Patent Description:
A wellbore is formed to access hydrocarbon bearing formations, e.g. crude oil and/or natural gas, by the use of drilling. Drilling is accomplished by utilizing a drill bit that is mounted on the end of a tubular string, such as a drill string. To drill within the wellbore to a predetermined depth, the drill string is often rotated by a top drive or rotary table on a surface platform or rig, and/or by a downhole motor mounted towards the lower end of the drill string. After drilling to a predetermined depth, the drill string and drill bit are removed and a section of casing is lowered into the wellbore. An annulus is thus formed between the string of casing and the formation. The casing string is temporarily hung from the surface of the well. The casing string is cemented into the wellbore by circulating cement into the annulus defined between the outer wall of the casing and the borehole. The combination of cement and casing strengthens the wellbore and facilitates the isolation of certain areas of the formation behind the casing for the production of hydrocarbons.

It is common to employ more than one string of casing in a wellbore. In this respect, the well is drilled to a first designated depth with the drill string. The drill string is removed. A first string of casing is then run into the wellbore and set in the drilled out portion of the wellbore, and cement is circulated into the annulus behind the casing string. Next, the well is drilled to a second designated depth, and a second string of casing or liner, is run into the drilled out portion of the wellbore. If the second string is a liner string, the liner is set at a depth such that the upper portion of the second string of casing overlaps the lower portion of the first string of casing. The liner string may then be fixed, or "hung" off of the existing casing by the use of slips which utilize slip members and cones to frictionally affix the new string of liner in the wellbore. The second casing or liner string is then cemented. This process is typically repeated with additional casing or liner strings until the well has been drilled to total depth. In this manner, wells are typically formed with two or more strings of casing/liner of an ever-decreasing diameter.

Once the hydrocarbon formations have been depleted, the wellbore must be plugged and abandoned (P&A) using cement plugs. This P&A procedure seals the wellbore from the environment, thereby preventing wellbore fluid, such as hydrocarbons and/or salt water, from polluting the surface environment. This procedure also seals sensitive formations, such as aquifers, traversed by the wellbore from contamination by the hydrocarbon formations. Setting of a cement plug when there are two adjacent casing strings lining the wellbore is presently done by perforating the casing strings and squeezing cement into the formation. This procedure sometimes does not give a satisfactory seal because wellbore fluid can leak to the surface through voids and cracks formed in the cement.

<CIT> describes a well conduit cutting and milling apparatus conveyable into and out of a well on coilable tubing using a tubing injection unit. The apparatus includes tubing or casing cutting arms which are radially extendable and retractable with respect to a cylindrical support body by pressure fluid operated pistons. The apparatus is made up of multiple end-to-end connected units including one which supports interchangeable cutting and milling arms and units which have radially extensible and retractable stabilizing arms which engage the inner wall of the tubing or casing to centralize and stabilize the apparatus during operation. The cutter and milling arms are modified for milling away a section of tubing by moving the apparatus in an upward or out of the well direction. The cutting elements may be cylindrical hard metal members which are supported on the arms for cutting away an end face of the tubing in a tangential direction of movement.

So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.

<FIG> illustrates a milling system for abandoning a wellbore <NUM>. The milling system may include a drilling or workover rig and workstring <NUM> deployed using the drilling rig. The rig may include a derrick <NUM> and drawworks <NUM> for supporting a top drive <NUM>. The top drive <NUM> may in turn support and rotate the workstring <NUM>. Alternatively, a Kelly and rotary table (not shown) may be used to rotate the workstring <NUM> instead of the top drive. The workstring <NUM> may include deployment string <NUM> and a bottomhole assembly (BHA) <NUM>. The deployment string <NUM> may include joints of threaded drill pipe connected together or coiled tubing. If the deployment string <NUM> is coiled tubing, the top drive <NUM> and derrick <NUM> may be omitted and the BHA <NUM> may include a mud motor (not shown).

