ELARGISSEUR

UNDERREAMER

Description

CA 02366134 2001-12-21
UNDERREAMER
Summary of the invention
When drilling a borehole through an earthen formation a pilot hole is drilled
by a pilot bit and the
hole can be enlarged by a underreamer. Underreamers have arms with cutters
thereon that cut
into the formation to enlarge the borehole to its intended guage.
An under reamer has been invented having at least one lower underreamer arm
and at least one
upper underreamer arm. The lower arm is designated as the one closest to the
lower end of the
tool, which is the end that will be positioned closest to the pilot bit and
the upper underreamer
arm is positioned further than the lower underreamer arm from the lower end of
the underreamer
and, in other words, closer to surface when the tool is positioned in the
borehole.
Underreamers are useful in casing drilling, wherein the pilot bit drills the
pilot hole into the
formation and underreamers enlarge the hole to permit advancement of the
casing.
Underreamers are also useful when extending a borehole below installed casing.
In such
embodiments, the arms are collapsible to permit the underreamer to be moved
through the bore
of the casing and are expandable downhole to permit drilling of a borehole to
a guage greater
than the outer diameter of the casing. Thus, one embodiment there is provided
an underreamer
comprising a housing including an upper end and a lower end, the lower end
being connectable
to a pilot bit-containing assembly; an upper underreamer arm carried by the
housing and being
moveable between an expanded position extending out from the housing to define
an upper
expanded diameter and a retracted position, wherein the upper arms define an
upper arm
retracted diameter less than the upper expanded diameter; a lower underreamer
arm carried by
the housing and being moveable between an expanded position extending out from
the housing
and defining a lower expanded diameter and a retracted position, wherein the
arms define a
lower arm retracted diameter which is less than the expanded effective
diameter, the lower
underreamer arm being positioned between the upper underreamer arm and the
lower end of the
housing; an actuator for driving the upper underreamer arm between the
retracted position and
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the expanded position; and an actuator for driving the lower underreamer arm
between the
retracted position and the expanded position.
The actuator for driving the upper underreamer arm and the actuator for
driving the lower
underreamer arm can be the same or separate mechanisms. They can be any
desired mechanism
including a fluid pressure or spring biased arrangement which acts against an
inner surface of the
arm, an arrangement of a pivotal connection on the arm and a drive rack and
pinion mechanism
or a cam mechanism.
There can be one or more lower arms and one or more upper arms. For example,
there can be a
plurality of lower arms spaced about the housing positioned substantially in a
common plane. As
another example, there can be a plurality of upper arms spaced about the
housing positioned
substantially in a plane.
In addition to the upper anm and the lower arm, there can be further
intermediate arms positioned
between the upper arm and the lower arm. Where the underreamer must be sized
to move
through the casing inner bore, the intermediate arms should be retractable.
The upper and lower arms when in their retracted positions can be in various
configurations such
as, for example, recessed into slots, indentations, etc. in the housing or
folded against the
housing. The retracted diameters of the upper arm and the lower arm can be the
same or
different, but each is less than the minimum drift diameter for the particular
application in which
the underreamer is to be used, such as the drift diameter for casing string
used in a drilling
operation.
The upper and lower arms can be axially aligned along the housing or offset
from each other.
The expanded diameters of the upper arm and the lower arm can be the same or
different, as
desired. In one embodiment, the lower arm expanded diameter is less than the
upper arm
expanded diameter. In another embodiment, the lower arm expanded diameter is
substantially
equal to the upper arm expanded diameter. It will be appreciated that the
actual expanded
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CA 02366134 2001-12-21
diameter of an underreamer arm can change with time due to wear on the cutting
surface or in
the connection to the housing. However, reference to the expanded diameter in
the application is
intended in reference to a underreamer wherein no wear has occurred.
The upper underreamer arm and the lower underreamer arm can vary in function.
In particular,
the lower underreamer arm generally performs the primary hole enlarging
function and,
therefore, includes cutters suitable for drilling into the formation, such as
those containing
diamonds, as for example polycrystalline diamond compact (PDC) material, to
enlarge the pilot
hole to the expanded diameter of lower arm.
The upper underreamer arm can also include cutters suitable for drilling into
the formation. In
such an embodiment, the upper underreamer arm can have an expanded diameter
substantially
similar to the lower arm expanded diameter, or, alternately, the upper arm
expanded diameter
can be greater than the lower arm expanded diameter and the upper arm acts to
further enlarge
the borehole to a desired guage.
