Note: Descriptions are shown in the official language in which they were submitted.
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DETECTION AND AUTOMATIC CORRECTION FOR DEPOSITION
IN A TUBULAR USING MULTI-ENERGY GAMMA-RAY
MEASUREMENTS
BACKGROUND
Field of the Disclosure
[00011 The disclosure relates generally to a method for detecting a
deposit in a tubular,
and particularly to a method for detecting a deposit in a tubular using multi-
energy
gamma-ray measurements.
Background Art
[0002] During production from an oil well, the formation of precipitates
and/or deposits
in tubulars through which fluids are flowing may occur, causing production
interruption
and intervention work. The precipitates and/or deposits may be of asphaltenes,
wax,
scale, etc. Deposition at the inner diameter of a tubular, such as tubing
strings, may occur
as a result of temperature and pressure changes, or after water evaporation
from the fluid
or pH-value changes of the fluid.
[0003] Deposits and/or precipitates affect the fluid production and
transportation in a
pipeline. It results, for example, in a decrease in oilfield productivity,
higher uncertainty
in production and reservoir monitoring data, and increased production cost due
to
frequent treatments required for prevention and removal of the deposit. In
particular, the
presence of these precipitates and deposits alters the performance of flow-
rate metering
devices, for example, for production allocation in reservoir management
applications.
[00041 To avoid these problems or to diminish the effect of the deposit,
upstream heating
devices may be installed, inhibitors may be injected in the pipe, or a pigging
device may
be inserted into the line.
[005] A method for detection and identification of a specific type of
deposit (scale) is
disclosed in WO 2003/04267. It is described from a theoretical point of view
how a
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correction could be done if scale was deposited inside a pipe. This solution,
however,
does not provide any detection and is based on the large contrast for specific
scale.
Therefore, it is difficult to apply this method to other types of deposit
materials, and in
particular, to any type of deposit material.
[0006] Another example of a typical deposit is sand. Sand particles
in the fluid, such
as a crude oil mixture, may be detected using multi-energy gamma- or X-ray
measurements. The document EP 236623 discloses photon-attenuation measurements
at more than two energy levels in order to obtain the mass flow rates of oil,
water,
gas, and sand. However, it does not provide a method for determining the
thickness
and composition of any kind deposit on the inner wall of the pipe.
[0007] A method that identifies the existence and the composition of
deposits within
the pipe and to measure its thickness as well as the evolution of the
deposit's thickness
with time would be highly desirable. Knowing the thickness of the deposit
reducing
the effective diameter of the pipe, it may then be possible to compensate for
the error
that is associated with the deposit when performing multiphase flow-rate
measurements. It may also be possible to obtain better production allocation
per well.
SUMMARY
[0008] In one aspect, there is provided a method to detect deposits
in a tubular in
which a fluid is flowing, the method comprising: measuring the water-liquid-
ratio of
the fluid as a function of time; and determining that a deposit exists if the
measured
water-liquid-ratio of the fluid as a function of time is linearly varying.
[0008a] In another aspect, there is provided an apparatus to detect a
deposit in a tubular
in which a fluid is flowing, the apparatus comprising: a measurement device
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configured to measure a water-liquid ratio of the fluid as a function of time;
and a
processor configured to determine that a deposit exists if the water-liquid
ratio as a
function of time is linearly varying.
[0009] Other aspects, characteristics, and advantages of the
invention will be apparent
from the following detailed description and the appended claims.
BRIEF DESCRIPTION OF DRAWINGS
[0010] Figures la and lb are schematic representations of an
apparatus for detecting a
deposit in a tubular according to embodiments disclosed herein installed to a)
a
straight pipe and b) a throat section of a venturi.
2a
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[0011] Figures 2a and 2b are schematic representations of an empty pipe
with an
apparatus according to embodiments disclosed herein, whereby in Figure 2a no
deposit is
present in the pipe, and in Figure 2b deposits are present in the pipe.
10012] Figure 3 is a graph representing the relation between mass
attenuation coefficients
of a deposit for higher and lower gamma-ray energies.
[0013] Figure 4 is a graph representing the relation between ratios of
attenuation
coefficients of a deposit.
DETAILED DESCRIPTION
[0014] Specific embodiments of the present disclosure will now be
described in detail
with reference to the accompanying Figures. Like elements in the various
Figures may
be denoted by like reference numerals for consistency.