A rig pump <NUM> may pump milling fluid 114f, such as drilling mud, out of a pit <NUM>, passing the mud through a stand pipe and Kelly hose to the top drive <NUM>. The fluid 114f may continue into the deployment string, through a bore of the deployment string <NUM>, through a bore of the BHA <NUM>, and exit the BHA. The fluid 114f may lubricate the BHA <NUM> and carry cuttings to surface. The milling fluid and cuttings, collectively returns, may flow upward along an annulus formed between the workstring <NUM> and an inner casing 119i, through a solids treatment system (not shown) where the cuttings are separated. The treated milling fluid may then be discharged to the mud pit for recirculation.

The drilling rig may further include a launcher <NUM> for deploying one or more closure members, such as balls 150a,b, and a pressure sensor <NUM> in communication with an outlet of the rig pump <NUM>. The wellbore may be land based (shown) or subsea (not shown). If subsea, the wellhead may be at the seafloor and the rig may be part of a mobile offshore drilling unit or intervention vessel or the wellhead may be at the waterline and the rig may be located on a production platform.

A first section of the wellbore <NUM> has been drilled. An outer casing string 119o has been installed in the wellbore <NUM> and cemented 111o in place. The outer casing string 119o may isolate a fluid bearing formation, such as aquifer 130a, from further drilling and later production. Alternatively, fluid bearing formation 130a may instead be hydrocarbon bearing and may have been previously produced to depletion or ignored due to lack of adequate capacity. A second section of the wellbore <NUM> has been drilled. The inner casing string 119i has been installed in the wellbore <NUM> and cemented 111i in place. The inner casing string has been perforated and hydrocarbon bearing formation 130b has been produced, such as by installation of production tubing (not shown) and a production packer. Once hydrocarbon bearing formation 130b is depleted, it may be desirable to plug and abandon (P&A) the wellbore <NUM>. To begin the P&A operation, the production tubing and packer may be removed from the wellbore. Alternatively, the production packer may be drilled or milled out.

<FIG> illustrates the BHA <NUM> of the milling system. The BHA <NUM> may include one or more radial cutout and window (RCW) mills 201i,o and one or more section mills 202i,o, 203i,o. As shown, the BHA <NUM> includes a first stage inner RCW mill 201i for milling the inner casing string 119i, such as seven inch diameter casing, and second 202i and third stage 203i inner section mills for milling the inner casing string and a first stage outer RCW mill 201o for milling the outer casing string 119o, such as nine and five-eighths inch diameter casing, and second 202o and third 203o stage outer section mills for milling the outer casing string. The BHA <NUM> may further include a disconnect <NUM>, catcher <NUM>, and a shoe, such as guide shoe or drill bit <NUM>. Each component of the BHA <NUM> may be connected to one another, such as by threaded couplings.

<FIG> is a radial cross section generic to any of the mills 201i,o-203i,o in a retracted position. <FIG> are a longitudinal section of the outer RCW mill 201o in a retracted position. <FIG> are a longitudinal section of the outer RCW mill 201o in an extended position.

The outer RCW mill 201o may include a housing <NUM>, one or more pistons <NUM>, 211a,b, a plurality of arms 215r, a biasing member, such as a spring <NUM>, and a flow tube <NUM>. The housing <NUM> may be tubular, have a bore formed therethrough, and include one or more sections 205a-d connected by couplings, such as threaded couplings. The upper 205a and lower 205d sections may each have threaded couplings, such as a box 206b and a pin 206p, formed at longitudinal ends thereof for connection to another mill, another BHA component, or the deployment string <NUM>.

