METHOD FOR DETERMINING THE LIQUID FACTOR OF A FLOWING MEDIUM USING A NUCLEAR MAGNETIC FLOW MEASURING DEVICE

Description

[0001] The invention relates to a method for determining the liquid content of a flowing medium using a nuclear magnetic flowmeter. The nuclear magnetic flowmeter comprises a measuring tube with a measuring volume, a measuring device, and a magnetization device. The medium flows through the measuring tube and has both a liquid and a gaseous component. A magnetic field along a magnetic field direction is generated in the measuring volume by the magnetization device, and the medium is magnetized by the magnetic field. This means that the medium is exposed to the magnetic field as it flows through the measuring volume and is magnetized by it.

[0002] The invention lies in the field of nuclear magnetic flowmeters. The measuring device of a nuclear magnetic flowmeter is designed to perform measurements on a medium flowing through the measuring volume of the measuring tube and to determine information about the medium from the measurements. When a measurement is performed by the measuring device, a precession of atomic nuclei of the medium is influenced in the presence of the magnetic field that previously magnetized the medium by exciting the atomic nuclei to nuclear magnetic resonances, and the nuclear magnetic resonances are measured. A nuclear magnetic resonance can occur, for example, in the form of a free induction decay (FID) or a spin echo. Such measurements are also referred to as magnetic resonance measurements. EP 2 910 907 A1 describes a corresponding nuclear magnetic flowmeter according to the prior art.

[0003] Precession is a property of atomic nuclei of elements that exhibit nuclear spin. Nuclear spin can be thought of as an angular momentum described by a vector, and accordingly, the magnetic moment caused by the nuclear spin can also be described by a vector parallel to the angular momentum vector. The presence of a macroscopic magnetic field causes an excess of atomic nuclei in a medium with magnetic moments aligned parallel to the macroscopic magnetic field, causing the medium to exhibit a macroscopic magnetization that can be described in its entirety by a vector. In the presence of a macroscopic magnetic field, the vector of the magnetic moment of an atomic nucleus precesses around the vector of the macroscopic magnetic field at the location of the atomic nucleus. This is the property of precession.The frequency of precision is called the Larmor frequency f L and is proportional to the magnitude of the magnetic flux density B of the macroscopic magnetic field. The Larmor frequency is calculated according to f L = (γ / 2). π ) B. Here, γ is the gyromagnetic ratio, which is maximum for hydrogen nuclei. The Larmor frequency is also referred to simply as frequency.

[0004] These magnetic moments and precessions are influenced by electromagnetic pulses that exert a torque on the magnetic moments, thereby rotating them by angles defined by the magnetic moment vector and the macroscopic magnetic field. When reference is made to pulses in the following, these pulses are meant.

[0005] Since the medium has a liquid and a gaseous component, it also has at least two phases. The medium is thus a multiphase medium. For example, the gaseous component is one phase, and the liquid component is another. Often, the liquid component also has more than one phase, for example, crude oil and (salt) water. The gaseous phase is, for example, natural gas.

[0006] To determine information about the individual phases of a multiphase medium, the atomic nuclei of the individual phases must be excitable to distinguishable nuclear magnetic resonances. For example, nuclear magnetic resonances can be distinguished from one another if the spin-lattice relaxations of the individual phases have different spin-lattice relaxation time constants T 1 . The spin-lattice relaxation time constant is a property of a phase. Other properties include a spin-spin relaxation time constant T 2 and a diffusion time constant D. Using at least one of the listed properties, information about the phases of the medium can be determined. Examples of information include flow velocities of individual phases of the medium through the measuring tube and a liquid fraction of the medium in the measuring tube. Properties of the phases are also counted as information about the individual phases.

[0007] The phases crude oil, (salt) water, and natural gas have already been mentioned. These are also the essential phases of a multiphase medium produced from an oil well. Since the atomic nuclei of all these phases contain hydrogen nuclei, and the crude oil and (salt) water phases in particular are usually characterized by different spin-lattice relaxation time constants T1, nuclear magnetic flowmeters are particularly suitable for determining information from media produced from oil wells.

[0008] One important piece of information is the previously mentioned liquid portion of the medium in the measuring volume of the measuring tube, which is also known as the pipe filling factor or liquid holdup. The liquid portion is, for example, the quotient of the volume occupied by the liquid portion in the measuring volume as the dividend and the measuring volume as the divisor. The measuring volume is completely occupied by the volume of the liquid portion and the volume of the gaseous portion. State of the art knows how to determine the liquid portion by first exciting a nuclear magnetic resonance in the entire liquid portion of the medium in the measuring volume with an excitation signal and then measuring this as a measurement signal. The amplitude of the measurement signal is then determined, and the liquid portion is derived from the amplitude. This method of determining the liquid portion has various disadvantages.

