Note: Descriptions are shown in the official language in which they were submitted.
CA 02704561 2013-03-08
HEATER STRING AND PROCESS OF WELL HEATING
FIELD OF THE INVENTION
The present invention generally relates to heating hydrocarbon recovery wells,
and more
specifically to heater strings and their application in hydrocarbon recovery
wells.
BACKGROUND
In order to recover hydrocarbons from geological formations, it is often
desirable or even
necessary to provide heat. Heating a hydrocarbon well may increase hydrocarbon
mobility,
reduce formation of unwanted compounds, pre-condition the hydrocarbons prior
to extraction,
facilitate pumping, enable or improve various in situ recovery methods,
generally improve the
quality, quantity and economics of production.
It is well known that hydrocarbon wells are expensive endeavours, both in
terms of capital
and operating costs. Hydrocarbon wells vary widely in design and operating
conditions,
depending on their age, the composition of hydrocarbons that are present or
recoverable
within the formation, the presence of unwanted compounds in the formation, and
their
geographical location.
In natural gas wells, for instance, well heating is important due to the
availability and
composition of the gas, the above- and below-ground temperatures and the
tendency of
hydrate formation.
Though wells may be constructed in various ways, one construction includes a
drilled
wellbore that may have vertical, slanted or horizontal portions, a casing
mounted within the
wellbore, a production tube mounted within the casing, and other optional
tubes mounted
within the casing along the production tube to provide heat, fluids, chemical
agents, steam,
and wellbore accessories, etc., as the case may be.
One way of heating a well is by providing a heater string within the casing
and along the
production tube. Known heater strings have mainly provided heat using
electricity,
combustion, pyrolysis or heater fluids.
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Heater strings that transmit heater fluids such as hot water or ethylene
glycol have been used
in the field and some designs have been described in United States patent Nos.
4,988,389
(Adamache et al), 3,627,047 (Leland et al), 3,207,219 (Mitchell), 2,705,535
(Waterman), and
2,914,124 (Ripley Jr).
Known heater strings for transmitting heater fluids are generally composed of
steel, due to its
mechanical and physical properties at low and high temperatures, its
widespread use in the
oil and gas industry, and its flow capacity, manufacturing sources and
availability.
However, the use of steel in heater string applications presents a variety of
disadvantages.
Heater strings are often desirable at profound depths, since heat is often
needed far below
the surface at or below the permafrost layer or at specific locations where
the pressure-
temperature conditions in the production tube may lead to a tendency toward
undesirable
hydrate "ice plug" formation. At such depths, steel heater strings are
particularly fraught with
drawbacks. Firstly, the friction coefficient of carbon and stainless steel
often necessitates
significant surface pressure in order to pump the heater fluid through the
lengthy heater
string. High surface pumping pressures require high capital cost pumps,
increase operating
costs and can be quite dangerous, also increasing the probability of ruptures,
leaks and
accidents.
Secondly, the thermal conductivity of steel leads to rapid heat transfer from
the heater fluid to
the well, which can be detrimental when heating is desired at great depths,
since the heater
fluid may be prematurely cooled by the time it reaches the desired heating
location. This
leads to higher energy costs - heating and pumping - necessary to transmit the
heater fluid to
the target zone. In order to avoid this premature cooling phenomenon, an
operator may
increase the fluid pressure, flow rate and temperature, which as mentioned
above further
exacerbates the disadvantages associated with high surface pumping pressures.
A section of
the steel heater string may also rupture, crack or break-off. Break-off of a
section of the
heater string leads to further complications, including increased "fishing".
When a broken-off
steel section falls down the wellbore at high speeds - sometimes reaching its
terminal velocity
- and then collides with the casing, the production tube or other well
components, it fractures
into many pieces that must be "fished" out of the well. Fishing can be very
dangerous and
results in shutdown time.
