Determining fluid properties

A fluid measurement system, comprising:

• a fluid flow circuit that comprises at least two liquid sources, a gas source, and a fluid mixer that mixes at least two liquids from the at least two liquid sources to form a liquid phase of a mixed-phase fluid that also includes a gas phase from the gas source;
• a pre-polarizing magnet positioned to receive the mixed-phase fluid flow from the fluid mixer;
• an Earth's field nuclear magnetic resonance (EFNMR) detector that comprises a radio-frequency (RF) coil, a first electromagnet, and a second electromagnet, the EFNMR detector positioned to receive the mixed-phase fluid from the pre-polarizing magnet;
• and
• a control system communicably coupled to the pre-polarizing magnet and the EFNMR detector and configured to perform operations comprising:
• controlling the pre-polarizing magnet to polarize at least the gas phase of the mixed-phase fluid flow to an initial polarization;
• controlling the EFNMR detector to measure a plurality of fluid induction decay (FID) values of the polarized gas phase;
• determining a velocity of the gas phase based on the plurality of FID values of the polarized gas phase;
• controlling the first electromagnet to produce a pulsed magnetic field gradient to suppress one or more signals acquired by the EFNMR detector;
• controlling the EFNMR detector simultaneously with the production of the pulsed magnetic field gradient to measure a plurality of FID values of the liquid phase of the mixed-phase fluid;
• controlling the second electromagnet to generate a homogeneous polarizing field to polarize the liquid phase of the mixed-phase fluid;
• determining a velocity of the liquid phase based on the plurality of FID values of the polarized liquid phase;
• and
• determining a content of the liquid phase based on the plurality of FID values of the mixed-phase fluid.