[0015] Embodiments disclosed herein provide a method for detecting a
deposit on an
inner wall of a tubular, determining its thickness, and identifying the
deposit material.
The tubular may be any kind of fluid-carrying conduit, such as a pipeline, a
production
tubing, surface facilities, or a venturi flow meter. Further, as the skilled
person will
appreciate, methods according to the present disclosure may be applicable
whenever
fluids are flowing in a tubular, giving rise to any kind of deposition, such
that the
applicability of the methods should not be restricted to the oilfield.
[0016] The applicant has shown that a specific relationship may be
established between
the temporal variation of the water-to-liquid ratio (WLR) of the fluid flowing
in the
tubular and the existence of deposit in the tubular. The WLR of a multi-phase
fluid may
indicate the proportion of water with respect to the proportion of water and
oil together.
Methods for detecting the presence of a deposit in a tubular and measuring its
thickness
according to embodiments disclosed herein may be based on this relationship.
[0017] The relation between the temporal variation of the WLR and the
presence of a
deposit will now be derived in detail. Referring to Figure la, a tubular 101
is shown
where a system 103, 105 is installed that measures the linear mass attenuation
of gamma
or X-ray radiation by a fluid flowing in the tubular 101. A radiation source
103 attached
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to one side of the tubular 101 emits, for example, gamma radiation that
propagates
through the fluid and is detected by a detector 105 situated at the opposite
side of the
tubular 101. Figure lb shows a similar configuration, whereby the tubular 101
has a
venturi section 107, illustrating the configuration in a venturi flow meter.
10018] The strength or count rate of the source 103 may be measured in a
reference
measurement where the tubular 101 is empty or filled with a known fluid 113
(e.g. air).
The known fluid 113, indicated by the subscript "k", may be characterized by
its mass
attenuation coefficient vk(x) for a given radiation energy Ex and its density
Pk. The count
rate of the reference measurement is as follows:
N (x) = N (x) (1)
exp(¨dpk vk (x)) '
where N(x) is the count rate at the detector 105, and No(x) is the count rate
at the radiation
source 103. In the case of Figure I a, d is the diameter 111a of the tubular
101.
[0019] When dealing with a multi-phase fluid and in order to determine the
fractions a;
(i = g, w, a) of the components of the multi-phase fluid (i.e., gas, water,
and oil), it may
be convenient to measure count rates at at least two radiation energy levels
Ex and Ey:
N o(x) = N (x)
exp clE (a p ,v (x))
(2a)
N 0 (y) = N (y)
exp dE (a p iv (y))
whereby the condition
a =1
(2b)
is fulfilled.
[0020] If a deposit is in the tubular, the count rate may be expressed as
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N (x , t) = N (x) dI(a ,(t) p iv f(x)) d cu(t)
p õv õ(x)1, (3)
wherein the subscript u indicates the unknown deposit, and o(t) is the time-
dependent
deposition rate.
[00211 It may then be assumed that within a certain period of time, At ¨
12 ¨ ti, the
fractions of the phases oil, water, and gas do not vary significantly compared
to the
fraction of the unknown component. The value of Al depends on the
characteristics of the
deposit material in the fluid and on the fluid characteristics (i.e., flow
rate, temperature,
and pressure). It is thus possible to calculate the ratio of the count rates
measured at the
times ti and /2:
N (x t)
in ' In 2¨At = -dpõvõ(x)(aõ(ti)- aõ(12)) .
(4)
N2 Cx,
The second term on the left-hand side takes into account the decay of the
source between
the first and the second measurement, whereby T is the half-life time of the
source.
Further, in most of the cases, it may be assumed that At/T << 1, so that this
term vanishes
and (4) becomes:
N,(x,t)
In ' -dPõvõ(x)(aõ(1])- an(12)) =
(5)
N2(x,t)
[0022] If, in a first case, it is assumed that the deposition rate aõ(t)
evolves linearly with
time, eq. (3) may be rewritten using eq. (5):
N (x ,t) = N ,(x) exp[,¨ dE (a p ,v (x)) ¨ Bt],
(6)
wherein
1 N (x ,t)
B(x) = ¨in ,
(7)
At
where B is defined in the interval of time At in which the variation of the
fractions of oil,
water, and gas, a, is negligible.
