Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02665921 2009-05-13
TITLE
METHOD AND APPARATUS FOR SEALING
ABANDONED OIL AND GAS WELLS
INTRODUCTION
This invention relates to a method and apparatus
for sealing abandoned oil and gas wells and, more
particularly, to sealing abandoned oil and gas well
utilising an eutectic alloy which expands upon passage from
the liquid to solid state.
BACKGROUND OF THE INVENTION
When oil and gas wells are shut in or abandoned, a
regulatory framework exists which mandates the procedures
and technology required to properly shut in or abandon the
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well. This is required to prevent so far as is possible the
leakage of gas from the underground formations to the
surface. Such leakage can have adverse consequences from
unpleasant smells and site contamination to creating a
possibly latent explosive condition or the release of a
toxic gas such as hydrogen sulfide.
Heretofore, following the addition of cement to
the production and surface casings following abandonment, a
steel cap was sealingly welded to the top of the outermost
casing. Such a cap forms a "last barrier" to the seepage of
any gas through the cement within the casing. The steel cap
is usually placed on the top of the casing beneath the
ground surface a certain distance, usually six feet or so,
to prevent the casing from being contacted by farm
implements and other earth moving or working equipment when
agricultural land is being worked following well
abandonment.
Because of significant real estate developments
caused by increasing population in urban areas, there may be
a number of previously abandoned wells in proximity to areas
being developed. Many operations may occur underground at
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depths considerably below the six feet level and the
possibility of underground machinery being used which can
contact and damage the casing and cap is much more likely
now than years ago. It has also been found that in many
abandoned wells, gas has migrated over time through the
cement upwardly within the casing and a pressure head is
formed directly below the welded steel cap. If the casing
or cap is damaged, this trapped gas may escape giving rise
to the aforementioned significant problems.
SUMMARY OF THE INVENTION
According to one aspect of the invention, there is
provided a method for sealing an oil or gas well comprising
positioning a melted bismuth/tin alloy material within a
casing to form a plug within said casing when said liquefied
alloy material solidifies within said casing.
According to a further aspect of the invention,
there is provided apparatus to allow a molten bismuth-tin
alloy material to be positioned as a plug within an oil or
gas well, said apparatus comprising means to add molten
eutectic material to said casing at a predetermined location
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within said casing so as to form a solid plug within said
casing when said liquefied alloy material cools and means to
add a force balancing material to said casing on-the top of
said molten material.
According to yet a further aspect-of the
invention, there is provided a method of sealing an
abandoned oil or gas well comprising removing the cap on the
top of the outer casing of said well, lowering and
positioning a melted bismuth/tin alloy material within said
casing to form a plug within said casing when said liquefied
alloy material solidifies within said casing, positioning a
force balancing material on the top of said melted
bismuth/tin alloy material and allowing said melted
bismuth/tin alloy to cool. a method for sealing an oil or
gas well comprising positioning a predetermined quantity of
melted bismuth/tin alloy material within a casing to form a
plug wholly within said casing when said liquified alloy
material cools and solidifies within said casing.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
Specific embodiments of the invention will now be
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described, by way of example only, with the use of drawings
in which:,
Figure 1A is a diagrammatic side and cutaway view
of the solidified alloy plug within the alloy heater which
5 has been lowered into the casing of an oil or gas well and
which is attached to the surface controlled wireline;
Figure 1B is a diagrammatic view similar to-Figure
1A but showing the alloy heater in operation with the alloy
in a molten state;
Figure 1C is a diagrammatic view similar to
Figures 1A and 1B but illustrating the alloy heater being
withdrawn from the well and leaving the molten alloy within
the well;
Figure 1D is a diagrammatic side view of the alloy
plug left in place within the well in its cooled and solid
state;
Figure 2 is a diagrammatic view of the components
utilised to form the alloy plug within the well casing; and
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Figure 3 is an enlarged side view of the heating
tool used,to melt the bismuth-tin alloy and to carry the
cement slurry which is deposited on the alloy material.