[0009] A disadvantage is that the gaseous component can also be excited to a nuclear magnetic resonance by the excitation signal and this can then also be measured, so that it is present in the measurement signal and increases the amplitude and thus distorts it.

[0010] A further disadvantage is that nuclear magnetic resonance cannot be measured immediately after the excitation signal, but only after it has decayed, as otherwise the excitation signal would distort the measurement signal. When determining the liquid fraction from the amplitude, the amplitude must be determined at the time of the excitation signal, which requires extrapolation from the time of measurement to the time of excitation. Various effects must be taken into account in this extrapolation. For example, it must be taken into account that the magnetized medium with nuclear magnetic resonance flows out of the measurement volume at a certain velocity. Furthermore, it must be considered that nuclear magnetic resonance decreases due to relaxation effects, which are described, for example, by the spin-spin relaxation time constant T 2.The accuracy of the extrapolation thus depends on the accuracy of the knowledge of the velocity and the spin-spin relaxation time constants. Other effects also affect the accuracy of the extrapolation.

[0011] A further disadvantage is that inaccuracies in the knowledge of the magnetization of the medium in the measuring volume lead to a corresponding inaccuracy in the amplitude and thus in the liquid content.

[0012] Therefore, it is an object of the invention to provide a method which at least mitigates at least one of the disadvantages mentioned.

[0013] The problem is solved by a method having the features of patent claim 1.

[0014] In a gradient step, the magnetization device generates a gradient magnetic field with a strength gradient along a first gradient direction in the measurement volume, and the medium is exposed to the gradient magnetic field in addition to the magnetic field. Thus, the flowing medium in the measurement volume is exposed to both the magnetic field and the gradient magnetic field simultaneously.

[0015] The following measuring steps are then carried out by the measuring device: Generating and radiating an excitation signal into the magnetized medium in the measurement volume to excite nuclear magnetic resonances in the magnetized medium along the first gradient direction. Due to the gradient magnetic field, the excited nuclear magnetic resonances differ from one another along the first gradient direction. Measuring nuclear magnetic resonances excited by the excitation signal in the magnetized medium along the first gradient direction in the measurement volume as a measurement signal. Thus, nuclear magnetic resonances in the medium that were previously excited by the excitation signal are measured. Determining frequencies of the nuclear magnetic resonances in the measurement signal along the first gradient direction. Determining the frequencies includes, for example, Fourier transformations. By superimposing the magnetic field with the gradient magnetic field, the frequencies are determined according to f L = (γ / 2 π) B depends on the position of the medium along the first gradient direction. B is the magnetic flux density resulting from the superposition of the magnetic field and the gradient magnetic field. An infinitesimally thin slice of the medium in the measurement volume, oriented orthogonally to the first gradient direction, exhibits a frequency. For example, if the strength of the gradient magnetic field increases in the first gradient direction, the frequencies in the first gradient direction in the slices of the medium also increase according to the gyromagnetic ratio.

[0016] The measuring steps are carried out while the flowing medium is essentially in the measuring volume.

[0017] The following evaluation steps are then carried out by the measuring device: Assigning the determined frequencies to positions along the first gradient direction. For example, thin slices of the medium in the measurement volume, which are arranged orthogonal to the first gradient direction, are assigned to the positions, whereby the slices are so thin that they are each assigned a frequency. Assigning the nuclear magnetic resonances at the positions to the liquid portion and the gaseous portion of the medium. Since the individual phases of the medium and thus also the liquid and gaseous portions of the medium are excited to distinguishable nuclear magnetic resonances, the nuclear magnetic resonances at the positions can also be assigned to either the liquid or the gaseous portion of the medium. The assignment of the nuclear magnetic resonances to the liquid and gaseous portions is preferably carried out by evaluating the strength of the nuclear magnetic resonances, such as amplitudes.The strength of a nuclear magnetic resonance of the liquid portion is greater than the strength of a nuclear magnetic resonance of the gaseous portion. Determine the liquid portion of the flowing medium in the measurement volume from the positions of the nuclear magnetic resonances of the liquid portion and the positions of the nuclear magnetic resonances of the gaseous portion. The measurement volume, especially its geometry, is also taken into account during the determination.