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Thirdly, although steel may be relatively anti-corrosive compared to some
other metals and
alloys, in the harsh conditions of gas recovery wells, where highly corrosive
compounds such
as dissolved carbon dioxide and hydrogen sulphide are present, steel incurs
unwanted
chemical and physical wear. External corrosion can lead to accelerated
deterioration, rupture
Furthermore, given the capital costs associated with drilling a wellbore and
installing the well
components, there are often dimensional constraints which must be considered
when
The chemical nature of the formation, the hydrocarbons and the heater fluid
also play a role
in the functioning of heater strings. For instance, when sulphur, hydrogen
sulphide or hydrate-
forming compounds are present in the hydrocarbon stream, corrosion, sulphur
deposition and
In view of the above, it should be clear that there is a need for a technology
that overcomes
at least some of the disadvantages of the heater strings and well heating
techniques that
have been used up to now in the field.
The present invention responds to the above-mentioned need by providing a
heater string, a
hanger assembly, and a unique process of heating a well.
In one aspect of the present invention, there is provided a heater string for
transmitting a
heater fluid within a well, which includes a wellbore, a casing arranged
within the wellbore
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and a production tube arranged within the casing. The heater string is
mountable within the
casing and adjacent to a portion of the production tube. The heater string
includes an inlet for
receiving the heater fluid, an elongate tubular body in fluid communication
with the inlet and
an inner tubular surface contiguous with the elongate body and contacting the
heater fluid,
and the inner tubular surface includes a plasticized polyamide blend.
The plasticized polyamide inner surface of the heater string allows a
reduction of the friction
between the heater fluid and the heater string and thermal conductivity of
heat from the
heater fluid toward the well, improving corrosion resistance, increasing the
ductility, reducing
the weight and reducing the likelihood of rupture, break-off and brittle
fracturing. This enables
improved heating at great depths by reducing the pressure required to pump the
heater fluid
as well as extending the heating capacity deep in to the well due to the
insulation properties.
The plasticized polyamide enables advantages that are specific to heater
string applications.
In optional embodiments, the heater string may be a one-piece tubular
extrusion and the
plasticizer may be a sulphonamide. The plasticized ployamide may optionally
include
tougheners and reinforcement members such as spines composed of aramid fiber.
In another aspect of the present invention, there is provided a process of
heating a well
having a wellbore, a casing arranged within the wellbore and a production tube
arranged
within the casing to define an annulus there between. The process includes:
providing a heater string comprising an inlet for receiving a heater fluid, an
outlet
for expelling the heater fluid, an elongate tubular body in fluid
communication with
the inlet and the outlet, and an inner tubular surface contiguous with the
elongate
body and contacting the heater fluid, the inner tubular surface comprising a
plasticized polyamide blend;
mounting the heater string within the annulus;
providing the heater fluid at the inlet at a first pressure;
allowing the heater fluid to flow through the heater string to transfer heat
to the
production tube; and
expelling the heater fluid from the outlet into the annulus.
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In yet another aspect of the present invention, there is provided a hanger
assembly for
suspending a heater string within a well. The heater string may be constructed
as one-piece
tubular extrusion comprising a plasticized polyamide blend and having upstream
and
downstream ends and an inlet at the upstream end for receiving a heater fluid.
The hanger
assembly includes an anchoring member secured to and extending radially
outward from the
upstream end of the heater string and having an upstream-facing surface and a
downstream-
facing surface. The hanger assembly also includes a nut mountable around the
heater string
and a connector body having an upstream section adapted to receive the heater
fluid and a
canalization in fluid communication with the inlet of the heater string. The
nut and the
connector body are securable together and abuttable against the upstream-
facing surface
and the downstream-facing surface of the anchoring member to enclose the
anchoring
member therebetween.
The hanger assembly allows a secure hold on a heater string while reducing or
avoiding
slippage or damage. The hanger assembly is preferably used for securing a
heater string
composed of a polymer material such as plasticized polyamide, as described
herein.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig 1 is a side plan partial cross-sectional view of a well having a heater
string therein,
according to an optional embodiment of the present invention.
Fig 2 is a side plan partial cross-sectional view of a well having a heater
string therein,
according to another optional embodiment of the present invention.
Fig 3 is a cross-sectional view of a hanger assembly including part of a
heater string,
according to an optional embodiment of the present invention.
Fig 4 is a perspective view of the hanger assembly of Fig 3.
Fig 5 is a perspective partial transparent view of an outlet section of a
heater string, according
to another embodiment of the present invention.
