant-laden slurry discharged from the blender through the discharge flow line 38 passes through a discharge flow rate sensor 50, which transmits a series of pulses through a wire 52 to be processed and displayed on a discharge flow-rate display 54. The suction and discharge flow rates, as displayed on the displays 48 and 54, are used by the operators to monitor the operation of the blender 34. 
     If used, a conventional nuclear densitometer (not shown) would be attached to the discharge flow line 38 between the blender 34 and the high-pressure pump 40 and would transmit a density signal to be displayed and used by the operators to monitor the amount of proppant in the fracturing fluid. 
     The present invention, however, employs the existing signals generated by the suction and discharge flow-rate sensors 44 and 50 to derive the density of the fluid discharged from the blender 34 relative to the density of the base fluid flowing into the blender 34 and display this information on a digital or other display 56 to be monitored by the operators. Additionally, an output of the densitometer 10 can be electrically interconnected by a cable 60 to a proppant control valve 62 as part of a closed-loop system to continuously and automatically regulate the amount of proppant in the slurry. 
     It has been found from empirical evaluations of suction and discharge flow rates and the amount of proppant added that, for a given amount of proppant, the ratio of the suction flow rate to the discharge flow rate is substantially constant. (Hereinafter, the relative density of the proppant-laden fluid will generally be referred to in units of &#34;pounds of sands added&#34; or &#34;PSA&#34;; it should be understood, however, that other units can also be used.) For example, given a PSA of ten, the suction:discharge ratio has been found to be about 0.686 when the discharge rate is 40 barrels per minute and also when it is 20 barrels per minute. Conversely, if the ratio is known, the PSA can be determined regardless of the suction and discharge flow rates. The present invention employs this relationship by determining the suction:discharge ratio and, based upon the ratio, calculating the amount of proppant added. This process is illustrated in the flow chart of FIG. 2. It will be appreciated that the method and apparatus of the present invention can be employed in any of a variety of applications and are not intended to be limited to use in the oil field service industry. Furthermore, the present invention can be implemented as an analog device or a digital device. 
     Tables 1 and 2 provide examples of the relationship between suction and discharge flow rates (as frequencies in Hz and as actual flow rates in barrels per minute) and the relative density of the slurry (in pounds of sand added). As can be seen, for any given S:D ratio, the PSA is the same whether the discharge rate is 20 barrels per minute or 40 barrels per minute. The PSA would also be the same for other discharge rates given the same S:D ratio. 
     
                       TABLE 1                                                     
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Discharge Rate: 40 bls/min (82.8 Hz)                                      
Suction Rate                                                              
           Suction Rate  Ratio   Density                                  
(Hz)       (bls/min)     (S:D)   (PSA)                                    
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79.16      38.24         0.956   1                                        
75.85      36.64         0.916   2                                        
72.78      35.16         0.879   3                                        
69.97      33.80         0.845   4                                        
67.40      32.56         0.814   5                                        
65.00      31.40         0.785   6                                        
62.76      30.32         0.758   7                                        
60.61      29.28         0.732   8                                        
58.71      28.36         0.709   9                                        
56.80      27.44         0.686   10                                       
55.06      26.60         0.665   11                                       
53.49      25.84         0.646   12                                       
51.92      25.08         0.627   13                                       
50.51      24.40         0.610   14                                       
49.10      23.72         0.593   15                                       
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                       TABLE 2                                                     
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Discharge Rate: 20 bls/min (41.4 Hz)                                      
Suction Rate                                                              
           Suction Rate  Ratio   Density                                  
(Hz)       (bls/min)     (S:D)   (PSA)                                    
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39.58      19.12         0.956   1                                        
37.92      18.32         0.916   2                                        
36.39      17.58         0.879   3                                        
34.98      16.90         0.845   4                                        
33.70      16.28         0.814   5                                        
32.50      15.70         0.785   6                                        
31.38      15.16         0.758   7                                        
30.31      14.64         0.732   8                                        
29.35      14.18         0.709   9                                        
28.40      13.72         0.686   10                                       
27.53      13.30         0.665   11                                       
26.74      12.92         0.646   12                                       
25.96      12.54         0.627   13                                       
25.25      12.20         0.610   14                                       
24.55      11.86         0.593   15                                       
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     Referring to FIG. 2, the densitometer 10 comprises an input stage 64 for generating two flow signals representative of the flow rates of the fluid flowing through the suction line 32 and through the discharge line 38, a calculating stage 66 for generating a density signal from the flow signals representative of the relative density of the fluid flowing through the discharge line 38, and an output stage 68 for indicating to the operators the density signal, such as with a visual display in appropriate units. In the calculating stage 66, the suction and discharge flow signals are received from the input stage 64, and their ratio, S:D, is derived 70. The S:D ratio is subtracted 72 from a zero adjustment reference to obtain a difference. The S:D ratio is also divided 74 into a full-scale adjustment reference to generate another ratio. Because the S:D ratio is not linear over the PSA range conventionally used in the stimulation services industry (from about 0 PSA to about 20 PSA), the second ratio is linearized 76 before the next step. The linearized second ratio is multiplied 78 by the difference of the zero adjustment reference minus the S:D ratio to generate the density signal. 
