Note: Descriptions are shown in the official language in which they were submitted.
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Prior art
Resman has a patent on a specific method and device for installing a
polymer carrier for a chemical tracer material wherein the polymer
carrier is formed as thin rods placed in a cavity within the well
completion tubing outside the central tube. Such a polymer carrier
is arranged for long-time release of the tracer materials and is not
desirable to use in the present method, as it is an advantage to
have a "clean shot" release of the chemical tracer material.
However, the experience gained with tracer flowback from more than
50 wells with such polymer carriers has been a necessary basic for
this new invention.
Problems related to the prior art.
The tracer carriers illustrated in Fig. 4 may be polymer carriers
with long-term release of tracer material. Annular wetting is fluids
from the annular space entering through a screen, wetting the tracer
carrier, and leaving to the annular space without local passage to
the central production tube. Tubing wetting is fluid arriving
through central pipe and deviating through a screen in the tubing
out to a closed sub enveloping a tracer carrier, and returning with
tracer material back through the internal screen to the production
tubing. Combined wetting may be obtained using a sub with a screen
for allowing influx from the annulus space and also allowing passage
through a screen from and to the central production pipe, with a
tracer carrier arranged between the central tubing screen and the
annulus screen.
In the present invention a downhole tracer release rate changes,
preferably in short pulses, while the well flow rate is constant
over time (or where the well flow rate changes slowly relative to
the short pulses of tracer release. Mechanical tracer release
chambers may be the source of such. If several chambers release
synchronously in a well the situation may be good as a basis for
extracting downhole inflow profile. This may correspond to the
situation of Fig. 1.2, with a mechanically or otherwise controlled
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instantaneously released tracer at a given point of time. This may
prove advantageous if the different influx zones have different
influx pressures. If different influx pressures exist, it is not
feasible to create the "shots" illustrated in Fig. 1.2 and 1.3 by
shutting in the well because cross-flow between the zones may arise
during shut-in.
There may be 20 to 30 influx zones in a well. The trend in the
technical field is that the number of influx zones is increasing,
and that one may arrive at SO or more separate influx zones. The
reason for this increase in influx zones is due to longer drilled
production wells and using generally horizontally drilled portions
of the well, and exploitation of more complex reservoirs.
According to an embodiment of the invention one may utilize
mechanically released so-called tracer shots. Groups of distinct
tracer materials are released in selected influx zones e.g. 4
different tracers at a time fired in each their separate zone. Then
one may calculate an image of the relative influx rates based on
sampling of a well flow which may have arisen such as illustrated in
Fig. 1. The total flux is expected to be measured topside.
Subsequently the same set of tracers, or a different set of tracers,
may be fired from other positions in the well at a later time. This
results in that one may do with a reduced number of unique tracers
than the number of influx zones. One may use tracer release
mechanisms which are installed in the production zone e.g. during
the completion of the well. It may be inappropriate or impossible to
set down tracer when the production has been started, e.g. in
subsea-wells wherein intervention is highly restricted due to price
or lack of access.
Another advantage using mechanical release according to the
invention is that it may take place at a desired point of time at a
desired place in the well. The installation of the completion may
take several days. A polymer carrier will usually start releasing
tracer immediately when in contact with the well fluids, and tracer
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will be smeared out along the entire well during the completion
installation.
Problems related to long term release in this context
- no sharp pulse.
- shut-in of the production required
- cross flow between zones in case of non-steady state production
flow.
Brief summary of the invention
The invention is a method for estimating influx volumes (qi) of
fluids to a production flow (F) in a well (Wr) with two or more
influx locations (3) along the well
- arranging tracer sources (4) with distinct tracer materials (4m) in
fluid communication with said influx zones (3),
- each said tracer material (4m) having a well'defined, comparatively
short duration release dose (Vt4) to the fluids in the well,
- allowing said tracer sources (4) to release said tracer material
(4m) to said fluids at a given release instant (tR),
- after said release instant (tR), consecutively collecting samples
(c1, c2, c3, ...) of said production flow (F) at the topside,
- analysing said samples (c1, c2, c3, ...) for identifying types of
tracer material (4m) and concentration of said identified tracer
materials (4,),
- based on said concentrations (4c, 41c, 42, 43) and their sampling
sequence and the well geometry, sequence of said separate influx
zones, calculating said influx volumes (ell) from transient tracer
flow models
- using the calculated influx volumes (q1) as parameters for
controlling the production flow or for characterizing the
reservoir.
