Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02645515 2008-12-01
SPLIT CASING WELLHEAD SEAL
FIELD OF THE INVENTION
The present invention relates to a seal for use in oil and gas applications.
BACKGROUND OF THE INVENTION
Wellbores are holes drilled into the ground through which oil and gas
("production
fluids") can be extracted. The process of extracting production fluids using a
wellbore is termed "production". Following drilling of a wellbore and prior to
using
the wellbore for production, the wellbore must go through a process called
"well
completion" by which the wellbore is prepared for production. Well completion
involves many steps. Two such steps are installing a "surface casing" in the
wellbore, which is a pipe string that lines the wellbore and that protects
water
sources around the wellbore from contamination by fluids within the wellbore;
and installing "production casing", within the surface casing, which surrounds
production tubing through which the production fluids are extracted.
An annular area exists between the production casing and the surface casing.
This annular area is typically filled to a certain degree with concrete. The
concrete aids in structurally stabilizing the well and prevents gases from
escaping up through the annular area and into the atmosphere. Problematically,
however, the concrete can sometimes develop fissures through which gases can
escape. Consequently, many wells utilize some type of "well cap" to seal the
top
of the well to prevent such gases from escaping. A problem common in such
well caps, however, is that they have to be installed during the well
completion
process. A completed well cannot easily be retrofit to include a well cap
following
well completion.
Accordingly, there exists a need for a seal that improves upon known seals in
the
art.
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CA 02645515 2008-12-01
SUMMARY OF THE INVENTION
According to an aspect of the invention, there is provided an apparatus for
sealing a cavity between production casing and surface casing. The apparatus
includes a seal having a plurality of sealing portions, each of the sealing
portions
dimensioned to fit around a portion of the perimeter of the production casing
and
to seal a portion of the cavity, the sealing portions circumscribing the
production
casing and sealing the entire cavity when secured in place around the
production
casing; and a retainer having a plurality of retaining portions, the retaining
portions configured to be clamped together around the surface casing into a
secured position and to securely retain the seal around the production casing
when in the secured position.
Each sealing portion may have a seal flange portion attached thereto, which is
configured to support the sealing portion on a surfacing collar enveloping the
surface casing.
The plurality of sealing portions may include first and second sealing
portions,
each having semi-circular inner and outer perimeters. Analogously, the
plurality
of retaining portions may include first and second retaining portions, each
having
substantially semi-cylindrical inner and outer surfaces.
The production casing may be eccentrically offset relative to the surface
casing
and each of the sealing portions may have an inner perimeter. The seal may
also include an aperture, for receiving the production casing, defined by the
inner
perimeters of the sealing portions, the aperture being eccentrically offset
relative
to the surface casing to correspond to the eccentrically offset production
casing.
Each sealing portion may include a top plate portion; a bottom plate portion;
and
a gasket portion, secured between the top and bottom plate portions.
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The apparatus may also include a vent in the walls of one of the retainer
portions
for venting gases contained within the cavity and for fluidly communicating
with
apertures in the surfacing collar and the surface casing; and a vent seal
fluidly
sealing the vent to the surface casing, the vent seal for preventing gases
from
leaking as they exit the surface casing through the apertures. A vent assembly
may be fluidly coupled to the vent. The vent assembly may include a conduit
having supply and exit ends, the supply end in fluid communication with the
vent;
and a valve disposed between the supply and exit ends and movable between
open and closed positions, the valve allowing gases to travel from the supply
end
to the exit end when in the open position and otherwise preventing gas from
traveling from the supply end to the exit end.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a perspective view of a completed wellhead;
Figure 2 is a perspective, partially exploded view of a first embodiment of an
apparatus for sealing a cavity between production casing and surface casing;
Figure 3 is a side elevation view of a seal, which forms part of the first
embodiment, being positioned around production casing;
Figure 4 is a perspective view of the seal in place around the production
casing;
Figure 5 is a perspective, partially exploded view of a retainer, which forms
part
of the first embodiment, being positioned around the seal;
Figure 6 is a side elevation view of the retainer, in place around the seal in
a
secured position;
Figure 7 is a perspective view of the retainer and the seal in the secured
position
and of a vent assembly; and
Figure 8 is a perspective view of the vent assembly in fluid communication
with
the retainer.
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DETAILED DESCRIPTION OF AN EXEMPLARY EMBODIMENT
The production casing and the surface casing that are installed into a
welibore
during well completion extend uphole to and can protrude from the surface of
the
earth. In Figure 1, there is depicted a completed wellhead. A "wellhead"
refers
generally to the top of the casings and to the equipment in connection
therewith.
