Note: Descriptions are shown in the official language in which they were submitted.
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WELLBORE SYSTEM AND METHOD OF COMPLETING A WELLBORE
In the industry of hydrocarbon fluid production from
a wellbore it is common practice to complete a lower
section of the wellbore, extending into the hydrocarbon
fluid-bearing formation, with a completion that
stabilises the wellbore wall and/or reduces sand
production from the wellbore. For example, screens or
gravel packs are generally placed in open-hole wellbore
sections to support the wellbore wall and prevent caving-
in of loose material, and to restrain sand from flowing
with the formation fluids to surface. Basically, gravel
packing includes the steps of installing a production
liner provided with small inlet openings, e.g. in the
form of slots or screens, in the wellbore and then
filling the annular space between the production liner
and the wellbore wall with particulate material such as
sand and gravel. The resulting gravel pack maintains
structural integrity of the wellbore in the absence of a
casing, while still allowing flow of fluid from the
reservoir into the wellbore. Screens and gravel packs
also control the migration of formation sands into
production tubulars and surface equipment, which can
cause washouts and other problems, particularly from
unconsolidated sand formations. After a flow path is
made, acids and fracturing fluids can be pumped into the
wellbore to fracture, clean, or otherwise prepare and
stimulate the reservoir rock to optimally produce
hydrocarbons into the wellbore. Finally the wellbore is
sealed-off above the reservoir section, inside the
casing, and connected to the surface via one or more
production tubings.
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In the description and claims hereinafter the terms
"wellbore" and "borehole" will be used interchangably,
and without intended difference of the meaning of such
terms.
Many wellbores are drilled such that a lower
wellbore section extends inclined or horizontally into
the reservoir formation to increase the contact length of
the wellbore with the reservoir formation. For example,
wells that are drilled from an offshore platform all
deviate in different directions so that hydrocarbon fluid
can be produced from a large surface area of the
reservoir formation. Although deviated and horizontal
wellbore sections significantly enhance the production
potential of a wellbore, particularly when compared to
vertical wellbores, it has been experienced that problems
may occur in properly installing completions in such
deviated or horizontal wellbore sections. One such
problem relates to the proper placement of a gravel pack.
Generally, gravel packs are installed using a liner
provided with a cross-over sub assembly to allow a slurry
of particulate material and viscous fluid to be pumped
through the liner and the cross-over sub assembly into
the annulus of a lower wellbore section where the
particulate material settles out of the slurry. The
viscous fluid is then circulated back via the cross-over
sub assembly and the annulus between the liner and the
wellbore wall (or casing), to surface. Experience has
shown that in an inclined or horizontal section it is
difficult, if not impossible, to fill the entire annular
space between the liner and the wellbore wall with the
gravel pack particulate material. This is due to the
particulate material that settles out of the slurry,
tending to fall to the bottom of the inclined or
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horizontal wellbore section so that an upper portion of
the wellbore section remains uncovered with particulate
material.
As a result, an undesired flow passage remains above
the gravel pack, which allows fluid to flow in
longitudinal direction through the wellbore section
thereby bypassing the gravel pack. This can lead to
several problems such as, for example, the ability of
locally produced sand from the formation to spread along
the length of the gravel pack thereby potentially
negatively affecting the permeability of the entire
gravel pack. Another problem becomes apparent if a
treatment fluid needs to be injected via the liner into
the open-hole section. The treatment fluid will tend to
flow through the flow passage above the gravel pack,
thereby rendering it impossible to accurately position
the treatment fluid at a desired location in the open-
hole section. For example, if a portion of the open-hole
section needs to be shut-off in order to reduce or
prevent formation water from flowing into the wellbore, a
treatment fluid is preferably used that reduces or
eliminates the permeability of the gravel pack at the
location where the water flows into the wellbore. However
it has been experienced that the injected treatment fluid
tends to flow through the flow passage above the gravel
pack thereby spreading in the open-hole section and
potentially affecting the permeability of the entire
gravel instead of at the desired location only.
