Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02432038 2003-06-12
BOREHOLE STABILIZATION WHILE DRILLING
Field of the Invention
The present invention relates to a method and an apparatus for borehole
drilling and in
particular a method and an apparatus for stabilizing a borehole during the
drilling
process.
Background of the Invention
Some oil-bearing formations, such as some of those found in Louisiana and
Texas and
in offshore waters of the Gulf of Mexico, are unstable iincluding
unconsolidated and/or
thief formations. When drilling oil and gas wells borehole instability
problems can occur
including sloughing, stuck pipe and lost circulation. Thus, drilling in these
formations
can be difficult, expensive and time consuming.
Methods suggested for dealing with some of the problems with unstable
formations
including the use of lost circulation materials, cementing loss zones and
setting casing
through the zones. However, it remains a challenge to drill in unstable
formations.
Summary of the Invention
A method and an apparatus for drilling a borehole has been invented. In one
embodiment, the method and the apparatus is useful for stabilizing the
borehole wall
when drilling through an unstable or loss zone. The method can improve
borehole wall
stability and reduce the risk of lost circulation, termed herein
stabilization, which is
desirable when drilling, especially in unconsolidated or thief formations.
In accordance with a broad aspect of the present invention the method includes
providing a drill string including a drill bit connected at a distal end
thereof and a pad
carried by the drill string to move axially and rotationally with the drill
bit; rotating the drill
bit to drill a borehole, the borehole including a borehole wall; and driving
the pad
substantially continuously against the borehole wall while rotating the drill
bit to apply a
lateral wall stress load helically about the borehole wall.
In accordance with another broad aspect of the present invention, there is a
borehole
drilling apparatus comprising: a drill string; a drill bit connected at a
distal end of the drill
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string; and a pad carried by the drill string to move axially and rotationally
with the drill
bit, the drill bit being rotatable to drill a borehole, the borehole including
a borehole wall
and the pad being drivable substantially continuously against the borehole
wall to apply
a lateral wall stress load helically about the borehole wall as it is moved
with the drill bit.
The drill string can be formed of tubing, for example of a continuous length
or formed of
interconnected joints. In one embodiment, the drill string is a string of
drill pipe
connected by threaded pipe connections. Drill pipe is used for drilling the
borehole and,
once the borehole is complete, is often replaced with a borehole liner, which
remains in
the hole. In another embodiment, the drill string is a string of casing joints
connected
together for both drilling the borehole and thereafter, remaining down hole to
fine the
borehole.
The bit can be selected based on various parameters and can include for
example
cone-type bits, fixed cutter bits with under reamers, etc. The drill bit can
be driven by a
mud motor or, in a preferred embodiment, by rotation of the drill string. In
this preferred
embodiment, the string rotation drives rotation of the bit and the pad is
carried by the
drill string to move rotationally and axially therewith. The drill! string can
be rotated by
employing various means such as by use of a kelly, a tap drive or other means.
As will
be appreciated, selection of the particular means for rotating the drill
string is often
dependent on the form of drill string being used.
The pad applies lateral wall stress load to the borehole wall as it is driven
against the
borehole wall and rotated and moved axially with the drill bit. The stress
load causes
compaction of the earthen materials exposed on the bor~:hole wall and plasters
materials against the borehole wall. Plastering mechanically works, as by
crushing,
applying and/or compacting borehole materials against, and into the porosity
and
fractures of, the borehole wall to create a strong layer or skin with reduced
permeability,
when compared to the untreated borehole wall. The borehole materials can
include, for
example, mud solids, in the form carried by the drilling mud or settled as a
filter cake,
which can be formed on porous permeable zones, and drilled solids. The lateral
wall
stress applied by the pad can be from direct contact with borehole wall or
contact of the
pad to drill cuttings or mud solids, which are plastered against the borehole
wall. This
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causes the hydraulic film between the pad and the solids of the borehole wall,
drill
cuttings or mud solids to be reduced such that there is solid to solid
contact.
Since the pad is in contact with solids as it is driven agaiinst and riding
over the borehole
wall, the pad preferably includes a means for enhancing its wear so that it
can
accommodate or resist wear and continue to act throughout the drilling
process. The
wear resistance need only be applied at the pad surface that has solid
contact.
The pad is intended to plaster and compact the borehole wall by application of
lateral
stress thereagainst. Thus, in a one embodiment, the pad is formed to ride over
the
surface rather than scraping or digging into it. To facilitate riding over the
solids of the
borehole wall or those solids disposed between the pad and the borehole wall,
pad can
be devoid of sharp edges that can dig into the formation, for example, the pad
leading
edge can be graduated as by radiusing or beveling. It is to be noted, however,
that the
pad should be selected to apply sufficient localized force, dirE=ctly or
indirectly, against
the borehole wall to compact the materials of the borehole wall and/or
mechanically
work mud solids and drilled solids against the borehole wall to thereby
enhance
borehole integrity by reducing wall permeability and sloughing. It is believed
that, in
poorly consolidated sandstones, beneficial effects can be achieved by line
load of 10 to
100 pounds/inch.
The pad is driven substantially continuously against the borehole wall while
drilling to
apply a lateral wall stress load helically about the borehole wall.
Preferably, the pad
sweeps the complete circumference of the borehole wall during each revolution
of the
drill bit, as the drill string advances, such that the pad alpplies stress
load in a
substantially continuous path to treat, with consideration to the drill bit
rates of
advancement and rotation and the size of the pad, at leasvt a major portion of
the
borehole wall in a selected interval of the borehole so that in treat
interval, there are few
zones over which the pad has not passed, which zones are therefore
unstabilized or
leaky. As discussed below, treatment of a major portion of the borehole wall
may also
be affected by providing a plurality of pads that together are driven against
the borehole
wall to increase, overlap and/or provide redundancy in borehole wall
stabilization.
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The pad is preferably also driven substantially continuously against the
borehole wall to
reduce damaging impact caused, for example, by the pad lifting off the surface
and
forcefully landing again, and thereby digging into, hammering or beating the
formation.
In one embodiment, the pad is mounted to be resilient to follow the contour of
the
borehole wall and to avoid easily lifting off the borehole wall. This
resiliency can be
provided through a biasing means such as a spring or hydraulic acting against
the pad.