Each arm 215r may be movable relative to the housing <NUM> between a retracted position and an extended position. The housing <NUM> may have a pocket 207p formed therein for each arm 215r. The housing <NUM> may also have a pair of ribs 207r formed in an outer surface thereof on each side of each pocket 207p and extending along the housing outer surface for at least a length of the pocket. One or more of the ribs 207r may slightly overlap the respective pocket 207p. A nominal outer diameter of the housing <NUM> may be slightly less than the drift diameter of the inner casing 119i. The ribbed outer diameter of the housing <NUM> may be essentially equal to the drift diameter of the inner casing 119i, such as a line fit having an allowance of less than or equal to one, three-fourths, one-half, or one-fourth percent of the drift diameter (and greater than or equal to zero). The ribs 207r may act as a stabilizer during milling, reinforcement for the housing <NUM>, and/or extend the sweep of the mill 201o.

Each arm 215r may be disposed in the pocket 207p in the retracted position and at least a portion of each arm may extend outward from the pocket in the extended position. Each pocket 207p may be eccentrically arranged relative to the housing <NUM> and each arm 215r may have an eccentric extension path relative to the housing resulting in a far-reaching available blade sweep (discussed below). Each arm 215r may have an inner body portion <NUM> and an outer blade portion 217r. The body portion <NUM> may have an actuation profile formed in one side thereof and a housing surface defining the pocket and facing the actuation profile may have a mating guide extending therefrom. The actuation profile may be a series of inclined grooves <NUM> spaced along the body portion <NUM>. For each groove <NUM>, the guide may be a set of fasteners <NUM>, such as pins, received by respective openings formed through a wall of the housing <NUM> between an outer surface of the housing and a respective pocket 207p. The fasteners <NUM> may be pressed, threaded, or bonded into each opening, such as by brazing, welding, soldering, or using an adhesive. Each set of fasteners <NUM> may be arranged along an inclined path corresponding to a respective groove <NUM>.

The actuation profile and guide may be operable to move the arm 215r radially outward as the arm is pushed longitudinally upward by the pistons <NUM>, 211a,b. The actuation profile and guide may also serve to mechanically lock the arms 215r in the extended position during longitudinal milling as longitudinal reaction force from the outer casing 119o pushes the blade portion 217r against an arm stop 230o fastened to the housing <NUM>, thereby reducing or eliminating any chattering of the blade portions due to pressure fluctuations in the milling fluid 114f. The actuation profile and guide may move each arm without pivoting.

Cutters <NUM> may be bonded into respective recesses formed along each blade portion 217r. The cutters <NUM> may be made from a hard material, such as a ceramic or cermet, such as tungsten carbide. The cutters <NUM> may be pressed or threaded into the recesses. Alternatively, the cutters <NUM> may be bonded into the recesses. Alternatively, the cutters <NUM> may be made from a super-hard material, such as polycrystalline diamond compact (PDC), natural diamond, or cubic boron nitride and the mill may be used as an underreamer instead. The cutters <NUM> may be disposed in the recesses to form a radial cutting face and a longitudinal cutting face.

Each blade portion 217r may have a short length relative to blade portions of the outer section mills 201o, 202o and relative to a length of a respective body portion <NUM>. An outer surface of each blade portion 217r may also taper <NUM> slightly inwardly from a top of the mill 201o to a bottom of the mill. The short blade portion 217r may advantageously provide increased cutting pressure when starting a window 160o (<FIG>) through the outer casing 119o, thereby reducing or eliminating any bearing effect. The taper <NUM> in the blade portion 217r may ensure that an upper portion of the blade portion engages the outer casing inner surface before the rest of the blade portion, thereby further increasing cutting pressure. The short blade portion 217r may also provide a relatively short cutting lifespan to form a relatively short window. The cutting lifespan may less than or equal to the length of a joint of the casing (typically forty feet), such as one-third, one-half, two thirds, or three-quarters the joint length and be greater than or equal to the length of the outer section mill blade portions. When extended, a sweep of the outer RCW mill 201o may be equal to or slightly greater than the outer casing coupling outer diameter and the outer RCW mill may be capable of cutting the window through both the outer casing 119o and the outer coupling.

