Note: Descriptions are shown in the official language in which they were submitted.
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MILLING SYSTEM FOR FORMING A WINDOW IN THE WALL OF A
TUBULAR
The present inven.tion relates to a system for milling
a window in a casing of a borehole formed in an earth
formation. In the technology of producing hydrocarbon
fluid from a wellbore it has become practice to drill
branch boreholes via corresponding openings formed in the
casing of the main wellbore, such openings being
generally referred to as windows. Conventionally a window
is created by fixedly positioning a whipstock having a
slanted guide-way, within the casing and guiding a
milling tool along the guide-way during milling of the
window. The downhole force required to progress the
milling tool along the guide-way is supplied from surface
via a pipe string connected to the milling tool. Fluid is
pumped through the pipe string to operate the milling
tool and to remove the milling cuttings from the
borehole.
A problem of the conventional system for milling a
window in a casing is that the dowrihole force required to
progress the milling tool along the guide-way is
insufficiently controllable. This is, for example, due to
frictional forces which counter-act the downward force
acting on the pipe string, and due to uncontrolled
bending of the pipe string within the casing.
It is an object of the invention to provide an
improved milling system which overcomes the problems of
the conventional system.
In accordance with the invention there is provided a
system for milling a window in a casing of a borehole
formed in an earth formation, the system comprising a
milling tool for milling said window, a guiding device
for guiding the milling tool along a selected path within
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the casing during milling said window, the guiding device
having means for fixedly arranging the guiding device within
the casing, wherein the guiding device is provided with
thrust means for thrusting the milling tool along the
selected path during milling the window, the milling tool
including a motor arranged to drive a mill of the milling
tool, characterised in that the thrust means is arranged to
thrust the milling tool inclusive of the motor along said
selected path.
The thrust force necessary for progressing the
milling tool during milling is provided from the guiding
device rather than from surface. For example, the guiding
device can be provided with a hydraulic motor or an electric
motor receiving power from surface via a suitable control
system. With the system of the invention it is achieved
that the problems of the conventional system are eliminated
as it is no longer required to provide the downward force
via an elongate element (such as a pipe string) extending
from surface through the casing.
It is to be understood that in the context of the
present invention the term "casing" refers to any elongate
hollow element which is arranged in the borehole and in
which a window is to be milled, such as a cemented casing or
a wellbore liner.
Suitably the guiding device is provided with a
guide-way defining said selected path and with means for
preventing movement of the milling tool transverse to the
guide-way. In this manner it is achieved that undesired
"walk" of the mill during milling is prevented and that an
accurate shape of the window is obtained.
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In a preferred embodiment of the system, the
selected path extends substantially parallel to the
longitudinal axis of the casing. In this manner another
problem of the conventional system, namely that the shape of
the window deviates from the planned circular or oval shape,
is solved. For example, in the conventional system whereby
the milling tool is arranged at an inclination angle
relative to the longitudinal axis of the casing, the
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upper window portion is milled by the upper side of the
mill and the lower window portion by the lower side of
the mill. As a result the upper window portion will be
biased in one direction (depending on the direction of
rotation of the mill) and the lower window portion in the
other direction. By selecting the path of the milling
tool to be substantially in longitudinal direction of the
casing it is achieved that only one side of the mill
contacts the casing during the entire milling procedure.
The invention will now be described in more detail
and by way of example with reference to the accompanying
drawing in which:
Fig. 1 schematically shows an embodiment of the
system according to the invention.
A borehole 1 has been formed in an earth formation 2
and provided with a steel casing 3. The embodiment 4 of
the system according to the invention includes a
retrievable downhole packer 5 fixedly positioned at a
selected location in the casing 3 and provided with an
upwardly extending latching element. 7 adapted to receive
and fixedly hold a support column 9 of a guiding
device 11 in a selected orientation. The guiding
device 11 includes a ramp element 14 rigidly connected to
the support column 9 and provided with a secondary
support element 16 which is radially extendible against
the inner surface of the casing 3. The ramp element 14
extends inclined relative to the longitudinal axis of the
casing 3 and defines inclined, opposite, directions 20,
22 of a path along which a milling tool 18 is movable,
direction 20 being inclined downwardly and direction 22
being inclined upwardly. The ramp element 14 is
internally provided with a thrust means in the form of a
chain (no: shown) movable in the directions 20, 22 and an
electric motor (not shown) for driving the chain. The
milling tcol 18 has legs 24, 26 which are connected to
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the chain 14 so that movement of the chain in
directions 20, 22 results in corresponding movement of
the milling tool 18. The milling tool is internally
provided with a hydraulic motor (not shown) for driving a
mill 27 of the milling tool, the hydraulic motor
receiving hydraulic fluid via a coiled tubing 30
extending through the casing 3 to a hydraulic pump (not
shown) at surface, the coiled tubing being connected to
the hydraulic motor by means of a flexible hose 31. The
mill 27 has a plurality of outlet nozzles (not shown) for
hydraulic fluid from the motor. The coiled tubing is
provided with a connector (not shown) for connecting the
coiled tubing to the guiding device 11 so as to lower and
retrieve the guiding device 11 and milling tool 18
through the wellbore 1.
An electric power supply cable 32 extends from a
control unit (not shown) at surface, through the coiled
tubing 30 and the hose 31, to the milling tool 18 and
from there via one of the legs 24, 26 to the electric
motor of the ramp element 14. The control unit is
provided with controls for selectively moving the chain
in direction 20 or 22. Furthermore, the control unit
includes a control for selectively extending or
retracting the secondary support element 16 relative to
the inner surface of the casing 3.
During normal use the packer 5 with the latching
element 7 connected thereto is fixedly positioned at a
selected location in the casing 3 at an orientation
corresponding to the desired orientation of the window to
be milled. The guiding device 11 is connected to the
coLled tubing 30 by means of the connector and the
cor.:bined auiding device 11/milling tool 18 are then
lowered bv coiled tubing 30 through the casing 3 until
the support column 9 latches into latching element 7.
Subsequently the control unit at surface is operated to
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provide a control signal via power supply cable 32 to the
guiding device so as to radially extended support
element 16 against the casing 3.
Hydraulic fluid is then pumped through the coiled
5 tubing 30 thereby driving the hydraulic motor so as to
rotate the mill 27. Simultaneously the control unit at
surface is operated so as to move the chain and the
milling tool 18 in the direction 20 in order to mill the
window. The cuttings resulting from the milling operation
are entrained and transported away by the hydraulic fluid
exiting through the nozzles of the mill 27. After milling
of the window has been completed, the control unit is
operated to move the milling tool 18 in direction 22 away
from the casing 3 and to retract the support element 16.
The combined guiding device 11/milling tool 18 are then
retrieved to surface by means of the coiled tubing 30.
The packer 5 and latching element 7 are subsequently
retrieved to surface in any suitable manner.
Instead of applying the milling tool and the guiding
device as separate units, these devices can be integrated
into one tool, optionally also including the packer and
latching element.
In an alternative embodiment of the system according
to the invention the thrust means is one of a worm-gear
having a spindle extending in the direction along which
the milling tool is guided and a hydraulic piston/
cylinder assembly extendible in said direction.
Furthermore, the milling device can be provided with
measuring means for measuring parameters such as the
torque delivered by the output shaft of the milling tool,
the thrust force delivered by the thrust means, and the
position of the milling tool along said path. Suitably
the measuring means is in communication with a data
receiving station at surface via a data transmission
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cable extending through a tubing, e.g. the coiled tubing
referred to above.