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[0023] In
a second case, it may be assumed that the deposition rate is an arbitrary
function of time, such as au(t) = atb, with a and b real numbers. Then eq. (6)
may be
rewritten as follows:
N(x,t)= N 0(x)exp dI (a rp iv ,(x))¨ 13(x)tbi, (8)
where B may be approximated by a first-order Taylor series around ti,
b 1, 1
B(x)t N(xt) + E) = in N1(x,t) t
(9)
At N2 (x, t)1 At N2 (x, t)1 '
and e is an arbitrarily small positive quantity, 6 = ______________________ B
may be defined for an interval
of time At that is sufficiently short so that the temporal variation of the
deposition rate
may be assumed as linear, as in the first case. It is therefore possible to
rewrite eq. (8) in
a general form under the hypothesis that s <- 1 as follows:
ln(N(x, t)\ + B(x)t dE(a p y ,(x)) ,
(10)
No (x)
Eq. (10) no longer depends on b so that it may be equivalent to eq. (6), where
b = 1.
j00241 In the case when there is no deposition in the pipe, the
fractions of the three
phases oil, water, and gas may be calculated by resolving the coupled
equations (2a, 2b).
If a summation is carried out, the eq. system (2a) may be rewritten in a
specific form,
1 In N(x) agPgVg (X) = ¨aopovo(x)¨ocooõviv(x),
d o(x)
(
m
1 , ( N(y) 11
I
gpgv g(y) ,¨aopovo(y)¨apvõ(y),
d No(y))
Ice, =1.
The equation system (11) may be understood as a system comprising two
unknowns, the
fractions of oil and water. The gas term on the left-hand side may be very
small compared
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to the oil and water terms. It may compensate for the presence of gas in the
measured
count rate (i.e., eq. system (11) may be applied to a fluid that contains gas
as well as to a
fluid that does not contain gas). For the purpose of demonstration and in
order to
simplify the notations, the fractions a, and cew of oil and water may be
expressed as
C(x)A,(y)¨ C(y)A0(x)
a 0,w = ___________________________________________________________________
(12)
A0(x)A(y)¨ A0(y)4(x) '
with
Ao,õ,(Y) = Pov 0,õ,(Y)
(13)
and
C(y) = lin( N(y) N 0(y)
d
+ a g p gv g (y)
(14)
10025] In the case when there is deposition in the pipe, the expression
(14) for C(y) may
be corrected using eq. (10), still provided that c << 1:
D(y,t)= C(y) B(y) t = C (y) ¨ G(y)t
(15)
Eq. (12) may then be rewritten for the case of deposition occurring in the
pipe so that it
reads:
C(x)A0(y) ¨ C (y) Ao(x) ¨ t[G(x)A0,1(y) ¨ G(y)A0(x)]
dep
a 0,õ(t) = ______________________________________________ =
A0(x)A(y) A0(y)A(x)
(16)
This leads to the following relation between the evolution of the oil and
water fractions in
the case of deposition and the oil and water fractions in the case of no
deposition:
aode,Pv (t) = a Po,wt
(17)
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whereby P0m, may be defined according to eq. (16). The fractions of oil and
water
ao`17(t) may thus vary linearly with time when a deposition occurs in the
pipe.
[0026] The WLR mentioned above may be expressed as
WLR = a w
a
(18)
a+ a0 lag
We may then assume that the flowing multi-phase fluid in the pipe consists
only of oil
a odep (0+ a 1(17 =
and water, i
(i.e., no gas is present in the fluid flowing in the pipe). If
a deposition occurs, a time-dependent WLRdeP (t) may be written as
WLRd' (t) = WLR ¨ Pwt ,
(19)
[0027] A
person having ordinary skill in the art will appreciate that even if gas is
present
in the fluid, the variation of the gas fraction ag due to a deposition may be
very small. It
does not significantly affect the calculation of the WLR because the
absorption of
radiation in the gas fraction of the fluid is negligible. The variation of the
WLR due to a
deposit may be mainly due to the variations of the oil and water fractions of
the fluid.