The invention may also be defined as a system for estimating influx
volumes (qj of fluids to a production flow (F) in a well (Wr) with
two or more influx locations (3) along the well, comprising
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- tracer sources (4) with unique tracer materials (4m) arranged in
fluid communication with said influx zones (3),
- each said tracer material (4m) having a predefined short duration
release dose (V") to the fluids in the well,
- said tracer sources (4) provided with a timer to release said
tracer material (4m) to said fluids at a given release instant (tR),
- a sampling device for consecutively collecting samples (c1, c2,
...) of said production flow (F) at the topside after said release
instant (tR),
- an analysing apparatus for said samples (c1, c2, c3, ...) for
identifying types of tracer material (4m) and concentration of said
identified tracer materials (4c),
- an algorithm for calculating said influx volumes (q,) from
transient tracer flow models based on said concentrations (4c, 41c,
42, 43) and their sampling sequence and the well geometry, sequence
of said separate influx zones,
- said calculated influx volumes (qj for being used as parameters
for controlling the production flow or for characterizing the
reservoir.
Advantageous embodiments of the invention are given in the dependent
claims.
Advantages of the invention
An advantage of the invention over prior art is that as the chemical
tracer according to the invention is released over a short period of
time compared to the characteristic time constants of the physics to
be monitored or the physics to be exploited during the monitoring
process. The tracer is released over a short period generally less
than one minute and in practice probably in about 10 seconds. The
tracer may advantageously be released under steady state flow of the
fluids in the well, and thus the method of the invention incurs no
or little disturbance to the well flow and that information
extraction may be done during relevant operating rate condition.
Thereby it is easier to understand details of the well flow such as
estimating the differences between different influx Zones'
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contribution to the total flow. If the calculation of the different
contributions to the well flow differ from what is a desired flow
pattern in the well, the operator may use the calculated
contributions from each influx zones as one of several parameters
for determining adjustments to the control of the well.
Figure captions
Embodiments of the method and device of the invention is illustrated
in the attached drawings, wherein
Fig. 1 shows a series of diagrams to visualize how the tracer
concentrations shots can be introduced in the production stream and
how the shots change as they are transported across the reservoir
interval. The downstream piping system and well path to the topside
equipment is not illustrated.
Nine frames are shown, Fig. 1-1 to 1-9 illustrating the technique.
Each frame is a time step and describe how the tracer shots move
after being built up as a result of a tracer shot. The diagrams
represents a horizontal well with four tracers of generally instant
release, installed at positions labelled A, B, C, D. For simplicity
in this example the distances between each subsequent tracer
position along the wellbore are equal.
The tracer release devices are exposed to the well fluids either
from the outside of the completion or inside depending on the
carrier system. The tracers are released to the fluids at a given
instant. When released as illustrated in Fig. 1-2, then the fluids
immediately surrounding the tracer develop a high concentration of
the tracer. Such volumes are referred to as a "tracer shot" and
typically start off as equal volumes.
In Fig. 1-3 the well influx has started and each vertical arrow in
this example represent a given flow for example 1000 bopd (barrels
of oil per day).
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As seen the influx from the zone between tracer C and D is three
times higher than the influx between zone A and B.
When the tracer slugs start moving with the well fluids as seen in
Fig. 1-5 these variations in influx between the zones will affect
the volume of fluids between each tracer slug and the concentration
of each slug as they pass across the zones.
The volume and hence time difference between the arrival of slug C
and D will be longer than between A and B due to the fact that there
will be three times more wellbore fluids that are entering in
between the two tracer slugs C and D. This is visually represented
in the Figs. 1-6 , 1-7, 1-8 and 1-9. Also the concentration of
tracer slug D will become more diluted and spread out as a result of
this higher influx, this is also visualized in Figs. 1-6 to 1-9.
