Systems and methods for multiphase flow metering accounting for dissolved gas

1. A system for metering flow of a multiphase fluid, the system comprising:

a vibratable flowtube for receiving the multiphase fluid flow;

a driver configured to vibrate the flowtube;

a pair of sensors configured to detect movement of the flowtube at different locations on the flowtube and output sensor signals indicative of the detected movement;

a pressure sensor configured to measure a pressure of the multiphase fluid;

a temperature sensor configured to measure a temperature of the multiphase fluid; and

one or more processors configured to receive the sensor signals from the pair of sensors configured to detect movement of the flowtube, determine a phase difference between the sensor signals from the pair of sensors configured to detect movement of the flowtube, and use the determined phase difference to determine a fluid flow rate through the flowtube, wherein said one or more processors are further configured to:

receive a signal from the pressure sensor indicative of the pressure of the multiphase fluid in the flowtube;

receive a signal from the temperature sensor indicative of the temperature of the multiphase fluid in the flowtube;

receive a signal from a meter indicative of relative amounts of multiple different liquids in the multiphase fluid; and

determine an amount of dissolved gas in the multiphase fluid using the pressure, the temperature, and the relative amounts of the multiple liquids in the multiphase fluid.

2. A system as set forth in claim 1 wherein said one or more processors are configured to determine the fluid flow rate by using a mapping between a plurality of inputs and at least one output related to the fluid flow rate, wherein the inputs comprise a raw mass fluid flow rate, pressure of the fluid, temperature of the fluid, and the relative amounts of the multiple liquids in the multiphase fluid and wherein said one or more processors are configured to determine the amount of dissolved gas according to a gas solubility model.

3. A system as set forth in claim 2 wherein the solubility model includes an adjustable tuning factor.

4. A system as set forth in claim 1 wherein said one or more processors are configured to determine an amount of free gas in the multiphase fluid.

5. A system as set forth in claim 4 wherein said one or more processors are configured to determine an amount of total gas in the multiphase fluid using the amounts determined for dissolved gas and free gas.

6. A system as set forth in claim 1 further comprising a flow conditioner configured to mix the multiphase fluid before it enters the flowtube.

7. A system as set forth in claim 6 wherein the flow conditioner comprises a plurality of tabs projecting inwardly from a wall of a conduit that is positioned to convey the multiphase fluid to the flowtube.

8. A system as set forth in claim 1 further comprising a conduit upstream of the flow tube and connected to the flowtube for conveying the multiphase fluid to the flowtube, the conduit being configured to have two substantially vertical segments and a substantially horizontal segment between the substantially vertical segments.

9. A system as set forth in claim 8 wherein one of the substantially vertical segments is between the substantially horizontal segment and the flowtube, wherein the flowtube is at a lower elevation than the substantially horizontal segment.

10. A system as set forth in claim 8 wherein the substantially horizontal segment is connected to each of the substantially vertical segments at upper ends of the substantially vertical segments.

11. A system as set forth in claim 1 further comprising a separator upstream of the flowtube and configured for separating at least some free gas in the multiphase fluid from the liquid phase, the separator having a gas output and a liquid output, wherein the liquid output is directed from the separator to the flowtube and the gas output is diverted from the flowtube to reduce the amount of free gas in the multiphase fluid before it flows through the flowtube.

12. A system as set forth in claim 1 wherein the multiple different liquids include water and oil and wherein said meter comprises a water cut meter.