Abstract:
An oil field separation facility utilizing a total organic carbon analyzer to help maintain consistently acceptable levels of oil and water separation. The facility separates a produced oil and water mixture into an oil-rich stream and a water-rich stream. The total organic carbon analyzer measures the total organic carbon content of the water-rich stream. The measured total organic carbon content of the water-rich stream can then be used to adjust an operating parameter of the separation facility.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to facilities for separating oil and water present in a fluid mixture produced from a subterranean well. In another aspect, the invention concerns a system for controlling an oil field separation facility by measuring the amount of oil present in the separated water stream. 
     2. Description of the Prior Art 
     Oil wells frequently produce water along with oil. In many instances the commonly produced oil and water phases are separated in the field prior to transporting the oil to a major pipeline or refinery facility. Each in-the-field oil separation facility may service a number of individual wells. Thus, large volumes of oil and water may be passed through the separation facility every day. If the separation facility does not properly separate the oil and water, either the oil stream exiting the facility will contain too much water or the water stream exiting the facility will contain too much oil. If too much water is contained in the separated oil stream exiting the facility, the oil stream may be rejected by the operator of the pipeline and/or the refinery. If too much oil is contained in the separated water stream exiting the facility, the value of the oil present in the water stream is lost because typically the separated water stream is simply discharged or reinjected into the subterranean formation for water flooding. For large separation facilities, an oil content of just 5,000 ppmw in the separated water stream can result in a yearly oil loss valued at several million dollars. 
     In the past, the oil content of the separated water stream from oil field separation facilities has been measured on a relatively infrequent (typically daily) basis. However, it has recently been discovered that the amount of oil in the separated water stream can vary greatly by the hour or minute. Thus, measuring the oil content of the separated water stream on an infrequent basis may not provide a reliable indication of oil content because limited duration spikes in the oil content may not be accounted for. Also, infrequent measurement of the oil content does not allow the separation facility to be controlled in a dynamic manner which can reduce or eliminate spikes in the oil concentration. An additional drawback of many conventional methods for measuring the oil content of a separate water stream is that past methods of sampling the separated water stream are somewhat suspect because they did not account for the tendency of the oil and water phases to separate from one another. Thus, conventional sampling methods drawing from a certain location in the separated water conduit may provide a poor representation of the actual content of oil in the separated water stream. 
     In the past, ultraviolet or visible fluorescence analyzers have been used to measure the oil content of diluted water discharged from separation facilities. These conventional analyzers are designed to measure much lower concentrations of oil (e.g., 10-50 ppmw) than are present in the undiluted separated water stream from the separation facility. Thus, because these conventional analyzers are not designed for the application proposed herein, they present a number of drawbacks. For example, these conventional analyzers are extremely sensitive to the amount of oil in the separated water stream and, therefore, would require high amounts of dilution (e.g., 100-10,000×) of the separated water sample in order to reduce the oil content of the analyzed sample down to a measurable level. Such high dilution rates result in a less accurate measurement due in part to trace amounts of organic contaminants present in the diluent. In addition, these conventional analyzers are designed for single phase fluids, rather than the 2-phase oil-in-water dispersions that are typical in the separated water stream. It is even more difficult for these conventional analyzers to provide an accurate measurement from a highly diluted sample because of the difficulty involved in providing uniform dispersions of oil in highly diluted water samples. Further, conventional analyzers must be recalibrated every time the composition of the oil in the separated water stream changes because conventional analyzers only measure a certain narrow range of oil compositions. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     It is, therefore, an object of the present invention to provide an oil field separation facility control system that frequently measures the oil content in the separated water stream and adjusts operating parameters of the separation facility in real time based on the measured oil content of the separated water stream. 
     Another object of the present invention is to provide a system for more accurately sampling the separated water stream produced from an oil field separation facility. 
     Still another object of the present invention is to provide a system for analyzing the amount of oil in a water stream that does not require high rates of dilution of the separated water sample. 
     Yet another object of the present invention is to provide a system for analyzing the amount of oil in a water stream that can accurately analyze a 2-phase oil-in-water dispersion. 
     A further object of the present invention is to provide a system for analyzing the amount of oil in a water stream that does not require recalibration of the analyzer when the composition of the oil is varied. 