In Figure 1, the wellhead includes surface casing S and production casing P
that
protrudes from the surface of the earth in this fashion, a surfacing collar C
that
partially envelops the surface casing S, and a valve assembly V. An annular
cavity A exists between the outer diameter of the production casing P and the
inner diameter of the surface casing S. A typical diameter for the production
casing P is about 4.5 inches and a typical diameter for the surface casing S
is
about 7 inches; consequently, a typical thickness for the cavity A is about
1.25
inches. Although the production casing P and the surface casing S are shown as
being concentric relative to each other in Figure 1, this is not the case for
all
arrangements of production casing P and surface casing S. The production
casing P may, for example, be eccentrically offset within the surface casing
S.
During well completion, the cavity A is filled, to a certain degree, with
concrete
(not shown). Over time, this concrete can develop fissures through which gases
contained downhole and within the cavity A can escape to the surface and into
the atmosphere. Such uncontrolled release of gas into the atmosphere is
undesirable.
A seal cannot easily be installed to seal the cavity A from the atmosphere
because of the presence of the valve assembly V. The valve assembly is
necessary during production and, once installed, cannot easily be removed. The
valve assembly V is bulky, and prevents a seal from easily being securely slid
on
top of the surface casing S and surfacing collar C.
Referring now to Figure 2, there is depicted a first embodiment of an
apparatus
10 for sealing the cavity A from the atmosphere. The apparatus 10 generally
includes a seal 22, a retainer 12, and a vent assembly 34. As illustrated in
more
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detail in Figures 3 and 4, below, the seal 22 is for sealing the cavity A
between
the production casing P and surface casing S. As illustrated in more detail in
Figures 5 and 6, the retainer 12 can be clamped over the seal 22, thus
securely
retaining the seal 22 in position over the casings P, S. As discussed in more
detail with respect to Figures 7 and 8, the vent assembly 34 is fluidly
couplable to
a vent 32 in the retainer 12, and can be used to sample gases contained within
the cavity A.
The seal 22 in the depicted embodiment is a "sandwich seal" that has three
layers: a top seal plate 40, a bottom seal plate 44, and a gasket 42
sandwiched
therebetween. The top and bottom seal plates 40, 44 may be manufactured from
a 0.25 inch thick A36 carbon steel plate. The gasket 42 may be manufactured
from a 0.50 inch thick piece of nitrile rubber having a hardness of about 70
durometer. When the outer diameter of the production casing P is about 4.5
inches and the inner diameter of the surface casing S is 7 inches, the inner
and
outer diameters of the top seal plate 40 may be about 4.63 inches and about
7.66 inches, respectively; the inner and outer diameters of the bottom seal
plate
44 may be about 4.63 inches and about 6.75 inches, respectively; and the inner
diameter and outer diameters of the gasket 42 may be about 4.63 inches and
about 7.00 inches, respectively. This is sufficient to seal the entire cavity
A from
the atmosphere.
Notably, the seal 22 is divided two portions: a first sealing portion 24 and a
second sealing portion 26. Each of the sealing portions 24, 26 has a top plate
portion, a gasket portion, and a bottom plate portion that corresponds to the
top
seal plate 40, gasket 42, and bottom seal plate 44, respectively. The first
and
second sealing portions 24, 26 are separable from each other. Consequently,
when installing the seal 22 on to a completed wellhead, the first and second
sealing portions 24, 26 may be clamped around the wellhead, thus avoiding the
bulky valve assembly V entirely. Similarly, the retainer 12 is composed of two
separable portions: a first retaining portion 24 and a second retaining
portion 26.
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Thus, when the retainer 12 is being positioned to clamp in place over the seal
22,
the retainer 12 can also avoid the valve assembly V.
Referring now to Figures 3 and 4, there are depicted views of the seal 22
being
installed such that the cavity A is sealed. The first and second sealing
portions
24, 26 each have semi-circular inner (which interfaces with the production
casing
P) and outer perimeters. Each sealing portion 24, 26 can be brought together
around the production casing P to clamp around it (Figure 3). Each sealing
portion 24, 26 also has a seal flange portion, which together form a seal
flange
28 when the sealing portions 24, 26 are brought together. The seal flange 28
results from the difference between the outer diameters of the top and bottom
seal plates 40, 44, that circumscribes the seal 22. The seal flange 28 allows
the
seal 22 to rest on and be supported by the surfacing collar C (Figure 4).