Alternately or in addition, the resilient nature of the pad can be provided by
means of
the resiliency in the drill string itself, on which the pad is carried. As
such the pad is
disposed, with reference to the borehole radius, to have an effective radius
such that
the pad would extend outside beyond the radius of the borehole if not
constrained within
the borehole.
In some embodiments, it may be beneficial to the apparatus or method to
provide a
plurality of pads, each carried by the drill string and disposed to rotate and
move axially
with the drill bit. The plurality of pads can act to together to plaster and
compact the
borehoie wall to improve borehole wall integrity. The plurality of pads may
also act to
balance forces on the drilling string generated at the pads so that they do
not counteract
rotation of the drill string or cause problematic twisting or deflection in
the drill string.
It may be desirable to stabilize the borehole wall as soon as possible after
it is drilled.
In such an embodiment, the pad should be positioned as close as possible to
the
bottom end of drill string. It may also be necessary to position further pads
along the
drill string to continue the effect. Alternately, the drill string may, in
some embodiments,
require stabilization to avoid damaging contact against the borehole wall to
destroy the
positive effect of the pad contact.
In view of the foregoing and in accordance with another broad aspect of the
present
invention, there is provided a borehole stabilizing apparatus for use with a
drilling
apparatus comprising: a pad installable into a drill string to move axially
and rotationally
with a drill bit of the drill string, the drill bit being rotatable to drill a
borehole including a
borehole wall, the pad being drivable substantially continuously against the
borehole
wall to apply a lateral wall stress load helically about the horehole wall
during drilling.
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The pad can be formed by various means carried on the drill string. The pad
can be
any apparatus that can be used to provide a smooth resilient contact stress
that sweeps
substantially the complete circumference of the boreholE; wall each revolution
of the drill
bit, or in the case of rotary drilling, each revolution of the drill string.
The pad can be
mounted onto the drill string tubulars or mounted on a sub to be installed, as
by
threading into the drill string. In one embodiment, the pad is installed on a
bend in the
drill string, the bend urging the pad outwardly to ride against the borehole
wall. The drill
string itself acts in an elastic manner about the bend.
In another embodiment, the pad can be incorporated to a drill string sub
connectable
between straight joints of pipe or drill collars. The pad can be formed on one
side of the
sub, such that when the element is installed in the drill string, the pad
surface extends a
radial distance from the drill string centerline that is greater than the
radius of the hole.
A plurality of these subs positioned at the junction between several joints of
pipe would
use the stiffness of the pipe to drive the pads with resiliency against the
borehole wall.
Of course, the subs must be oriented such that their pads do not line up
axially along
the drill string.
An alternate embodiment includes a sub including a plurality of smooth blades,
each
forming a pad, biased outwardly by resilient members such as springs, elastic
inserts, or
hydraulics. In one embodiment, the sub includes three pad-carrying blades
supported
by springs so that they each have an effective radius greater 'than the
borehole radius,
when the sub is not constrained by the borehole. The lateral wall stress load
applied by
the blades to the borehole wall can be easily controlled in this embodiment,
as can the
pad surface contour to prevent digging into the borehole wall. The blades and
sub must
be durable to withstand operation wherein they are moved rotationally and
axially with
the drill bit, such as by installing in the drill string in a rotary drilling
operation. The
blades are biased to ride over the borehole wall and to follow the surface
contour
thereof.
Brief Description of the Drawings
A further, detailed, description of the invention, briefly described above,
will follow by
reference to the following drawings of specific embodiments of the invention.
These
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drawings depict only typical embodiments of the invention and are therefore
not to be
considered limiting of its scope. In the drawings:
Figure 1 A is a schematic sectional view along a wellbore illustrating a
method according
to the present invention.
Figure 1 B is a sectional view along lines I-I.
Figure 2A is a schematic sectional view along a wellbore illustrating a method
according
to the present invention.
Figure 2B is a sectional view along path F'2 of Figure 2A, as shown by section
lines II-II.
Figure 3A is a front elevation of a drill string useful in the present
invention, using casing
as the drill string.
Figure 3B is a schematic sectional view along a wellbore illustrating another
method
according to the present invention, using the drill string of Figure 3A.
Figure 4 is a perspective view of a wear resistant casing connection useful in
the
present invention;
Figure 5 is a sectional view through the sidewall of the connection shown in
Figure 4
wherein a shoulder ring is included to provide improved torque capacity;
Figure 6 is a front elevation of a pair of connected casing Joints showing a
bend angle
formed by the connection shown in Figure 4; and
Figure 7 is a partially cut away view through another connection, where the
coupling
bend angle is controlled.
Description of the Invention
A method and apparatus for conditioning the borehole wail of wells drilled
through earth
materials has been invented. The method can improve borehole integrity, which
is
useful when drilling through formations of earth material susceptible of
sloughing, such
as unconsolidated sandstone. The method can also reduce the risk of lost
circulation
when drilling through formations difficult to seal using conventional
practices primarily
relying on the action of filter cake, deposited by invasion of drilling
fluids.
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In one embodiment, the method includes: providing a drill string and a drill
bit
connected at a distal end thereof; rotating the drill string while
simultaneously moving
the drill string axially into or out of the borehole of a well containing
drilling fluid as
required to drill the well or maintain the borehole over at least one selected
well interval,
the borehole including a borehole wall; providing the drill string with one or
more
contoured pads having outward facing generally convex surfaces and means to
laterally
force the contoured pads into continuous contact with the borehole wall to
create one or
more rotating local contact stress zones having a length and circumferential
distribution
of contact stress determined by the shape of the contoured pad and applied
lateral
force; the magnitude and distribution of local contact stress selected
according to
properties of the earth materials occurring in the at least one selected well
interval to
promote borehole wall stability and sealing against loss circulation but avoid
excess
stress causing spalling or other failure of the earth material.; the
combination of rotation
and axial movement of the drill string causing each local contact stress zone
to move on
a generally helical sweep pattern having a local pitch; the axial length of
local contact
stress zone, as determined by the contact length of the contoured load pad or
pads, in
combination with the position and number of pads is selected to ensure the
entire
borehole wall surface is swept, at least once, even where the local pitch is a
maximum;
and the leading edge, with respect to the direction of rotation, of the
contoured load pad
outward facing surface is preferably shaped to provide a converging drilling
fluid filled
channel with the borehole wall in front of the zone c>f coni:act so as to
encourage
trapping and compaction of solids entrained in the drilling fluid and
discourage scraping
of the borehole surface; together seeking to ensure that the entire surface of
the
borehole wall over the selected well interval is at least once subjected to
application of a
local contact stress riser, preferably applied in combination with compacted
solids
particles otherwise carried in the drilling fluid to thus simultaneously
plaster the borehole
hole wall with a solids pack and compact this material with that of the near
well bore
earth material tending to form a generally compressive skin of reduced
permeability.