     It should be understood that the above-listed objects are only exemplary, and not all the objects listed above need be accomplished by the invention described and claimed herein. 
     Accordingly, in one embodiment of the present invention, there is provided a method comprising the steps of: (a) separating a mixture of oil and water into an oil-rich stream and a water-rich stream, the water-rich stream containing organic compounds; (b) measuring the total organic carbon content of the water-rich stream; and (c) adjusting the manner in which step (a) is performed based on the total organic carbon content measured in step (b). 
     In accordance with another embodiment of the present invention, there is provided a method of controlling an oil field separation facility that is operable to separate a produced oil and water mixture into a predominately oil stream and a predominately water stream. The method comprises the steps of: (a) agitating the predominately water stream to thereby increase the turbulence of the predominately water stream; (b) continuously conducting a sample portion of the agitated predominately water stream to an analyzer system comprising a total organic carbon analyzer; (c) periodically measuring the total organic carbon content of the sample portion using the total organic carbon analyzer; and (d) adjusting at least one operating parameter of the separation facility based on the measured total organic carbon content of the sample portion. 
     In accordance with yet another embodiment of the present invention, there is provided a method comprising the steps of: (a) conducting a predominately water stream through a conduit, the predominately water stream comprising organic compounds; (b) continuously withdrawing a sample portion of the predominately water stream from the conduit; (c) continuously conducting the sample portion to an analyzer system comprising a defined sample volume, a defined diluent volume, and a total organic carbon analyzer; (d) periodically operating the analyzer system in a by-pass mode wherein the defined sample volume, the defined diluent volume, and the total organic carbon analyzer are fluidly isolated from one another; and (e) periodically operating the analyzer system in a sampling mode wherein the defined sample volume, the defined diluent volume, and the total organic carbon analyzer are in fluid flow communication with one another, the defined sample volume being fluidly disposed between the defined diluent volume and the total organic carbon analyzer. 
     In accordance with a still further embodiment of the present invention, there is provided an apparatus comprising separation equipment operable to separate a mixture of oil and water into an oil-rich stream and a water-rich stream, a water conduit for carrying the water-rich stream away from the separation equipment, and an analyzer system fluidly coupled to the water conduit and operable to measure the total organic carbon content of the water-rich stream. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
       A preferred embodiment of the present invention is described in detail below with reference to the attached drawing figures, wherein: 
         FIG. 1  is a schematic representation of an oil field separation facility servicing several different oil fields that each contain a number of individual oil wells; 
         FIG. 2  is a more detailed representation of the oil field separation facility of  FIG. 1 , particularly illustrating the configuration of the separation equipment as well as a system for measuring the oil content of the separated water stream; 
         FIG. 3  is a schematic flow diagram of the analyzer system used to sample the separated water stream and determine the amount of oil in the separated water stream sample, particularly illustrating the analyzer system in a by-pass mode where a sample of the separated water stream is not being charged to the total organic carbon analyzer; and 
         FIG. 4  is a schematic flow diagram of the analyzer system of  FIG. 3 , particularly illustrating the analyzer system in a sampling mode where a sample of the separated water stream is being charged to the total organic carbon analyzer. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring initially to  FIG. 1 , an oil field separation facility  10  is illustrated as servicing several different oil fields, each including a number of individual wells. Produced oil and water from a first oil field  12 , a second oil field  14 , and a third oil field  16  are combined in a main conduit  18  and carried to separation facility  10 . Although  FIG. 1  shows separation facility  10  servicing only 16 individual wells, it is more typical for oil field separation facility  10  to service a greater number of individual wells (e.g., 50-500 wells). In separation facility  10 , the produced oil and water mixture is separated into an oil-rich stream exiting separation facility  10  via an oil conduit  20  and a water-rich stream exiting separation facility  10  via a water conduit  22   a.    
