Water cut monitoring system

Abstract: Fluid from a producing well is accumulated in an accumulator (10) and allowed to separate into free water, a water-continuous phase and an oil-continuous phase; the separated fluid flows through a monitor (30) having electrodes (104-106;152-157) exposed to the fluid; a power source (125) provides an AC voltage and current to the electrodes and provides an injection voltage signal (Vi) and an injection current signal (Ii); a generator (125) provides a phase angle signal (PH), a sensor connected to another said electrode provides a sensed voltage signal (VS), and a sensor provides a temperature signal (T); and a processor (22) provides a water content signal (WC) in accordance with signals Vi,Ii,PH,VS and T and depending on whether the fluid is in the water-continuous or in the oil-continuous phase.

Description: The present invention relates to water cut monitors, i.e. to monitors for providing a signal representative of the percentage water content in a fluid stream. The present invention as defined in claim 1 provides a water cut (percentage) monitor which includes a settling tank in which a quantity of fluid from a producing oil well is accumulated. The fluid is removed as a stream of fluid after a predetermined interval from the accumulation of the fluid by the settling tank so as to allow the accumulated fluid to separate into three phases: free water, water-continuous and oil-continuous. The flow rate of the stream of fluid is measured and a corresponding flow rate signal is provided as well as a temperature signal representative of a sensed temperature of the stream of fluid. The water cut monitor includes a plurality of electrodes in contact with the fluid. Injection electronics connected to at least one of the electrodes provides an injection voltage and injection current to the fluid stream and also provides signals corresponding to the injection voltage, injection current and the phase angle between the injection voltage and injection current. A voltage in the fluid stream is measured and a measured voltage signal provided. A computer determines the water cut of the petroleum stream in accordance with the temperature signal, the injection voltage, the electrical phase angle and injection current signals and the measured voltage signal. Advantageous features are defined by the dependent claims. 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 are illustrated by way of example. It is to be expressly understood, however, that the drawings are for illustrative purposes only. Figure 1 is a drawing of a water cut monitoring means and method constructed in accordance with the present invention. Figure 2 is a flow diagram of the operation of the water cut monitoring means shown in Figure 1. Figure 3 is a partial simplified block diagram and a partial schematic diagram of one embodiment of the water cut meter shown in Figure 1. Figure 4 is a partial simplified block diagram and a partial schematic diagram of another embodiment of the water cut meter shown in Figure 1. Oil field production is defined in terms of barrels of oil per day (BOPD) and barrels of water per day (BWPD). These figures are determined by the measurement of flow rate and water cut. Many commercial flowmeters are available which yield satisfactory flow values. A number of commercial water cut meters are also available. Most of these meters, however, are designed for use only when the oil/water mixture is oil continuous, i.e., when any water present is suspended within the oil. These meters function purely as a capacitance measurement and operate properly only when a non-conductive (high impedance) path exists across the measurement electrodes. In water-continuous emulsions, a conducting path exists across the probe which essentially "shorts out" the capacitance measurement. The shorting effect is highly sensitive to salinity and temperature changes and renders standard capacitance probes useless in water-continuous operation when only a small amount of salt is present. The present invention is inexpensive and can operate over the entire 0 to 100 percent range of water cut, even where high water salinities and heavy oil emulsions are encountered. With reference to Figure 1, there is shown a well under test 3 providing a production stream to the settling tank 10 via line 14. After a predetermined quantity of the production stream has entered settling tank 10, further filling of settling tank 10 is stopped. Over a predetermined period of time the fluid in settling tank 10 will partially separate to provide free water at the bottom of the tank 10; above this, the water-continuous emulsion exists, and finally at the top of tank 10 the fluid is an oil-continuous emulsion. After the predetermined period of time has elapsed, a pump 20, controlled by the AWT process controller 21, pumps the fluid from the bottom of tank 10 through a flowmeter 25 and thence through a water cut meter (water content monitor) 30, and finally discharges the fluid to the production system, all via a line 23. Water cut meter 30 provides five signals to process means 22: the injection voltage V i the injection current I i , the phase angle PH, the sensed voltage V s , and the temperature T. The obtaining of those signals and their utilization will be explained more fully hereinafter. Initial fluid pumped from the bottom of tank 10 is practically always water. The water cut system of the present invention measured the impedance of the water (Z water) and retains this value. When the water-continuous emulsion begins to flow through water cut meter 30 the impedance (Z emulsion) increases. Dividing Z emulsion by Z water gives Z ratio, which is proportional to the percent water cut of the water-continuous emulsion. When the emulsion switches to the oil-continuous phase, the electrical phase angle between V i and I increases dramatically. This increase is used to change the measurement method to an inverse relationship between the injection impedance and percent water cut. The present invention can switch back and forth between oil and water-continuous phases with no loss of accuracy. In the event no free water is present in tank 10, a default value of Z water typical of low water wells is assigned. Referring to Figure 2, which is a flow chart, block 50 represents pump 20 being turned on by the pump signal. The pump signal is used to initialize all the coefficients of values required for all calculations as represented by block 51 entitled "INITIALIZE". The following values are initialized: water-continuous equation coefficients A, B, C, D and E; oil-continuous equation