Fig. 2 is an idealised illustration of concentration of identified
tracers sampled topside, with time or cumulative production volume
(since the injection) as the abscissa.
Fig. 3 is an illustration of an approach for matching the unknown
downhole influx rates in the downhole production zones with the
modelled influx rates. The model influx rates are adjusted until the
calculated concentrations of model tracers compare well with the
measured concentrations of identified tracers.
Fig. 4 gives a few rough examples of well fluids wetting tracers
generally according to prior art.
Fig. 5a is a simplified section through a petroleum well. Influx
volumes of fluids enters from the reservoir rocks to end up in a
production flow in a central production pipe in the well provided
with two or more separate influx locations. In this situation the
influx zones may not be precisely known and it is not taken for
granted that the tracers are placed where the influx exactly occurs.
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Fig. 5b is a simplified section through a petroleum well wherein
packers are arranged for mutually isolating the influx zones. In
this situation the tracers are also placed each in its separate
influx zone. There may be many more influx zones and tracer carriers
than what is illustrated in Fig. 5a and b.
Fig. 6 comprises illustrations of embodiments of the invention. In
this example, the injection of the tracer shot performed during
steady-state flow so only the tracer forms a transient in time, and
the fluids with the tracer is eventually flushed out from the
isolated zone's completion void.
Fig. 6a illustrates one insulated influx zone insulated by a lower
(right) and an upper (left) packer defining a zone of influx of
petroleum fluids (and / or water) entering the annulus about the
production tubing, the fluids passing a mechanical tracer release
sub, (not yet released) and the fluids with more or less tracer
material leaving the annulus space through apertures in the central
production tube to the production flow which passes towards the
topside. A steady state flow rate is advantageous.
Fig. 6b illustrates the same setup, now with the mechanical tracer
release triggered and tracer material released into the annulus
space. A tracer shot of short temporal duration is created. The
dispersion of the tracer material will be a function of turbulence
and flow geometry in the annulus space.
Fig. 6c illustrates the subsequent step wherein the fluids with the
tracer shot with a more or less distributed tracer material is
flushed out the annulus space through apertures in the central
production tube to the production flow which passes towards the
topside. Again, a steady state flow rate is advantageous.
Fig. 7 shows curves of tracer shot release into the base pipe from
the annulus void of Fig. 6c into the central production pipe (base
pipe) as a function of time. The "rate 2Q" curve indicates a twice
as high influx rate as the "rate Q" curve, both washing out the same
amounts of tracer delivered by equal shots. Please notice that the
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area under the 2Q and Q rate curves are equal. Please also notice
that both curves approach nil concentration as the doses released
are finite at the short term. The higher rate will flush out fastest
and die out faster, while the lower influx rate will wash out at a
lower rate and sustain at a detectable level for longer.
Fig. 8 comprises illustrations of a possible problem. In this
situation the tracer shot is built over time from the tracer leak-
out from polymers into still shut-in fluids. The flow-back of the
shot to surface is then done during production ramp-up. In this
example, not only the tracer forms a transient in time, but there
may also occur a cross-flow of fluid from the shown zone to another
zone during the build-up of the shot, which will complicate the
backf low pattern and obscur the measurements obtained.
Fig. 8a illustrates a situation similar to what is illustrated in
Fig. 6a, with the difference that tracer is released more or less at
a constant rate over long time, e.g. tracers from a polymer rod
arranged in the annular space outside the central production tube.
The fluid carries tracer with it at a generally even rate with the
production flow.
Fig. 8b illustrates the result of a shut-in downstream (topside) in
order to build a concentration in the annulus space, called to build
a "shot". This method may work well in case there is no cross flow,
but the method of using a continuous release of tracer is sensitive
to crossf low between zones (this is one zone) through the central
production pipe, if a downstream (topside) shut-in is used. If the
shut-in occurs downhole between all influx zones and the production
pipe, this is not a problem, but requires a more elaborate well
control apparatus.