To obtain the most immediate benefit from the present invention during
drilling, it will be
apparent that it is preferable to provide contact pads near the bit as this
will minimize
the time between drilling and application of the rotating contact stress to
the borehole
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wall, thereby simultaneously minimizing the time for rock failure or
'washouts' to develop
and the interval length exposed to greater risk of lost circulation. The
contoured pads
may be provided in various shapes, however since the borehole is generally
cylindrical,
the outward facing generally convex pad surfaces are most readily provided
also as
generally cylindrically shaped, with a radius of curvature somewhat less than
that of the
borehole wall to be contacted. This shape also readily provides a converging
channel at
the leading edge of the pad when in contact with the borehole wall. The length
of pad is
selected in combination with the applied force and local curvature to ensure
continuous
contact occurs between the pad and borehole wall during conditions of
rotation, as
required by the method of the present invention, and is not prevented by
hydrodynamic
forces tending to push the rotating pad away from the borehoie wall creating a
film of
sufficient thickness to prevent compaction of the solids entrained in the
drilling fluid.
The method of the present invention is thus seen to differ from the teachings
of the prior
art with respect to contact stress between drill string components and the
wellbore wall
arising from lateral forces in two principle respects. First, tile majority of
drill string
components are designed to minimize wear associated with just such a reduction
in
hydrodynamic film to the point where continuous and therefore abrasive contact
occurs.
Second, even where contact is allowed for, such as at stabilizers, the contact
is not
intended to sweep the entire well surface but is directed toward centering the
drill string,
preventing lateral impact loading causing failure of the borehole wall
material and
continuous circumferentially statically positioned contact causing keyseating.
Instead, the method of the present invention employs application of a
controlled rotating
contact stress field to the entire surfaces of certain borehole rocks, in the
presence of
typical solids carrying drilling fluids, tends to mechanically work the near
borehole wall
material to improve both its strength and ability to seal betvveen the
typically higher
pressure of the wellbore fluid relative to the formation fluid. lnlhile the
required stress
level to achieve optimum results will vary depending on rock properties,
beneficial
results have been obtained in unconsolidated sandstones, for cylindrical pad
contact
line loads in the range of 10 to 100 Ib/inch. As already taught the lower
bound load to
achieve at least some beneficial solids compaction is limited by the load
required to
develop hydrodynamic film separation. The maximum load is limited by risk of
spalling
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or fatigue failure of the formation rock as well as practical limitations
imposed by
available torque as it wilt be evident that the contact force occurring at
each pad
produces an associated drag force adding to the torque required to rotate the
drill string.
While many possible means of applying lateral force exist in principle, in the
drilling
environment two broad sources of force are more readily available and
therefore
preferred: radial deflection of resilient elastic members and hydraulic
pressure typically
existing across the drill string tubular wall. Hydraulic pressure may be
exploited by
providing hydraulic actuators acting between the contoured pads and the drill
string.
Radial deflection of resilient elastic members required to develop a lateral
force is
readily provided by interference between the drill string components carrying
the
contoured pads and the borehole wall. The resilient elastic members may be
provided
as separate components, for example bow springs carrying the contoured pads,
or the
drill string pipe may itself be pressed into service where the tubular members
loaded in
opposition at spaced axial position act as long resilient beams.
Figures 1A and 1 B are schematic views illustrating an apparatus and method
according
to the present invention. Figures 2A and 2B are schematic views illustrating
another
method and apparatus according to the present invention, wherein the pads are
carried
at bends in the drill string. Figures 3A and 3B are schematic views
illustrating another
method and apparatus according to the present invention wherein casing is
employed
as the drill string.
Referring first to Figures 1 A and 1 B, a method and apparatus is shown
schematically for
drilling a well and stabilizing a borehole wall in an unstable formation 15~7y
for example
of unconsolidated sand stone wherein problems of sloughing and fluid loss have
been
encountered. The apparatus includes a drill string 150 with a drill bit 152
connected at a
distal end thereof. The drill bit is rotatably driven to drill a borehole 154
by rotation of
the drill string. The borehole being drilled includes a borehole wall 156,
which are the
exposed earth materials from the formation. The borehole can extend from
surface
158, as shown, or can be a iateral well extending from another borehole. The
borehole
can be vertically or horizontally oriented, straight or deviated, etc.
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Drill string 150 carries a sub 160 including three smooth blades 162, each
forming a pad
surface 164, biased outwardly by resilient members 166 such as springs or
elastic
inserts. The blades, at their pad surface 164, define an effective radius
greater than the
borehole radius, when the sub is not constrained by the borehole. However,
when the
drill string is confined in borehole 154, having a radius according to the
drill bit being
used, the blades will be constrained by and apply a lateral wall stress, arrow
F, against
the borehole wall as they are biased outwardly. When the string including sub
160 is
rotated, arrow R, within the confinements of a borehole, the pads 164 cause an
axisymmetric to helical plastering action against the borehole wall. The
plastering
action, thus provided, results in axisymmetric to helical paths P of
consolidation in the
well bore earth material exposed on and adjacent to the borehole wall, as the
pads are
driven thereover. The blades also mechanically work, as by crushing, applying
and
compacting, solids 165 such as mud solids or drilled solids, into the porosity
and
fractures of the borehole wall to create a strong layer or skin with decreased
permeability and increased surface strength, when compared to the borehole
wall over
which the blades have not ridden. The blades are biased to ride over the
borehole wall
and to follow the surface contour thereof.
The lateral wall stress load applied by the blades to the borehole wall can be
easily
controlled, as by selection of spring force, as can the pad surface contour to
prevent
digging into the borehole wall. For example, the blade leading edges can each
be
ramped, as shown. The blades and sub must be durable to withstand operation
wherein they are moved rotationally and axially with drill string 150 in a
rotary drilling
operation. In addition, the length of the pad surfaces of the blades can be
selected, with
consideration as to the rate of drill string advancement and rate of rotation,
so that
substantially all or at least a major portion of the borehole wall over a
selected interval
of the well is plastered as the sub passes thereby.
