Note: Descriptions are shown in the official language in which they were submitted.
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T 5587 PCT
FLUID COMPOSITION METER
The invention relates to an apparatus for measuring a
composition of a fluid flowing through a flowline. When a multi-
component fluid is transported through a flowline to a processing
facility, it is generally required to measure the composition of
the fluid either at a location along the flowline or at the
upstream end thereof. In the oil and gas industry, wellbore fluids
can be transported from an onshore or offshore wellhead through a
flowline to a processing facility where the different components
of the fluid are separated from each other. The wellbore fluids
can include oil, gas and water in fractions which can vary in time
and can be different for different wells producing from the same
subsurface reservoir. Measurement of fluid composition directly at
the wellheads or further downstream where streams from different
wells are joined enables specific reservoir characteristics to be
updated during the producing lifetime of the reservoir, and allows
timely corrective measures to be taken if necessary.
European patent No. 269 432 discloses an apparatus for
measuring a composition of a fluid flowing through a flowline,
comprising a conduit suitable to be connected to the flowline in a
manner that a stream of said fluid flows through a conduit, a
radioactive source arranged so that radiation from the source
passes through a wall of the conduit and through the stream of
fluid, a radiation detector arranged so as to detect said
radiation passing through the wall of the conduit and through the
2J stream of fluid, and means to generate a signal representative of
said radiation detected by the detector. The amount of radiation
passing through the stream of fluid and which is detected by the
c'~tector provides an indication of the composition of the fluid.
The wall of the conduit through which radiation passes forms a
radiation window made of beryllium, which wall is surrounded by a
tubular guard provided with diametrically opposed holes for
Ai~IENDcD SHEET
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passage of the radiation. A drawback of the known device is that
toxic beryllium oxide will form on the surface of the beryllium
wall. Furthermore, for high pressure applications a relatively
thick beryllium wall is required, resulting in a significant
attenuation of radiation in the beryllium wall and a corresponding
reduction of accuracy of the composition measurement.
It is an object of the present invention to provide an
apparatus for measuring a fluid composition of a stream of multi-
component fluid, which overcomes the drawbacks of the prior art
apparatus.
It is another object of the invention to provide an apparatus
for measuring a fluid composition of a stream of multi-component
fluid, which apparatus is compact and provides accurate
measurements.
In accordance with the present invention the apparatus for
measuring a composition of a stream of fluid flowing through a
flowline is characterized in that the detector comprises a solid
state detector configuration with at least two detection surfaces
and a filter which is located between the radiation source and a
first of said detection surfaces.
In general, the efficiency of a solid state detector
decreases with increasing radiation energy level. If for example
an Americium (Am-241) radiation source is used which radiates at a
low energy level of 18 KeV and at a high energy level of 60 KeV,
2J the detector efficiency at the low energy radiation level is about
50-60$, while the detector efficiency at the high energy radiation
level is about 5$. In the apparatus according to the invention the
efficiency of the detector is improved by providing a solid state
detector configuration with at least two radiation detecting
surfaces, whereby a filter is located between the radiation source
and a first of said detecting surfaces.
Preferably, the filter substantially prevents the low energy
radiation to pass therethrough, and substantially allows the high
energy level radiation to pass therethrough. Thereby it is
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achieved that the low energy level portion of the radiation
passing through the fluid mixture and the wall of the conduit is
substantially detected by a second of said two detecting surfaces.
Furthermore, the high energy level portion of the radiation which
passes through the fluid mixture and the wall of the conduit is
substantially detected by said first detecting surface at an
increased efficiency since by the arrangement of the filter the
count-rate for the high energy level portion is significantly
increased. Preferably the filter comprises Copper.
It is to be understood that in the context of the present
invention the term solid state detector refers to a semiconductor
diode detector, for example as described in publication ISBN 0-
471-49545-x entitled "Radiation detection and measurement",
chapter 11. Such a detector is compact and has a high resolution
in the energy range between 15-100 keV which is suitable for the
measurement. The resolution and efficiency of the semiconductor
diode detector can be increased by providing the detector with
cooling means which maintain the temperature of the detector
between 0-15 °C, or preferably between 5-10 °C.