Fig 6 is a transverse cross-sectional view schematic of a heater string
showing embedded
spines, according to an embodiment of the present invention.
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DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Figs 1 and 2 illustrate wells 10 in which embodiments of the heater strings 12
may be
installed. The illustrated wells 10 include a drilled vertical wellbore,
although various
embodiments of the present invention may be used in connection with slanted or
horizontal
wellbores or portions thereof. The well 10 further includes a casing 14
mounted within the
wellbore, a production tube 16 mounted within the casing 14, and other tubes
mounted within
the casing. The outside of the production tube 16 and the inside of the casing
14 define an
annulus 17 therebetween. It should be understood that the various tubes,
strings and other
components illustrated in Figs 1 and 2 are not to scale.
Referring to Fig 2, a chemical injection string 18 may also be installed
within the annulus 17
to extend down the wellbore.
The heater string 12, chemical injection string 18 and production tube 16,
each have a
subsurface safety valve (SSSV). Figs 1 and 2 illustrate a SSSV control line 19
which may be
arranged to a depth of about 50m. The chemical injection string 18 also has
profiles 20 along
its length.
Referring back to both to Figs 1 and 2, the production tube 16 also includes
profiles 22
provided along its length. There may also be a packer 23 provided at a middle
or lower
portion of the well to provide isolation from the reservoir below.
Still referring to Figs 1 and 2, the heater string 12 is preferably installed
to extend closely
along an upper portion of the production tube 16 for providing heat to it. The
heater string 12
is preferably suspended from a hanger assembly 24 at the top of the wellbore,
and there may
also be a string catcher 25 mounted below the heater string 12, in case of
break off.
More particularly, the heater string 12 is installed for transmitting a heater
fluid within the well
10. The heater fluid may be water, ethylene glycol or another type of heat
transfer fluid or a
combination thereof. Preferably, the heater fluid flows down the heater string
and is expelled
into the annulus 17 defined between the production tube 16 and the casing 14.
Also
preferably, the heater fluid is then cycled back toward the surface and
remains in a closed
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system. Spent heater fluid recovery and recycling equipment (not illustrated)
may be provided
at the surface of the well.
Referring briefly to Fig 6, the heater string 12 includes an elongate tubular
body 26 and an
inner tubular surface 27 contiguous with the elongate body 26 and contacting
the heater fluid.
The inner surface defines a canalization having a preferred diameter between
about 1/4 inch
and about 2 inches. The tubular body may be generally cylindrical and the
canalisation
through which the heater fluid travels may also be generally cylindrical.
Alternatively, at least
a portion of the wall or the canalization of the heater string may be tapered
to provide specific
fluid flow or heat transfer properties. The tapered portion may be, for
instance, at the inlet or
outlet of the string.
The inner tubular surface of the heater string 12 includes a plasticized
polyamide blend. In
one embodiment, the plasticized polyamide blend is provided as an internal
liner and the
elongate tubular body of the heater string is composed of a different material
chosen from
metals, rigid polymers or ductile polymers. Optionally, the heater string 12
may be
manufactured by co-extruding polymers in contiguous melt-bonded layers.
In another embodiment, the entire heater string includes a plasticized
polyamide blend and
thus the elongate body and the inner surface form a one-piece tubular
extrusion including the
same plasticized polyamide blend. In this case, the heater string may be
ductile and
spoolable to facilitate transportation, storage and installation.
The heater string may comprise a plasticized polyamide blend produced by
various methods.
The heater string may be extruded from pellets. The polyamide pellets may
contain a variety
of added components, such as plasticizer or a toughener or combinations
thereof. In
addition, thermal and oxidative stabilizers may be incorporated into the
polymer as needed
for the particular conditions of the heater string application. The pellet
moisture content of the
polyamide may be adjusted by drying or adding additional water. The polyamides
used to
produce the particles may be polyamide 11, 12, 6, 10, or 6,12. Other additives
such as fillers
and reinforcing agents may be incorporated into the pellets.
The heater string may be manufactured by using pellets as per the above
general description,
by forming the blend into a tubular shape. The heater string may be made of a
polymer sold
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by DuPont under the trade name PipeIon . Alternatively, other plasticized
polyamide blends
may be employed in the production of the heater strings described herein.