Each body portion <NUM> may have a groove <NUM> formed along an exposed portion (not having the blade portion) of an outer surface thereof. A pad <NUM> (see <FIG>) may be bonded or pressed into the groove <NUM>. The pad <NUM> may be made from the hard or super hard material. The pads <NUM> may serve to stabilize the outer RCW mill 201o by engaging an inner surface of the outer casing after the outer RCW blade portion <NUM> has cut through the casing. Once the blade portions 217r have worn off, the body portion <NUM> may continue to serve as a stabilizer for the outer section mills <NUM>, 203o. A slight inner portion of the blade portion 217r may or may not remain to serve as a scraper. Alternatively, the groove and/or the pad may extend along only a portion of the body portion outer surface. Alternatively, the pad may be the exposed outer surface of the body portion instead of an insert and the exposed outer surface may be surface hardened or coated.

Each blade portion 217r may have two sets of cutters <NUM>, the sets staggered to form a lead cutting surface 221ℓ for the casing and a trail cutting surface 221t for the coupling. The blade sweep of the outer RCW mill 201o may be substantially greater than a nominal outer diameter of the housing, such as greater than fifty percent, sixty-seven percent, seventy-five percent, or eighty-five percent greater. For example, for the seven inch diameter inner casing, the housing may have a nominal outer diameter equal to five and three-quarter inches and the blade sweep may be equal to ten and five-eighths inches or greater. The blade sweep may be adjusted by modification of the arm stop 230o.

An upper surface of each arm 215r may be inclined for engaging the inner casing string (upper surface of an inner window 160i (<FIG>)) and partially or fully retracting the arms 215r once the milling operation is complete. The retraction inclination may be perpendicular to the inclination of the actuation profile and the guide. A lower surface of the body portion <NUM> and a slight inner portion of the body portion upper surface may be inclined corresponding to the actuation profile and guide.

The flow tube <NUM> may disposed in the housing bore and be longitudinally movable relative to the housing <NUM>. The flow tube <NUM> may include one or more sections 225a-d connected by couplings, such as threaded couplings. The blade piston <NUM> may be connected to the flow tube at an upper end thereof by having a shoulder engaging a top of the flow tube <NUM> and one or more fasteners, such as set screws. Each booster piston 211a,b may be connected to the flow tube <NUM>, such as by a threaded connection. The flow tube <NUM> may have one or more ports 214a-c formed through a wall thereof corresponding to each piston <NUM>, 211a,b. An extension <NUM> may be connected to the housing <NUM>, such as by a threaded connection.

A blade piston chamber may be formed in a wall of the housing <NUM> and between the housing and the extension <NUM> and be sealed at a lower end by a blade partition 212p connected to the housing <NUM>, such as by a threaded connection. An upper end of the blade piston chamber may be in fluid communication with the pockets 207p. An upper end of the flow tube <NUM> may sealingly engage an outer surface of the extension <NUM> and a first set of ports 214a may provide fluid communication between the flow tube bore and the blade piston chamber.

The blade piston <NUM> may have one or more passages 210p formed longitudinally therethrough for diverting a portion of the milling fluid 114f to flush cuttings from the pockets 207p and cool the blade portions 217r. A seat <NUM> may be connected to the blade partition 212p and may sealingly engage an outer surface of the flow tube <NUM> in the retracted position, thereby closing the ports 214a and preventing flow through the passages 210p until the outer RCW mill 201o is being extended. Opening of the ports 214a may result in a slight pressure decrease in the housing bore when the ports open due to flow through the pockets 207p which may or may not be detectable at the rig. As the arms 215r fully extend, the bore pressure may increase due to the arms obstructing flow through the pockets 207p, thereby providing a pressure increase detectable at the rig (using the sensor <NUM>).