A suitable cooling means forms a Peltier element since such
element is compact, whereby the compactness of the apparatus is
even enhanced when the detector is directly attached to the
Peltier element.
It is preferred that the wall of the conduit comprises a
2J fibre reinforced resin. It has been found that the wall of fibre
reinforced resin forms an adequate radiation window with low
radiation absorption, while such a wall allows relatively high
internal pressures to be applied. Furthermore, in the absence of
beryllium there is no danger that beryllium oxide will form, and
future problems of disposing beryllium are eliminated.
Absorption of radiation in the conduit wall is particularly
1-ow when the wall is reinforced with carbon fibres. In view
thereof it is preferred that the wall comprises a relatively high
percentage of carbon fibres, for example between 50-70~ wt carbon
3~ fibres.
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A suitable matrix material to be applied for the wall forms a
polyether resin, such as an epoxy resin.
The composition of the fluid flowing through the flowline is
determined from the signal representing the radiation detected by
the detector according to the invention in accordance with the
following method:
The absorption of gamma rays in matter is described by the
following equation:
I (e) = I (e) .exp(-~(e) .h,
0
where
I (e) - initial intensity of the gamma rays at energy e;
0
I(e) - intensity of the gamma rays after absorption at
energy e;
~(e) - linear absorption coefficient at energy e;
h = thickness of material.
Given an oil/water/gas mixture in a pipe of internal diameter
d and wall thickness t, the following equations can be derived for
two energy levels e1 and e2:
I (e1) = I (e1) .exp.(-N.mix(el) .d) .exp(-~tpipe(el) .2t)
0
and
I(e2) - I (e2).exp(yix(e2).d).exp(-~pipe(e2).2t),
0
where
~mix(el) _ ~oil(el)~xoil + ~water(el)~°~water + ugas(el).~cgas
and
~x(e2) _ ~oi1(e2).°~oil + ~water(e2).°~water +
~gas(e2)~°~gas
The symbol oc refers to the volume fraction of the subscripted
phase; the sum of all the three fractions should be equal to 1:
°~oil + °~water + °~gas = 1
if the absorption coefficients for oil, water and gas are measured
in separate calibration procedure, then, by measuring the
radiation absorption through a pipeline filled with oil, water and
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~G
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gas, at two gamma-ray energy levels e1 and e2 the three fractions
°~oil~ °~water and ocgas can be determined using the above
equations.
It is observed that the International Journal of Applied
Radiation and Isotopes, Vol. 34, No. 1, January 1982, discloses on
pages 309-331 that a solid state detector which is cooled by a
Peltier element can be used in an apparatus for on-stream analysis
of metalliferous ore slurries, that European patent specification
No. 0 187 460 discloses the use of several gamma ray detectors for
measuring the composition of a multiphases fluid stream in a
flowline along several chords and that European patent
specification 0 236 623 discloses the use of a single radiation
detector together with a differential pressure gauge in a
multiphase flowmeter.
1J The invention will now be described by way of example in more
detail with reference to the drawings in which:
MCS9/T5587PCT
WO 94125859 PCT/EP94I01320
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Fig. 1 shows schematically a longitudinal cross-section of the
apparatus according to the invention;
Fig. 2 shows cross-section 2-2 of the apparatus of Fig. 1;
The apparatus of Figs. 1 and 2 includes a steel conduit 1
having a central part 3 of reduced internal diameter. A carbon fibre
reinforced epoxy (CFRE) cylinder 5 is located in a recess 7 provided
at the inside of the central part 3 of the conduit 1. The wall of
the central part 3 of the conduit 1 is provided with four ooeninqs
10, 11, 12, 13, whereby openings 11, 12, 13 are arranged opposite
opening 10 and at spaced intervals along the circumference of the
conduit 1. Each opening 10, 11, 12, 13 extends from the outer
surface of the conduit 1 to the outer surface of the CFRE cylinder
5.