US patent 4,995,456 discloses a wellbore completion
assembly whereby a horizontal wellbore section is
provided with a fluid-permeable liner provided with a
cross-over sub and vanes for imparting a spiralling flow
to a gravel pack slurry which is pumped into the
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horizontal wellbore section. The spiralling flow is
intended to enhance the distribution of gravel pack
particulate material in the horizontal wellbore section.
However there remains a need for an improved
wellbore system and completion method, which overcomes
the problems of the prior art.
In accordance with the invention there is provided a
wellbore system comprising a borehole formed in an earth
formation, the borehole having a borehole section
containing a volume of gravel pack particles and at least
one body of a swellable material, each body of swellable
material being adapted to expand from an unexpanded state
to an expanded state upon contact of the swellable
material with a selected fluid, wherein a flow passage is
present in said borehole section allowing fluid to bypass
the volume of gravel pack particles when the body of
swellable material is in the unexpanded state, and
wherein the body of swellable material is arranged to
substantially close the flow passage upon expansion of
the body of swellable material to the expanded state.
Thus, by swelling of the swellable material, the
flow passage becomes closed or vanishes, so that fluid no
longer can flow unhindered in longitudinal direction
through the borehole section. Also, locally produced sand
is thereby prevented from spreading along the entire
gravel pack, but instead remains in the wellbore location
where it was produced. Furthermore, treatment fluid that
is injected into the wellbore is confined to the
injection location rather than spreading along the gravel
pack.
In an advantageous embodiment, the body of swellable
material is arranged to push the volume of gravel pack
particles into the flow passage upon swelling of the
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swellable material, so that the flow passage gets
blocked. Also, the body of swellable material, after
expansion, can be arranged to completely fill the cross-
section of the borehole section and thereby block the
flow passage.
Suitably, the body of swellable material includes a
sleeve arranged around a tubular element extending into
said borehole section. The tubular element is, for
example, a production liner provided with slots, openings
or screens for the inflow of hydrocarbon fluid from the
formation.
Movement of the volume of gravel pack particles into
the flow passage is optimal if the sleeve is at least
partly covered by the volume of gravel pack particles.
Preferably the tubular element is provided with a
plurality of said sleeves mutually spaced along the
tubular element. In this manner it is ensured that the
annular space between the tubular element and the
wellbore wall is formed into compartments which prevent
fluid or formation sand from bypassing the gravel pack.
In such arrangement the tubular element is suitably
provided with fluid inlet means arranged at a portion of
the tubular element located between a pair of adjacent
sleeves.
In an alternative application, said at least one
body of swellable material includes a plurality of
particles of swellable material. Such application has the
advantage that the particles of swellable material can be
pumped into the wellbore section, and are allowed to flow
into irregular wellbore portions. Preferably the
particles of swellable material are intermixed with the
gravel pack particles. To achieve adequate intermixing,
the particles of swellable material and the gravel pack
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particles suitably have about equal density. This can be
achieved, for example, by providing the particles of
swellable material with a weighting material so as to
increase their density. A suitable weighting material is
Iron powder or a similar material. Since the function of
the weighting material is to adapt the density of the
swellable particles to the density of the gravel pack
particles, a weighting material may be applied that
lowers the density of the swellable particles in case the
density of the swellable particles, absent the weighting
material, exceeds the density of the gravel pack
particles.
The wellbore system of the invention is most
advantageous for application in wellbore sections that
extend inclined or substantially horizontally. This is
because it is generally difficult, if not impossible, to
fill the entire cross-section of such inclined or
substantially horizontal wellbore section with gravel
particles. In most such applications an undesired flow
passage remains above the volume of gravel pack
particles.
Furthermore, the selected fluid can be fluid from
the earth formation flowing into the wellbore section,
such as water or oil, or fluid that is pumped from
surface into the wellbore section.
In another aspect of the invention there is provided
a method of completing a borehole formed in an earth
formation, the method comprising:
- inserting a volume of gravel pack particles into a
borehole section of the borehole;
- inserting at least one body of swellable material into
the borehole section, each body of swellable material
being adapted to expand from an unexpanded state to an
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expanded state upon contact of the swellable material
with a selected fluid, wherein a flow passage is present
in said borehole section allowing fluid to bypass the
volume of gravel pack particles when the body of
swellable material is in the unexpanded state, and
wherein the body of swellable material is arranged to
substantially close the flow passage upon expansion of
the body of swellable material to the expanded state; and
- allowing the body of swellable material to expand due
to contact of the swellable material with the selected
fluid, thereby substantially closing the flow passage.