Each booster piston 211a,b may be disposed between the housing <NUM> and the flow tube <NUM>. A first booster piston chamber may be formed between the blade partition 212p and a first booster partition 213a connected to the housing <NUM> and a second booster piston chamber may be formed between the first booster partition and a second booster partition 213b connected to the housing <NUM>. A second set of ports 214b may provide fluid communication between the flow tube bore and the first booster piston chamber and a third set of ports 214c may provide fluid communication between the flow tube bore and the second booster piston chamber. An upper portion of each booster piston chamber may be vented by one or more equalization ports formed through a wall of the housing.

The spring <NUM> may be disposed between the second booster partition 213b and a shoulder of the flow tube <NUM>, thereby longitudinally biasing the pistons <NUM>, 211a,b and the flow tube <NUM> away from the arms 215r and toward the retracted position. The spring <NUM> may be disposed in a spring chamber formed between the second booster partition 213b and a shoulder of the housing <NUM>. The spring chamber may be in fluid communication with the ports 214c via a gap formed between the second booster partition 213b and the flow tube <NUM>. The flow tube <NUM> may initially be fastened to the housing <NUM> by one or more frangible fasteners, such as shear screws <NUM>.

<FIG> is an offset section of an arm 215r of the inner RCW mill 201i in an extended position. <FIG> is a cross section of middle portion of the inner RCW mill 201i in a retracted position. The inner RCW mill 201i may be similar or identical to the outer RCW mill 201o except for a few differences. The arm stop 230o may be replaced by arm stop 230i extended to adjust the sweep of the blade portions 217r to correspond to the inner casing 119i. When extended, a sweep of the inner RCW mill 201i may be equal to or slightly greater than the inner casing coupling outer diameter and the inner RCW mill may be capable of cutting the window 160i through both the inner casing 119i and the inner coupling. The seat <NUM> may be omitted so that the ports 214a are open in the retracted position. Further, the shear screws <NUM> may be omitted from the inner RCW mill 201i. Alternatively, the inner RCW mill may include one or more of the shear screws <NUM>.

Referring specifically to <FIG> and applicable to any of the mills 201i-203i, 201o-203o, the second booster piston 211b, housing section 205c, flow tube section 225c, and first booster partition 213a may form a booster module <NUM>. Depending on the desired actuation force for the particular application of the particular mill, the booster module <NUM> may be omitted, a single module may be used, or additional modules (not shown) may be added to any of the mills.

<FIG> is an offset section of an arm <NUM> of one of the inner section mills 202i, 203i in an extended position. <FIG> is an offset section of an arm <NUM> of one of the outer section mills 202o, 203o in an extended position. The outer section mills 202o, 203o may be similar or identical to the outer RCW mill 201o except that arms 215r may be replaced by arms <NUM>. The inner section mills 202i, 203i may be similar or identical to the outer section mills 202o except that arms 215r may be replaced by arms <NUM> and the arm stops 230o may be replaced by the arm stops 230i. Further, as discussed above, the section mills 202i,o, 203i,o may have less (including zero) booster modules <NUM> than the outer RCW mill 201o. As such, one of the mills may be converted to any other mill by simply replacing the arms 215r,s, stops 230i,o, adding or removing booster modules <NUM>, and adding or removing the seat <NUM> (not all required depending on which mill is being converted to which other mill).

The section mill blade portions <NUM> may be substantially longer than the RCW mill blade portions 217r, such as two to six times the length of the RCW blade portions and may have a length corresponding to a length of the body portion <NUM>. A length of the section mill blade portions <NUM> may ensure a long cutting lifespan, such as greater than or equal to one hundred feet of casing (including couplings). As with the RCW blade portions 217r, once the section mill blade portions wear off, the body portions <NUM> (with or without a slight remaining portion of the blade portion) may serve as a stabilizer for the next section mill of the particular size.

An outer surface of the section mill blade portions <NUM> may be straight. A sweep of the section mill blade portions <NUM> may correspond to the respective casing coupling outer diameter so that the blade portion may mill both the outer casing 119o and the outer casing coupling. A sweep of the inner section mill blade portions <NUM> may extend to the drift diameter of the outer casing 119o so that cement and centralizers located between the casing strings 119i,o may also be milled.