     Water conduit  22   a  carries the water-rich stream to an agitating means  24  which is operable to increase the turbulence of the water-rich stream. Agitating means  24  can be any apparatus for enhancing the physical mixing of the oil present in the water-rich stream to thereby provide a more even dispersion of oil droplets in the water-rich stream. In a preferred embodiment of the present invention, agitating means  24  is a booster pump; however, agitating means  24  could also be a conventional static mixer. After being agitated in agitation means  24 , the water-rich stream is conducted through water conduit  22   b . A sample line  26  is fluidly coupled to water conduit  22   b  and is operable to withdraw a small portion of the water-rich stream from water conduit  22   b . Sample line  26  carries the sampled portion of the water-rich stream to an analyzer system  28 . Analyzer system  28 , described in detail below with respect to  FIGS. 3 and 4 , includes a total organic carbon analyzer that measures the total organic carbon content of the sampled portion of the water-rich stream. The total organic carbon content measured in analyzer system  28  provides a reliable and accurate indication of the amount of oil present in the water-rich stream, regardless of the composition of the oil in the water-rich stream. The total organic carbon content value measured in analyzer system  28  can be communicated to a controller  30  via a communication means  32 . Controller  30  is operably coupled to separation facility  10  and functions to control at least one operating parameter of separation facility  10 . Preferably, the operating parameter or parameters of separation facility  10  that is/are controlled by controller  30  can be adjusted to vary the amount of oil present in the water-rich stream exiting separation facility  10  via water conduit  22   a . Communication means  32  can be any system for communicating information between two locations. For example, communication means  32  could be electrically conductive wires, fiber optic lines, or a wireless transmission system. 
     In a preferred embodiment of the present invention, a sample of the water-rich stream is continuously withdrawn from water conduit  22   b  via sample line  26 . As used herein, the term “continuous” shall denote an operation that is continually performed for uninterrupted periods of at least 12 hours. The total organic carbon content of the continuously withdrawn water-rich sample can then be either continuously or periodically measured in analyzer system  28 . As used herein, the term “periodically” shall denote an operation that is performed at intervals of less than 12 hours. Preferably, analyzer system  28  periodically measures the total organic carbon content of the sampled water-rich stream at intervals of less than about 6 hours, more preferably less than about 1 hour, still more preferably less than about 0.25 hours, and most preferably less than 0.1 hours. Thus, a signal indicating the current oil content in the water-rich stream can continually, or at least frequently, be provided to controller  30 , which can then vary the manner in which separation facility  10  operates to thereby optimize the separation process. 
     Typically, the total organic carbon content measured by analyzer system  28  will be compared to a predetermined maximum total organic carbon content value in order to determine whether or not an operating parameter of separation facility  10  needs to be adjusted. If the total organic carbon content measured by analyzer system  28  exceeds the predetermined maximum total organic content value, controller  30  will adjust separation facility  10  so that less oil is present in the water-rich stream exiting separation facility  10  via water conduit  22   a . Preferably, the predetermined maximum total organic content value is equivalent to an oil content in the water-rich stream in the range of from about 100 to about 10,000 ppmw (parts per million by weight), most preferably 1,000 to 5,000 ppmw. 
     After a portion of the water-rich stream in water conduit  22   b  is withdrawn via sample line  26 , make-up water from conduit  34  can be added to the water-rich stream in water conduit  22   b . The combined make-up water and water-rich stream can then be carried to an injection pump  36  which increases the pressure of the stream to a level sufficient for injection into injection wells via water conduit  22   c . The water stream conducted to the injection wells is typically used for water flood operations in oil fields  12 ,  14 , and  16 , or other oil fields. 
     Referring to  FIG. 2 , a preferred configuration of separation equipment in separation facility  10  is illustrated in greater detail. It should be understood that  FIG. 2  illustrates just one of many configurations of the separation equipment that can be employed in separation facility  10 . The embodiment illustrated in  FIG. 2  shows the produced oil and water mixture entering separation facility  10  via main conduit  18 . The produced oil and water mixture is first carried to a primary separator  38  where the oil and water are separated, with the water portion exiting primary separator  38  via conduit  40  and the oil portion exiting primary separator  38  via conduit  42 . Optionally, a wet oil stream in conduit  44  can be combined with the separated oil-rich stream in conduit  42  and then sent to a crude heater  46 . From crude heater  46 , the heated oil-rich stream can then be conducted via conduit  48  to a secondary separator  50 . The separated water exits secondary separator  50  via conduit  52 , while the separated oil exits secondary separator  50  via conduit  54 . The oil-rich stream in conduit  54  is then introduced into an electrostatic coalescer  56  wherein a remaining portion of water is separated from the oil. The separated water exits electrostatic coalescer  56  via conduit  58 , while the separated oil-rich stream exits electrostatic coalescer  56  and separation facility  10  via oil conduit  20 . Preferably, the separated oil-rich stream in oil conduit  20  contains less than about 5 volume percent water, more preferably less than about 2 volume percent water, still more preferably less than 1 percent water, and most preferably less than 0.5 volume percent water. 