[0018] The method according to the invention for determining the liquid fraction at least mitigates the disadvantages outlined above. Unintentional excitation of nuclear magnetic resonances in the gaseous fraction does not occur. The fact that a nuclear magnetic resonance cannot be measured immediately after the excitation signal, but only after it has begun to decay, has no negative effects. Furthermore, inaccuracies in the knowledge of the medium's magnetization do not lead to falsified measurements of the liquid fraction.

[0019] In a first embodiment of the method, the measuring tube is oriented horizontally and the first gradient direction is oriented vertically. The information horizontal and vertical refers to the gravitational field vector. A horizontal orientation is perpendicular to the gravitational field vector and a vertical orientation is parallel to it. The horizontal alignment of the measuring tube causes a horizontal stratification of the liquid and gaseous components in the measuring tube due to gravity. Due to the vertical orientation of the first gradient direction, planes perpendicular to the first gradient direction, which are also horizontal, pass through the positions along the first gradient direction. Because the liquid and gaseous components of the medium are layered horizontally and the planes are also aligned horizontally, a transition between the liquid and gaseous components can be determined more precisely.

[0020] The magnetic field has a certain homogeneity and is, in this sense, homogeneous. This means that the magnetic field exhibits small fluctuations. For example, the magnetic field has a magnetic flux density of 0.2 T and the fluctuations amount to 1000 ppm. This results in a frequency uncertainty of Δf L = (γ / 2 π ) ΔB = 42.58 MHz / T · 0.2 T · 1000 ppm = 8.5 kHz. The gradient magnetic field, for example, has a strength gradient of G = 0.25 mT / cm. The frequency uncertainty then corresponds to a position uncertainty of Δd = ΔB / G = 0.8 cm. If the measuring tube has an inner diameter of, for example, 10 cm and the first gradient direction coincides with this, then the position uncertainty of 0.8 cm corresponds to a precision of 0.8 cm / 10 cm = 8%.

[0021] To achieve the required precision, a further embodiment of the method provides for determining the homogeneity of the magnetic field and determining the strength of the strength gradient based on the determined homogeneity and a required precision along the gradient direction. The required precision is preferably within 10%, more preferably within 5%, and particularly preferably within 1% of a distance along the gradient direction in the measurement volume. The homogeneity of the magnetic field is preferably determined by the magnetization device. The distance along the gradient direction is, for example, a diameter of the measuring tube.

[0022] According to the invention, the excitation signal comprises at least one excitation pulse. An excitation pulse, which is radiated into the magnetized medium in the measurement volume, excites nuclear magnetic resonances in the magnetized medium along the first gradient direction. For this purpose, the excitation pulse must have a specific bandwidth. The bandwidth is determined such that an excitation pulse completely excites the medium along the first gradient direction in the measurement volume to nuclear magnetic resonances. Furthermore, the invention provides that the magnetization device determines a bandwidth for the at least one excitation pulse based on a homogeneity of the magnetic field and a strength of the strength gradient. Preferably, the at least one excitation pulse is generated to rotate magnetic moments of the magnetized medium by angles between 5° and 45°, and more preferably by angles of 90°.Preferably, a hard, soft, adiabatic, CHIRP or SAUSAGE pulse is used as the at least one excitation pulse.

[0023] An excitation pulse typically does not excite nuclear magnetic resonances that have different frequencies with the same efficiency. Accordingly, the efficiency of the excitation of nuclear magnetic resonances is frequency-dependent. As a result, nuclear magnetic resonances of different frequencies also have different strengths, which represents a disadvantage in the evaluation steps. Therefore, a further development of the above embodiment provides that the magnetization device determines the strength of the nuclear magnetic resonances with respect to their frequencies and that the strength of the nuclear magnetic resonances is normalized with respect to their frequencies, so that a frequency-dependent efficiency of the excitation of the nuclear magnetic resonances by the at least one excitation pulse is compensated.

[0024] In a further embodiment, the excitation signal comprises at least one refocusing pulse. A refocusing pulse, which is irradiated into the magnetized medium in the measurement volume, refocuses a previously excited nuclear magnetic resonance. Preferably, the at least one refocusing pulse is generated to rotate magnetic moments of the magnetized medium by angles of 45° and more preferably by angles of 180°. Preferably, a hard, a soft, an adiabatic, a CHIRP, or a SAUSAGE pulse is used as the at least one refocusing pulse.