In another embodiment, the upper underreamer arm does not include cutters and
can act to clean
the hole, stabilize the drill string behind the lower underreamer arm and/or
provide an indication
that the borehole is under guage, for example due to wear of the lower
underreamer arm, as
determined by an increase in torque on the drilling assembly. To enhance the
use of the upper
underreamer arm, surface hardening can be used at least on the surface that
can come into
contact with the formation. Surface hardening includes hard facing or
application of fine or
coarse abrasion resistant materials such as tungsten/carbide coatings. It will
be appreciated that
while surface hardened components can be used to cut into a formation in a
limited way, such
components would not be useful for drilling into a formation. Surface
hardening can be used
alone or in addition to cutters.
Brief Description of the Drawings
Figure 1 is a schematic sectional view through a portion of a borehole and
disposed therein a
drill string for use in drilling with casing.
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Figure 2 is schematic view of another underreamer.
Figure 3 is an isometric view of another underreamer.
Figure 4 is a quarter section, isometric view of the underreamer of Figure 3.
Figure 5 is a section along line V-V of Figure 3.
Figure 6 is a section as in Figure 5 with the underreamer arms in the
retracted position.
Figure 7 is a section along line B-B of Figure 6.
Figure 8 is an isometric view of an underreamer arm useful in an underreamer.
Detailed Description of the Invention
Referring to Figure l, a schematic section through a borehole is shown. The
borehole is
extended by a drilling assembly disposed therein. The drilling assembly is
engaged at the lower
end of a string of casing, which is acting as the drill string. The drilling
assembly includes a lock
housing including lock means (not shown) for releasably securing the drilling
assembly to the
casing string, a pilot bit, which is drilling a pilot hole and an underreamer
disposed therebetween.
A drilling assembly can alternately include many other components, such as
bent subs, MWD
devices, etc. However, those devices are not included in the illustrated
assembly for simplicity.
The drilling assembly is rotated either by rotation of the casing string from
surface, as would be
the case in the illustrated embodiment, or by inclusion of a mud motor in the
drilling assembly.
The underreamer includes a housing and is connected into the drilling assembly
through an upper
end and a lower end. The lower end is positioned adjacent the pilot bit. The
underreamer
includes a pair of upper underreamer arms and a pair of lower underreamer
arms. While in the
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illustrated embodiment each of the sets of upper and; lower underreamer arms
includes a pair of
arms, it is to be understood that other numbers, for example, one or more arms
can be used in
each position.
The lower pair of arms and the upper pair of arms spaced longitudinally along
the housing and,
in particular, the lower underreamer arms are positioned adjacent the lower
end of the
underreamer and between the lower end and the upper arms. In the expanded
position, as shown,
the lower arms act to enlarge the pilot hole to borehole guage. The expanded
diameter of the
upper arms Du is substantially equal to the expanded diameter Dl of the lower
arms. The upper
arms in the illustrated embodiment include cutters thereon and when expanded,
as shown, clean
out the hole behind the lower arms, provide a stabilization effect close to
the primary hole
enlarging underreamer arms and act as a backup to the lower underreamer arms
should they
begin to wear and drill under guage. The upper. arms also may provide an
indication that the
lower arms are operating under guage by observing the drill string torque at
surface.
The upper arms and the lower arms are axially aligned along the housing. When
it is desired to
pull the drilling assembly uphole for repair or replacement of any of the
components or in
preparation for completion of the borehole, the underreamer arms can be
retracted against the
housing to permit the drilling assembly to fit through the casing inner bore.
An actuator, cannot
be seen, within the housing is operated to move the underreamer arms between
their expanded
positions and their retracted positions. The actuator can take various forms,
but is preferably
actuated remotely such as, for example, by a change in fluid pressure, an
acoustic pulse or by
engagement against the casing shoe.
Referring to Figure 2, an underreamer is shown in a schematic form having a
housing, an upper
end, a lower end, and four pairs of underreamer arms (all shown in their
expanded position). The
underreamer arms include, from the lower most arms to the upper most arms, a
lower pair of
cutting arms, a lower pair of stabilizer arms, an upper pair of cutter arms
and an upper pair of
stabilizer arms. Each pair of underreamer arms operates in a single plane and
the plane of each
of the pairs is spaced longitudinally along the underreamer from the planes of
its adjacent pairs.
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Each of the cutting arms includes cutters selected to drill through the
formation being penetrated
by the borehole. The cutters can be, for example, formed of PDC. Each of the
stabilizing arms
are coated at their outer end with hard facing formed of a matrix containing
carbide/tungsten
particles. This coating prevents erosion and hardens the surface to decrease
the rate of material
degradation by engagement against the rock of the formation.