Alternatively, as illustrated in Figures 14D and 15D of the '<NUM> provisional, a second pad (not shown) may be disposed in an outer surface of each of the section mill blade portions for engaging an inner surface of the outer casing for the inner section mills and for engaging an inner surface of cement or wellbore wall for the outer pads. The second pads may serve as stabilizers during section milling. The second pad may be made from the hard or super hard material.

<FIG> illustrates a catcher <NUM> and drill bit <NUM> of the BHA <NUM>. The catcher <NUM> may receive a plurality of balls 150a,b so that the mills may be selectively operated (discussed below) during one trip of the workstring. The catcher <NUM> may include a tubular housing <NUM> and a ball seat <NUM>. The housing <NUM> may have couplings 55b formed at each longitudinal end thereof for connection with other components of a workstring. The couplings may be threaded, such as a box 55b and a pin (not shown). The housing <NUM> may include one or more sections <NUM>, <NUM> connected by couplings, such as threaded couplings. The housing <NUM> may have a flow path formed therethrough for conducting milling fluid.

A lower portion of the upper housing section <NUM> may form a cage <NUM>. The cage <NUM> may be made from an erosion resistant material, such as a tool steel or cermet, or be made from a metal or alloy and treated, such as a case hardened, to resist erosion. The cage <NUM> may be perforated, such as slotted <NUM>. The slots <NUM> may be formed through a wall of the cage <NUM> and spaced therearound. A length of the slots <NUM> may correspond to a ball capacity of the catcher <NUM>. A lower end of the cage <NUM> may form a nose 60n. A port 60p may be formed through the nose 60n and have a diameter substantially less than a diameter of the smallest ball 150a,b. An annulus may be formed between the cage <NUM> and the lower housing section <NUM>. The annulus may serve as a fluid bypass for the flow of milling fluid 141f through the catcher <NUM>. The first caught ball may land on the nose 60n. Milling fluid 141f may enter the annulus from the housing bore through the slots <NUM>, flow around the caught balls along the annulus, and reenter the housing bore below the nose 60n.

Each of the balls 150a,b may include a core and cladding. The cladding may be made from a resilient material, such as a polymer, and the cladding may be made from a high density material to control buoyancy (i.e., negative). The seat <NUM> may be fastened to the upper housing section <NUM>, such as by a threaded connection. The seat <NUM> may have a conical inner surface to accommodate a plurality of differently sized balls and to facilitate squeezing therethrough. A liner <NUM> may be made from the erosion resistant material and may be fastened to the seat. The liner <NUM> may facilitate using of the seat <NUM> as a choke to increase pressure in the BHA <NUM> (above the catcher <NUM>) and relative to the annulus pressure (discussed below). Each of the balls 150a,b may have a diameter greater than a minimum diameter of the seat <NUM> such that the ball will land and seal against the seat when dropped or pumped through the deployment string <NUM> and the portion of the BHA <NUM> (above the catcher <NUM>). Pressure may then be increased to operate one of the section mills 202i,o, 203i,o or the outer RCW mill 201o. Pressure may then be further increased to a predetermined threshold (dependent on the diameter of the particular ball) to squeeze the ball through the seat <NUM>. A diameter of the ball core may be less than the minimum diameter of the seat <NUM> so that the core does not obstruct squeezing of the ball through the seat.

<FIG> is a cross section of a disconnect <NUM> of the BHA <NUM>. In the event that the BHA <NUM> becomes stuck in the wellbore, the disconnect <NUM> may be operated to release the BHA <NUM> from the deployment string <NUM> so that the deployment string may be retrieved from the wellbore <NUM>. The disconnect <NUM> may include a housing <NUM>, a mandrel <NUM>, an actuator <NUM>, <NUM>, and threaded dogs <NUM>. The mandrel <NUM> and the housing <NUM> may each be tubular and the each may have a threaded coupling formed at a longitudinal end thereof for connection with other components of the workstring. Each of the housing <NUM> and mandrel <NUM> may include a plurality of sections 5a,b, 10a,b, each section connected, such as by threaded connections, and sealed, such as by O-rings.

