Reverse section milling method and apparatus

A method and apparatus for milling a section of casing in an upward direction, utilizing a downhole hydraulic thrusting mechanism for pulling a section mill upwardly. A downhole motor and torque anchor can be used to rotate the section mill, or the mill can be rotated by a work string. A stabilizer above the section mill can be used to stabilize the mill relative to the casing being milled. A spiral auger below the section mill can be used to move the cuttings downwardly.

Not Applicable

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention is in the field of methods and apparatus used to remove a window or section of piping from a casing pipe in an oil or gas well.

2. Background Art

Section milling of pipe, that is, removing a section of pipe installed down hole in an oil or gas well, by milling it away, has been known in the art for a long time. However, passing a section milling tool through a smaller diameter pipe in order to section mill a larger diameter pipe farther downhole has always been more difficult, and the known methods have not met with much success. Typically, the procedure has relied upon an attempt to mill the larger diameter pipe from above, proceeding in the downhole direction. In milling downwardly, the weight of the drill string, possibly including drill collars, is used to apply downward force to the mill to cause it to progress through the pipe being milled. This application of force to the mill by weight applied from above creates a wobble in the milling work string, which has a tendency to fracture the cutting inserts on the section mill blades. This, in turn, causes the mill to wear out sooner, resulting in the removal of less pipe footage before replacement of the mill is required. Further, since milling progresses downwardly, cuttings must be removed from the well bore as they are formed, to avoid forming a ball of cuttings around the mill and reducing its effectiveness. Specialized formulation of milling fluid, and maintenance of proper fluid flow rates, are required in order to circulate the cuttings out of the hole.

One example of a situation in which these section milling problems are important is in the resolution of a gas migration problem. Many oil and gas well producers are faced with the problem of wells that have gas migration between casing strings, and this gas may ultimately migrate back uphole to the wellhead system. This leakage could pose a serious problem in that the gas could be ignited, causing a well explosion. Consequently, in the interest of safety, such wells must be repaired. In doing so, it is generally considered necessary to provide a means of removing one or more inner strings of casing pipe, at a location downhole, and exposing an outer string of casing pipe for cementing, to seal off the gas migration path.

As an example, a 16 cased hole may have a 10 casing and a 7 casing inside, in a more or less coaxial arrangement. Gas migration may occur between the 10 casing and the 16 casing. Heretofore, the typical repair has been to pilot mill all the 7 and 10 casings completely away, from the top, down to a selected location downhole. A packer is then set against the 16 casing, and cement is installed on top of the packer. This is a time consuming and costly endeavor. Further, management of cuttings, cuttings disposal, and milling mud properties all have to be planned for in this program.

BRIEF SUMMARY OF THE INVENTION

The method and apparatus of the present invention provide a better solution to this problem, as described in the following. In a first embodiment, a section mill is used in combination with an up-thruster tool and a downhole motor. The apparatus is tripped into the hole to position the section mill at the lower end of the downhole interval where a window is to be cut. The section mill is at or near the bottom of the apparatus, with a stabilizer, an up-thruster, a mud motor, and an anti-torque anchor positioned above that, in order. A spiral auger with a left hand twist can be positioned below the section mill, to assist in moving the cuttings downhole.

The anti-torque anchor is set against the innermost casing, the mud motor is run, and an upward force is exerted on the section mill with the up-thruster. The casing is cut through, and a portion of the casing is milled out, as the mill progresses upwardly. When the up-thruster reaches its full travel, the apparatus is released and re-set at a higher location, with the mill positioned at the upper end of the milled opening, and with the up-thruster extended. The process is then repeated. After milling of the desired window, other operations through the window can take place, such as cementing.

In a second embodiment, the same type of section mill is used in combination with an up-thruster tool and a rotating work string. The difference between this and the first embodiment is that the mill is rotated by a rotating work string, rather than a downhole motor, and no anti-torque anchor is needed. Here again, a spiral auger with a left hand twist can be positioned below the section mill.

Use of this invention increases the life of the mill, resulting in the milling of more footage with each mill, reducing the number of trips of the work string, and reducing rig costs. In either embodiment, the work string is always in tension while milling. Cuttings can be left down hole, which eliminates the need for special mud and the need for handling and disposing of the cuttings. A relatively constant force is exerted on the cutting blades. Pump pressure is regulated to keep a regulated upward force on the cutter, by means of the up-thruster. Better centralization of the drilling string and the cutter are achieved, with less wobble. Especially in the mud motor embodiment, there is much less wobble in the work string than with downward milling. Where used, the anti-torque tool eliminates back torque and results in a stiffer milling assembly. Drill collars are not needed; smaller pipe and smaller rigs can be used. Coil tubing can even be used in the downhole motor embodiment.