[0025] In a further embodiment, the excitation signal comprises a multi-echo pulse sequence, and nuclear magnetic resonances excited in the magnetized medium by the multi-echo pulse sequence are measured and evaluated. A multi-echo pulse sequence comprises at least one excitation pulse and at least one refocusing pulse, preferably at least two refocusing pulses. The measurement signal comprises the nuclear magnetic resonances excited in the magnetized medium by the multi-echo pulse sequence. By using the multi-echo pulse sequence, the measurement signal contains more information than is required to determine the fluid content. Preferably, the nuclear magnetic resonances excited in the magnetized medium by the multi-echo pulse sequence are evaluated by determining at least one flow velocity of the magnetized medium. Thus, further evaluation steps are performed.The analysis thus yields the fluid fraction and, in this example, the flow velocity. A CPMG, SSFP, or MMME pulse sequence is preferably used as the multi-echo pulse sequence. CPMG stands for Carr-Purcell-Meiboom-Gill, SSFP for Steady-State Free Precession, and MMME for Multiple Modulation Multiple Echoes.

[0026] In a further embodiment, the excitation signal comprises an excitation pulse followed by a refocusing pulse, and a spin echo excited by the refocusing pulse is measured. Preferably, the refocusing pulse is generated to rotate magnetic moments in the magnetized medium by angles of 180°.

[0027] In a further development, the excitation signal comprises a CPMG pulse sequence, and the excitation pulse and the refocusing pulse are part of the CPMG pulse sequence. While the excitation pulse and the refocusing pulse provide the information in the measurement signal that enables the determination of the liquid content, the additional components of the CPMG pulse sequence provide further information in the measurement signal that can be evaluated. Accordingly, the further development also provides for the measurement and evaluation of nuclear magnetic resonances excited by the CPMG pulse sequence. In particular, the nuclear magnetic resonances are evaluated by determining at least one flow velocity of the magnetized medium. Thus, further evaluation steps are carried out by the measuring device.

[0028] In a further embodiment, the excitation signal comprises a first excitation pulse followed by a first refocusing pulse and a second refocusing pulse followed by a second excitation pulse. The second excitation pulse serves to compensate for any remaining nuclear magnetic resonances. Furthermore, a spin echo between the first refocusing pulse and the second refocusing pulse is measured as a measurement signal or as part of the measurement signal. Preferably, the second excitation pulse has a phase shift of 180° compared to the first excitation pulse. This part of the measurement signal contains the information for determining the fluid content.

[0029] In a further development of the above embodiment, the excitation signal comprises a CPMG pulse sequence, and the CPMG pulse sequence follows the second excitation pulse. Between the second excitation pulse and the CPMG pulse sequence, the gradient magnetic field is reduced, preferably to zero. Furthermore, nuclear magnetic resonances excited by the CPMG pulse sequence are measured and evaluated, in particular evaluated by determining at least one flow velocity of the magnetized medium. The nuclear magnetic resonances excited by the CPMG pulse sequence are a further part of the measurement signal, and thus, further evaluation steps are performed by the measuring device.

[0030] In an alternative development of the above embodiment, the excitation signal comprises an MMME pulse sequence, and the MMME pulse sequence follows the second excitation pulse. Between the second excitation pulse and the MMME pulse sequence, the gradient magnetic field is initially reduced to zero and then increased again in a second gradient direction. Preferably, the second gradient direction is oriented perpendicular to the first gradient direction, in particular parallel to a flow direction of the medium. Furthermore, nuclear magnetic resonances excited by the MMME pulse sequence are measured and evaluated, in particular evaluated by determining at least one flow velocity of the magnetized medium. The nuclear magnetic resonances excited by the CPMG pulse sequence are a further part of the measurement signal, and thus, further evaluation steps are carried out by the measuring device.

[0031] In a further embodiment of the method, the gradient step is performed again with a third gradient direction that differs from the first gradient direction. The measurement steps and the evaluation steps are then performed again. Furthermore, the precision of the determination of the liquid fraction is improved by combining the two determinations of the liquid fraction. Preferably, the third gradient direction is perpendicular to the first gradient direction.

[0032] In detail, there are numerous possibilities for designing and developing the method. Reference is made to the claims subordinate to claim 1 as well as to the following description of preferred embodiments in conjunction with the drawing. The drawing shows Fig. 1 shows an embodiment of a nuclear magnetic flowmeter for carrying out an embodiment of a method for determining a liquid content, Fig. 2 shows a first embodiment of a signal sequence of the method, Fig. 3 shows a second embodiment of a signal sequence of the method, Fig. 4 shows a third embodiment of a signal sequence of the method and Fig. 5 shows a fourth embodiment of a signal sequence of the method.

[0033] Fig. 1 shows an embodiment of a nuclear magnetic flowmeter 1, which is designed to carry out the embodiment of a method for determining a liquid content of a flowing medium described below. The illustration of the nuclear magnetic flowmeter 1 is partially sectioned.