     FIG. 3 illustrates a block diagram of an analog embodiment of a densitometer 80 of the present invention, comprising the input stage 64, the calculating stage 66, and the output stage 68 described in conjunction with FIG. 2. The input stage 64 comprises a signal processor having a first input shaper 82 with an input electrically interconnected with the discharge flow-rate sensor 50 and a second input shaper 84 having an input electrically interconnected with the suction flow-rate sensor 44. Each of the input shapers 82 and 84 generates an output signal in the form of a substantially uniform series of square waves having the same frequency as the frequency of the input pulses from the flow-rate sensors 44 and 50. The input stage 64 also comprises first and second frequency-to-voltage converters 86 and 88, each of which generates an output voltage proportional to the frequency of the corresponding square waves. The two voltage flow signals from the converters 86 and 88 are transmitted to the calculating stage 66. 
     The calculating stage 66 includes a divider 90 which receives the two voltage signals from the input stage 64 and outputs a signal representing their ratio (i.e., suction flow rate divided by discharge flow rate). The output of the divider 90 is electrically interconnected with one input of a second divider 92; a second input of the divider 92 is electrically interconnected with a full-scale reference comprising a potentiometer 94 interconnected between a power supply and ground. 
     The output of the first divider 90 is also electrically interconnected with an inverting input of a comparator 96, the non-inverting input being interconnected with a zero adjustment reference, comprising a second potentiometer 98 interconnected between the power supply and ground. 
     The output of the second divider 92 is linearized in a linearizer 100 and is electrically interconnected with an input of a multiplier 102; the output of the comparator 96 is electrically interconnected with a second input of the multiplier 102. The output of the multiplier 102 is the density signal and is transmitted to the output stage 68. It can be displayed on the display 56 and, if desired, incorporated into a feedback loop (as illustrated in FIG. 1) to both monitor and control proppant density. 
     Various circuit components in the calculating stage 66 of the densitometer 80 cause the density signal output from the multiplier 102 to have a lower amplitude than is generally desired for satisfactory operation of the display 56. Consequently, the output stage 68 preferably includes a buffer 104 to amplify the density signal before it reaches the display 56. A range from about zero volts to about five volts is typically used to provide satisfactory resolution for a conventional three-digit LED display. Similarly, the linearizer 100 can comprise two inverting operational amplifiers 106 and 108 to increase the amplitude and linearity of the second ratio signal. By changing the gain of the buffer 104 and/or the linearizer 100, the densitometer 80 can be calibrated to display the density signal in any desired units. 
     The potentiometer 94 of the full-scale adjustment reference is preferably set only once, during factory calibration, to provide low-scale (e.g., 1 PSA) and full-scale (e.g., 15 PSA) readings on the display 56 when signals simulating appropriate flow rates are transmitted to the input stage 64. 
     The potentiometer 98 of the zero adjustment reference is field adjustable before each use of the densitometer 80. With the densitometer 80 connected to the suction and discharge flow-rate sensors 44 and 50, clean base fluid is pumped through the system and no proppant is added. Thus, the flow rates in the suction portion 32 and discharge portion 38 of the flow line will be the same (a ratio of one, indicating no proppant added), and the potentiometer 98 is adjusted until the display 56 reads zero PSA. This adjustment can compensate for minor variations in the flow rate sensors and turbines as well as changes in component values due to age and temperature variations. 
     FIG. 4 illustrates a densitometer in which the steps of the calculating stage 66, illustrated in FIG. 2, are implemented by a digital device, such as a computer 116. The signals from the suction and discharge flow rate sensors 44 and 50 are transmitted to a signal processor 110 which transmits digital flow signals to the computer 116. The computer 116 processes the signals and generates a density signal, which can be displayed on a digital display, such as LED display 112, on a CRT 114, or on both. The density signal can also be incorporated as part of a feedback loop and transmitted by the cable 60 to the valve 62 which controls the amount of proppant added to the blender 34. Thus, once a value for the desired amount of proppant has been entered into the computer 108, the system can maintain a relatively constant proppant density with significant speed and accuracy. 
     The signal processor 110 can be any means by which the signals from the flow rate sensors 44 and 50 are converted into appropriate digital signals for processing by the computer 108. For example, the input stage 110 can comprise circuitry similar to that of the input stage 64 of the embodiment illustrated in FIG. 3, including, for example, input shapers and frequency-to-voltage converters to generate uniform square waves to be processed by an analog-to-digital converter or processed directly by the computer 108. 
     The density signal can be calculated mathematically from the steps of FIG. 2 or can be obtained from a look-up table in memory based upon empirically derived data similar to that in Tables 1 and 2 above. If the density signal generated by the computer 108 is desired in analog form, a digital-to-analog converter can be employed for this purpose. 
     Although the present invention has been described in detail, it should be understood that various changes, substitutions, and alterations can be made herein without departing from the spirit and scope of the invention, as defined by the appended claims.