Lower cutting arms, when expanded, define a diameter Dlca, to which a pilot
hole would be
enlarged, when the lower cutting arms are drilled therethrough. The upper
cutting arms are
formed to have a diameter Duca, when expanded, that is greater than diameter
Dlca. Thus, when
the illustrated underreamer is operated behind a pilot bit, the hole will be
enlarged to a diameter
Dlca and then increased to the diameter Duca by the upper cutting arms. Duca
is generally the
selected final guage for the borehole. Thus the hole will be enlarged in two
stages and the
operation will be divided between two sets of cutters. This can extend the
useful life of the
underreamer cutters, can result in torque reduction and can ease steering in a
directional drilling
operation.
The lower stabilizing arms are formed to have substantially the same expanded
diameter as
diameter Dlca and the upper stabilizer arms have an extended diameter
substantially equal to
Duca. As such, the stabilizer arms do not operate to enlarge the hole behind
their respective
cutting arms, but bear at their guage contact face against the borehole wall
to stabilize the
underreamer adjacent the cutting arms.
Refernng to Figures 3 to 7, there is shown another underreamer. Some
underreamer arms are
illustrated in Figures 8 that are useful in the underreamer of Figures 3 to 7.
The underreamer includes a housing having an outer surface, an inner bore, a
threaded lower end
for connection directly or indirectly to a pilot bit and a threaded upper end
for connection
directly or indirectly to the lock housing or to a drill string A pilot bit
stabilizer is formed on the
outer surface of the housing adjacent its lower end.
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The housing supports a plurality of underreamer arms including three lower
cutting arms and
three upper stabilizer arms. The underreamer arms are pivotally ~ connected in
slots in the
housing by pivot pins secured in holes by bolts. The slots for the upper
underreamer arms are
spaced about the circumference of the housing and the slots for the lower
underreamer arms are
spaced about the circumference of the housing. The lower underreamer arm slots
are spaced
along the long axis and radially offset from the slots for the upper
underreamer arms such that
the lower arms are positioned below and are oriented between the upper arms.
The arms are each actuated to move between an expanded position, shown in
Figures 3 and 4,
and a retracted position, shown in Figures 5 and 6. The actuator includes a
mandrel slid ably
disposed in the inner bore of the housing. The mandrel includes a plurality of
rack gears, which
each mesh with a pinion gear formed on the rear surface of one of the arms. As
will be
appreciated, longitudinal movement of the mandrel within the inner bore will
drive the arms to
rotate about their pivot pins, in unison. The mandrel is prevented from
rotating about its long
axis within the bore by a pin that engages in an elongate keyway. The pin and
the keyway are
correspondingly sized such that the mandrel can slide a selected distance
within the bore but
cannot rotate therein.
The mandrel is normally biased into a position that maintains the arms in a
retracted position in
their slots. Biasing is provided by a spring acting between a shoulder on the
mandrel and a
shoulder on the inner bore of the housing. The spring is positioned in a
spring chamber with
ports that provide communication with the ambient pressure about the
underreamer.
The mandrel is driven to move within the bore by increasing the fluid pressure
in the inner bore
of the housing. In particular, mandrel includes a piston face against which
fluid pressure can act
to compress the spring. When moved in this way, the rack gears are driven
relative to the arm
pinion gears to rotate the arms about their pivot pins. The mandrel includes a
bore for passage of
drilling fluid to the pilot bit. Seals are disposed between the mandrel and
the inner bore to seal
against passage of drilling fluid therebetween. If desired, fluid jet ports
can be provided adjacent
the arms to provide for cleaning of the slots. A choke jet can be provided to
regulate fluid flow
to the pilot bit and to provide a greater fluid pressure thereabove, about the
mandrel.
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The slots are sized relative to the arms to permit each arm to be retracted
such that its upper
surface is generally flush with the outer surface of the housing. The slots
also preferably provide
support for the arms when they are in the expanded position and in operation.
In particular, the
arms and slots are generally correspondingly shaped about their tops and
sides. The edges of the
slots can be built up, as desired, to increase the area of the support
surface.
Underreamer arms that are useful in the present invention can take various
forms. In one
embodiment, the arms include a rear pinion gear and an opening for insertion
therethrough of the
pivot pin. The distance from the connection point to the housing, in this case
the pivot pin
opening, to the guage contact face will affect the expanded diameter of the
underreamer arm.
Thus, it will be appreciated that an upper arm can have a greater distance
from the pivot pin
opening to the guage contact face.
In one embodiment, a shoulder is formed on the underreamer arm that acts to
increase the
thickness of the arm at its outer end. Thus, although a smaller thickness can
be used at the
connection end, the cutting or stabilizing face of the arm can be increased.
Preferably, the
shoulder is positioned along the arm so that it corresponds with the outer
edge of the slot in
which the arm is mounted. This provides that the shoulder can seat against the
outer edge of the
slot and transfer a portion of the force applied to the arm to the housing at
this point. This takes
some of the stress off of the pivot pin.
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