DESCRIPTION OF SPECIFIC EMBODIMENT
Referring now to the drawings, an oil or gas well
is, shown generally in enlarged form at 100 in Figure 1. It
comprises production casing illustrated generally at 101
which is cemented in and surrounded with cement 102. A
solid cement plug 103 is illustrated in place at the
position of interest within the production casing 101. The
setting and formation of the cement plug 103 is well known
in the art.
The components generally used for setting and
forming the bismuth-tin alloy plug are best illustrated in
Figure 2. Such components comprise a power control unit 104
located on the surface 114 which also serves as the source
of input power, generally 480 volt three-phase alternating
current which is subsequently rectified to adjustable
voltage DC current for transmission to a heating tool 111.
The required power connections 105 are connected to a
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wireline spool 110 and extend from the spool 110 downhole by
way of wireline cables 113 to an attachment 115 of the
heating tool 111 which is diagrammatically illustrated in
position within the production casing 101 with the cement or
bridge plug 103 illustrated as being in place. A lubricator
112 to maintain a pressure seal may also be required as the
cables 113 and heating tool 111 move up and down within the
well casing 101.
The longitudinal and circular heating tool 111 is
illustrated in greater detail in Figure 3. The power
carrying cables 113 extend through an attachment point 115
on the tool 111 and terminate at an instrument pod 120. The
instrument pod 120 contains the necessary electronics to
monitor downhole performance of the heating tool 111 and
also provides for power transfer from the cables 113 to the
circumferential alloy heating heaters 121 to be described in
greater detail hereafter. The heating tool 111 contains a
first circular cavity 122 (see also Figure 1A) adapted to
hold the bismuth-tin alloy in solid form and to allow the
alloy to melt and run from the circular cavity 122. The
heating tool 111 further contains a second circular cavity
123 which is generally concentric to and of identical
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internal'configuration to first circular cavity 122 although
the length of second cavity 123 may be increased or
decreased in order to hold a required amount'of cement
slurry 124. A loading port 130 is also provided to allow
the loading of alloy billets (not illustrated) or of liquid
alloy material as 131 as well as a force balancing material
such as a cement slurry 124 although other materials may be
water, sand, gravel or other suitable and fluid materials.
The pressure sealing material that is preferred in
the present operation is a bismuth-tin alloy mixture having
58% by weight bismuth and 42% by weight tin alloy. Bismuth
is the essential ingredient inasmuch as it is non-toxic and
exhibits the valuable property that it expands
volumetrically upon solidification from the liquid phase.
This expansion causes an effective fluid seal when placed
within a well casing in molten form. Tin is also non-toxic,
hence the mixture can be tolerated in direct contact with
fresh groundwater which is a desirable characteristic for a
well plugging material. Whereas any composition of bismuth
-tin alloy could be used, the most favorable is the
aforementioned 58/42 composition because this mixture is a
eutectic mixture melting and solidifying at 137 deg.C. This
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is the minimum temperature at which a bismuth-tin alloy can
exist entirely as a liquid and, therefore, facilitates the
process of in situ melting and placement of the alloy plug.
The use of bismuth material to form the sealing
plug 132 illustrated in Figure 1C is desirable for the
principal reason that bismuth expands as it solidifies.
This is advantageous since while the alloy is in liquid
form, it will fill and run into interstices that might be
used as eventual passageways for fugitive gas transmission
and, as, the alloy cools and solidifies, it expands to fill
the constrained volume of the well casing and therefore
forms a far better seal than that of a material that may
contract or remain at the same volume upon cooling.
The size of the plug 132 which is required will
15. generally be known in order to utilise the correct quantity
of alloy. A rule of thumb generally used in the art is that
the plug 132 will be approximately three times in length as
compared to the diameter of the casing 101. Clearly, this
dimension will vary particularly if the wellhole is deep and
pressures downhole are high in which event a plug of greater
lengthwise dimension would be desirable. But because the
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plug 132 is wholly within the casing 101, the magnitude of
alloy required will be far more accurate than when the alloy
material is being used to seal a geological formation
outside the wellbore by way of perforations in the casing.