[0034] The nuclear magnetic flowmeter 1 comprises a measuring tube 2 with a measuring volume 3, a measuring device 4, and a magnetization device 5. The measuring tube 2 is oriented horizontally.

[0035] The nuclear magnetic flowmeter 1 is in operation, which is why a medium 6 flows through the measuring tube 2. The flow and its direction are indicated by an arrow. The medium 6 has a liquid portion 7 and a gaseous portion 8. The liquid portion 7 is one phase, and the gaseous portion 8 is another phase. The horizontal orientation of the measuring tube 2 causes a horizontal stratification of the liquid portion 7 and the gaseous portion 8 in the measuring tube 2 due to gravity. Furthermore, the nuclear magnetic flowmeter 1 performs the process described below.

[0036] The magnetization device 5 generates, on the one hand, a magnetic field 9 along a magnetic field direction 10 in the measurement volume 3 and, on the other hand, a gradient magnetic field 11 with a strength gradient along a first gradient direction 12 in the measurement volume. The magnetic field direction 10 and the gradient field direction 12 are oriented parallel to each other and perpendicular. Fig. 1 A superposition of the magnetic field 9 and the gradient magnetic field 11 is shown. The medium 6 is exposed to both the magnetic field 9 and the gradient field 11 in the measuring volume 3, whereby it is magnetized by the magnetic field 9, so that nuclear magnetic measurements can be performed on the magnetized medium 6.

[0037] The embodiments of the method described below are each based on the steps described above and carried out by the magnetization device 5.

[0038] In one embodiment, see Fig. 2, the measuring device 4 performs the following steps: Generating and radiating an excitation signal into the magnetized medium 6 in the measurement volume 3 to excite nuclear magnetic resonances in the magnetized medium 6 along the first gradient direction 12. The excitation signal comprises a CPMG pulse sequence 13 with an excitation pulse 14 and a refocusing pulse 15. The refocusing pulse 15 follows the excitation pulse 14. The refocusing pulse 15 is generated to rotate magnetic moments in the magnetized medium 6 by angles of 180°. Measuring nuclear magnetic resonances excited by the excitation signal, i.e., the CPMG pulse sequence 13, in the magnetized medium 6 along the first gradient direction 12 in the measurement volume 3 as a measurement signal. In particular, a spin echo 16 excited by the refocusing pulse is measured. Determining frequencies of the spin echo 16 along the first gradient direction 12. Assigning the determined frequencies to positions along the first gradient direction 12. Assigning the spin echo 16 at the positions to the liquid portion 7 and to the gaseous portion 8 of the medium 6. Determining a liquid portion of the flowing medium 6 in the measuring volume 3 from the positions of the spin echo 16 of the liquid portion 7 and the positions of the spin echo 16 of the gaseous portion 8 of the medium 6. In the present case, the medium 6 flows laminarly and the liquid portion is 50%, see Fig. 1 . Determining a flow velocity of the magnetized medium 6 from the measured nuclear magnetic resonances.

[0039] In a first section 17, the fluid fraction is determined, and in a second section 18, the flow velocity is determined. The intervals between the individual signals are denoted by τ HE and τ CPMG. The strength of the gradient magnetic field is indicated by G z. Signals include, in particular, any excitation pulses, refocusing pulses, FIDs, and spin echoes.

[0040] Instead of spin echo 16, a subsequent spin echo can also be used to determine the fluid content, which is excited by a subsequent refocusing pulse. The use of an FID is also possible. However, determining the fluid content with an FID is less accurate than with a spin echo.

[0041] A disadvantage of this embodiment is that the refocusing pulse 15 often has a smaller bandwidth than the excitation pulse 14, which reduces the quality of the measurement signal.