For installation, the heater string 12 may be provided spooled around a coil
tubing unit or reel.
The heater string is then uncoiled and fed down the wellbore to the desired
depth, usually
around to about 500-1500m depending on well flow properties.
Referring now to Fig 6, in one optional embodiment, the elongate body 26 may
also include
reinforcement members 28. The reinforcement members 28 may be spines embedded
within
the elongate body 26 and extending axially there along or in a spiral pattern
at an angle from
the longitudinal axis of body 26. The reinforcement members 28 may be up to 12
separate
and discrete bundles consisting essentially of aramid fibres.
In another optional embodiment, the plasticized polyamide blend further
includes at least one
toughener. The tougheners may be grafted rubbers or ionic polymers or another
suitable
type, and may be anhydride-functionalized.
The heaters string 12 may also include one or more fibre optic cable 29. In
the embodiment
illustrated in Fig 6, the fibre optic cable 29 is embedded in the elongate
body 26 of the heater
string 12. The fibre optic cable 29 could alternatively be arranged inside the
heater string after
installation. The fibre optic cable 29 functions as a sensor and thus may be
arranged
according to the particular well flow properties and may be coupled with an
automatic or
manual control system (not illustrated) for modulating the well heating as
desired. For
example, when the sensor indicates that the temperature of a given portion of
the well is near
or has gone below a critical threshold of hydrate formation, the heater fluid
temperature or
inlet pressure may be increased as a counter measure.
Referring now to Fig 5, the elongate body is preferably generally linear and
further includes
an outlet for expelling the heater fluid the casing and the production tube.
The outlet may be
flat, curved or have a mule-shoe shape or include modified outlet guide.
According to the
illustrated embodiments, the heater string is non-branched, but various
designs and
alternative arrangements are possible.
The heater string may also include, upstream and proximate the outlet (0), at
least one
aperture (A) for expelling at least a portion of the heater fluid. In fact,
there may be one or
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more apertures (A) provided along the heater string, for expelling a portion
of the heater fluid
at deliberate points along the heater string to prevent hydrate formation. The
location of the
apertures may be coordinated with the heater fluid inlet pressure and
temperature to reduce
the formation of hydrates at pre-determined or calculated locations along the
production tube.
The heater sting embodiments according to the present invention may be used in
a variety of
applications for well heating. In one preferred application, the heater
strings are used in
natural gas wells. They may also be used in gas wells in general, condensate
gas wells,
crude oil wells with associated gas, and other wells. The heater strings are
preferably used in
applications requiring heat to prevent or mitigate the formation of hydrates.
In operation, the heater string is preferably used in conjunction with a
process for well heating
as will be further described below.
According to one aspect of the present invention, the process of heating a
well includes step
(a) of providing an embodiment of the heater string as described herein.
The process also includes step (b) of mounting the heater string within the
annulus of the
well. It should be understood that more than one heater string may be used,
but in most
cases a single heater string will be used as a replacement of an old steel
version, thereby
retrofitting an existing well. The heater string is mounted and arranged so as
to heat
exchange with the production. Optionally, the heater string may be suspended
freely within
the annulus.
The process also includes step (c) of providing the heater fluid at the inlet
of the heater string
at a first pressure. Optionally, the first pressure of the heater fluid is
above about 0 MPa and
below or equal to about 21 MPa. The heater fluid may be water, ethylene
glycol, diesel, or a
combination thereof, and the temperature of the heater fluid provided at the
inlet may be
between about 10 C and about 100 C. As will be explained hereinbelow, the
pressure and
temperature of the heater fluid may be controlled during the heating
operation.