Fig. Sc illustrates that the tracer concentrated fluid (the "shot")
is flushed out with resumption of the production by opening the
topside valve, and the partially leaked-out shot will be flushed out
as a longer pulse than strictly desirable because of the potentially
non-ideal build-up of the shot.
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Fig. 9 shows ideal curves of tracer shot release into the base pipe
from the annulus void of Fig. 6c into the central production pipe
(base pipe) as a function of time, in the situation described for
Fig. 8c, but without cross-flow, i.e. it is the best imaginable
situation of shut-in with long term release of tracer. Please notice
that both curves cannot approach nil concentration as the doses are
continually released. The higher rate will flush out fastest and die
out faster, while the lower influx rate will wash out at a lower
rate, but both may be at a detectable level for very long time.
Fig. 10 illustrates a mechanical tracer release sub according to an
aspect of the invention and for use with the method according to the
invention. A tracer dose is in this case arranged in a breakable
ampoule, e.g. in a glass bulb and to release through holes open to
the central production pipe. A release mechanism comprising such as
a small explosive charge or a puncturing needle is arranged for
breaking the breakable ampoule controlled by a timer of an
electronic unit. The electronic unit, please see section B-B, is
preferably provided its own electric battery and is preferably
arranged to trigger the release mechanism at a given date and time
of day. There may be arranged a series of such breakable ampoules
around the perimeter of the release sub, please see section A-A, in
order for enabling a series of measurement rounds over time, each
ampoule predestined to break at long intervals, such as one each
month, each six months, or more. The entire release sub may be
provided with end rings such as friction slip rings for being
mounted into the central production tubing and inserted with the
completion into the production zone. The setup in Fig. 10 will
typically be used in the context described in Fig. 1 where venting
towards the central base pipe is needed.
Fig. 11 shows a similar embodiment of the mechanical tracer release
sub according to a slightly different embodiment of the invention,
for use with the method according to the invention. The tracer doses
are arranged in breakable ampoules which are arranged with vent
holes of the sub open to the annulus space and not to the production
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pipe directly. Otherwise the mechanical release sub is similar to
what is described under Fig. 10. This mechanical embodiment thus
releases into the void outside the central production pipe and
should be used in the context shown in Fig. 6 with flush-out from
the insulated zone in the annulus void in the completion, and will
work along the lines of Fig. 7.
Fig. 12 illustrates this mechanical embodiment which releases into
the void outside the central production pipe and used in the context
shown in Fig. 6 with flush-out from the insulated zone in the
annulus void in the completion.
Fig. 13 relates to a setup with tracer shots being injected into the
central base pipe, as also explained in Fig. 1. Fig. 13 shows curves
of tracer concentrations as function of cumulative production volume
topside. In the upper portion of the drawing there is illustrated
highly simplified illustrations of two parallel production zones
called "zone 1" and "zone 1 & 5" (which may produce into the same
main well) or two wells on the same tie-back, leading to the same
topsides sampling site. The vertical coloured lines are the
positions of tracers in insulated influx zones to the two branches.
The different coloured lines in the curves indicate measured
concentrations (interpolated). The vertical bars of same colours
indicate peak arrivals (as function of cumulative volume) if even
influx rates had existed and this is calculated from models. One
will see that the first (heel) production of zone 1 and zone 1 & 5
arrive almost as predicted from the even rate model, but that the
toe marker of zone 1 arrives far too early and its influx must be
higher than presumed, and the nearer toe of zone 1 & 5 arrives far
too late and must be due to a lower influx than presumed. This
indicates that the influx model should be adjusted significantly.
Fig. 14 shows the same measured curves and well models as for Fig.
13 above. A general scheme of comparison between the Real World and
the model world as shown in Fig. 3 may be used. The difference is
that here the influx model of "zone 1" and "zone 1 & 5" are heavily
corrected to indicate influx rates downhole "zone 1" of 18%, only
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1%, and as high as 43% contributions to the combined total flow
topside, and for zone 1 & 5 contributions of 9% at the heel, 10%,
and 18% at the toe. Here we see that the middle production zone of
"zone 1" contributes insignificantly and may be shut down or
considered as a candidate for an overhaul. One will now see that the
predicted peak arrivals coincide with the actual peaks.