Therefore, departing from eq. (18), the time dependent WLR when a deposition
is
occurring may be written generally as
anuct
WLRfluct = a4,
"' ___________________________________________ WLR PWt =
(20)
aõ,+a, 1¨ag
[0028] Expressions (12) ¨ (19) may be more complex if the
simplification of eq. (11) is
not made (i.e., if the three components of the fluid oil, water, and gas are
considered
equally). However, a similar solution as (19) will be obtained. If a
deposition occurs, the
WLRdeP (t) varies linearly with time. Therefore, it is shown that it is
possible to
determine the presence of deposit in the tubular by measuring the WLR (or the
fractions
of oil and water) over a period of time to establish the dependency of the WLR
(or the oil
and water fractions) on time.
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[0029] If the thickness of the deposit is much smaller than the diameter
of the tubular, the
differential pressure (i.e., pressure drop) may be measured. One method to
determine the
differential pressure is to measure the fluid pressure at two different
locations in the
tubular to obtain the natural pressure lost DPI, as shown in Figure I a.
Another method is
to use a V-cone or a venturi (as shown in Figure lb), or any other device for
measuring
pressure drop, to obtain the differential pressure DP1 or DP2. This pressure
drop may be
related to the total volumetric or mass flow rate passing through the pipe. As
persons
skilled in the art will appreciate, any appropriate multiphase flow-metering
device may be
used to measure the flow rate. Applicant has shown that if there is a constant
volumetric
flow rate for a given period of time, and during this period a linear
variation of a
measured fraction or the WLR is noticed, then there may be a high probability
that a
deposition is occurring inside the pipe.
[0030] Therefore, if the variation of the gas fraction is small compared
to the variations
of the oil and water fractions or if no gas is present in the fluid, if the
differential pressure
or flow rate are nearly steady, and if the WLR (or the water fraction of the
fluid) varies
linearly with time, then we may conclude that a deposition has occured in the
pipe. As
will be understood by persons skilled in the art, the deposit may be of any
material, such
as wax, asphaltene, or scale, because no assumption regarding the deposit type
has been
made.
[0031] In methods to detect the presence of a deposit in a pipe according
to embodiments
of the present disclosure, a first step may be to measure the WLR(t) using any
appropriate
sampling techniques in the art. For example, one technique is to radiate
photon beams
with at least two levels of energies through the multi-phase fluid, whereby
the photon
beam is usually generated by an X- and/or gamma-ray source. The deposit in the
tubular
may be understood as a thin layer that increases the thickness of the wall of
the tubular.
[0032] If using gamma rays, an additional layer formed by the deposit, as
observed
within a relative short interval of time, on the wall of the tubular may cause
an
attenuation of the gamma radiation in addition to the attenuation caused by
the fluid
flowing in the tubular. This behavior may be treated as an artificial aging of
the nuclear
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radiation source. As it appears from eqs. (11) ¨ (20), it may be justified to
consider that
the contribution of the gas fraction of the fluid to the fluctuation of the
WLR due to a
deposit is small compared to the contributions of the oil and water fractions.
The gas
contribution to the WLR was therefore mathematically approached by a linear
compensation to the count rates measured in the fluid. Therefore, using
methods
according to embodiments disclosed herein, it is not only possible to correct
for the gas
fraction in fraction measurements, but also to correct for the decreased count
rate due to
the deposit. Therefore, during a count rate measurement, the count rate for an
empty
tubular may be artificially increased.
Example 1
[0033] A
first embodiment to measure the thickness of a deposit will now be described.
Referring to Figures 2a and 2b, an empty pipe 101 with a measuring device
including a
radiation source 103 and a radiation detector 105 is shown, whereby in Figure
2a no
deposit is present in the pipe, and in Figure 2b deposit 109 is present at the
wall of the
pipe 101. The scenario of Figure 2b may be achieved by shutting off the well
(i.e.,
stopping the flow), and depressurizing the measuring device. The deposit 109
shown has
a thickness 1/2 du. With an empty pipe, the ratio of the count rates No(x) and
Ni(x) without
and with deposit, respectively, may be written as
(
ln N1(x)1 = ¨da p iv (x) = d p iv (x)
(21)
No(x))
where d is the distance between the radiation source 103 and the detector 105,
4=
and j = g, w, o, u. The count rates No(x) and Ni(x) without and with deposit,
respectively,
differ from each other by AN(x),
N (x) = N (x) + AN (x)
(22)
which may be considered as the artificial aging of the radiation source 103.