An X-ray or gamma-ray radiation source 15 is located in opening
10 which forms a collimator for the source 15. Three solid state
detectors 16, 17, 18 are located in openings 11, 12, 13
respectively, which openings 11, 12, 13 form respective collimators
for the detectors 16, 17, 18 so that the latter can detect radiation
from the source 15. The detectors 16, 17, 18 are attached to
respective Peltier elements 20, 21, 22 which serve to maintain the
temperature of the detectors 16, 17, 18 between 5-10 °C. An
electronic processor Inot shown) is linked to the detectors 16, 17,
18 to receive and process signals representing the radiation
detected by the detectors 16, 17, 18.
The radioactive source 15 is selected such that suitable energy
levels e1 and e2 are emitted so that ~oil~ °~water and ocgas can be
solved using the above equations.
A differential pressure meter 29 is connected to a pressure tap
26 located at the inner surface of the conduit 1 adjacent the
central part 3 thereof, and to a pressure tap 28 located at the
inner surface of the central part 3 of the conduit 1.
The conduit 1 is at both ends provided with flanges 30, 32 to
connect the conduit 1 to a flowline (not shown) for transporting
fluid containing oil, water and gas produced from a wellbore in an
earth formation.
WO 94/25859 21 b 13 4 l PCT/EP94/01320
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During normal operation of the apparatus shown in Figs. 1 and
2, the conduit 1 is connected to the flowline and a stream of fluid
containing oil, water and gas produced from the wellbore flows
through the conduit 1. X-ray or gamma-ray radiation emitted by
radiation source 15 passes through the wall of the carbon fibre
reinforced cylinder 5 and through the stream of fluid towards the
detectors 16, 17, 18, whereby the radiation is partly absorbed by
the wall of the CFRE cylinder 5 and by the stream of fluid. Thus,
the radiation emitted by the source 15 is attenuated by the wall of
the CFRE cylinder 15 and by the stream of fluid. The attenuated
radiation is detected by the detectors 16, 17, 18 and signals
representing the attenuated radiation are received by the electronic
processor which determines the composition of the fluid using the
above equations. Furthermore, the differential pressure meter 29
provides signals representing the flow rate of the fluid mixture, so
that the flowrates of the individual fluid components can be
calculated.
Application of several detectors at circumferentially spaced
locations around the conduit has the advantage, compared to applying
only one detector, that the count-rate is thereby increased and that
the homogeneity of the fluid can be checked by comparing the fluid
composition as determined from the individual detectors. In the
embodiment shown in Figs. 1 and 2 three detectors are applied,
however less or more than three detectors can be applied in the
apparatus according to the invention depending on the
characteristics of the application.
A suitable material for constructing a CFRE cylinder for use in
the apparatus described with reference to Figs. 1 and 2 comprises
Araldit LY556 resin with HY917 hardener and DY070 hardening
accelerator manufactured by Ciba Geigy, and Tenax IM-400-12000
carbon fibres manufactured by AKZO. A CFRE cylinder was constructed
from this material with an internal diameter of 43 mm and a wall-
thickness of 2mm, and was positioned within a steel conduit provided
with one 16 mm diam opening and three 8 mm diam openings for
locating the radiation source and the three detectors respectively.
WO 94/25859 PCT/EP94101320
2161347
A pressure test revealed that the CFRE cylinder could withstand an
internal pressure of 120 MPa.
To reduce attenuation of the radiation in the wall of the
conduit, in an alternative embodiment of the apparatus according to
the invention at least one of the source and the detector is
arranged within the conduit.
In such embodiment, it is preferred that the source is arranged
within the conduit and the detector is arranged outside the conduit.
Thus the radiation passes through the wall of the conduit only once.
Suitably the source is surrounded by a sleeve located within the
conduit, which sleeve preferably is arranged concentrically within
the conduit.