Preferably the body of swellable material pushes the
volume of gravel pack particles into the flow passage
upon swelling of the swellable material.
To allow accurate placement of a treatment fluid in
the borehole section, the method suitably further
comprises injecting a treatment fluid into the volume of
gravel pack material after the volume of gravel pack
material is pushed into the flow passage. For example, if
the purpose of the treatment fluid is to shut-off a
selected portion of the wellbore, the treatment fluid
suitably is adapted to locally reduce or eliminate the
permeability of the gravel pack material in such portion.
Suitably the swellable material is an elastomer
adapted to swell when in contact with water and/or oil.
Examples of materials that swell upon contact with
hydrocarbon fluid are natural rubber, nitrile rubber,
hydrogenated nitrile rubber, acrylate butadiene rubber,
poly acrylate rubber, butyl rubber, brominated butyl
rubber, chlorinated butyl rubber, chlorinated
polyethylene, neoprene rubber, styrene butadiene
copolymer rubber, sulphonated polyethylene, ethylene
acrylate rubber, epichlorohydrin ethylene oxide
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copolymer, ethylene-propylene-copolymer (peroxide
crosslinked), ethylene-propylene-copolymer (sulphur
crosslinked), ethylene-propylene-diene terpolymer rubber,
ethylene vinyl acetate copolymer, fluoro rubbers, fluoro
silicone rubber, and silicone rubbers. Preferred
materials are EP(D)M rubber (ethylene-propylene-
copolymer, either peroxide or sulphur crosslinked), EPT
rubber (ethylene-propylene-diene terpolymer rubber),
butyl rubber, brominated butyl rubber, chlorinated butyl
rubber, or chlorinated polyethylene.
Instead of, or in addition to, the swellable
material being adapted to swell upon contact with
hydrocarbon fluid, the swellable material suitably is
adapted to swell upon contact with water. Suitably such
water-swellable material is selected from rubber based on
NBR, HNBR, XNBR, FKM, FFKM, TFE/P or EPDM. In order to
enhance the swelling capacity of the water-swellable
material, even for saline water conditions, said material
suitably is a matrix material wherein a compound soluble
in water is incorporated in the matrix material in a
manner that the matrix material substantially prevents or
restricts migration of the compound out of the swellable
seal and allows migration of water into the swellable
seal by osmosis so as to induce swelling of the swellable
seal upon migration of said water into the swellable
seal. Said compound suitably comprises a salt, for
example at least 20 weight % salt based on the combined
weight of the matrix material and the salt, preferably at
least 35 weight % salt based on the combined weight of
the matrix material and the salt. In order to prevent, or
reduce, leaching of the compound out of the matrix
material, it is preferred that the matrix material is
substantially impermeable to said compound or to ions of
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said compound. The compound can be present in the matrix
material, for example, in the form of a plurality of
compound particles dispersed in the matrix material. If
the matrix material is an elastomer, the compound can be
mixed into the matrix material prior to vulcanisation
thereof.
The invention will be described hereinafter in more
detail and by way of example, with reference to the
accompanying drawings in which:
Fig. 1 schematically shows a borehole extending into
an earth formation, provided with an embodiment of the
wellbore system of the invention;
Fig. 2 schematically shows detail A of Fig. 1;
Fig. 3 schematically shows cross-section 3-3 of
Fig. 2;
Fig. 4 schematically shows detail A of Fig. 1 after
swelling of a body of swellable material; and
Fig. 5. schematically shows cross-section 5-5 of
Fig. 4.
