GB2286686A

GB2286686A - Fluid density measurement

Info

Publication number: GB2286686A
Application number: GB9402544A
Authority: GB
Inventors: Eric John Atherton
Original assignee: Individual
Current assignee: Individual
Priority date: 1994-02-10
Filing date: 1994-02-10
Publication date: 1995-08-23
Anticipated expiration: 2014-02-10
Also published as: GB2286686B; GB9402544D0

Abstract

A device for measuring the density of a subject fluid 13 flowing in a well bore 11 comprises a sealed container constituted by a tube 14 and two bellows 17, 19 containing a working fluid column made up of two fluids 21, 22 of different densities (e.g. oil and water) providing an interface C. Insulated plates 23, 24 within the tube form an electrical capacitor and are connected to a meter 25. The position of the interface C will change with changes in the density of the subject fluid 13, and will thus change the output of the capacitor. The device may be installed in a protective housing provided with ports so that at the bellows the subject fluid may be flowing at a minimal velocity. <IMAGE>

Description

FLUID DENSITY MEASUREMENT This invention relates to the measurement of fluid density. In particular it is concerned with the provision of a device for measuring fluid density remotely over a relatively limited density range.

Among such density measuring devices are gravimetric devices that use the fact that a vertical column of fluid will display an increasing pressure with depth, proportional to the density of the fluid, the depth and the size of local gravity.

Such devices depend on a pressure sensitive transducer to detect the small pressure differentials generated. A problem arises when the density range of interest is very small, for example when it is required to sense the fraction of oil in an oil/water mix from a well. In this case the density ranges from the density of oil to the density of water. Typically this could be 900 kg/cubic metre to 1000 kg/cubic metre (or put another way a specific gravity between 0.9 and 1.0). This puts further stringent requirements on the performance of the pressure transducer as the range of interest is only a small fraction of the full range of the transducer.

A use of fluid density is to measure the fraction of water in the fluid in oil wells.

Gravimetric instruments are well known that perform this task though they suffer from the above mentioned problem regarding the narrow range of densities of interest, and subsequently stringent performance criteria on the pressure transducer. In addition oil wells are often deviated to a considerable degree from the vertical and so when the instrument containing the gravimetric density device travels through such deviated sections the effective fluid head is decreased as the cosine of the deviation from the vertical. This causes a drop in the pressure differential. This may be erroneously be computed as a density change in the angle of deviation is not accurately known.

The present invention is concerned with precise fluid density measurement over a narrow range to be made in deviated oil wells of unknown deviation.

According to the present invention there is provided a fluid density measuring device comprising: I a longitudinal containment for an internal working fluid column having a longitudinal axis and first and second ends; 2 first and second local pressure equalising means for respectively the first and the second ends whereby when the containment is immersed in a subject liquid whose density is to be measured the first and second means function to equalise local pressure between the end of the internal working fluid column and the contiguous subject liquid so that the overall density of the internal working fluid column matches the density of the subject liquid; and 3 means for measuring the density of the internal working fluid column.

According to a first preferred version of the present invention each pressure equalising means comprises a bellows adapted to expand and contract along an axis substantially parallel to the longitudinal axis and the internal cross sectional area of each bellows measured at right angles to the longitudinal axis of the containment is greater than the internal cross sectional area of the containment measured at right angles to the longitudinal axis.

According to a second preferred evrsion of the present invention or the first preferred version thereof the fluid column is made up of two immiscible fluids of differing densities; one of the fluids occupying the longitudinal containment in the region of the first end; the other of the liquids occupying the longitudinal containment in the region of the second end; the two fluids having a common interface located in the longitudinal containment; and the means for measuring the density of the internal working fluid includes means for sensing the location of the interface.

According to a third preferred version of the present invention or the first or the second preferred version thereof the fluid column is made up of two fluids of differing densities; one of the fluids occupying the longitudinal containment in the region of the first end; the other of the liquids occupying the longitudinal containment in the region of the second end; the two fluids having a common interface region located in the longitudinal containment and the means for measuring the density of the internal working fluid includes means for sensing the bulk properties of the working fluids.

Typically the internal fluid column is made up of two fluids of differing densities, and the device is aligned substantially vertically in the subject liquid with the denser liquid in the working column in the lower part and the less dense liquid in the upper part of the column.

When the subject liquid is the same density as the denser liquid in the working column the denser liquid rises in the tube driving the interface upwardly towards the upper bellows.

When the subject liquid is the same density as the less dense liquid in the working column the less dense liquid drives the interface downwardly towards the lower bellows.

When the subject liquid is of an intermediate density between the denser and less dense liquids of the working column then the interface is driven to an intermediate location in the intermediate containment. The location of the interface relative to a column datum provides a measure of the density of the subject fluid. If the denser and the less dense liquids in the working column are of different colour, or of different electrical properties then the interface position can be located by optical, electronic or electrical means.

An exemplary embodiment of the present invention will now be described with reference to the accompanying drawing of a density measuring device of which the sole figure is a sectional elevation.

