Method of interpreting NMR signals to give multiphase fluid flow measurements for a gas/liquid system
A method for assessing a gas phase in a flowing multi-phase fluid comprises flowing the fluid through magnetic resonance and pre-polarization modules and applying to the fluid a radio-frequency pulse sequence at least once with and at least once without a magnetic field gradient. The method further includes measuring an NMR signal. The method also includes using a calibration between the ratio of slope and intercept of the NMR signal and flow velocity for at least one non-gas phase with the gradient applied to determine that phase's velocity. A calibration between the signal intensity of the liquid phases as function of flow velocity is used, with and without gradient, to correct the gradient-induced attenuation of the liquid signals and to calculate a gradient-corrected signal intensity of the liquid phases without a magnetic field gradient. Additionally, the method includes subtracting the gradient-corrected signal intensity from the NMR signal to calculate the volumetric fraction of the liquid phase.
1. A method for determining flow rates of gas and liquid phases in a multi-phase fluid flowing in a pipe without using time-of-flight measurements, comprising:
a) providing a measurement and analysis magnetic resonance module through which the multi-phase fluid flows;
b) providing a pre-polarization module through which the multi-phase fluid flows before entering the magnetic resonance module;
c) flowing the multi-phase fluid through the pre-polarization module and the magnetic resonance module;
d) as the multi-phase fluid flows through the magnetic resonance module, applying to the multi-phase fluid a radio-frequency pulse sequence at least once in the presence of a magnetic field gradient and at least once in the absence of a magnetic field gradient and measuring the intensity of a pre-determined number of spin echoes that are produced by the RF pulse sequence;
e) using a first calibration between the ratio of slope and intercept of the measured spin echoes and flow velocity for at least one non-gas phase with the magnetic field gradient applied to determine the flow velocity of that non-gas phase;
f) using a second calibration of the signal intensity of the non-gas phases as a function of flow velocity, with and without magnetic field gradients applied, to correct the gradient-induced attenuation of the non-gas signal and to calculate a gradient-corrected signal intensity of the non-gas phase;
g) subtracting the gradient-corrected signal intensity of the non-gas phase from the NMR signal of the multi-phase fluid measured at the liquid flow velocity determined in step e) to determine a signal that corresponds to the gas phase; and
h) determining the volumetric fraction and flow velocity of the gas phase, wherein the gas flow velocity is determined using a flow rate calibration for pure gas in conjunction with the slope and intercept of the gas signal determined in step g).
2. The method of claim 1 wherein the radio-frequency pulse sequence is a CPMG pulse sequence.
3. The method of claim 1 wherein the magnetic field gradient is constant during the RF sequence.
4. The method of claim 1 wherein the magnetic field gradient is pulsed during the RF sequence.
5. The method of claim 1 wherein at least steps e) through h) are performed for a plurality of segments of the cross-sectional area of the flow.
6. The method of claim 5 wherein the segments of the cross-sectional area of the flow are horizontal segments.
7. The method of claim 6 wherein step h) further comprises multiplying the volume fraction of the gas by the gas flow velocity to yield a volumetric flow rate of the gas phase.
8. The method of claim 1 where in the gas of the multi-phase fluid flows through the magnetic resonance module at a pressure less than 15 MPa.