     The separated water-rich streams in conduits  40 ,  52 , and  58  are combined and carried to a produced water tank  60  via conduit  62 . In produced water tank  60 , oil and water phases are separated, with the skimmed oil exiting produced water tank  60  via conduit  64  and the water-rich stream exiting produced water tank  60  and separation facility  10  via water conduit  22   a . The skimmed oil in conduit  64  is carried to a slop oil tank  66  where the oil is separated from sand present therein. The separated oil from slop oil tank  66  can be conducted to main conduit  18  via conduit  68 , where it is combined with the produced oil and water mixture entering primary separator  38 . Slop oil tank  66  can also receive trucked-in recycle hydrocarbon fluids via conduit  70 . 
     As described above with reference to  FIG. 1 , the water-rich stream in water conduit  22   a  is first agitated in agitating means  24 , then sampled via sample line  26 , then combined with make-up water from make-up water conduit  34 , and finally pumped from injection pump  36  to injection wells via water conduit  22   c .  FIG. 2  shows that the total organic carbon analyzer system  28  can communicate with controller  30  via communication means  32 . In the embodiment illustrated in  FIG. 2 , controller  30  is operable to vary the manner in which primary separator  38  functions, to thereby adjust the amount of oil in the water-rich stream exiting separation facility  10  via water conduit  22   a . Although controller  30  is illustrated in  FIG. 2  as varying an operating parameter of primary separator  38 , it should be understood that controller  30 , or a number of different controllers, can be used at a number of different locations in separation facility  10  to control the amount of oil present in the water-rich stream which ultimately exits separator facility  10  via water conduit  22   a.    
     Referring now to  FIGS. 3 and 4 , the components that make up analyzer system  28  are illustrated in greater detail.  FIG. 3  shows analyzer system in a by-pass mode, while  FIG. 4  shows analyzer system  28  in a sampling mode. In both modes, a quill  72  is disposed in water conduit  22   b  and continuously samples a portion of the water-rich stream. Preferably, quill  72  extends into water conduit  22   b  to thereby withdraw the sample of the water-rich stream from about the middle of water conduit  22   b . The sampled portion of the water-rich stream is then carried to analyzer system  28  via sample line  26 . 
     Referring to  FIG. 3 , when analyzer system  28  is operated in the by-pass mode, the sampled portion of the water-rich stream in sample line  26  is continuously conducted through a first sample valve  74 , a second sample valve  76 , a defined sample volume  78 , a third sample valve  80 , and a by-pass conduit  82 . By-pass conduit  82  can be routed back to water conduit  22   b . Alternatively, by-pass conduit  82  can be routed to a drain. First, second, and third sample valves  74 ,  76 , and  80  are preferably electronically controlled 3-way valves. Defined sample volume  78  can be any container or conduit that defines a specific volume through which the sampled portion of the water-rich stream can continuously pass during the by-pass mode. Referring to  FIG. 3 , when analyzer system  28  is operated in the by-pass mode, a diluent, such as water, is passed through a first diluent valve  84 , a second diluent valve  86 , a defined diluent volume  88 , a third diluent valve  90 , and a drain conduit  92 . First diluent valve  84  is preferably an electronically controlled 2-way valve. Second and third diluent valves  86  and  90  are preferably electronically controlled 3-way valves. Defined diluent volume  88  can be any container or conduit that defines a specific volume through which the diluent (e.g., water) can be passed when analyzer system  28  is operated in the by-pass mode. It is preferred for the diluent to comprise substantially no organic compounds. It is further preferred for the volume of defined diluent volume  88  to be about 1 to about 100 times greater than the volume of defined sample volume  78 , more preferably about 4 to about 25 times greater, and most preferably 6 to 12 times greater. 