As an improvement, further curve analysis could be conducted in
order to determine the assumed continuous curve peak arrivals from
the non-continuous measurement results, as the peak of a non-
continuous series is not necessarily the real peak. Anyway, the
illustrated match is far better than for Fig. 13.
Embodiments of the invention
The invention is a method for estimating influx volumes (qj of
fluids to a production flow (F) in a well (Wr), please see Fig. 5.
The well is provided with two or more separate influx locations (3,
31, 32, 33) along the well. The actual positions of the influx
locations are not necessarily precisely known. Tracer sources (4,
41, 42, 43) with distinct tracer materials (4m, 41m, 42m, 43m) are
arranged upstream/downstream said influx zones (3, 31, 32, 33). Each
said tracer material (4m) has a predefined, comparatively short,
quickly released dose (V") (short release time dose) to the fluids
in the well.
With the term "comparatively short" we here mean significantly short
compared to subsequent sampling intervals, compared to the time
required for the well flow's transit time from the influx zones to
the topside of the well, compared to possibly the leak-out time
constant from the annulus to the central production pipe if the
tracer is released into an external void in the well completion and
compared to the characteristic time constant of the physics we are
monitoring. The influx zone furthest from the topside is called the
"toe" and the nearest influx zone is called the "heel". The method
aims at extracting information from tracer transients in the
petroleum fluid (or water) flowback of tracers to the surface.
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The tracer sources (4, 41, 42, 43) are according to the invention
allowed to release the tracer material (4m, 41m, 42m, 43m) to the
fluids each belonging zone at a given release instant (tR). The
tracer sources according to the invention are arranged to release
the tracers at a given instant in time in order for the subsequent
topsides sampling to be conducted rationally. In an embodiment of
the invention this is done by providing each tracer source downhole
with a timer which is set for triggering the release at a given date
and time. The release may be repeated at one or more later given
date and time in order to conduct further measurement series.
After the release instant (tR), samples (c1, c2, c3, ...) of the
production flow (F) are consecutively collected at the topside. The
sampling may simply be conducted by tapping small amounts of the
petroleum flow (F) at registered times. An alternative to sampling
as a function of time is to collect sample at intervals based on
cumulative petroleum volumes (fl, f2, f2, f4), if the flow is not a
steady-state flow. (One may collect samples at regular time
intervals and plot and analyze the measurements as a function of
cumulative productin.)
After sampling, the samples (c1, c2, c3, ...) are analysed for
identifying the types of one or more tracer material (4m, 41m, 42m,
43m) and their corresponding concentrations (4c, 41c, 42c, 43c) of
the identified tracer materials.
In an embodiment of the invention the analysis is conducted on site
during the sampling period using field analysis instruments topside
in order to provide results rapidly. The analysis may be conducted
in a chemical laboratory in order to provide more precise
measurements or for verifying or refining field measurements.
Rough calculation
Based on the measured concentrations (4c, 4lc, 42, 43) and their
sampling sequence, i.e. sampling times or cumulative production
volumes (fl, f2, f3, f4) and the well geometry, one may calculate the
influx volumes (qj from transient tracer flow models (in a
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preferably, but not necessarily steady state flow). The well
geometry comprises the sequence and positions of the separate influx
zones, and the length and geometry such as pipe diameters
corresponding lengths of the sections of production pipe, possibly
including tie-back pipes, all the way from the influx zones to the
topside sampling point.
Utilizing calculated influx volumes
The calculated influx volumes (qj are used as parameters for
comparison indirectly with the real measurements so as for
controlling changes to the production flow, such as increasing or
decreasing the total flow topside or adjusting the influx from the
separate influx zones using valves between the influx zones and the
central production pipe, or adjusting the flux ratios from well
branches' production pipes into a main well.