Referring to Fig. 1 there is shown a borehole
extending into an earth formation 2, in the form of
wellbore 1 having a vertical upper wellbore section 4
provided with a scheme of casings and an open-hole lower
section 8 that extends substantially horizontally into a
reservoir zone 10 containing hydrocarbon fluid. For ease
of reference, the scheme of casings is referred to
hereinafter as casing 6. A tubular production liner 12
extends from a wellhead 14 at surface 16 through the
upper wellbore section 4 and into the open-hole lower
section 8, whereby a production packer 18 seals the
production liner 12 to the lower end of the casing 6. The
production liner 12 has a lower part 20 provided with a
plurality of sleeves 22a, 22b, 22c, 22d of elastomer
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material susceptible of swelling with a selected fluid,
such as water and/or oil. In the present example, the
elastomer material is selected to swell upon contact with
oil from reservoir zone 10. The sleeves 22a, 22b, 22c,
22d are spaced from each other in longitudinal direction
of the liner 12, whereby liner portions 24 inbetween the
sleeves 22a, 22b, 22c, 22d are provided with small
openings or slots 23 (Fig. 2) which provide fluid
communication between the interior and the exterior of
the liner 12. The liner portions 24 can be provided in
the form of sandscreens, slotted pipes or other devices
suitable for inflow of produced hydrocarbon fluid into
the liner 12, or outflow of treatment fluid from the
liner 12 into the wellbore 1. The open-hole section 8 of
wellbore 1 is furthermore provided with a gravel pack 26
containing particulate material such as gravel, sand and
the like, as is well known in wellbore completions. For
ease of reference the volume of gravel pack particles 26
is referred to hereinafter as "gravel pack 26".
Referring further to Figs. 2 and 3 there is shown
detail A of Fig. 1, including open-hole section 8
provided with the gravel pack 26 and liner 12. Only one
elastomer sleeve 22b is shown for ease of reference, the
other elastomer sleeves 22a, 22c, 22d being similar to
sleeve 22b. The gravel pack 26 does not occupy the entire
cross-sectional area of the open-hole section 8, but
instead leaves a flow passage 30 in the open-hole section
8 through which fluid can flow in axial direction of the
open-hole section 8 and thereby bypass the gravel
pack 26.
In Figs. 4 and 5 is shown detail A of Fig. 1 after
swelling of the elastomer of sleeve 22b due to contact
with water or oil from the earth formation, whereby the
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sleeve 22b has increased in diameter and thereby has
pushed the gravel pack 26 into the flow passage 30. As a
result the flow passage 30 is blocked, or perhaps better
to say the flow passage vanishes at the location opposite
the sleeve 22b so that fluid no longer can bypass the
gravel pack 26.
During normal operation the wellbore 1 is drilled
from surface 16 using a drilling rig (not shown), and the
casings 6 are installed in the vertical wellbore
section 4. The production liner 12 is then installed in
the wellbore so that the sleeves 22a, 22b, 22c, 22d of
swellable elastomer are located in the reservoir zone 10
of the earth formation 2. Thereafter a slurry of gravel
pack particles and a viscous fluid, such as crude oil or
a polymer-type water-based fluid, is pumped into the
open-hole section 8 of the wellbore 1. For this purpose,
the end part 20 of the production liner 12 is provided
with a cross-over sub assembly (not shown) which packs
off the open-hole section 8 and allows the gravel pack
slurry to be pumped via the liner 12 into a portion of
open-hole section 8 below the cross-over assembly. There
the gravel pack particles settle out from the slurry in
the open-hole section 8 to form the gravel pack 26, while
the viscous fluid is circulated back to surface via the
cross-over sub assembly and the annulus formed between
liner 12 and the wellbore wall or casing 6. The cross-
over sub assembly will not be described in more detail
since it does not form part of the invention, and since
it is a well known tool for completing wellbores. The
production packer 18 is installed between the liner 12
and the lower end of casing 6 after the gravel pack 26
has been placed in the wellbore 1.
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Although it is aimed that the gravel pack 26
occupies the entire annular space between the liner part
20 and the wall of the open-hole section 8, it has proved
difficult, or even impossible, to fill the entire annular
space with gravel pack particles. The problem is more
pronounced in horizontal, or inclined, wellbore sections
where the particles have a tendency due to gravity to
fall to the lower side of the wellbore section. Thus, in
the present instance of substantially horizontal open-
hole section 8, it is almost inevitable that the flow
passage 30 remains between the volume of gravel pack
particles 26 and the wellbore wall.
