GB2128756A - Flow measurement of multi- phase fluids - Google Patents

Abstract

A method and apparatus for characterising a two-phase or multi- phase fluid flowing in a pipe, comprising subjecting the fluid to homogenisation at a given point in the pipe so that a substantially homogenised mixture is present in a portion of the pipe downstream of the homogenisation point, and providing one or more meters within the said portion of the pipe for measuring the volumetric and/or mass flow, and/or mean fluid-density and/or on-line water content wherein one of the phases is water, and/or sample extracting means, for extracting a sample for analysis from said portion of the pipe. The invention is particularly applicable to the oil industry.

Description

SPECIFICATION Measurement of two-phase or multi-phase fluid flow in pipes This invention relates to the measurement of two-phase or multi-phase fluid flow in pipes, and is particularly, but by no means exclusively, applicable to the oil industry.

By a "two-phase or multi-phase fluid" is meant a fluid containing matter in at least two different states, i.e. liquid/gas, solid/liquid, solid/gas or solid/liquid/gas. It includes cases in which two or more liquid phases are present as well as, say, a gas phase and/or one or more solid phases, or two or more solid phases are present as well as a gas phase and/or one or more liquid phases.

The invention finds particular application in the oil industry where it is important to regulate the relative rates at which natural gas and oil are drawn off from production wells. If the gas is allowed to escape too quickly, there is the danger that insufficient gas will eventually be left to pump up from the deposit oil that would otherwise be exploitable. Alternatively, if the proportion of gas drawn off is too low, the rate of oil production from the deposit is unnecessarily slow, which is economically disadvantageous. There is therefore a need for a metering method and system which will give at least the ratio MA/MB of the mass flowrate M of the two phases, gas (A) and liquid (B).

Sometimes there is a requirement for just the total mass flowrate MA + MB, or even a more stringent requirement to determine both the individual mass flowrates MA and MB.

Measurement is made difficult or even impossible because two-phase or multi-phase mixtures can take a variety of different forms.

For example, a liquid/gas system, such as oil/natural gas, may consist of relatively small gas bubbles in the liquid, or relatively large 'slugs' of gas in the liquid, or a mist of liquid droplets in the gas, or (in a horizontal pipe) a layer of gas above a layer of liquid. A combination of these forms can occur simultaneously, or the form can change from moment to moment in an unpredictable way.

Sometimes, more than two phases may be present. For example, natural hydrocarbon deposits frequently comprise three separate phases, i.e. oil, water and natural gas, and in such cases it would be useful to know the mass flow rate of the water, as well as the mass flow rate of the oil and the gas.

Where the fluid consists of separate gas bubbles in a liquid, the two phases will have the same velocity in a horizontal pipe. Otherwise, however, the gas will in general flow faster, and the velocity rate VA/VB will be unknown. Under these conditions it is with the present technology impossible to determine accurately the values of MA and MB, cr the ratio MA/Ms. No volumetric flowmeter will give accurate readings of total volumetric flow in such conditions: it is arguable whether any mass flowmeter will give accurate readings of total mass flowrate: and although overall (mean) density of the flowing fluid can be measured it cannot be measured accurately in such conditions, and in any case this information is useless without a knowledge of the velocity ratio.

We have now developed apparatus and a method which enable the two-phase or multiphase mixture to be homogenised so effectively that the various phases are certain to be moving at substantially the same velocity.

Once this has been achieved, for a two-phase mixture the flow can be characterised fully and accurately by measuring any two of the following three variables: (a) total mass flowrate (b) total volumetric flowrate (c) overall (mean) fluid density, assuming that the densities of the individual phases are known. Moreover, the ratio MAIMS (very important in oil-well operation, where gas/oil ratio for every producing well has to be monitored, as mentioned above), can be determined from measurement of overall density alone, which can be done easily with present technology, e.g. with a gamma-ray absorption device (available as a commercial instrument) put on the outside of the pipe.

For three-phase flow, where the third phase is water, a third instrument could be added in the form of a water-content meter, but even that would not necessarily give all the required information. For full characterisation of the flow it would also be necessary to obtain a hydrocarbon analysis of the homogeneous flowing fluid. To obtain this it would be necessary to extract from the region of temporary homogenisation a sample of the homogeneous fluid and analyse it. The analysis could be carried out by an on-line analyser-for example, an on-line chromatograph---or by taking discrete samples for laboratory analysis.

It should be noted that other information besides hydrocarbon analysis can be obtained from such a laboratory sample. For example, water content can' be determined from such samples, and so could gas/oil ratio.

In certain circumstances, partial characterisation of the flowing fluid may be sufficient.

In such cases any one of the measuring or analytical techniques described above, or any combination of them, may be applied to the temporarily homogenised multi-phase fluid.

Thus, in accordance with the present invention there is provided a method of characterising a two-phase or multi-phase fluid flowing in a pipe, comprising subjecting the fluid to homogenisation at a given point in the pipe so that a substantially homogeneous mixture is present in a portion of the pipe downstream of the homogenisation point, and providing one or more meters within the said portion of the pipe for measuring volumetric flow and/or mass flow and/or mean fluid density and/or on-line water content (where one of the phases is water), and/or sample extracting means for extracting a sample for analysis from said portion of the pipe.

There is also provided apparatus for carrying out this method.