[0042] This disadvantage is in another embodiment, see Fig. 3 , at least mitigated. In this embodiment, the measuring device 4 performs the following steps: Generating and radiating an excitation signal into the magnetized medium 6 in the measurement volume 3 to excite nuclear magnetic resonances in the magnetized medium 6 along the first gradient direction 12. The excitation signal comprises a first excitation pulse 19 followed by a first refocusing pulse 20 and a second refocusing pulse 21 followed by a second excitation pulse 22 to compensate for any remaining nuclear magnetic resonances. The second excitation pulse 22 has a phase shift of 180° relative to the first excitation pulse 19. The excitation signal further comprises a CPMG pulse sequence 13, which follows the second excitation pulse 22 and may be different from that of the preceding exemplary embodiment. Between the second excitation pulse 22 and the CPMG pulse sequence 13, the gradient magnetic field 11 is reduced to zero.The strength of the gradient magnetic field 11 is shown as G z and the reduction to zero occurs within a time period T. . Measuring nuclear magnetic resonances excited by the excitation signal in the magnetized medium 6 along the first gradient direction 12 in the measurement volume 3 as a measurement signal. In particular, a spin echo 16 between the first refocusing pulse 20 and the second refocusing pulse 21 is measured as part of the measurement signal. Determining frequencies of the spin echo 16 along the first gradient direction 12. Assigning the determined frequencies to positions along the first gradient direction 12. Assigning the spin echo 16 at the positions to the liquid portion 7 and the gaseous portion 8 of the medium 6. Determining a liquid portion of the flowing medium 6 in the measurement volume 3 from the positions of the spin echo 16 of the liquid portion 7 and the positions of the spin echo 16 of the gaseous portion 8 of the medium 6.Determining a flow velocity of the magnetized medium 6 from nuclear magnetic resonances excited by the CPMG pulse sequence 13, which are part of the measurement signal.

[0043] In the first section 17, the liquid fraction is determined, and in the second section 18, the flow velocity is determined, with the two sections separated by the time period T. The intervals between the individual signals are marked with τ HE and τ CPMG.

[0044] Instead of spin echo 16, a subsequent spin echo can be used to determine the fluid content, which is excited by a subsequent refocusing pulse. The use of an FID is also possible.

[0045] A disadvantage of this embodiment is that many spin echoes are required to determine the flow velocity with a CPMG pulse sequence, which means that the temporal resolution is low and changes in the flow velocity to be measured during the measurement lead to errors.

[0046] This disadvantage is in another embodiment, see Fig. 4 , at least mitigated. In this embodiment, the measuring device 4 performs the following steps: Generating and radiating an excitation signal into the magnetized medium 6 in the measurement volume 3 to excite nuclear magnetic resonances in the magnetized medium 6 along the first gradient direction 12. The excitation signal comprises a first excitation pulse 19 followed by a first refocusing pulse 20 and a second refocusing pulse 21 followed by a second excitation pulse 22 to compensate for any remaining nuclear magnetic resonances. The second excitation pulse 22 has a phase shift of 180° compared to the first excitation pulse. The excitation signal further comprises an MMME pulse sequence 23 following the second excitation pulse 22. Between the second excitation pulse 22 and the MMME pulse sequence 23, the gradient magnetic field 11 is reduced to zero and then increased again in a second gradient direction. The second gradient direction is perpendicular to the first gradient direction 12, specifically parallel to the flow of the medium 6.The strength of the gradient magnetic field 11 in the first gradient direction 12 is plotted as G z and in the second gradient direction as G x . The reduction to zero and the increase occur within a time period T. Nuclear magnetic resonances excited by the excitation signal in the magnetized medium 6 along the first gradient direction 12 in the measurement volume 3 are measured as a measurement signal. In particular, a spin echo 16 between the first refocusing pulse 20 and the second refocusing pulse 21 is measured as part of the measurement signal. Determining frequencies of the spin echo 16 along the first gradient direction 12. Assigning the determined frequencies to positions along the first gradient direction 12. Assigning the spin echo 16 at the positions to the liquid portion 7 and the gaseous portion 8 of the medium 6. Determining a liquid portion of the flowing medium 6 in the measurement volume 3 from the positions of the spin echo 16 of the liquid portion 7 and the positions of the spin echo 16 of the gaseous portion 8 of the medium 6. Determining a flow velocity of the magnetized medium 6 from nuclear magnetic resonances excited by the MMME pulse sequence, which are part of the measurement signal. The time required to determine the flow velocity with the MMME pulse sequence 23 is significantly less than with a CPMG pulse sequence.

[0047] In the first section 17, the liquid fraction is determined, and in the second section 18, the flow velocity is determined. The two sections are separated by the time period T. The intervals between the individual signals are marked with τ HE and τ .

[0048] Instead of spin echo 16, a subsequent spin echo can be used to determine the fluid content, which is excited by a subsequent refocusing pulse. The use of an FID is also possible.