The novel features of this invention, as well as the invention itself, will be best understood from the attached drawings, taken along with the following description, in which similar reference characters refer to similar parts, and in which:

DETAILED DESCRIPTION OF THE INVENTION

In a first embodiment of the apparatus 10 of the present invention, shown in FIG. 1 , a section mill 14 designed for upward milling, in combination with an up-thruster tool 16 , an anti-torque tool 24 , and a downhole motor 22 , are mounted to a work string 12 . The apparatus 10 is tripped into the hole to position the section mill 14 at the lower end of the interval where a window W is to be cut. For clarity, FIG. 1 actually shows the apparatus 10 after the inner casing C 1 has been cut through, and after the milling of the window W has begun. The section mill 14 is at the bottom of the apparatus 10 , with a stabilizer 18 , an up-thruster 16 , a mud motor 22 , and an anti-torque anchor 24 positioned above that, in order. A spiral auger 20 with a left hand twist can be positioned below the section mill 14 , to assist in moving the cuttings downhole, as shown by the lower arrows.

Torque anchor. A torque anchor 24 , better seen in FIG. 10 , is run above the up-thruster 16 , or lift cylinder, in the mud motor embodiment 10 . The upper end 100 of the torque anchor 24 is attached to the work string 12 , and the mud motor 22 is attached to the lower end 102 of the torque anchor 24 . The torque anchor 24 prevents the drill string 12 from overreacting to the torque generated by the mud motor 22 . Often, without the torque anchor 24 , the drillstring 12 would torque up and reduce in length as the motor 22 stalls, causing the milling tool blades to quickly degrade. The torque anchor 24 eliminates this condition. The torque anchor 24 is a downhole torque barrier, or anti-torque tool, which engages the wall of the borehole or casing C 1 in which it is positioned, with at least one gripping member 74 therein. The gripping member 74 is designed to prevent rotation of the torque barrier 24 relative to the borehole wall or casing wall. The gripping members 74 are preferably hydraulically displaced in a generally outward direction by a plurality of cylinders 78 , transverse to the longitudinal axis of the tool 24 , until they engage the wall of the borehole or casing. The cylinders 78 are pressurized by fluid from the fluid flow path 80 through the center of the tool. An outwardly facing surface 76 of at least one of the gripping members 74 has gripping contours designed to engage the borehole or casing wall and prevent rotational movement relative thereto, such as teeth, ridges, or ribs. The tool 24 can be actuated by increasing the pressure of fluid being pumped downhole through a fluid flow path 80 in the center of the tool, to displace the gripping members 74 outwardly until they engage the borehole wall or casing. Thereafter, the downhole motor 22 or other downhole rotating tool can be operated, with all of the reactive torque being absorbed by the anti-torque tool 24 . This isolates the downhole torque from the work string 12 .

The gripping members 74 can be configured to allow movement of the anti-torque tool 24 in either longitudinal direction, or only in the uphole direction, to prevent longitudinal movement of the torque anchor 24 during the upward advance of the section mill 14 . This can be done by implementing one or more wheels 82 , or other rolling devices, in the gripping member 74 , as shown in FIG. 11 . The rolling device 82 can include a mechanism such as a ratchet to allow longitudinal movement in only the uphole direction. Alternatively, the gripping members 74 can be configured to prevent any longitudinal movement of the torque barrier 24 relative to the borehole or casing wall, as well as preventing rotation of the torque barrier 24 relative thereto. A blade without wheels would be an example of such a longitudinally stationary gripping member 74 .

Up-thruster. The purpose of the up-thruster or lift cylinder 16 is to supply a constant upward load on the section mill 14 . If a mud motor 22 were used to drive the mill 14 without the up-thruster 16 , the loading imparted by the drilling operator, using the drilling rig to lift the mill 14 and cut into the casing C 1 , would be too erratic. The operator would have to be extremely careful not to overload the mill 14 , otherwise the mud motor 22 would stall out. In a preferred embodiment as shown in FIG. 3 , the up-thruster 16 is a hydraulic cylinder pressurized by the mud flow which is pumped through a fluid flow path in the anti-torque anchor 24 , the mud motor 22 , the up-thruster 16 , and on down through the section mill 14 . Drilling mud passes through the section mill 14 below the up-thruster 16 , as described below, through a flow restriction which creates a back pressure in the apparatus 10 . This back pressure is used to cause the up-thruster 16 to lift upwardly on the section mill 14 . With a lifting cylinder 16 in the apparatus 10 , the pump pressure can be controlled in such a fashion that loading on the mill 14 is very constant, and loading can be imparted with much more precision.