If mean fluid density is measured, for a two-phase fluid, the relative proportions of the two components flowing in the pipeline can be determined, provided that the density of the individual components is known. Furthermore, for a two-phase fluid, from any two of the mean fluid density, total mass flowrate, and total volumetric flowrate, it is possible, again given the individual densities, to calculate the absolute values of the mass flowrates of the two components.

According to another aspect of the invention, apparatus for measuring the ratio of the mass flowrates of the two phases of a twophase fluid (as hereinbefore defined) flowing through a pipe comprises means for homogenising the fluid flowing through the pipe and a mean density meter in the pipe in the vicinity of the homogenising means. The mean density meter may, for example, be of the conventional gamma-ray absorbtion type.

According to a further aspect of the invention, apparatus for accurately measuring the mean density, and/or the total mass flowrate, and/or the total volumetric flowrate and/or the individual flowrates of a two-phase fluid or multi-phase fluid (as hereinbefore defined) flowing through a pipe, comprises means for homogenising the fluid flowing through the pipe, and meters in the pipe close to the point at which homogenisation takes place for measuring at least one of the parameters: mass flow, volumetric flow, mean fluid density, online water content, and/or sample extracting means for extracting a sample for the purpose of analysis.

A density meter should preferably be located downstream of the homogenisation point, but if necessary it may be located immediately upstream. This is because the immediate effect of homogenisation is the formation of a 'plug' of fluid of which the individual components are so well mixed that they must be moving at the same velocity. Consequently, this plug has a retarding influence of the component that would otherwise be the faster moving, which influence extends for some distance upstream of the homogenisation point.

Flowmeters, on the other hand, should be sited downstream of the homogenisation point, since they require a homogeneous fluid if they are to give accurate results.

Preferably, homogenisation is carried out in such a way that a system of twin-cell or multi cell rotation is imposed on the pipe contents.

Such a system generates intense local turbu lence which assists in achieving homogenisa tion, and may for example, be achieved using a jet mixer or mixers, e.g. as described in our EPC applications Nos. 0844652 and 0060634. Alternatively, homogenisation is achieved by means of the invention disclosed in our further co-pending British Patent appli cation No. 8306977, which provides means for homogenising a gas-liquid mixture flowing through a pipe, comprising a restriction in the pipe providing a pressure drop across the restriction, and at least cns passage bypassing the restriction, the or each bypass entering the pipe downstream of the restriction in such a way that fluid discharged from it agitates and homogenises the fluid in the pipe in the vicinity of its point of entry.

When jet mixers of the type used in the two aforementioned EPC applications are used, where the mixing jet is provided by a propor tion of the pipe contents withdrawn from the pipe pip and then re-injected under pressure, the jet will preferably comprise either gas (pres surised by a compressor) or liquid (pressurised by a pump), but not both, and gas/liquid separating means will therefore have to be provided upstream of the compressor or pump.

It is to be noted that the power required to homogenise a gas/liquid mixture is much greater than that required to homogenise a two-phase liquid/liquid mixture. For example, experiments have shown that only 20 watts of hydraulic power were needed to homogenise oil containing 4 per cent of water in a 4-inch pipe, whereas to homogenise water and air in a a 2-inch pipe required 1 50 watts of hydraulic power.

As mentioned above, homogenisation is preferably carried out so that a system of twin-cell or multi-cell rotation is imposed on the the pipe contents. A system of multi-cell rota- tion may be imposed by an even number of jets symmetrically disposed around the pipe circumference, each pair of jets giving rise to one pair of cells. Thus, for example, a system of four cell rotation may be imposed by using four jets.

Claims (8)

1. A method of characterising a two-phase or multi-phase fluid flowing in a pipe, com prising subjecting the fluid to homogenisation at a given point in the pipe so that a substan tially homogenised mixture is present in a portion of the pipe downstream of the homo genisation point, and providing one or more meters within the said portion of the pipe for measuring volumetric flow and/or main flow, and/or mean fluid density and/or on-line water content where one of the phases is water and/or sample extracting means, for extracting a sample for analysis from said portion of the pipe.

2. A method according to claim 1 for characterising two-phase flow, wherein meters are provided for measuring any two of volumetric flow, mass flow, and mean fluid density.

3. A method according to claim 1 for characterising three-phase flow, where one of the phases is water, wherein said meters are provided for measuring water content and any two of the volumetric flow, mass flow and mean fluid density.

4. A method according to claim 2 or claim 3, wherein a sampler is also provided to enable the percentage composition of the homogenised mixture to be determined.

5. A method according to any previous claims, wherein homogenisation is achieved by imposing a system of twin-cell or multi-cell rotation on the pipe contents.

6. A method according to claim 1 or 5, wherein homogenisation is achieved by using at least one jet mixer.

7. A method according to claim 1 or claim 5, wherein homogenisation is achieved by means of a mixer which comprises a restriction in the pipe providing a pressure drop across the restriction, and at least one passage bypassing the restriction, the or each bypass entering the pipe downstream of the restriction in such away that fluid discharged from it agitates and homogenises the liquid in the pipe in the vicinity of its point of entry.

8. Apparatus for enabling a two-phase or multi-phase fluid flow in a pipe to be characterised, comprising means for homogenising the fluid within the pipe so that a substantially homogenised mixture is present in a portion of the pipe downstream of the homogenisation point, and a meter or meters within said portion of the pipe for measuring volumetric flow and/or mass flow and/or mean fluid density and/or on-line water content, (where one of the phases is water) and/or sample extracting means for extracting a sample for analysis from said portion of the pipe.