[0049] In another embodiment, see Fig. 5 , the measuring device 4 performs the following steps: Generating and radiating an excitation signal into the magnetized medium 6 in the measurement volume 3 to excite nuclear magnetic resonances in the magnetized medium 6 along the first gradient direction 12. The excitation signal comprises a CHIRP pulse sequence 24 with an excitation pulse 14 followed by a refocusing pulse 15. In Fig. 5Only the envelopes of the excitation pulse 14 and the refocusing pulse 15 are shown. The envelopes have the essentially rectangular shape typical for hard pulses. A frequency of the excitation pulse 14 and a frequency of the refocusing pulse 15 increase during their respective durations, in this case linearly. While the excitation pulse 14 has a duration of 2τ and a normalized amplitude of 1, the refocusing pulse 15 has a duration of τ and a normalized amplitude of 3. Measurement of an FID 25 excited by the excitation signal, i.e., the CHIRP pulse sequence, in the magnetized medium 6 along the first gradient direction 12 in the measurement volume 3 as a measurement signal. The measurement takes place for a duration τ after the refocusing pulse 15 to avoid problems with dead times. Determining frequencies of the FID 25 along the first gradient direction 12. Assigning the determined frequencies to positions along the first gradient direction 12.Assigning the FID 25 at the positions to the liquid portion 7 and to the gaseous portion 8 of the medium 6. Determining a liquid portion of the flowing medium 6 in the measuring volume 3 from the positions of the FID 25 of the liquid portion 7 and the positions FID 25 of the gaseous portion 8 of the medium 6. .

[0050] The Fig. 2 to 5 show schematic excerpts from signal sequences. This means, in particular, that in addition to those shown, additional signals may be included in the signal sequences. An explicit assignment of the described steps to the measurement and evaluation steps has not been made, but can be derived from the description preceding the exemplary embodiments.

[0051] In the above embodiments, reference is made to a single spin echo, from which the fluid content is determined. These embodiments are expanded, for example, by determining the fluid content from multiple spin echoes, making the determination more reliable and / or precise. The additional spin echoes are generated by additional refocusing pulses in the excitation signal. Reference symbol

[0052] 1 nuclear magnetic flowmeter 2 measuring tube 3 measuring volume 4 measuring device 5 magnetization device 6 medium 7 liquid portion of the medium 8 gaseous portion of the medium 9 magnetic field 10 magnetic field direction 11 gradient magnetic field 12 first gradient direction 13 CPMG pulse sequence 14 excitation pulse 15 refocusing pulse 16 spin echo 17 first section 18 second section 19 first excitation pulse 20 first refocusing pulse 21 second refocusing pulse 22 second excitation pulse 23 MMME pulse sequence 24 CHIRP pulse sequence 25 FID

Claims

1. Method for determining a liquid portion of a flowing medium (6) with a nuclear magnetic flowmeter (1), wherein the nuclear magnetic flowmeter (1) comprises a measuring tube (2) with a measuring volume (3), a measuring device (4) and a magnetizing device (5), wherein the medium (6) is made to flow through the measuring tube (2) and has a liquid portion (7) and a gaseous portion (8), wherein a magnetic field (9) is generated along a magnetic field direction (10) in the measuring volume (3) by the magnetizing device (5) and the medium (6) is magnetized by the magnetic field (9), wherein, in a gradient step, a gradient magnetic field (11) with a strength gradient along a first gradient direction (12) is generated in the measuring volume (3) by the magnetizing device (5) and the medium (6) is exposed to the gradient magnetic field (11) in addition to the magnetic field (9), wherein the following measurement steps are carried out by the measuring device (4): - generating and irradiating an excitation signal into the magnetized medium (6) having at least one excitation pulse (14, 19, 22) in the measuring volume (3) for exciting nuclear magnetic resonances in the magnetized medium (6) along the first gradient direction (12), - measuring nuclear magnetic resonances excited by the excitation signal in the magnetized medium (6) along the first gradient direction (12) in the measuring volume (3) as a measuring signal, - determining frequencies of the nuclear magnetic resonances in the measuring signal along the first gradient direction (12), wherein the following evaluation steps are then carried out by the measuring device (4): - assigning the determined frequencies to positions along the first gradient direction (12), - assigning the nuclear magnetic resonances at the positions to the liquid portion (7) and to the gaseous portion (8) of the medium (6), characterized in that the following evaluation step is carried out by the measuring device: - determining a liquid portion of the flowing medium (6) in the measuring volume (3) from the positions of the nuclear magnetic resonances of the liquid portion (7) and the positions of the nuclear magnetic resonances of the gaseous portion (8) of the medium (6), and that a bandwidth for the at least one excitation pulse (14, 19, 22) based on a homogeneity of the magnetic field (9) and a strength of the strength gradient of the gradient magnetic field (11) is determined by the magnetizing device.

2. Method according to claim 1, characterized in that the measuring tube (2) is oriented horizontally and the first gradient direction (12) is oriented vertically.