Due to the
temporal linearity of the deposition rate discussed above, we may then assume
that during
a time interval Al = 12 ¨ /1 that is sufficiently short (i.e., c << 1), the
source 103 "ages" by
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AN(x). Accordingly, the term 6,N(x) may be determined by measuring the count
rate in
the empty pipe at two different times ti and t2 (once before the deposition
occurred and
once the deposit is present). It is then possible to calculate N(x,t)
N (x, t) N 0(x) + N (x) t ,
(23)
At
with N(x,t) being the count rate of the empty pipe recalculated at any period
of time t
based on two consecutive measurements done of the empty pipe. Through
expression
(23), it may be possible to build a relation for a deposition rate varying
linearly with time.
It may further be possible to obtain the empty-pipe count rate if no
deposition is
occurring (No(x)).
[0034]
The total thickness of the deposition in the tubular that is traversed by the
radiation may then be approximated by the following expression:
1 AN(x)
du =t
(24)
põvu At
It should be noted that in this embodiment, the type of deposition should be
known in
order to be able to determine the density and the mass attenuation coefficient
of the
deposit component.
[0035]
As persons skilled in the art will appreciate, the measurements in this
example
may also be carried out in a tubular that is filled with a known fluid after
the well fluid
flow has stopped. This embodiment may also be applied if the tubular, in which
the
count-rate measurements are carried out, includes a bypass that allows for not
stopping
the flow in the system.
Example 2
[0036]
In another example, the fluid flow may not have to be stopped. First, the
presence
of a deposit may be detected by observing the temporal evolution of the WLR.
Then, the
WLR may be measured before and after detection of the deposit at two energy
levels Ex
and E. The WLR of the fluid must be the same before and after the deposition
has
occurred. The ratio of the count rate with deposit and the empty-pipe count
rate without
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deposit, 1n[Ni(x)/No(x)], is then tuned until the WLR measured after the
deposit detection
is the same as without the deposit in the tubular (i.e., the fractions of oil
and water are the
same before and after the detection of the deposit).
10037] An estimation based on a reversed model may then be used to
calculate the
expected count rate of the empty pipe for each energy, i.e., No(x) becomes
No (x) = (x) exp dI a p rv ,(x) ,
(25)
whereby the fractions a, are the ones calculated from measurements without
deposit. The
same applies to No(y).
100381 The following two equation systems (before and after the
deposition) may then be
used to calculate the oil and water fractions using an iterative method to
obtain the same
WLR in both cases:
N(x)
In = ¨d1(a, piv ,(x)),
\N 0(x) )
InN (Y)
( N (y) = ¨ dE p ,v (y)), (26)
a 1
and
¨ dL oorv, (x)),
N 1(x) )
(
N (y)
in ¨d1(a1p1v (Y)),
(27)
\ 1(y)
Ea, = 1 .
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No(x) is the empty-pipe count rate before a deposition occurs, and NI (x) is
the empty-pipe
count rate after a deposition has occurred. Further, from the systems (26) and
(27), the
following expression may be derived:
(
In Ni(x) I in 7 N (y) = v" (x)
(28)
No (x) No (y) (y)'
where vu are the mass attenuation coefficients of the deposit material for the
radiation
energies E and E. In tuning the ratio of the count rates in (28), the
attenuation
coefficients vu of a deposit material for the two radiation energies Ex. and
Ey may be found
using a database (lookup table) that lists attenuation coefficients for these
energies for
several kinds of deposit. For illustration, table 1 lists the mass attenuation
coefficients for
typical materials at 32 keV and 81 keV. The database needs to be implemented
with the
system.
keV
BaSO4 SrSO4 Mg$04 aCO3 404 MgC12 ZnSO4
bSP4,
1120'
0.3514 5.029 8.9641 0.8033 1.5163 1.542 2
4.4845 17.274
0.18396 2.2699 0.7683 0.194 0.2393 0.2405 0.2416 0.4364 1.5922
keV oil NaCl ZnS PbS Ca( 7.6 Fe304 S2 asphaltene
vv'ax
_3414. 0.2466 1.4429 7.2015 21.802 2,4557 4.926 1
731 0.35724 0.2639
81
0.1704 0.2286 0.6125 1.973 0.29185 0.4531 0.2505 0.17898 0.1817
Table 1: Mass attenuation coefficients for 32 keV and 81 keV for several
materials.