Drill string 150 can be formed of tubing, for example of a continuous length
or formed of
interconnected joints. Bit 152 can be selected based on various parameters and
can
include, for example, a cone-type bit. The drill string can be rotated by
employing
various means such as by use of a kelly, a top drive or other means. As will
be
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appreciated, selection of the particular means for rotating the drill string
is often
dependent on the form of drill string being used.
The illustrated sub 160 not only operates to plaster and consolidate the
borehole wall,
but also acts to centralize the drill string to prevent it from hammering
against and
thereby damaging the formation and the skin formed by the plastering effect.
Note also,
that the annulus about the drill string is not considerably obstructed to
adversely effect
mud flow.
Referring to Figures 2A and 2~, another method and apparatus for drilling a
well and
stabilizing a borehole wall in an unconsolidated formation 207 are shown. The
method
includes providing a drill string 200 with a drill bit 202 connected at a
distal end thereof,
rotating the drill string to drive the drill bit to drill a borehole 204. The
borehole being
drilled includes a borehole wall 206, which are the exposed earth materials
from the
formation. The borehole can extend from surface 208, as shown, or can be a
lateral
well extending from another borehole. The borehole can be vertically or
horizontally
oriented, straight or deviated, etc. The method further includes driving pads
210, 212
along the drill string against the borehole wall while rotating, arrow R, the
drill string to
apply a lateral wall contact load, arrow F, in an axisymrr~etrical to helical
path about the
borehole wall. In the illustrated embodiment, drill string 200 bears at two
pads 210, 212
against the borehole wall to define two paths P2', P2" in ~uvhich a lateral
wall contact load
is applied against the borehole wall. The applied load consolidates and
plasters the
surface of the borehole wall.
The pads are formed at bends in the drill string. Each bend is a deviation in
the long
axis of the drill string and causes the drill string to operate in an elastic
sense tending to
bias the pads against the borehole wall. This bend can be achieved by forming
the drill
string tubing, as by bending, milling, etc. Alternately, the bend can be
formed through a
connection between lengths of tubing. For the purpose of this invention, a
connection
should be understood broadly to mean any arrangement or device that joins the
ends of
drill string tubulars to create a section over which a structural union is
arranged so that
the axes of the joined tubulars is substantially continuou s across the
connection interval
but includes a bend inherent or deliberately introduced at the connection such
that the
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axis although continuous changes direction. Understood thus, the term
"connection"
includes, but is not limited to, welded connections, integral connections and
threaded
and coupled connections. For example the connection can be a welded joint
between
adjacent lengths of tubing, which are brought together to create the bend. As
another
example, the bend can be located at a threaded connection along the drill
string. Such
threaded connections are, for example, known as tool joints in drill pipe
strings and as
couplings in casing strings. The bend can be intentionally generated or
inherent in the
form of the connection. The vast majority of well bores are drilled with metal
tubing
strings comprising for example drill pipe, drill collars, casing joints,
having threaded
connections. Therefore to be most readily implemented, the bends of the drill
string can
be provided at the threaded connections of the drill string. While some casing
strings
may have inherent bends at the threaded connections k~etween joints of casing,
it may
be desirable to intentionally increase or control the angular deviation at
selected
connections or intentionally introduce angular deviations at further
connections. Drill
strings of drill pipe and drill collars are generally, due to the tolerances
of the drill pipe
threaded joints, more straight. Thus, to be useful in the present invention,
it may be
necessary to modify the drill pipe or drill collar connections to introduce
bends into the
string. Connections are best provided in a manner that accorramodates existing
thread-
forms, sealing geometries and operational tolerances.
The bend can be gradual or abrupt. For example, the bend can be in the form of
a
gradual axial deviation (i.e. a curve), a sharp axial deviation or a stepped
deviation
formed by spaced apart deviations in the long axis of the drill string wherein
an
intermediate section is formed at the bend.
Since the drill string may experience considerable abrasive forces at the
bend, it may be
advantageous to provide wear protection at the bend. In one embodiment, wear
protection is applied to the drill string bend at the circumferential location
corresponding
to the outside of the bend, since contact with the borehole wall will occur
only on the
outside of the bend.
Excess wear can be avoided by use of various forms of wear protection such as
for
example the introduction of wear resistant material to the bend. The wear
resistant
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material may be integral to the material of the bend, obtained by surface
hardness
treatment such as boronizing, nitriding or case hardening, or applied thereto
such as by
use of a coating such as hardfacing. The wear resistant material can be
introduced at
the bend in various ways. For example, the tubulars of the string can have the
wear
resistant material directly on them. Alternately or in addition, a separate
device can be
used such as, for example, a wear band, as disclosed in Cdn. Patent App
2,353,249,
filed July 18, 2002. The disclosed wear band includes a band of wear resistant
material
and is structurally attached to the tubing string by, for example, crimping.
This solution
is effective and provides a readily implemented means to enhance the wear
resistance
of the tubing string at the bend. As another example, a wear resistant
coupling or sub
can be used. For application of the present invention to a casing string, the
bend can
be formed at a casing coupling and the wear resistance can be applied or
formed
integral thereto, as disclosed in applicant's corresponding International
application
WO/03/008755, published January 30, 2003. Thus, a useful casing connection
includes
a wear resistant material on at least a portion of its outer surface. The wear
resistant
material can be arranged to at least overlap the circumferentially oriented
location
forming the outside of the bend at which the coupling is positioned.
When a string including a bend therein is rotated within the confinements of a
borehole,
the bend rotates with the drill string and the drill string rotates with
respect to the
borehole. When driven against the borehole wall, the bend causes an
axisymmetric to
helical 'wiping' action on the interior of the borehole wall, the wiping
action thus
provided, results in an axisymmetric to helical path of consolidation in the
well bore
earth material exposed on and adjacent to the borehole wall, as the bend is
driven
thereover. This consolidation increases the stability of the bor~ehole wall
earth material
by packing the earth materials and closing fissures and pores therein, thereby
reducing
risk of sloughing and of lost circulation.