Although only two sealing portions 24, 26 are depicted in the figures, any
number
of sealing portions can be used to seal the cavity A from the atmosphere.
Additionally, as mentioned above, the surface casing S and production casing P
may be eccentric relative to each other. In such embodiments, the shape of the
seal 22 and retainer 12 can be accordingly modified to be able to fit around
and
clamp to such surface casing S and production casing P. The apertures defined
by the inner perimeters of the seal 22 and the inner surfaces of the retainer
12
may, for example, be eccentrically offset just as the production casing P is
eccentrically offset relative to the surface casing S.
Referring now to Figures 5 and 6, there are depicted views of the retainer 12
being positioned over the seal 22 and being used to securely clamp the seal 22
in place. The first and second retaining portions 14, 16 are substantially
semi-
cylindrical in shape. By "substantially semi-cylindrical", it is meant that
the
retaining portions 14, 16 are semi-cylindrical in shape except for the
presence of
bushings 31, 33 and fastening tabs 20 that are used to securely clamp the two
retaining portions 14, 16 together. Each retaining portion 14, 16 can be
brought
together around the surfacing collar C and the seal 22 to clamp down on the
seal
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22. Fasteners 18 extend through the fastening tabs 20 to clamp the two
retaining
portions 14, 16 together. In order to exert pressure on the seal 22, the
retainer
12 has a retainer flange 30 that fits over the seal 22 and compressively
secures
the seal 22 to the surfacing collar C. When the seal 22 is so secured to the
surfacing collar C, the apparatus 10 is in a secured position. In this
exemplary
embodiment, the retainer 12 is made from 8 5/8 inch J-55 casing weight 24.00
lbs/ft. The retainer 12 is about 9.38 inches in height, has an inner diameter
of
about 8.10 inches, and an outer diameter of about 8.63 inches. The retainer
flange 30 is made from a 0.25 inch thick A36 carbon steel plate and has an
inner
diameter of about 6.50 inches and an outer diameter of 8.63 inches. The flange
30 can be welded to an end of the retainer 12. Both ends of the retainer 12
can
have a flange. The fasteners 18 can be any suitable nut and bolt.
Each retaining portion 14, 16 is manufactured with a hole through which a
spacer
bushing 31 and a thread-o-let 33 are inserted. The spacer bushing 31 contacts
the surfacing collar C and helps to stabilize the retainer 12 about the
surfacing
collar C. The bushings and thread-o-let 31, 33 can be welded to the retaining
portions 14, 16. Prior to clamping the retainer 12 over the surfacing collar
C, an
aperture through to the cavity A can be drilled through the surfacing collar C
and
the surface casing S. By aligning the thread-o-let 33 with the apertures
through
the surfacing collar C and surface casing S when the retainer 12 is in the
secured
position, gases from within the cavity A can escape through the thread-o-let
33.
Disposed between the thread-o-let 33 and the surface of the surfacing collar C
is
a vent seal 36 that fluidly seals the connection between the thread-o-let 33
and
the surfacing collar C. The thread-o-let 33 and drilled apertures can thus be
used as a vent 32 between the cavity A and a vent assembly 34. In this
exemplary embodiment, the vent seal 36 is an o-ring. The thread-o-let 33 has
about a 2 inch inner diameter and about a 3 inch outer diameter, and has a
length of about 1.50 inches. The spacer bushing 31 has about a 3 inch outer
diameter and a length of about 1.50 inches.
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Referring now to Figures 7 and 8, there is depicted a vent assembly 34 that
can
be fluidly coupled to the vent 32. The vent assembly 34 is composed of tubing
that has supply and exit ends. The supply end of the vent assembly 34 couples
to the thread-o-let 33. The vent assembly 34 also includes a valve 38 that is
movable between open and closed positions. In the open position, gas from the
cavity A can travel through the vent assembly 34 and exit the assembly 34 at
the
exit end of the tubing. Otherwise, gas cannot exit through the vent assembly
34.
During typical usage, the valve 38 is in the open position and a technician
can
determine whether any gas is leaking out of the cavity A by performing, for
example, a bubble test at the exit end of the tubing of the vent assembly 34.
Consequently, the technician can easily determine whether gas is leaking from
the wellhead and, if so, corrective measures can be taken.
While illustrative embodiments of the invention have been described, it will
be
appreciated that various changes can be made therein without departing from
the
scope and spirit of the invention. The invention is therefore to be considered
limited solely by the scope of the appended claims
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