coefficients F, G and H; sensed impedance constant KA; temperature curve coefficients AA, BB, and water impedance initial value SWINIT. From block 51 we go to block 53 entitled "CONTINUE MODE" which means process means 22 continues to the next step which is block 55. Block 55, entitled "ACQUIRE DATA", represents the acquisition of data signals V i , I i , PH, V s and temperature. Block 59 is the next step which calculates averages of the acquired data. From block 59 we proceed to block 62 which raises the question: is number of samples ten? If the answer is no, the system is recycled back to "CONTINUE" block 53 and measurements made again and the calculation goes on again until ten samples are collected and averaged, at which point block 62 provides a "yes" answer to block 63 which asks "is the phase angle greater than 80 degrees?" If PH> 80 the fluid is oil-continuous and block 88 is used to calculate the injection impedance Z i and block 90 calculates water cut using the equation: 1. WC = F+G (Z i ) + H (Z i )² If PH< 80, block 65 calculates sense impedance as: 2. Z s = (V s /I i ) x KA Z s is then corrected for temperature change using the equations: 3. Z s cpr = Z s + KB x Z s x (DEGC - 40) 4. DEGC = AA + BB x Temp All of this is represented by block 67 entitled "Correct Z s for temperature". From block 67 we proceed to block 72 which makes the statement: If Z s corrected is less than Z water, set Z water equal to Z s corrected. This procedure determines the water impedance Z water to be the lowest impedance measurement. If Z s corrected is greater than Z water, the value of Z water is not changed and the next step is represented by block 75 entitled "Calculate Impedance Ratio Zrat". Zrat equals Zscor/Zwater. Finally as represented by block 78, the value of Zrat is used to calculate the water cut of the water-continuous fluid in accordance with the following equation 5: 5. WC = A+B(Zrat) + C(Zrat)² + D(Zrat)³ + E(Zrat)⁴ This calculated-value WC is provided by process means 22 in the form of signal WC to the AWT process controller 21. Block 83 inquires if the pump is still on. If so, the processor loops back to block 53 where a new measurement begins. If the pump has gone off, the measurement process is halted (block 95). Coefficients for the water-continuous water cut equation are empirically determined by recording values of Zrat as fluid samples are taken. Samples are then analyzed to determine the actual percentage water cut. A plot of water cut WC versus Zrat is then made using computer curve-fitting software. This software can curve fit the plotted points and give the coefficients of the resultant polynomial curve. Data are taken for several different water salinities and a curve is determined which best fits all the data points. Coefficients from this curve are then used in the water-continuous polynomial equation. Coefficients for the oil-continuous water cut equation are determined in a similar manner by simultaneously recording values of the injection impedance, Zi, and taking water cut samples when the emulsion is known to be oil-continuous. A curve fit for the WC versus Zi data is then obtained from the curve-fitting program and the coefficients determined. These coefficients are then entered into oil-continuous WC equation 1. In one embodiment of the present invention, water cut meter 30 is arranged as shown in Figure 3. Test cell 100 includes a housing 101 with flow line adapter pipes 103. Housing 101 has a square cross-section to the fluid flow. Flow line adapters 103 permit connecting of housing 100 to line 23. Test cell 100 also contains three electrodes 104, 105, and 106, mounted along with insulator 108 to form a sensing body 110 having end caps 119. Excitation electronics 125 is connected between ground 118 and electrode 104. Excitation electronics (power source) 125 has within it an oscillator which provides an AC injection voltage across to an electrode 104 and ground 118 causing an injection current to flow within the fluid. Although the injection voltage in the present invention may be used at substantially lower frequencies or at higher frequencies, a preferred frequency range is from 1 MHz to 50 MHz. Also within excitation electronics 125 is circuitry that provides signals V i and I i , corresponding to the injection voltage and the injection current, respectively. The injection signal provided to electrode 104 creates an electric field within the fluid. Electrodes 105 and 106 are electrically connected to sensing electronics 130 which senses the voltage picked up across electrodes 105 and 106 and provides a corresponding signal Vs. A temperature sensor 137 senses the temperature of the fluid flowing in housing 23 and provides a corresponding temperature signal T. Water cut meter 30 further includes a phase angle generator to provide a signal PH representative of the phase angle between the injection voltage and the injection current. Referring now to Figure 4 there is shown another embodiment of water cut meter 30 which is non-intrusive in nature. That is, electrodes as hereinafter explained, do not intrude into the path of the fluid flow. As shown, a test cell 150 is connected inline with line 23. Although the actual fittings are not shown, obviously test cell 150 includes means for being connected inline to line 23. Test cell 150 may be basically made of metal except where noted otherwise. In this regard, test cell 150 includes four electrodes 152, 154, 156 and 157, mounted so that they are inline with the inner surface of line 23. As can be seen in Figure 4, electrodes 152 through 157 are circular in nature and are in contact with the fluid passing through the meter. Electrodes 152 through 157 are separated from a metallic body 160 of test cell 150 by polytetrafluoroethylene (Teflon) spacers 165 through 171 which are concentric within Teflon body 180 which is contained within metal cylinder 185. Further, cylinder 185 has openings 190 which allow the electrical connections of 152 and 154 to excitation electronics 125 and also allows the connections of electrodes 156 and 157 to sensing electronics 130. Further, there are O-rings 195 between each interface of an insulator ring and an electrode and an insulator spacer and metallic body 160. The operation of the second embodiment is the same as the first embodiment with the difference being that the injection current between electrodes 152 and 154 produces an electric field which is parallel to the fluid flow instead of perpendicular to the flow as in the first embodiment. The temperature is sensed by a temperature sensor 137 as previously described for the other embodiment.