The degree of consolidation and associated benefits depends on the lateral
force
generated by the drill string as it bears against the borehole wall. The
lateral force
generated by the bend bearing against the borehole wail can be controlled by
selection
of the bend angle magnitude and/or bend direction. The lateral forces
generated at a
bend wilt tend to increase with increasing bend angle. The control of bend
direction,
DMSLega1~03236110015611537204v3 13
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especially with consideration to the drill string configuration and
positioning within the
confines of the borehole, for example as caused by other bends along the drill
string,
provides control of lateral forces generating when compared to random
orientation of
connection bend direction. Thus in one embodiment, the method further includes
selecting the angle magnitude of at least one bend along the drill string to
control the
lateral reaction force of the drill string against the borehole wall in which
the drill string is
intended to extend and in another embodiment, the method further includes
selecting
the angular direction of at least one bend along the drill string to control
the lateral
reaction force of the drill string against the borehole wall in which the
drill string is
intended to extend. Control of the connection bend magnitude, and preferably
also the
bend direction, enables control of said lateral force exerted and is, thus, a
means to
balance the benefits gained by wiping action on the borehole wal6 against
other factors
such as resistance to drill string advancement and the eccentric wear rate of
the string
at the bend.
Where a plurality of spaced apart bends are positioned along an interval of
the drill
string and the drill string is rotated to drill a borehole such that the
interval becomes
positioned in the borehole, the bend angle and direction of the plurality of
bends control
the local lateral wall contact load applied by the interval against the
borehole wall as it is
positioned within the confines of the borehole. The bend angle and direction
of at least
some of the bends can advantageously therefore be arranged to deflect some or
all of
the bends into selected contact, for example generally radially opposed
contact, with the
borehole wall over the interval.
When considering the use of casing connections to control bend angle magnitude
and
direction, the bend magnitude and direction occurring across a threaded
connection is a
function, for example, of one or more of the casing joint end straightness,
joint thread
axis angle alignment with the joint axes and the coupling thread axes. For
industry
typical casing connections, the bend magnitude or axis misalignment is not
tightly
controlled, as for example described in the API Specification 5CT and Standard
5B.
Furthermore, the bend direction is randomly oriented. Thus to select band
angle or
direction, it is necessary to either review and select the particular casing
joints and
casing connections to be used in the drill string of casing or~ form the
casing string
DMSLegal\032361100156\1537204v3 14
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components to be used to form the bends. Thus, in one embodiment, where casing
is
used as a drill sting, the method further includes providing casing string
components for
assembling a casing string having controlled bend angle magnitude or
direction, for
example, including a casing connection formed to provide a controlled bend in
the axes
of the casing tubulars joined by the casing connection. The controlled bend
can be
selected such that when said casing connection is employed to assemble the
casing
tubulars to form at least one interval of a casing drill string placed in a
borehole, the
resulting bend of a selected bend magnitude and/or direction in the interval
such that
the bend contacts the borehole wall. Where a plurality of said casing
connections are
used, the resultant bends can be selected to induce selected bend angle
magnitudes or
directional variations to induce some or all of the bent wear resistant
connections to at
least contact the borehole wall and induce selected lateral wall contact
forces, such as
generally radially opposed contact forces between the plurality of bends.
As a means to more predictably control said radially opposed contact forces,
in a further
embodiment, said wear resistant casing cannection of controlled bend is
provided
having the circumferential direction of the bend controlled with respect to a
casing string
assembled from such connections. Such control of circumferential direction is
preferably selected to provide a repeating pattern between beat connections
comprising
an interval of an assembled casing string.
In the illustrated embodiment of Figures 2A and 2B, drill string 200 includes
at least two
bends carrying pads 210, 212. The bends are selected such that, when the drill
string is
confined in a borehole sized according to the drill bit being used, the bends
will be
constrained by and bias the pads against the borehole wall. When the string
including
bends is rotated within the confinements of a borehole, the fiends cause the
pads to
effect a continuous, axisymmetric to helical 'wiping' action on the interior
of the borehole
wall, the wiping action thus provided, results in an axisymmetric to helical
path of
consolidation in the well bore earth material exposed on and adjacent t~ the
borehole
wall, as the bend is driven thereover. This consolidation increases the
stability of the
borehole wall earth materiaB by packing the earth materials, closing fissures
214 and
pores therein and mechanically working materials from the mud stream, filter
cake into
the porosity of the borehole wall, thereby reducing risk of sloughing and of
lost
DMSi.egal\032361 \OO15G\1537204v3
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circulation when compared to a borehole hole wall surface over which the bend
has not
passed.
Since it is desirable to stabilize the borehole wall as soon as possible after
it is drilled, it
is preferred therefore, that pads 210, 212 in the drill spring be positioned
as close as
possible to the bottom end of the drill string. It is also desirable that as
much as
possible of the borehole wall be compacted by application of lateral wall
contact load.
Therefore, with consideration to the rate of advancement of tt~e drill string,
the surface
contact area of the pads with the borehole can be adjusted or further contact
areas ~i.e.
pads) can be provided along the drill string.
Drill string 200 can be formed of tubing, for example of a continuous length
or formed of
interconnected joints. In the illustrated embodiment, dlrill string 200 is
formed of drill
pipe connected by threaded pipe connections 216. Bit 202 can be selected based
on
various parameters and can include, for example, a cone-type bit. The drill
string can
be rotated by employing various means such as by usE, of a Kelly, a top drive
or other
means. As will be appreciated, selection of the particular means for rotating
the drill
string is often dependent on the form of drill string being used.
Although the drill string has a continuous long axis 200x, the bends are
deviations in the
long axis of the drill string. In the illustrated embodiment of Figures 2, the
bends are
positioned along the drill string by threading bent drill pipe sections into
the string.
Pads 210, 212 and bends can be formed in various ways, as by bending, milling,
casting, etc. In the illustrated embodiment, pad 210 t s carried on a sub
including an
insert elbow 218 welded between pipe section ends 22ta. Pad 210 is formed on
elbow
218 as a generally flat region with a degree of radial curvature. Pad 210 has
applied
thereon a wear resistant surface 222. While the wear resistant: surface could
have been
applied over a greater area, to reduce costs surface 222 is only applied to
the drill string
bend at the circumferential location corresponding to the outside of the bend,
since
contact with the borehole wall will occur only on the outside of the bend.
Pad 212 is formed on an eccentric extension 223 radially extending from the
drill string.