10039]
Once a deposit material has been found that best matches eq. (28), the density
of
the deposit material may be known. Using then eq. (21), an estimate of the
total distance
du traversed by the radiation within the deposit may be obtained through the
relation:
1 in( (x)
2ddep = d = _____________________________________________________________
(29)
p õv õ (x) N 0(x) )'
where we assume that the thickness cid,p of the deposit is uniform around the
circumference of the tubular so that the radiation passes it twice when it
propagates
through the tubular. Advantageously, this second example only requires that a
database
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be present within the system that lists the attenuation coefficients for the
gamma and/or
X-ray energies used for the measurements. No further knowledge of the type of
deposition is required.
Example 3
[0040] In a third realization example, two different radiation energies
are also used for
the count rate measurements. Here, it may be assumed that the mass attenuation
coefficient of the deposit for the higher energy may be fitted by a polynomial
function of
the mass attenuation coefficient of the deposit at the lower energy. This
assumption is
true for most of the deposit materials, except, amongst others, BaSO4. For
example, in
the case of Barium, the polynomial function may be written as
v (y) = h 1/2 (x) + k v (x) + 1 ,
(30)
whereby h, k, and I are integers, Ey = 81 keV, and Ex = 32 keV. Relation (30)
is depicted
in the graph shown in Figure 3.
To identify the deposit material, the mass attenuation coefficient v(x) may be
calculated
by combining eqs. (28) and (30),
ln N1(Y)
N (y) )
v (x)a, = v2(x)+bv(x)+c
(31)
in(N1(x)
N 0(x) )
The thickness of the deposit may then be found following the method detailed
in the
second example. In this embodiment, v(x) and v(y) may be measured and not
extrapolated from a database. The deposit density may be found using a lookup
table
with the densities at different radiation energies.
10041] Persons skilled in the art will appreciate that, in Examples 2
and 3 described
above, more than three radiation energies may be used.
Example 4
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[00421 In
a fourth realization example, the count rate measurements may be carried out
using three different radiation energy levels Ex, Ey, and Eõ. The three
radiation energies
may be provided by combining, for example, several radioactive sources or a
radioactive
source and an X-ray source.
10043] In order to obtain the mass attenuation coefficient of the
deposit material,
equation systems similar to (26) and (27) (for the case of two energies in
Example 2) may
be used, whereby every equation system contains three relations for the count
rate ratios.
From this, two coupled equations for the mass attenuation ratios may be
derived:
võ (x)
No (x)) No (y) ) võ (y)
(32)
In ( N (y) /1n( N1(z) võ (y)
No (y) No (z)) võ (z)
10044] As in Example 2 described above, the ratios (32) may be tuned in
order to
determine the deposit material using a database as in table 2 that lists the
attenuation
coefficients for different materials and radiation energies. The relation
between the two
ratios of the attenuation coefficients in eqs. (32) (low/high energy versus
low/medium
energy) is depicted in the graph shown in Figure 4 for Barium with E- 32 keV,
Ey = 81
keV, and Ez = 356 keV.
key H20 BaSa; SrSai NgSO4 CaCO3
CaSO4 Mgcb Znsat I bs04.
0.3514 5.029 8.9641 0.8033 1.5163 1.542 2 4.4845 17.274
,81_- 0.18396 2.2699 0.7683 0.194 0.2393 0.2405 0.2416 0.4364 1.5922
0,10565 0.1259 0.1025 0.0963 - 0.09899 0.09906 0.095577
0.09953 0,2215
µµ.
keV I
oil MCI ZnS PbS CaC12 Fel , S2 asphaltene "silax
. I '
0.2466 1.4429 7.2015 21.802 2.4557 4.926 1,731 0.35724 0.2639
6'81 0.1704 0.2286 0.6125 1.973 0.29185 0.4531 0.2505 0.17898 0.1817
_
354,v: 0.1105 0.09645 0.1023 0.2552 0.09907 0.09886 0.1 0.105 0.1105
table 2: Mass attenuation coefficients for 32 keV, 81 keV, and 356 keV for
several materials.
CA 02725061 2010-07-27
WO 2009/095876 PCT/1B2009/050365
100451
However, at high radiation energies, for example 356 keV, the absorption of
the
radiation may become dependent on the density of the deposit. The electronic
density pei
of the deposit material may be linked to the ratio of the count rates with and
without
deposit by the following relation:
¨Kin( N' (z)
0(z) )= 1.9 el ,
(33)
where K is a constant that depends essentially on the geometry of the system.