Claims: A water content monitoring system comprising: an accumulator (10) for accumulating a quantity of fluid from a producing well whereby to permit the fluid to at least partially separate into three phases: free water, a water-continuous phase and an oil-continuous phase; outlet means (20) operable to cause the fluid to flow as a stream from the accumulator (10) after a predetermined time interval to allow said separation; a temperature sensor (137) operable to sense the temperature of the fluid stream and to provide a temperature signal (T) representative thereof; and a water content monitor (30) comprising: a plurality of electrodes (104-106;152-157) exposed to said fluid stream, a power source (125) connected to provide an AC voltage and current to a said electrode (104;152,154) for injection into said stream and to provide signals representative of the injection voltage and injection current (Vi;Ii), a phase angle generator operable to provide a signal (PH) representative of the phase angle between the injection voltage and the injection current, a voltage sensor (130) connected to another said electrode (105,106;156,157) to provide a signal (Vs) representative of the sensed voltage, and a signal processor (22) connected to said temperature sensor (137), said power source (125), said phase angle generator and said voltage sensor (130) to provide a signal representative of the water content in accordance with the temperature signal (T), the injection voltage signal (Vi), the injection current signal (Ii), the phase angle signal (PH) and the sensed voltage signal (Vs). A system according to claim 1 characterized in that said signal processor (22) is operable to derive the injection impedance (Zi) in accordance with the injection voltage signal (Vi) and the injection current signal (Ii), and to provide said water content signal (WC) when the fluid is in the oil-continuous phase in accordance with the equation: WC = F + G (Zi) + H (Zi)², where F, G and H are empirically determined coefficients. A system according to claim 1 or claim 2 characterized in that said signal processor (22) is operable to determine the sensed impedance (Zs) in accordance with the sensed voltage signal (Vs) and the injection current signal (Ii), to correct the sensed impedance (Zscor) in accordance with the temperature signal (T), and to determine the impedance ratio (Zrat) in accordance with the equation: Zrat = (Zscor)/(Zwtr), where Zwtr is the impedance of free water and may be sensed or predetermined. A system according to claim 3 characterized in that said signal processor (22) is operable to provide said water content signal (WC) when the fluid is in the water-continuous phase in accordance with the equation: WC=A+B (Zrat)+C (Zrat)² + D (Zrat)³ + E (Zrat)⁴, where A, B, C, D and E are coefficients. A system according to any one of claims 1 to 4 characterized in that said signal processor (22) is responsible to said phase angle signal (PH) to determine that the fluid is in the oil-continuous phase when the phase angle signal is greater than a predetermined value and is in the water-continuous phase when the phase angle signal is below said predetermined value. A system according to any one of claims 1 to 5 characterized by a flow rate sensor (25) to provide a signal representative of the flow rate of said stream from the accumulator (10), and said signal processor (22) is responsive to the flow rate signal and the water content signal (WC) to determine the total water content of the fluid accumulated in the accumulator (10). A system according to any one of claims 1 to 6 characterized in that said power source (125) is operable to provide said AC voltage at a frequency in the range of from 1 MHz to 50 MHz. A system according to any one of claims 1 to 7 characterized in that said water content monitor (30) includes a test cell (100) connected in a fluid flow line from said outlet means (20), and said electrodes (104-106;152-157) are mounted within said test cell. A system according to claim 8 characterized in that said power source (125) is connected to a first pair of said electrodes (152,154) to cause said injection current to flow through the fluid stream from one to the other electrode of said first pair, and said voltage sensor (130) is connected to a second pair of said electrodes (156,157) spaced apart from said first pair to sense the voltage in the stream across the electrodes of the second pair. A system according to claim 8 characterized in that said power source (125) is connected to a first said electrode (104) and to ground (118) to cause said injection current to flow through the fluid stream from said first electrode to ground, and said voltage sensor (130) is connected to second and third said electrodes (105,106) spaced apart from said first electrode and from ground to sense the voltage in the stream across the second and third electrodes.

Inventors: COX PERCY T, GRAY CHARLES L JR, NUSSBAUM THEODORE W, COX, PERCY TERRELL, GRAY, CHARLES LOUIS, JR., NUSSBAUM, THEODORE WILLIAM
Applicants: TEXACO DEVELOPMENT CORP, TEXACO DEVELOPMENT CORPORATION
Application year: 1991
Main classification: E21B49
IPC codes: G01N27, G01F1, G01N33, G01F15, B01D17, E21B47