As shown in FIG. 3 , the up-thruster 16 is a tensioning device which is attached at its upper end 26 to the lower end of the mud motor 22 , and a stabilizer 18 can be attached to the lower end 28 of the up-thruster 16 . The up-thruster 16 can include an upper mandrel 30 , and an intermediate mandrel 32 , with a piston 34 therebetween. A lower mandrel 36 can be joined to the intermediate mandrel 32 by means of a mandrel cap 42 , with the lower mandrel 36 protruding in a sliding fashion from the lower end of the housing 46 . Initially, the lower mandrel 36 can be pinned to the housing 46 by a shear pin 44 , retaining the lower mandrel 36 in its fully extended position. It can be seen that this also results in the fully extended condition of the overall up-thruster 16 .

As shown in FIG. 4 , the piston 34 , along with the mandrels 30 , 32 , 36 , is slidingly mounted within the housing 46 , forming an annular hydraulic cylinder 51 between the piston 34 and the housing 46 . At least one fluid passage 38 conducts fluid from the fluid flow path 50 near the axis of the tool to the annular cylinder 51 , for the purpose of driving the piston 34 and the mandrels 30 , 32 , 36 upwardly. This can only occur after shearing of the shear pin 44 . When the piston 34 is driven upwardly, it can be seen that the lower end 28 of the up-thruster 16 is drawn upwardly toward the upper end 26 , and toward the work string 12 .

Section Mill. The primary design feature of the section mill 14 , better seen in FIG. 5 , is that the arms 54 are held in the open position by an upward moving wedge block 56 that supports the arms 54 and prevents them from collapsing under heavy loading. The upper end 92 of the section mill 14 is attached to the lower end of the up-thruster 16 , via a stabilizer 18 if desired. The section mill 14 used in the present invention has a plurality of pivotable arms 54 mounted in longitudinal slots in a tool body 52 . As seen in FIGS. 5 and 6 , the arms 54 pivot about pins 60 near the upper ends of the arms 54 . A piston 57 below the arms 54 , within the tool body 52 , is slidably disposed to move the wedge block 56 upwardly against the lower ends and inner sides of the pivotable arms 54 . A fluid flow passageway 90 for drilling fluid is provided through the tool body 52 and through the piston 57 , to a space 59 within the tool body 52 below the piston 57 . Application of fluid pressure to this space 59 below the piston 57 exerts an upward hydraulic force, moving the piston 57 and wedge block 56 upwardly against the arms 54 . This upward motion of the piston 57 exerts an upward and outward force against the lower ends of the arms 54 , thereby exerting a maximized outward force on the blades 58 on the outer surfaces of the arms 54 . Alternatively, the piston 57 and arm 54 can have an angled slot-and-pin mechanism (not shown) which exerts this upward and outward force. Further alternatively, the piston 57 can have a pin or roller (not shown) which engages the lower edge and the inner edge of the arm 54 at an angle.

The piston 57 can have a fluid inlet port through which the drilling fluid flows to reach the space 59 below the piston 57 . A ball or other closure member can be pumped downhole with the drilling fluid to close this fluid inlet port, resulting in the subsequent application of downward hydraulic pressure against the piston 57 , driving it downwardly. Alternatively, a spring can be arranged to drive the piston 57 downwardly, and the arms 54 inwardly, upon release of hydraulic pressure. Downward driving of the piston 57 can be used to retract the arms 54 and the blades 58 .

A fluid outlet port can be provided in the lower end of the tool body 52 , below the piston 57 . A nozzle 62 can be mounted in this port in the lower end 94 of the body 52 , as seen in FIGS. 5 and 7 . The nozzle 62 can be sized to create the desired backpressure in the drilling fluid system.

The section mill arm 54 can be fitted with a casing cutter type blade (not shown), for penetration of a casing, or the arm 54 can be fitted with the square type blades 58 typically found on a pilot mill, to provide for milling an extended length of casing. The section mill 14 can first be operated to penetrate the casing with the casing cutter type blade, then the arms 54 can be exchanged for arms 54 having the pilot mill type blades 58 , for the remainder of the procedure.