       FIG. 3  depicts analyzer system  28  as including a total organic carbon analyzer  94  which is fluidly connected to second sample valve  76  via sample line  96 . When analyzer system  28  is operated in the by-pass mode, second sample valve  76  prevents any of the sampled portion of the water-rich stream from entering total organic carbon analyzer  94 . It can be seen from  FIG. 3  that when analyzer system  28  is operated in the by-pass mode, defined diluent volume  88 , defined sample volume  78 , and total organic content analyzer  94  are fluidly isolated from one another. Total organic carbon analyzer  94  can be any conventional total organic carbon analyzer known in the art. Preferably, total organic carbon analyzer  94  is operable to receive a defined volume of a liquid sample, oxidize the organic components of the liquid sample, and measure the carbon dioxide formed via oxidization of the organic compounds in the liquid sample. The measured total organic carbon content of the liquid sample can then be communicated to a control device via communication means  32 . Various total organic carbon analyzers are well known in the art. A particularly preferred total organic carbon analyzer is available from Star Instruments, Inc., League City, Tex. under the commercial designation of “Ultra Pure Pumpless Total Organic Carbon Analyzer Model 4000.” As shown in  FIG. 3 , analyzer system  28  further includes a gas valve  98  which controls the flow of a gas, typically air, to second diluent valve  86 . Gas valve  98  is preferably an electronically controlled 2-way valve. When analyzer system  28  is operated in the by-pass mode, gas valve  98  prevents the gas from flowing to second diluent valve  86 . 
       FIG. 4  shows analyzer system  28  operating in the sampling mode. When analyzer system  28  is switched from the by-pass mode in  FIG. 3  to the sampling mode in  FIG. 4 , each of valves  74 ,  76 ,  78 ,  80 ,  84 ,  86 ,  90 , and  98  are adjusted. When analyzer system  28  operates in the sampling mode, the sampled portion of the water-rich stream from water conduit  22   b  is continuously carried through sample line  26 , first sample valve  74 , and by-pass line  82 . The sampled portion in by-pass line  82  can then be routed back to conduit  22   b  for combining with the water-rich stream. Alternatively, the sampled portion in by-pass conduit  82  can be routed to a drain. It can be seen from  FIG. 4  that when analyzer system  28  operates in the sampling mode, defined diluent volume  88 , defined sample volume  78 , and total organic content analyzer  94  are in fluid flow communication with one another, with defined sample volume  78  being fluidly disposed between defined diluent volume  88  and analyzer  94 . 
     When analyzer system  28  is switched from the by-pass mode ( FIG. 3 ) to the sampling mode (FIG.  4 ), a gas is conducted through gas valve  98 , second diluent valve  86 , defined diluent volume  88 , third diluent valve  90 , third sample valve  80 , defined sample volume  78 , second sample valve  76 , and sample line  96 . As such, the gas effectively pushes the diluent in defined diluent volume  88  and the water-rich sample in defined sample volume  78  into total organic carbon analyzer  94 . The diluent from defined diluent volume  88  is used to flush the water-rich sample out of the defined sample volume  78 . In total organic carbon analyzer  94 , the diluent from defined diluent volume  88  and the water-rich sample from defined sample volume  78  are combined to produce a diluted sample. The volumetric ratio of defined sample volume  78  to defined diluent volume  88  can be determined by the amount of the diluent required to adequately flush the sample from defined sample volume  78 , as well as the sensitivity of total organic content analyzer  94 . It is preferred for the volume of defined diluent volume  88  to be 1 to 100 times greater than the volume of defined sample volume  78 , more preferably 4 to 25 times greater, and most preferably 6 to 12 times greater. It is also preferred for analyzer system  28  to continuously switch back and forth between the by-pass mode and the sampling mode during normal operation. Preferably, analyzer switches between the by-pass and sampling modes at least every 12 hours, more preferably at least every 6 hours, still more preferably at least every 1 hour, yet still more preferably at least every 0.25 hour, and most preferably at least every 0.1 hour. 
     The preferred forms of the invention described above are to be used as illustration only, and should not be used in a limiting sense to interpret the scope of the present invention. Obvious modifications to the exemplary embodiments, set forth above, could be readily made by those skilled in the art without departing from the spirit of the present invention. 
     The inventors hereby state their intent to rely on the doctrine of equivalents to determine and assess the reasonably fair scope of the present invention as it pertains to any apparatus not materially departing from but outside the literal scope of the invention as set forth in the following claims.