The mass
density may then be obtained through
Pel 2Z u
(34)
A
whereby Z is the atomic number and A is the mass number of the material. For
most of
the deposit materials, with the exception of CH4, we may estimate 2 x ZIA I.
[0046]
To calculate the mass attenuation coefficients of the deposit material, the
density
of the deposit material may first be estimated using the high-energy count
rate ratio (33)
and then eq. (34). Then, using one of the methods described in Examples 2 or
3, a set of
solutions
pei võ (x)/
(35)
/võ (y)
or
v1 (x)/
(36)
(Y)
may be obtained. From this, the thickness may be calculated similarly to the
method in
the second example. Persons skilled in the art will appreciate that more than
three
radiation energies may be used.
100471
In this embodiment, due to the use of a third radiation energy, an additional
characteristic of the deposit may be obtained from the count rate
measurements. The
16
CA 02725061 2010-07-27
WO 2009/095876 PCT/1B2009/050365
additional characteristic may be, for example, the density of the deposit.
Using this
information with the associated ratio of the mass attenuation coefficients as
expressed in
(36), fewer assumptions on the type of the deposit may be needed, and a better
discrimination of the deposit composition may be obtained.
[0048] If the approach proposed in Example 3 is used (e.g., eq. (31)), the
deposit may be
completely described with Cod, vu(x), vu(y)). In this case, if we then apply
eq. (29), it may
even be possible to describe the deposit (mass attenuation coefficient,
density) without
any further knowledge (i.e., no mass attenuation database has to be present).
It may then
be possible to calculate directly the deposit thickness.
[0049] Persons skilled in the art will appreciate that in each of the
examples, once the
mass attenuation coefficient and the density of the deposit have been
determined, the
linear attenuation coefficient /6 may be easily obtained through the relation
)6 = VP.
Thus, methods according to the embodiment described in the fourth example
enable
obtaining any combination of characteristics of the deposit (e.g., linear
attenuation
coefficient, mass attenuation coefficient, density, thickness). Embodiments
disclosed
herein may cover any type of output combination (pd, p, võ(x), vu(y)).
[0050] Having access to the deposit thickness, it may be possible to look
at the rate of
deposition. This may be done by several count-rate measurements at at least
two
different times separated by varying time intervals. This step will allow the
operator to
know when a tubular should be "pigged" if the restriction due to the deposit
becomes
too large. It may also allow for monitoring the effectiveness of a chemical
product
injected into the pipe to dissolve the deposition. Thus, as the maintenance
program of
the well may be optimized, production deferral may be avoided.
[0051] In a second aspect, embodiments disclosed herein relate to
apparatus to detect
deposits in tubulars. The apparatus are partially represented in Figures 1 and
2, where
the apparatus include a radiation source 103 to radiate a photon beam through
the fluid
and a radiation detector 105 to measure the radiation absorption in the fluid
to obtain
absorption data. The radiation source 103 may be any appropriate radiation
source
known in the art, such as, but not limited to, gamma-ray or X-ray sources or
combined
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WO 2009/095876 PCT/1B2009/050365
gamma/X-ray sources. Similarly, the detector 105 may be any appropriate
detector
known in the art, such as, but not limited to, Geiger or scintillation
counters. The
apparatus may further include processors to carry out the methods according to
embodiments disclosed herein, and output units to output the deposit
information for a
user.
100521 Scale deposition with a thickness of less than 0.3 mm has been
detected with
methods according to embodiments disclosed herein. Further, the scale has been
identified without any problem. Persons skilled in the art will appreciate
that the
methods are applicable to any other kind of deposit.
[0053J Methods according to embodiments disclosed herein may also be
applied to any
system in which monitoring a deposition is necessary, whereby the deposit may
be
composed of any material, having any concentration. For example, methods to
identify
and characterize deposits are applicable in the food industry as well as in
the oilfield.
[0054] While the disclosure has been described with respect to a limited
number of
embodiments, those skilled in the art, having benefit of this disclosure, will
appreciate
that other embodiments may be devised which do not depart from the scope of
the
invention as disclosed herein. Accordingly, the scope of the invention should
be limited
only by the attached claims.
18
