Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Application for Patent
Inventor: ANDREW R. STRILCHUK
Title: METHOD OF CEMENTING A CASING IN A WELLBORE
Specification
Field of the Invention:
This invention relates to a method of cementing a
casing in a wellbore. More particularly, this invention
relates to a method of decentralizing a casing in a
wellbore, especially a deviated wellbore, to facilitate
the displacement of the drilling mud during cementing
operations.
Background of the Invention:
In the rotary drilling of wells,~a drilling
circulation fluid is circulated down the drill string,
past the bit and back to the surface in the annulus
between the drill string and the wellbore wall. The
typical drilling fluid, or mud, contains water and/or oil,
finely divided solids, and various additives which impart
the desired gel strength, viscosity, weight and filtrate
loss properties to the mud.
Typically, after the well has been drilled to a
desired depth, a permanent casing is cemented into place.
Cementing is accomplished by pumping a cementatious slurry
down the casing and up the annulus between the casing and
the wellbore wall. The cement sets and binds the casing
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to the formation. In order for a cementing operation to
be successful, it is imperative that there be no channels
between the casing and the wellbore wall which would allow
communication from one formation to another.
Obtaining a complete displacement by the cement
slurry of the circulation fluid in place in the annulus is
a primary consideration in cementing operations.
Inadequate displacement of the mud leaves channels of
gelled mud remaining in the annulus after the cement is in
place. This residual mud greatly reduces the
effectiveness of the cementing operation because it
prevents the cement from sealing the annulus between the
casing and the wellbore. Moreover, the mud may contain
components which prevent or delay the setting of the
cement or reduce the strength of the set cement.
The problems associated with cementing a casing in a
wellbore become more complicated in highly deviated or
horizontal wellbores because the effect of gravity tends
to cause the casing to lay along the lower surface of the
wellbore. During cementing operations of highly deviated
wellbores, it is difficult to displace the mud in that
portion of the annulus between the casing and the lower
surface of the wellbore because the annulus on the
opposite side of the casing is correspondingly wider and
allows for freer movement of the cement. Since the moving
cement slu~ry tends to take the path of least resistance,
it displaces the mud from the wide portion of the annulus
but not from the narrow portion along the lower surface of
the wellbore.
Various methods have been employed to cement the
casing in the wellbore in an effort to improve the
cementing operation. One such method is disclosed in U.S.
Defensive Publication T978,002 to Messenger on January 2,
1979. Messenger disclosed using a plurality of wipers and
standard centralizers in the "zone of interest" to
facilitate the displacement of mud and improve the
cementing operation in that zone. The centralizers were
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concentrated in this zone of interest, and the region
above the zone of interest was allowed to rest on the
lower side of the deviated portion of the wellbore.
Another method for centralizing a casing prior to
cementing is disclosed in U.S. Patent No. 3,526,280 to
Aulick on September 1, 1970. Aulick used a combination of
centralizers and a buoyancy chamber to "float" the casing
in a cement slurry. While the method employed by Aulick
may help keep the casing from resting on the bottom side
of the borehole, it is extremely inefficient in that it
reguires that the borehole be filled with cement which is
subsequently substantially displaced by the casing.
Another problem encountered with Aulick is the resistance
to the placement of the casing in the wellbore and the
potential that the cement will harden prior to the casing
being installed. The problems associated with Aulick far
outweigh any advantages associated with centralizing the
casing in the wellbore.
There still exists a need for a method of cementing
which more effectively removes the drilling mud during
cementing operations. In contrast to the methods
heretofore known, the present invention provides a method
of decentralizing a casing in a wellbore, during cementing
operations especially a highly deviated or horizontal
wellbore, to permit better mud displacement. The method
of the present invention thus improves the efficiency of
cementing operations in any wellbore, but especially in a
highly deviated wellbore.
Summary of the Invention:
The present invention is a method for decentralizing
a casing in a wellbore, particularly a highly deviated
wellbore, during cementing operations. The method
utilizes "offset" centralizers which do not provide equal
spacing of the casing in the wellbore. In the preferred
embodiment an offset centralizer is placed on each length
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of casing, preferably 180 out of phase with the adjacent
offset centralizer.
During cementing operations, a casing string having a
plurality of offset centralizers is lowered into the
wellbore just as in a conventional cementing operation.
While the cement is being pumped into the annulus to
displace the drilling mud, the casing is reciprocated,
i.e., stroked in and out of the wellbore, usually one full
cycle. This procedure is standard practice in a cementing
operation. However, the motion from the reciprocation of
the casing having offset centralizers will cause the
annulus to vary in width along the entire length of the
wellbore. The cement will tend to flow preferrentially in
the wider portions of the annulus. Therefore, the
reciprocation of the casing will cause the cement to more
completely displace the drilling mud all around the casing
and increase the likelihood of complete mud removal. Of
course, any movement of the casing string having offset
centralizers would cause the same effect. Therefore, this
invention could be practiced by rotating the casing string
or by combining rotation and reciprocation.