Stabilizer. An expandable stabilizer 18 is used to stabilize the mill 14 once it has passed through a smaller casing C 1 , such as the 7 casing, if milling of a larger casing C 2 , such as the 10 casing, is needed. Basically, the stabilizer 18 is identical to the section mill 14 , except that the arms 68 are dressed with hard facing material, to the size of the casing inner diameter. The arms 68 pivot about pins in the stabilizer housing 66 , when driven by a wedge block 70 . Extension and retraction of the arms 68 of the stabilizer 18 are shown in FIGS. 8 and 9 , respectively. When the stabilizer 18 is used, the upper end 96 of the stabilizer 18 can be attached to the lower end of the up-thruster 16 , and its lower end 98 can be attached to the upper end of the section mill 14 .

Spiral Auger. The spiral auger 20 is simply a short drill collar dressed with aggressive left hand spiraled ribs. The ribs tend to force or auger the cuttings to the bottom of the well, as shown by the arrows, moving them away from the cutter blades 58 , and preventing the cuttings from balling up around the mill 14 .

In a second embodiment of the apparatus 10 , the same type of section mill 14 , designed for upward milling, is used in combination with an up-thruster tool 16 and a rotating work string 12 . The apparatus 10 is tripped into the hole to position the section mill 14 at the lower end of the interval where a window W is to be cut. The section mill 14 is at or near the bottom of the apparatus 10 , with a stabilizer 18 and an up-thruster 16 positioned above that, in order. A spiral auger 20 with a left hand twist can be positioned below the section mill 14 , to assist in moving the cuttings downhole.

Method of Operation

The anti-torque anchor 24 is set against the innermost casing C 1 as the milling fluid pressure is increased, which also starts the mud motor 22 running and exerts an upward force on the section mill 14 with the up-thruster 16 . Fluid pressure extends the arms 54 and blades of the mill 14 , and the mill 14 is rotated by the downhole motor 22 . The torque anchor 24 , mud motor 22 , up-thruster 16 , stabilizer 18 , and section mill 14 can have the sizes and shapes of their fluid flow paths designed to initiate their respective operations at selected progressive pressure levels, to insure the desired sequence of activation of the various tools. The section mill 14 can be set to extend its arms 54 at a relatively low pressure, so that the arms 54 will extend before the up-thruster 16 begins to lift the arms 54 into cutting contact with the casing. Additionally, the motor 22 can be designed to bypass fluid before it begins to rotate. As a result, the cutter arms 54 extend, then the torque anchor blades 74 contact the casing wall, then the mud motor 22 begins to rotate, and finally, the up-thruster 16 begins to lift the section mill 14 . On the first cut, the casing is cut through, and then a portion of the 7 casing is milled out, until the up-thruster 16 reaches its full travel, or bottoms out . This opens the piston valves 40 , and a pressure drop will be noted in the milling fluid at this time.

Then, the milling fluid pressure is reduced, to stop rotation of the mud motor 22 , release the anti-torque tool 24 , retract the mill arms 54 , and allow the up-thruster 16 to extend to its original length. The work string 12 is then lifted to raise the section mill 14 until its arms 54 are positioned next to the milled lower end of the 7 casing, at the top of the window W. Pressure is then increased to extend the mill arms 54 , reset the anti-torque anchor 24 , rotate the mud motor 22 , apply upward pressure to the mill 14 , and resume milling. This process is then repeated as required. In this way, a window W of desired length, for example, 250 feet, is cut out of the 7 casing. Use of this method insures that the drill pipe is held in tension at all times, thereby eliminating wobble in the work string 12 . Pump pressure is regulated to keep a regulated upward force on the cutters 58 , by means of the up-thruster 16 . Cuttings can also be dropped down hole, since milling is moving in the upward direction, eliminating the necessity to circulate the cuttings out of the hole. The procedure is continued until milling of the desired section length is complete, or until new cutting blades are needed.

When the rotating work string is used, the anti-torque anchor 24 and mud motor 22 are not used, so rotation of the section mill 14 is accomplished by rotation of the work string and the other components. Otherwise, the procedure is essentially the same.

In the example given earlier, a suitable underreamer is then installed to remove the cement from the window W, out to the inside diameter of the 10 casing C 2 . A larger section mill 14 and anchor 24 can then be installed, and the process can be repeated to remove a shorter section, for example, 150 feet, of the 10 casing. The lower end of the 150 foot window in the 10 casing is preferably located at the lower end of the 250 foot window in the 7 casing. After removal of the cement in the 150 foot window, out to the inside diameter of the 16 casing, an inflatable packer (not shown) is set at the lowest point where the 16 casing has been exposed and cleaned of cement. Once set, the packer is then covered with approximately 100 feet of cement. This effectively stops the gas migration in the well.

While the particular invention as herein shown and disclosed in detail is fully capable of obtaining the objects and providing the advantages hereinbefore stated, it is to be understood that this disclosure is merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended other than as described in the appended claims.