Refining the calculations
A model of the well may be established. The model may be adjusted
with regard to influx volumes in the distinct zones until there is
correspondence in the measured concentration curves and the modeled
calculation curves.
Decisions based on many parameters
The well operator will usually not decide on controlling the well
flow only based on the estimates of influx volumes, but use
additional relevant parameters such as pressure and fluid
composition and other operational parameters.
Simultaneous release all or in groups
In an advantageous embodiment of the invention each tracer source
(4) arranged for releasing preferably simultaneously, at least in
groups, at a given release instant (tR) in time. One may release in
all influx zones in the well simultaneously if so many different
tracers are available. If the number of influx zones in the well is
high, say 30 to 50 or more, one may release a limited number of
different tracers, say 4 to 6, in a corresponding number of isolated
influx zones at a time, and repeat the release with the same set of
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tracers in subsequent sets of zones with a required sufficient delay
until the entire well is covered.
Release duration
In an advantageous embodiment of the invention, the release dose
(V") is released during less than one minute, preferably less than
seconds. The release time rather short so that the release of
tracer is a pulse compared with characteristic time constants of the
flow events that we are monitoring, and, in case we have a delay
10 chamber, significantly shorter than the characteristic time constant
of the delay chamber.
Release instant control
In an advantageous embodiment of the invention, the given release
instant (tR) is an advance determined instant in time. The release
instant may be set while installing the mechanical release sub in
the completion, before the entire completion is inserted into the
well production zone. The mechanical release sub may be provided
with a self-powered timer in a tracer release unit so as for to
avoid any external power supply, and also for avoiding control lines
from the surface: one knows the date and time the tracers are
released, and sampling must be conducted in a required number of
samples through a sufficiently long time after the release, and one
will have a good set of samples to analyze.
The given release instant (tR) may be being one of a series of
release instants. In an advantageous embodiment of the invention all
the release instants may be predetermined before assembly and
installation of the production pipe assembly. Thus one may conduct a
release of tracers, sampling and analysis according to the invention
short after the start-up of production or test production in a well,
and then conduct another round of release, sampling and analysis
after one month, after two months, and so on for a long time, and
obtain improved control over the well.
In an embodiment of the method of the invention the given release
instant (tR) may be commanded from the surface. A signal transmitter
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at the topside may be arranged to send a "request to release" signal
to the sub containing a corresponding signal receiver in the
mechanical tracer dose release unit of the invention. In an
embodiment, the actual tracer release point of time may still be set
in the electronic control module in the sub to be delayed to a
predefined hour, minute and second, so one may be certain that the
tracer is released at an exactly known point in time.
In an advantageous embodiment of the invention, the method is
conducted in a well wherein the two or more influx locations are
separated by e.g. packers, so as for the influx locations to be
mutually isolated in the annulus around the production pipe. In this
way one may be sure that there is no mixing of the contributions
from the different influx zones before the fluids enter the central
pipe, and one may expect the samples topsides to be better for
distinguishing the different influx zones. In such a model the
dispersion of the tracer materials will be dominated by the physical
conditions and geometry of the well above the influx zones on the
fluids' way to the sampling site topside.
In an advantageous embodiment of the invention the tracer sources
are arranged in fluid communication with the influx zones. More
specifically, the tracer sources are preferably, if possible, each
arranged within or very near its corresponding influx zone so as to
have a relatively short flux path from the influx zone, past the
tracer source, and out through vents to the production pipe, such as
illustrated particularly in Fig. 8 and Fig. 12.
In an embodiment of the invention one may have knowledge of the
influx locations' (3) positions along the well from well logs. This
may improve the certainty of the modelling of the tracers'
propagation to the surface. Alternatively, the real positions are
unknown, but may be varied in the model well in order to better
match the modelled tracer arrivals topside.
In and embodiment of the method of the invention, the sampling is
conducted after said predefined release instant (tR), after a time
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reasonably comparable to the minimum transit time for a first of the
tracers to reach the sampling site. This is in order to avoid
starting sampling before the first tracer may actually arrive
through e.g. the tie-back length of several kilometres of pipe to
the topside location.
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