Wells – Processes – Perforating – weakening – bending or separating pipe at an...
Reexamination Certificate
1999-10-25
2002-04-23
Dang, Hoang (Department: 3672)
Wells
Processes
Perforating, weakening, bending or separating pipe at an...
C166S055000, C175S320000, C175S327000
Reexamination Certificate
active
06374916
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to oil field tools. More specifically, the invention relates to a system for and a method of using cutting tools disposed in wellbores.
2. Background of the Related Art
Historically, oil field wells are drilled as a vertical shaft to a subterranean producing zone forming a wellbore, the wellbore is lined with a steel tubular casing, and the casing is perforated to allow production fluid to flow into the casing and up to the surface of the well. In recent years, oil field technology has increasingly used sidetracking or directional drilling to further exploit the resources of productive regions. In sidetracking, a slot or “window” is cut in a steel cased wellbore typically using a mill and drilling is continued at angles to the vertical wellbore. In directional drilling, a wellbore is cut in strata at an angle to the vertical shaft typically using a drill bit. The mill and the drill bit are rotary cutting tools having cutting blades or surfaces typically disposed about the tool periphery and in some models on the tool end.
FIG. 1
is a schematic cross sectional view of a typical vertical wellbore
10
. A casing
12
is disposed in the wellbore with a cutting tool
14
, such as a mill, having cut a portion of a window
16
in a sidewall of the casing. The cutting tool
14
can be coupled to tubing
32
, such as coiled tubing or a drill string, by a motor
19
having a shaft
18
that rotates the cutting tool. In such instance, the shaft
18
is known as an output shaft. Alternatively, the cutting tool can be coupled to a shaft
18
that forms a portion of a drill string that is attached to a surface rig. A motor disposed on the surface rig rotates the drill string which rotates the cutting tool and cuts the casing or other downhole components.
To direct the cutting tool
14
toward the side of the casing
12
, a whipstock
22
is inserted into the wellbore. The whipstock
22
is used to direct the cutting tool or other tool in a direction that is angularly offset to the original wellbore by using a whipstock face
24
, that is, a sloped surface which progressively narrows the open cross sectional area in the casing
12
. The whipstock
22
is set in position in the casing at a given depth and the cutting tool
14
engages the whipstock face
24
as the cutting tool traverses downward. The cutting tool
14
is progressively deflected laterally toward the casing
12
as the cutting tool cuts the window
16
. After the window
16
is cut and the cutting tool is removed, the whipstock
22
can remain in position to guide subsequent operations, such as directional drilling with drill bits.
FIG. 2
is a schematic cross sectional view of a cutting tool
14
coupled to the motor
19
at joint
26
. The motor
19
includes an output end
46
and a shaft
18
, where the motor transmits torque to the cutting tool
14
through the shaft
18
. The cutting tool
14
is coupled to an end
34
of the shaft
18
at an engagement section
36
internally disposed in a bore
39
of the cutting tool
14
. The shaft
18
has threads
35
which engage corresponding threads
37
on the cutting tool
14
. A portion of the end
34
of the shaft
18
is surrounded by a peripheral wall
40
of the cutting tool disposed upstream from the engagement section
36
and defines an annular space
42
, known as a box relief. The shaft
18
has a hexagonal shaft portion
52
, which provides engagement surfaces for a wrench (not shown). By convention, an end
44
of the peripheral wall
40
is typically aligned with the downstream end
53
of the shaft portion
52
, leaving exposed a portion of the shaft
18
. A passageway
48
is formed in the shaft
18
and the cutting tool
14
, where the passageway allows fluid to flow through the shaft and the cutting tool and then to exit through nozzles
50
in the cutting tool for washing away the debris as the cutting tool is rotated.
FIG. 3
is a schematic cross sectional view through the shaft, showing the peripheral wall
40
of the cutting tool surrounding the shaft
18
and the shaft portion
52
. The peripheral wall
40
disposed about the perimeter of the shaft
18
defines the annular space
42
.
One challenge with cutting a window with a mill or drilling an angled wellbore with a drill bit is the stress imparted to the cutting tool
14
and the shaft
18
. The stress imparted from cutting the side of the casing
12
for a mill or the strata for a drill bit is not evenly displaced about a circumference of the rotating components. For instance, as best seen in
FIG. 1
, at joint
26
defining the connection between the shaft and the cutting tool, a first portion
28
of the joint
26
on the side of the cutting tool that cuts the window is placed under a longitudinal compressive load, but the portion
30
of the joint
26
that is opposite the window
16
is placed under a longitudinal tensile load. As the joint rotates, each portion
28
,
30
is subjected to alternating compressive and tensile stresses. Additionally, the stresses on the joint
26
are proportional to the distance between the cutting surfaces of the cutting tool and the joint. A longer distance proportionally increases the stresses. The alternating stresses, especially using long cutting tools, create cyclical bending of the members, such as the cutting tool
14
and the shaft
18
, and can produce stress fatigue and failure of one or more of the members. It is believed that at least a portion of the failures are due to stress concentrations in a stress failure region
41
near an upstream end of the threads
35
on the shaft
18
. The downtime can be costly for retrieving a broken shaft
18
that involves fishing the parted assembly from the wellbore, replacing the shaft and reinserting the assembly down the wellbore.
There remains a need for an improved system and method for using a cutting tool at an angle in a wellbore, particularly for stiffening a cutting tool system to avoid the cyclical bending.
SUMMARY OF THE INVENTION
The present invention generally provides a system and method for stiffening a cutting tool assembly used in cutting laterally relative to a wellbore axis to reduce stresses and cyclical bending of the cutting tool assembly during cutting. The system includes a cutting tool attached to a shaft such as an output shaft of a motor or a drill string. A sleeve is disposed in an annular space, known as a box relief, defined between the shaft and a peripheral wall of the cutting tool. The sleeve is preferably fixed in the annular space by a sleeve ring surrounding a recess in the shaft, but can be coupled to the peripheral wall and/or shaft by, for example, a threaded engagement. As the cutting tool attempts to bend at a connection with the shaft during cutting, creating stresses at the connection, the stresses are distributed throughout the increased contact area of the sleeve with the cutting tool, causing less stress per unit area and distributing at least a portion of the stress away from the threaded engagement between the shaft and the cutting tool. The reduced stresses cause less fatigue and thus lower failure rate of the members. Also, the walls of the cutting tool surrounding the shaft are lengthened to engage even more surface area of the sleeve and further reduce the bending stresses. Also, the distance between the cutting portion of the cutting tool and an engagement portion between the cutting tool and the shaft can be shortened to reduce stresses on the engagement portion by forming a shorter cutting tool.
In one aspect, the invention provides a window milling system, comprising a shaft, a mill coupled to the shaft having walls at least partially surrounding a portion of the shaft. defining an annular space between the walls and the shaft, and a sleeve disposed in the annular space. The sleeve is preferably a split sleeve that fits snugly in the annular space. In another aspect, the invention provides a method of cutting a casing with a window mill system, comprising engaging a shaft wi
Dang Hoang
Moser, Patterson & Sherdian, L.L.P.
Weatherford / Lamb, Inc.
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