Eccentric extension 223 can be fixed, as shown, hydraulically driven or biased
outwardly by some means. While eccentric extension 223 is formed on a bent
sub, in
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another embodiment, the eccentric extension can be incorporated to a drill
string sub
connectable between straight joints of pipe or drill collars. The pad can be
formed on
one side of the sub, such that when the element is insi:alled in the drill
string, the pad
surface extends a radial distance from the drill string centerline that is
greater than the
radius of the hole. A plurality of these subs positioned at the junction
between several
joints of pipe would use the stiffness of the pipe to drive the pads with
resiliency against
the borehole wall, rather than requiring a bend in the drill string. Of
course, the subs
must be oriented such that their pads do not line up axially along the drill
string.
The degree of consolidation and associated benefits depends on the lateral
force
generated by the drill string as it bears against the borehole wall. The
lateral force
generated by the bend bearing against the borehole walll can be controlled by
selection
of the bend angle magnitude and/or bend direction. The lateral forces
generated at a
bend will tend to increase with increasing bend angle. The control of bend
direction,
especially with consideration to the drill string configuration and
positioning within the
confines of the borehole, for example as caused by other bends along the drill
string,
provides control of lateral forces generated when compared to random
orientation of
connection bend direction. Thus in one embodiment, the method further includes
selecting the angle magnitude of at least one bend along the drill string to
control the
lateral reaction force of the drill string against the borehole wall in which
the drill string is
intended to extend and in another embodiment, the m~ahod further includes
selecting
the angular direction of at least one bend along the driBl string to control
the lateral
reaction force of the drill string against the borehole wall in which the
drill string is
intended to extend. Control of the connection bend magnitude, and preferably
also the
bend direction, enables control of said lateral force exerted and is, thus, a
means to
balance the benefits gained by wiping action on the borehole wall against
other factors
such as resistance to drill string advancement and the eccentric wear rate of
the string
at the bend.
Vilhere a plurality of spaced apart bends are positioned along an interval of
the drill
string and the drill string is rotated to drill a borehole such that the
interval becomes
positioned in the borehole, the bend angle and direction of the plurality of
bends control
the local lateral wall contact load applied by the interval againsi; the
borehole wall as it is
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positioned within the confines of the borehole. The bend angle and direction
of at least
some of the bends can advantageously therefore be arranged to deflect some or
all of
the bends into selected contact, for example generally radially opposed
contact, with the
borehole wall over the interval.
Referring to Figures 3A and 3B, a method and apparatus is shown using a drill
string of
connected casing joints including joints 230, 232, 234, 236. The joints are
joined by
threaded connection into casing couplers including couplers 233, 235, 237. The
drill
string of Figure 3A is useful for drilling a borehole 238, wherein after the
borehole is
drilled, the casing string can be left in to line the hole. A bit (not shown)
is mounted at
the distal end of the casing string to provide drilling action. For drilling
with casing the
bit is selected to be trippable through the casing inner bore and generally
includes a
pilot bit and an expandable under reamer.
Figure 3A shows the drill string outside the borehole and Figure 3B shows the
drill string
when it is confined in a borehoie drilled by the bit. Bends are formed along
the casing
string at connections 233, 235, 237. In particular, the overall string axis is
defined by
the axis 230x, 232x, 234x, 236x of the individual joints. These axis intersect
at the
connections. However, it can be seen that the connections tend to deviate the
long axis
of the drill string, for example by an angle a'.
Due to the angular deviation along the long axis of the casing string, when it
is
constrained in the borehole to be drilling using the string, the connections
233, 235, 237
will be forced to flex to reduce the angle of deviation. For example angle a'
will be
reduced to cc". This permits the string to extend in the borehole, but will
cause the
connections to be driven against borehole wail 238 such that a lateral wall
contact force
F1, F2, F3 is applied to the wall, at each connection. Due to the forces
applied, the
borehole wall is consolidated and, thereby, stabilized, as the drill string
and connections
233, 235, 237 pass therethrough.
It is possible that a standard casing connection will, when used to join
casing joints,
create a deviation. However, the angle of deviation for example at connection
235 can
be selected by inspection or forming, to provide a particular angle of
deviation between
the joints.
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When considering the use of casing connections to control bend angle magnitude
and
direction, the bend magnitude and direction occurring across a threaded
connection is a
function, for example, of one or more of the casing joint end straightness,
joint thread
axis angle alignment with the ,joint axes and the coupling thread axes. For
industry
typical casing connections, the bend magnitude or axis misalignment is not
tightly
controlled, as for example described in the API Specification 5CT and Standard
5B.
Furthermore, the bend direction is randomly oriented. Thus to select bend
angle or
direction, it is necessary to either review and select the particular casing
joints and
casing connections to be used in the drill string of caging or form the casing
string
components to be used to form the bends. Thus, in one embodiment, where casing
is
used as a drill sting, the method further includes providireg casing string
components for
assembling a casing string having controlled bend angle magnitude or
direction, for
example, including a casing connection formed to provide a controlled bend in
the axes
of the casing tubulars joined by the casing connection. The controlled bend
can be
selected such that when said casing connection is employed to assemble the
casing
tubulars to form at least one interval of a casing drill string placed in a
borehole, the
resulting bend of a selected bend magnitude andlor direction in the interval
such that
the bend contacts the borehole wall. Where a plurality of said casing
connections are
used, the resultant bends can be selected to induce selected bend angle
magnitudes or
directional variations to induce some or all of the bent wear resistant
connections to at
least contact the borehole wall and induce selected lateral wall contact
forces, such as
generally radially opposed contact forces between the plurality of bends.
As a means to more predictably control said radially opposed contact forces,
in a further
embodiment, said wear resistant casing connection of conf;rolled bend is
provided
having the circumferential direction of the bend controlled with respect to a
casing string
assembled from such connections. Such control of circumferential direction is
preferably selected to provide a repeating pattern between bent connections
comprising
an interval of an assembled casing string.
The connection bend angles were calculated from sample of typical 7inch
(178mm) API
buttress threaded and coupled (BTC) casing joints. These magnitudes were used
to
calculate the possible maximum lateral load arising from this load mechanism,
were
D:~ISLega11032361\00156\1537204v3
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such casing joints assembled into a casing string and placed in a borehole
drilled with a
bit size of 8.5inches (216mm). It was found that, with negligible axial load,
a lateral force
of at least 1000 Ibf (4450N) could be applied agairost the borehole wall at
each
connection if at least an interval of such casing joints were assembled into a
casing
string and the string was confined in the borehole. As will be appreciated,
axial load
such as by the weight of the drill string or weight on bit will increase the
lateral force at
each connection.