5 OPERATION
In operation, there are several techniques. that
may be used to set up the heating tool 111 for downhole.
operation. Preferably, a cap 133 (Figure 4) is attached to
the bottom of heating tool 111. Cylindrical bismuth-tin
10 billets (not illustrated) may be added through the loading
port 130 and positioned one on top of the other within the
billet magazine 134 with the lowermost billet being in
contact with the cap 133. Thereafter, heat is applied to
the heaters 121 which surround the circular billets in order
to melt the billets within the cavity 122. The heating is
preferably of resistive or inductive nature but surface
heating of the billets may conveniently be performed using
other heating techniques. The heating is terminated
following the melting of the billets and the bismuth-tin
alloy solidifies with the cylindrical heating cavity 122 as
also seen in Figure 1A. The cap 133 is then removed and the
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tool lli is ready for downhole operation.
When it is desired to plug a well, the-tool 111 is
attached to the power and wireline cables 113 which lower
and raise the heating tool 111 within the casing 101. At
this juncture, a predetermined quantity of force balancing
material such as cement slurry 124 (Figure 3) is added to
the tool 111 through the loading port 130. The cement
slurry is positioned on top of the solid alloy material 131
and is intended to remain in fluid form until it exits the
heating tool 111 when the alloy plug is being formed. The
quantity of cement slurry or other force balancing material
is dependent upon the pressure which is acting on the plug
but the quantity required need not be highly accurate since
the cost of the force balancing material is not great.
15. The heating tool 1L1 is lowered into the wellhole
as best seen in Figures 1A and 2 until it contacts the
bridge or cement plug 103. Heat is then applied to the
solid alloy material by the heaters 121 of the alloy heater
111 until its melting point is reached at which point
gravity will tend to move the liquid alloy out of the cavity
122 (Figure 1A) of the heating tool 111 as seen in Figure
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1B. The weight of the heating tool 111 is monitored and as
the alloy 116 runs out of the heating tool 111, the tool 111
is raised within the casing away from cement plug 103 and
the liquid alloy forms a plug 132 within the casing 101 as
best seen in Figure 1C. The fluid force balancing material
(not shown) such as the liquid cement slurry will follow the
liquefied alloy out of the heating tool 111 and forms a
counterforce type layer on top of the alloy plug 132. It
will be observed that the action of the force balancing
material on the molten alloy will have at least two
interesting characteristics. First, since the alloy is at a
temperature greater than the force balancing material, the
contact between the force balancing material and the top of
the liquefied alloy plug 132 will cool the top of the alloy
faster than at the bottom. Thus, expansion of the alloy at
the top of the plug 132 will occur before the alloy expands
as it cools in the lower portions of the plug 132. This
enhances the seal at the top of the alloy plug 132 and forms
resistance to any subsequent creep in the plug 132 caused by
well pressure. Second, as the force balancing material
solidifies on top of the plug 132, it also serves as a
barrier to any subsequent creep of the alloy material in the
plug 132 over time.
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Following the release of the alloy material and
the force, balancing material, the heating tool 111 is
withdrawn from the casing 101 by use of the wireline cables
113 and the heating operation is terminated as seen in
Figure 1C. A significantly improved plug 132(Figure 1B) is
formed in the casing 101 which will reduce or eliminate the
migration of gases to the surface though the well casing
101.
While the use of alloy billets has been described,
it is envisioned that molten alloy material may be added
through the loading port 130 rather than in solid billet
form which liquefied alloy material will then run down
within the heating cavities to the temporary cap 133. The
alloy material is then allowed to cool and the cap 133 is
removed as described earlier.
While the force balancing material has been
described as being added to the heating tool and
subsequently released by the tool upon the exit of the alloy
plug material, it is also envisioned that the force
balancing material could be added to the wellhole in other
manners such as simply lowering an automatically or manually
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opening bucket or other container. Sand, for example, could
simply be,poured down the wellhole following the
installation of the alloy plug.
Although the force balancing material described
herein is preferably a cement slurry, other-materials such
as sand, gravel, water or other suitable materials could
conveniently be used.
Many modifications will readily occur to those
skilled in the art to which the invention relates and the
particular embodiments described herein should be taken as
illustrative of the invention only and not as limiting its
scope as defined in accordance with the accompanying claims.