Brief Description of the Drawinqs:
Figures lA and lB are a schematic view of a deviated
wellbore showing the casing with a decentralizing means
embodying the present invention.
Figure 2 is a side elevational view of an "offset"
centralizer.
Figure 3 is a top view taken along line 3 of
Figure 2.
Figures 4A and 4B are top views of an "offset"
centralizer taken along lines 4A and 4B of Figures lA and
lB, respectively.
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Detailed DescriPtion of the Preferred Embodiments:
The present invention is a method for cementing a
casing in a wellbore, especially a highly deviated
wellbore. This method uses an "offset" centralizer or
"decentralizer" to provide unequal spacing of the casing
in the wellbore. During cementing operations the casing
is reciprocated, usually one length of casing, and the
offset centralizers cause the annulus between the casing
and the wellbore to vary in width at any given point.
This variance will allow the cement to more completely
displace the drilling mud all around the casing because it
will create a relatively wide channel around the entire
casing as the casing is reciprocated. This wide channel
will facilitate the removal of the drilling mud.
Fig. lA illustrates a standard casing string made up
of individual lengths of casing 14 connected at a joint
12. In the preferred embodiment, the offset centralizers
are alternated along each section of the casing such that
each consecutive decentralizer is 180 out of phase with
the adjacent decentralizer, as illustrated by
decentralizers 2, 4 and 6 in Fig. lA. This preferred
embodiment causes the maximum possible amount of variance
in the annulus 16 as the casing is reciprocated through
one length of casing. Figures lA and lB show the position
of the casing as it is reciprocated one length of casing
in the wellbore. The position in the wellbore occupied by
decentralizers 2, 4 and 6 in Figure lA is occupied by
decentralizers 4, 6 and 8 in Figure lB during reciprocation
of the casing.
The offset centralizer or decentralizer used to
practice this invention is any type of conventional
centralizer that has been modified to provide unequal
spacing of the casing in the borehole. Figure 2
illustrates a bow-spring type decentralizer having large
members 10 and small members 20. In this example, members
20 on one side of the decentralizer provide for a narrow
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annulus while members 10 on the opposite side create a
wide annulus.
For example, the minimum acceptable cement sheath
thickness in an annulus is about 1.5 cm. Therefore, in a
standard 22.2 cm borehole using a standard 17.8 cm casing,
the wide side of the annulus would be about 2.9 cm, and
the narrow side would be about 1.5 cm. The large members
10 of the decentralizer are set to provide an annulus
width of 2.9 cm while the small members 20 of the
decentralizer would be set for about 1.5 cm. Of course,
any type of decentralizer could be used, even a solid bar
type decentralizer. The only requirement of the
decentralizer is that it cause the portion of the casing
where the decentralizer is attached to be offset from the
center axis of the wellbore.
Figure 2 illustrates a preferred decentralizer which
has six bow-spring type members. For example, in this
embodiment three adjacent members provide for a narrow
annulus while the three opposite members create the wide
side of the annulus. Obviously, other types of
decentralizers, e.g., solid bar type, could be used to
provide the same results during reciprocation.
Figure 3 is a top view of Figure 2 taken along line
3, and it shows one type of configuration where members 10
are of equal size and members 20 are of equal size. Of
course, these members would be of varying sizes, e.g.,
graduated from smallest to largest, and they could have a
variety of configurations.
Figures 4A and 4B illustrate the cross-section of
Figures lA and lB, respectively, taken along lines 4A and
4B. These figures show the variance in the annulus that
takes place during reciprocation of one complete length of
casing.
In a deviated wellbore, the casing should be rotated
until the greatest offset is in a vertical plane to ensure
that the low side of the annulus experiences the maximum
change in the flow pattern when the casing is
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reciprocated. One method that can be used to check the
alignment of the offset centralizers is to paint a short
radioactive strip on the casing in line with the maximum
offset. After the casing is in place, a gamma ray tool
could be run downhole and the casing could be rotated
until the gamma ray tool indicated that the maximum offset
was aligned in the vertical plane.
Of course, there are other possible combinations that
would provide the same effect. Since this invention
depends on two features, namely offset centralizers and
casing movement, it would be possible to rotate the casing
during the cementing operation, instead of reciprocating
it through one length of pipe. If the casing were rotated
at least 180~, the offset centralizers on the casing would
create the same effect, i.e., variance of the annulus
width at any given point in the annulus. Of course, this
invention could also be practiced with both reciprocation
and rotation, either simultaneously or sequentially. A
person skilled in the art can devise other schemes to
decentralize the casing during a cementing operation
without departing from the true scope of the invention
defined in the following claims.
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