In a method wherein casing joints are connected and the bends are formed at
their
connections, a wear resistant casing connection can be employed to address the
problems arising from wear. In its preferred embodiment, the wear resistant
casing
connection is generally of a threaded and coupled nature and more preferably
employs
a thread-form geometry compatible with a buttress connec~:ion as specified by
the
American Petroleum Institute (API).
Such existing threaded connections include the thread-forms and sealing
geometries
comprising so called premium connections, in addition to both integral and
threaded and
coupled American Petroleum Institute (API) specified geometries. (Reference
herein to
a 'thread-form' is generally understood to include the seal geometry if
present, unless
these two components of the connection geometry are specifically separated in
the
context.) This accommodation of existing geometry extends to the connection
diameter
where it is preferable to provide wear resistance without a significant
increase in outside
diameter to avoid correlatively increasing the annular flow resistance, where
such a
wear resistant connection is deployed in a casing string within a well bore.
It is advantageous to adapt existing threaded connection geometries to provide
locations where wear resistant materials can be most economically and least
invasively
applied to the connection, i.e., without significantly altering the existing
connections with
respect to seal and structural performance, while providiing adequate
protection against
wear from rotation while drilling. In particular, preferably the wear
resistant material is
provided at the lower or leading end of the coupling (leading is defined with
respect to
the axial direction of travel while drilling), as the upset diameter change
from the pipe
DMSLegal\0323G110015G\1537204v3 20
CA 02432038 2003-06-12
body to the coupling occurring at this location tends to promote preferential
wear while
drilling with casing.
Threaded and coupled connections useful in the present irlvention can include
an
internally threaded coupling having an upper end, a lower end and generally
cylindrical
exterior surface, as typically provided for such couplings, where wear
resistant surface
treatment or coating material is disposed axisymmetrically on said external
surface over
one or more axial intervals to form one or more hardbands of diameter somewhat
greater than the diameter of the generally cylindrical exterior surface. Said
axial interval
length and coating thickness are chosen, based on appliication requirements,
to provide
sufficient volume of material to resist wearing through to the base metal.
VOlear resistant
surface treatment or coating material is axisymmetrically distributed to
accommodate
the random distribution of bend angle and hence circurrrferential location of
connection
contact with the well bore.
For most of these geometries, wear resistance can be provided by applying
coatings
resistant to abrasive wear to the exterior surface of the connection. Such
coatings are
commonly referred to as hardfacing. These coatings are applied using a variety
of
techniques and materials, but typically the bond chemistry and mechanics
require heat
input to obtain the elevated temperature required to create a strong bond
between the
coating and metal substrate. It is therefore necessary to consider the effects
of this heat
input and bond chemistry on the metal substrate, and in particular to allow
for any
changes in structural or mechanical performance the heat input and bond
chemistry
might have.
In addition, the choice of axial interval location where wear resistant
surface treatment
or coating is provided is preferably selected to occur at locations where
stresses
induced by structural and pressure loads are lowest. Such choice of location
reduces
the risk of connection failure due to crack initiation within the typically
brittle coating
material.
However such a suitable region of low stress is often not available for many
of the
threaded and coupled connection geometries employed by industry. A suitable
region
of low stress at one or both ends of the coupling can be provided by a
coupling having
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its length and interval of internal threading arranged so that the end
hardband interval
does not overlap with the internal threaded interval of the coupling.
Otherwise stated,
relative to the 'standard' non-wear resistant coupling geometry a coupling is
provided
where at least one end and preferably the lower end is modified to provide a
generally
cylindrical extension which extension or extensions having external and
internal
surfaces without load bearing threads on which said exirernal surface or
surfaces wear
resistant surface treatment or coating material such as hardfacing is applied
to create a
hardband or hardbands of upset diameter. Where only one hardband is required,
the
lower end is preferred as this end forms the leading edge of the coupling
while drilling
with casing and protects this region from preferential wear.
Application of these teachings for placement of wear resistant surface
treatment or
coating material on the couplings of threaded and coupled connections may be
extended to integral connections and externally upset integral connections. As
commonly understood in the industry, an integral connection is comprised of an
externally threaded pin formed on the end of one tubular screwed into a mating
internally threaded box formed on the end of a second tubular. Said internally
threaded
box having an external largely cylindrical surface and proximal end.
Particularly where
the connection design is arranged to shoulder on said proximal end when made
up to
the pin, the stress state in this region is less prone to crack initiation and
propagation. A
wear resistant integral connection can be used that has a hardband of wear
resistant
surface treatment or coating material disposed on its proxirnal end. Relative
to the
'standard' non-wear resistant geometry of an integral connection box it is
more
preferable if the proximal end of the box is modified to provide a generally
cylindrical
extension which extension having external and internal surfaces without load
bearing
threads on which said external surface wear resistant: surface treatment or
coating
material such as hardfacing is applied axisymmetrically to create a hardband
or
hardbands of upset diameter.
Where the integral connection is formed on externally upset tubulars, such
externally
upset interval typically extends beyond the depth required to carry the box or
pin
threaded connection geometry, and in certain applications it may be preferable
to
provide a hardband on the connection exterior surface at or near the leading
end of the
DMSI,egal\032361\00156\1537204x3 22
CA 02432038 2003-06-12
upset interval either separately or in combination with a hardband placed at
the proximal
end of the box. The leading end of the upset interval, thus carrying the
hardband,
occurs at a location of significantly greater thickness than the pipe body and
therefore of
significantly reduced stress, but having the further advantage of being
positioned at the
location of preferential wear. Therefore, a wear resistance externally upset
tubular
connection can be provided having an externally upset interval with leading
and trailing
ends comprising the connection, and having at least once hardband positioned
on said
leading end.