When oil starts flowing from the reservoir zone 10
into the open-hole section 8, such oil contacts sleeves
22a, 22b, 22c, 22d thereby inducing the elastomer of the
sleeves to swell. As a result the sleeves 22a, 22b, 22c,
22d expand in diameter and thereby push the gravel pack
26 into the flow passage 30 which, as a result, gradually
vanishes at the location of sleeves 22a, 22b, 22c, 22d.
After the sleeves 22a, 22b, 22c, 22d have expanded, the
gravel pack 26 completely fills the annular space between
each sleeve 22a, 22b, 22c, 22d and the wellbore wall in
open-hole section 8. In this manner the gravel pack 26
divides the open-hole section 8 in compartments which
prevent free flow of fluid and rock particles through the
open-hole section 8 in longitudinal direction thereof.
Thus, sand particles from the rock formation can only
locally flow into the gravel pack 26 rather than flowing
along the whole length thereof as in the prior art. It is
thereby achieved that any negative effect on the
permeability of the gravel pack 26 as a result of such
inflow of sand particles, is confined to local spots of
the gravel pack. Oil from the reservoir zone 10 flows
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through the gravel pack 26 into the openings or slots 23
and from there through the liner 12 to surface.
The method of the invention also enables better
placement of treatment fluid in the open-hole section 8
of the wellbore. For example, if such fluid is pumped via
the liner 12 and the openings 23 into the open-hole
section 8, the fluid can no longer freely flow in
longitudinal direction through the open-hole section 8 by
virtue of the compartments formed in the gravel pack 26.
This allows the treatment fluid to be placed more
accurately in the open-hole section 8. In an exemplary
application, it may be desired to shut-off a selected
portion of the open-hole section 8 if after some time of
continued oil production, formation water starts flowing
into such portion of the open-hole section 8. A treatment
fluid that substantially reduces, or eliminates, the
permeability of the gravel pack 26 is then pumped via
production liner 12 and openings 23 into the gravel
pack 26 at the selected location. Due to the compartments
formed in the gravel pack 26, the treatment cannot freely
flow in longitudinal direction through the open-hole
section 8, so that the treatment fluid can be accurately
placed at the desired location of the gravel pack 26. As
a result, only the desired portion of the open-hole
section 8 is shut-off while other portions of the open-
hole section 8 remain unaffected by the treatment fluid.
In an alternative embodiment of the wellbore system
of the invention, particles of swellable material
susceptible of swelling upon contact with water and/or
oil are intermixed with the particulate material of the
gravel pack. Suitably such particles of swellable
material are made of one or more of the swellable
elastomers described hereinbefore. The elastomer
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particles can be mixed into the gravel pack slurry at
surface and pumped with the slurry into the wellbore
section. Also the gravel pack slurry can be pumped first
into the wellbore, whereafter the elastomer particles are
pumped into the gravel pack. Upon flow of oil or water
from the earth formation into the wellbore section, the
elastomer particles start swelling. As a result the
volume of the combined gravel pack particles and
elastomer particles increases so that the volume is
pushed into the flow passage which thereby gradually
becomes blocked and eventually completely vanishes. In
this manner it is achieved that injected fluid, such as
treatment fluid, and sand particles from the formation
can no longer bypass the gravel pack.
In the above detailed description it is indicated
that the body of swellable material or the swellable
particles swell by contact with oil or water from the
earth formation. However it is envisaged that swelling of
the swellable body or the swellable particles also can be
triggered by inducing the selected fluid to flow from
surface into the borehole, for example by pumping oil or
water into the borehole to contact the body of swellable
material or the swellable particles.
Furthermore, it is to be understood that the
procedure described hereinbefore, whereby a slurry of
gravel pack particles and a viscous fluid is pumped into
the wellbore, includes applications whereby the gravel
particles not only are pumped into the open-hole section
of the wellbore, but also into fractures of the earth
formation which are in communication with the wellbore.
Such applications are sometimes referred to as "Frac &
Pack".