3. Method according to claim 1 or 2, characterized in that a homogeneity of the magnetic field (9) is determined, that a strength for the strength gradient is determined based on the determined homogeneity and a required precision along the gradient direction (12) and that, preferably, the required precision is within 10%, more preferably within 5% and particularly preferably within 1% of a distance along the gradient direction (12) in the measuring volume (3).

4. Method according to any one of claims 1 to 3, characterized in that the at least one excitation pulse (14, 19, 22) is generated for rotating magnetic moments of the magnetized medium (6) by angles between 5° and 45° and more preferably by angles of 90° and that, preferably, a hard, a soft, an adiabatic, a CHIRP (24) or a WURST pulse is used as the at least one excitation pulse (14, 19, 22).

5. Method according to claim 4, characterized in that a strength of the nuclear magnetic resonances with respect to their frequencies is determined by the magnetization device (4) and that the strength of the nuclear magnetic resonances with respect to their frequencies is normalized so that a frequency-dependent efficiency of the excitation of the nuclear magnetic resonances by the at least one excitation pulse (14, 19, 22) is compensated.

6. Method according to any one of claims 1 to 5, characterized in that the excitation signal comprises at least one refocusing pulse (15, 20, 21) and that, preferably, the at least one refocusing pulse (15, 20, 21) is generated for rotating magnetic moments of the magnetized medium by angles of 45° and, more preferably by angles of 180° and that, preferably, a hard, a soft, an adiabatic, a CHIRP (24) or a WURST pulse is used as the at least one refocusing pulse (15, 20, 21).

7. Method according to any one of claims 1 to 6, characterized in that the excitation signal comprises a multi-echo pulse sequence (13, 23), that nuclear magnetic resonances excited by the multi-echo pulse sequence (13, 23) in the magnetized medium (6) are measured and evaluated, in particular by determining at least one flow rate of the magnetized medium (6), and that preferably a CPMG, an SSFP or an MMME pulse sequence is used as the multi-echo pulse sequence (13, 23).

8. Method according to any of claims 1 to 7, characterized in that the excitation signal comprises the excitation pulse (14, 19, 22) followed by a refocusing pulse (15, 20 21), that a spin echo (16) excited by the refocusing pulse (15, 20 21) is measured, and that, preferably, the refocusing pulse (15, 20 21) is generated for rotating magnetic moments in the magnetized medium (6) by angles of 180°.

9. Method according to claim 8, characterized in that the excitation signal comprises a CPMG pulse sequence (13), that the excitation pulse (14) and the refocusing pulse (15) are part of the CPMG pulse sequence (13), that nuclear magnetic resonances excited by the CPMG pulse sequence (13) are measured and evaluated, in particular by determining at least one flow rate of the magnetized medium (6).

10. Method according to any one of claims 1 to 9, characterized in that the excitation signal comprises a first excitation pulse (19) followed by a first refocusing pulse (20) and a second refocusing pulse (21) followed by a second excitation pulse (22) for compensating for nuclear magnetic resonances still present, that a spin echo (16) is measured between the first refocusing pulse (20) and the second refocusing pulse (21) as a part of the measuring signal, and that, preferably, the second excitation pulse (22) has a phase shift of 180° with respect to the first excitation pulse (19).

11. Method according to claim 10, characterized in that the excitation signal has a CPMG pulse sequence (13), that the CPMG pulse sequence (13) follows the second excitation pulse (22), that, between the second excitation pulse (22) and the CPMG pulse sequence (13), the gradient magnetic field (11) is reduced, preferably to zero, that nuclear magnetic resonances excited by the CPMG pulse sequence (13) are measured and evaluated, in particular by determining at least one flow rate of the magnetized medium (6).

12. Method according to claim 10, characterized in that the excitation signal comprises an MMME pulse sequence (23), that the MMME pulse sequence (23) follows the second excitation pulse (22), that, between the second excitation pulse (22) and the MMME pulse sequence (23), the gradient magnetic field (11) is first reduced to zero and then increased again in a second gradient direction, that, preferably, the second gradient direction is perpendicular to the first gradient direction (12), that nuclear magnetic resonances excited by the MMME pulse sequence (23) are measured and evaluated, in particular by determining at least one flow rate of the magnetized medium (6).

13. Method according to any one of claims 1 to 12, characterized in that the gradient step is carried out again with a third gradient direction different from the first gradient direction (12), that, then, the measurement steps and the evaluation steps are carried out again, that a precision of the determination of the liquid portion is improved by combining both determinations of the liquid portion and that, preferably, the third gradient direction is perpendicular to the first gradient direction (12).