The process also includes step (d) of allowing the heater fluid to flow
through the heater
string to transfer heat to the production tube and step (e) of expelling the
heater fluid from the
outlet into the annulus. In some preferred embodiments of the process, the
heat transfer
mechanisms at work may include conduction and convection. More particularly,
when the
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heater string is in direct physical contact with the production tube,
conduction of heat occurs
from the heater string's body through it contact surface with the production
tube, which then
heats the hydrocarbon stream within it. In addition, the heater fluid expelled
from the outlet
fills the annulus and flows upward, thereby heating the production tube on
many if not all
temperature and pressure conditions of the production tube so as to
substantially reduce or
avoid the formation of hydrates. The control may be based on pre-acquired
measurements,
modelling, experience, or real-time measurements on the particular wellbore
location and
depth parameters. For instance, the fibre optic cable or other sensors may be
used with the
The process may further include, in response to hydrate formation or an
elevated risk of
hydrate formation at a given location within the well, providing at least one
aperture in the
heater string for expelling a portion of the heater fluid at the given
location. The apertures
It is also preferable to cycle the heater fluid back up the annulus toward to
the surface of the
well to be recycled in a closed system.
Turning more particularly to the installation of various embodiments of the
heater string 12, a
hanger assembly 24 such as the one shown in Figs 3 and 4 may be employed. This
hanger
CA 02704561 2010-05-18
friction polymer material such as one composed of a plasticized polyamide
blend. Such
heater strings can be secured and suspended without slippage or damage.
More particularly, Fig 3 illustrates the hanger assembly 24 and its
components. In describing
the hanger assembly 24 it is useful to define the heater string 12 as
including upstream 30
and downstream 32 ends and an inlet 34 at the upstream end for receiving a
heater fluid. The
hanger assembly 24 includes an anchoring member 36 secured to and extending
radially
outward from the upstream end of the heater string 12 and having an upstream-
facing
surface 38 and a downstream-facing surface 40. The anchoring member 36 may be,
for
example, a slip-type configuration, such as a LenzTM split ring on the outside
surface of the
heater string 12. The hanger assembly 24 also includes a nut 42 mountable
around the
heater string 12 and a connector body 44 having an upstream section 46 with
outer threads
47. The upstream section 46 is adapted to receive the heater fluid. The
connector body 44
also has a canalization 48 in fluid communication with the inlet 34 of the
heater string 12. The
nut 42 and the connector body 44 are securable together and abuttable against
the
upstream-facing surface 38 and the downstream-facing surface 40 of the
anchoring member
36 to enclose the anchoring member 36 between them.
In one optional embodiment of the hanger assembly 24, the nut 42 may have a
head 50
abuttable against the downstream-facing surface 40 of the anchoring member 36
and have a
projection 52 extending upstream from the head 50. The connector body 44 may
have a
downstream section 54 opposite the upstream section 46, the downstream section
54
comprising a neck 56 mountable around the heater string 12, abuttable against
the upstream-
facing surface 38 of the anchoring member 36 and securable to the projection
52 of the nut
42, in order to enclose the anchoring member 36. The projection 52 of the nut
42 is preferably
secured around an outer surface of the neck of the connector body, and the
projection and
the outer surface are screwed together via cooperative threads.
The anchoring member 36 may also include an annular portion 58 secured to the
heater
string and a flange portion 60 extending outward from the annular portion 58,
the flange
portion 60 defining the upstream-facing 38 and downstream-facing 40 surfaces.
The annular
portion 58 may be frustro-conical shaped tapering radially outward in the
upstream direction.
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The head 50 of the nut 42 may have an upstream-facing rim 62 abuttable against
the
downstream-facing surface 40 of the anchoring member 36, and an inner surface
64 having a
frustro-conical shape corresponding to the annular portion 58 to abut
thereagainst.
The connector body 44 may further include a peripheral edge 66 extending
outward from the
neck and being axially abuttable against the projection 52 of the nut 42. The
neck of the
connector body may also have inner annular grooves 68 and further comprising 0-
rings 70
mounted within the inner annular grooves 68 and contacting the heater string.
The hanger assembly may also include a backup member 72 integrated into the
connector
body 44 and inserted into the inlet of the heater string. The backup member,
he nut, the
connector body and the anchoring member may all be composed of stainless
steel.
It should be understood that "upstream-facing" and "downstream-facing" are
meant for
general reference and not to limit the geometry of the hanger assembly or
heater string to
mathematical exactitude. Thus, many angles and variations in orientation are
possible for
embodiments of the hanger assembly.
It should also be understood that the embodiments described herein are
exemplary and are
not meant to narrow or limit of what has actually been invented.
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