Fluid density measuring device 11 is located in a well bore 12 in which a fast moving flow of an oil water mixture, subject fluid 13. passes upwardly in the direction of arrow F. The device 11 is made up of a longitudinal containment tube 14 with axis 15. Upper end 16 of the tube 14 is closed by way of bellows 17 adapted for expansion and contraction along axis 17' which is co-axial with axis 15. Lower end 18 of tube 14 is closed by way of bellows 19 adapted from expansion and contraction along axis 19t which is co-axial with axis 15. The tube 14, bellows 17 and bellows 19 together form a sealed system of variable volume containing a working fluid 20 made up of two substantially immiscible components: an oil volume 21 and a water volume 22.The oil volume 21, being less dense than the water volume 22, floats on the water volume resulting in an common interface C whose position can vary as will be described hereafter.

It will be understood that the device 11 can be inserted into well bore 12 either end up. Since the liquids are not separated from one another by a barrier at their common interface C in the event the device is located in a bore with the water volume 22 uppermost this will drop downwardly through the less dense liquid oil volume 21 so that the working fluid 20 will end up in an equilibrium position with the oil volume 21 on top and the water volume 22 at the bottom.

The bellows 17, 19 are designed to be readily displaced along their respective axis 17', 19' relative to the tube 14 and contribute a negligible pressure drop.

In effect the device 11 constitutes an internal working fluid column that is in balance with the subject fluid 13. If the subject fluid 13 becomes more dense than the average density of the working fluid 20 water is forced out of bellows 19 and the average density of the column of working fluid 20 increases until it matches the density of the subject fluid 13 at that time flowing past the device 11.

Insulated plates 23, 24 are mounted concentrically within tube 14 to form an electrical capacitor connected to capacitance meter 25 by leads 26. As the dielectric constants of the water and oil in the working fluid 20 are different the capacitance measured by way of the meter 25 is a function of the relative amounts of oil volume 21 and water volume 22 which in turn is a function of the density of the subject fluid 13 so providing a measure of that density and of changes in it.

The density of the oil in oil volume 21 is selected so that is less dense than the component of minimum density of interest in the subject fluid 13.

The density of the water in the water volume 22, adjusted by additives if necessary, is selected so that it is more dense than the maximum density of interest in the subject fluid 13.

An advantage of the embodiment is that the inclination of the well bore 12 has no substantial effect on the fluid column density balance between the internal fluid column 20 and the subject fluid 13 as the inclination has the effect of reducing both the working column head and the subject fluid head by the same amount.

Hence there is no effect on the electrical output to the meter 25 for moderate to large deviations. However very large deviations, approaching the horizontal, will progressively render the device 11 less sensitive to density changes as there is less fluid head to drive the device into equilibrium.

It is not necessary to have a sharp common fluid interface between the oil volume 21 and the water volume 22. The capacitance measured by way of plates 23, 24 depend on the bulk properties of the fluid volume contained in the region of the plates 23, 24. Thus bubbles in the oil or water volumes will not cause unacceptable degradation in operation.

The specific embodiment refers to use in an oil well. In such a location the velocity of the subject fluid 13 past the device 11 is high and consequently it is desirable that any velocity head changes over the length of the device should not lead to anomalous pressure differentials. In order to protect the device 11 from such effects the device is installed in a protective housing with pressure ports P enabling subject liquid pressure acting on the bellows to be travelling at minimal velocity.

Claims

CLAIMS 1A fluid density measuring device comprising:

1 a longitudinal containment for an internal working fluid column having a longitudinal axis and first and second ends;

2 first and second local pressure equalising means for respectively the first and the second ends whereby when the containment is immersed in a subject liquid whose density is to be measured the first and second means function to equalise local pressure between the end of the internal working fluid column and the contiguous subject liquid so that the overall density of the internal working fluid column matches the density of the subject liquid; and

3 means for measuring the density of the internal working fluid column.

2 A device as claimed in Claim 1 wherein each pressure equalising means comprises a bellows adapted to expand and contract along an axis substantially parallel to the longitudinal axis and the internal cross sectional area of each bellows measured at right angles to the longitudinal axis of the containment is greater than the internal cross sectional area of the containment measured at right angles to the longitudinal axis.

3 A device as claimed in Claim 1 or Claim 2 wherein the fluid column is made up of two immiscible fluids of differing densities; one of the fluids occupying the longitudinal containment in the region of the first end; the other of the liquids occupying the longitudinal containment in the region of the second end; the two fluids having a common interface located in the longitudinal containment; and the means for measuring the density of the internal working fluid includes means for sensing the location of the interface.

4 A device as claimed in Claim 1 or Claim 2 wherein the fluid column is made up of two fluids of differing densities; one of the fluids occupying the longitudinal containment in the region of the first end; the other of the liquids occupying the longitudinal containment in the region of the second end; the two fluids having a common interface region located in the longitudinal containment and the means for measuring the density of the internal working fluid includes means for sensing the bulk properties of the working fluids.

5 As device as hereinbefore described with reference to, and as illustrated in, the accompanying drawings.