Referring to Figures 4 and 5, an assembled threaded and coupled wear resistant
connection 1 is shown that is useful in one embodiment of the invention
including a
lower joint 2 with threaded ends 5a, 5b, an internally threaded coupling 3 and
an upper
joint 4 with threaded ends 7a, 7b. As commonly understood in the industry, the
connection is assembled or 'made up' by screwing the externally threaded mill
end pin
5a of lower joint 2, into the mill end box 6 of coupling 3 and screwing the
field end pin 7b
of upper joint 4 into the field end box 8 of coupling 3 to form a. sealing
structural union.
The generally cylindrical coupling 3 includes an upper end 0, a lower end 10
and a
hardband 13 formed from application of hardfacinc~ axisymmetrically about the
circumference of the coupling on the exterior surface 111 adjacent lower end
10. In the
illustrated embodiment, the hardfacing is applied in a substantially uniform
thickness to
form the hardband.
The main body of coupling 3 is arranged to generally match tPre thread-form
geometry,
tolerancing and length of an API specified buttress connection, where the
lower end 10
is formed as a generally cylindrical extension of the main body. The extension
extends
out beyond the threads 6a of the mill box end a sufficient length to carry the
hardband
13 such that the hardband does not radially overlap the threaded interval of
the mill end
box 6.
The outer diameter of the coupling at hardband 13 is preferably selected to be
greater
than the diameter of the coupling outer surface 11 to such that the hardband
preferentially contacts the borehole wall when connection 1 is employed in a
casing
string. However, when selecting the outer diameter of the hardband, care
should be
DMSLegal\032361\00156\1537204v3 23
CA 02432038 2003-06-12
taken, with consideration as to the borehole diameter in which the coupling is
to be used
to reduce adverse effects on annular flow.
A multi-lobe shoulder ring 15 is disposed in the coupling center region,
between the mill
and field end pins 5a, 7b. Under application of sufficient torque the mill and
field end
pins 5a, 7b are caused to abut ring 15 to thus increase torque capacity in
support of
drilling with casing as described in Canadian Patent Application 2,311,156.
The illustrated embodiment of Figures 4 and 5, thus shows a wear resistant
connection
where the manufacturing of the pin and box thread-forms is compatible with
existing
industry practice with respect to geometry, tolerance and make-up practice.
However, it
is to be understood that other connections can be used.
The bend angle and direction formed across the assembled Connection 1 depends
on
the cumulative effect of the thread axis angle misalignments and the relative
direction of
the misalignments for the pins 5a, 7b and boxes 6, 8 afts~r make-up. With
reference now
to Figure 6, the bend angle a is defined as the angle change between a first
line 2a
extending though the center points Sax, 5bx at the ends of the lower joint 2
and a
second line extending though the center points lax, 7bx at the ends of the
upper joint 4
in the connection. The bend angle or connection straightness is dependant on
variables
generally controlled by specifications known to the industry such as: pipe
straightness,
pin geometry parameters such as imperfect thread limits for buttress threads,
coupling
thread angular misalignment and make-up position. Prevalent industry practice
for
control of these variables results in randomly controlled casing connection
bend
magnitudes, where a significant number of connection bend angles are greater
than
allowed by comparable drill pipe specifications. Therefore, when a plurality
of such
connections are employed to form a tubular casing string placed in a bore
hole, joint to
joint local directional variations interfering with the borehole confinement
are likely. As
noted hereinbefore, this interference is frequently great enough to cause
large radial or
lateral reaction loads between the connection outside bend surface 16 and the
confining
borehole wall and, thus, there is a need to protect the connections against
excess rates
of wear under conditions of extended rotation, such as in drilling with said
tubular casing
string.
DMSLegal\032361 \001 SG11537204v3 24
CA 02432038 2003-06-12
While the wear resistant connections shown in Figures ~ and 5 are useful for
applications where the bend angle a is allowed to vary randomly in accordance
with
typical industry practice for manufacture and assembly of threaded and coupled
casing
connections, in certain applications it is desirable to control the magnitude
of said lateral
reaction force in at least one interval of an assembled casing string, which
lateral
reaction force is dependent on several design variables including: casing
flexural
stiffness, spacing between contacting bent connections, axial load, relative
radial
orientation of connection bends and radial interference of local bent section
as
controlled by the magnitude of the connection bend angle a.
To control of lateral load arising in an interval of a casing string, it is
useful to control the
bend angle geometry and spacing along that string interval. This can be done
by
surveying couplings and casing joints to determine the bend angle magnitude at
a
connection of selected ones of the couplings and casing joints and selecting
the
couplings and casing joints to be used in the string interval.
Referring now to Figure 7, in an alternate embodiment of the present invention
a bent
wear resistant connection 101 can be used where the center axis 6x of the mill
end box
6 and the center axis 8x of the field end box 8 are offset out of alignment to
form a bent
coupling 103 having an angle (3 between axes 6x and 8x. A wear pad 113 is
positioned
on the outer surface of the coupling about the circumferential location
defined by the
outside bend 16 of bent coupling 103. Coupling 103 accommodates a shoulder
ring
115 which substantially conforms to the bend of the coupling. In particular,
shoulder
ring 115 includes end faces 115a, 115b defining planes that are not parallel,
such that
the width of the ring varies from a narrow wall 115c to a long wall 115d. The
ring is set
within the coupling bore having its long wall 115d positioned radially
inwardly of outside
bend 16 of the bent coupling 103. The planes of end faces 115x, i 15b there
between
define an angle selected to be similar to that of angle ~.
In use, the bent coupling can be employed to achieve further control of said
lateral force
arising from confinement within a borehole, by selecting the frequency of bent
connections and, thereby the spacing there between, and by controlling the
relative
orientation of outside bend position 16 between sequential bent couplings
employed to
DMSIxga1\032361\00156\1537204v3 25
CA 02432038 2003-06-12
connect a plurality of tubular joints forming an interval in a casing string.
To conveniently
select the bend orientation of the connection during make up of a string,
means, such
as a power tong, can be used to apply torque to the coupling for control of
mill end
make-up position. Final mill end make-up position may then be selected to
align the
outside bend position of sequential connections at, for example, positions
180° apart or
other similar pattern as required.
In a further embodiment, the casing joint pin ends used can have the
misalignment
tolerance of their thread axes reduced from typical industry practice to
further improve
control of their bend angle.
It will be apparent that many other changes may be made to the illustrative
embodiments, while falling within the scope of the invention and it is
intended that all
such changes be covered by the claims appended hereto.
DMSI,egal\032361\0015611537204v3 26