In a locked position, the dogs <NUM> may be disposed through respective openings formed through the mandrel <NUM> and an outer surface of each dog may form a portion of a thread corresponding to a threaded inner surface of the housing <NUM>. Abutment of each dog <NUM> against the mandrel wall surrounding the opening and engagement of the dog thread portion with the housing thread may longitudinally and rotationally connect the housing <NUM> and the mandrel <NUM>. Each of the dogs <NUM> may be an arcuate segment, may include a lip (not shown) formed at each longitudinal end thereof and extending from the inner surface thereof, and have an inclined inner surface. A dog spring (not shown) may disposed between each lip of each dog <NUM> and the mandrel, thereby radially biasing the dog inward away from the housing <NUM>.

The actuator may include a sleeve <NUM> and a biasing member <NUM>, such as a spring. The sleeve <NUM> may be longitudinally movable between the locked position (shown) and an unlocked position (not shown). The actuator spring <NUM> may be disposed in a chamber formed between the sleeve <NUM> and the mandrel <NUM> and act against a shoulder of the sleeve and the mandrel, thereby biasing the sleeve into engagement with the dogs <NUM>. An upper portion of the actuator sleeve <NUM> may have a conical outer surface and an inner surface of each dog <NUM> may have a corresponding inclination. Engagement of the sleeve <NUM> with the dogs <NUM> may push the dogs radially into engagement with the housing thread. An inner surface of the actuator sleeve <NUM> may form a seat <NUM> for receiving a closure member, such as a ball (not shown). The seat may have a minimum diameter greater or substantially greater than a maximum diameter of the balls 150a,b so that the disconnect seat <NUM> does not interfere with the balls 150a,b.

In operation, if it becomes necessary to operate the disconnect <NUM>, the BHA <NUM> may be set on a bottom of the wellbore <NUM> and the disconnect ball may be pumped/dropped through the deployment string <NUM> to the disconnect seat <NUM>. Milling fluid 141f may be pumped or continued to be pumped into the deployment string <NUM>. Pressure exerted on the seated ball may move the actuator sleeve <NUM> longitudinally against the actuator spring <NUM>, thereby disengaging the actuator sleeve from the dogs <NUM> and allowing the dog springs to push the dogs radially inward away from the housing <NUM>. The deployment string <NUM> may then be raised from surface, thereby pulling the housing <NUM> from the mandrel <NUM>.

<FIG> illustrate operation of the inner RCW mill 201i. To begin the P&A operation, a BHA (not shown, see BHA <NUM> in <FIG>) including the disconnect <NUM>, inner section mills 201i-203i, catcher <NUM>, and shoe <NUM> may be assembled and deployed into the wellbore <NUM> using the deployment string <NUM> through the inner casing 119i and to the hydrocarbon formation <NUM>. A section of the inner casing 119i lining the hydrocarbon formation <NUM> may be milled and the workstring removed from the wellbore <NUM>. Cement may be pumped into the wellbore, thereby forming a plug <NUM> (<FIG>). Although a top of the plug <NUM> is shown aligned with a top of the formation <NUM>, the plug may have an excess amount extending above the formation top. The BHA <NUM> may then be assembled and connected to the deployment string <NUM>. The workstring <NUM> may then be deployed into the wellbore <NUM> through the inner casing 119i. Alternatively, if the formation 130a is hydrocarbon bearing, both formations 130a,h may be milled in the same trip or in separate trips as for the aquifer.

During deployment of the workstring <NUM>, milling fluid may be circulated at a flow rate less than a predetermined threshold. The BHA <NUM> may be deployed to a top of the plug <NUM>. The workstring <NUM> may then be rotated and the drill bit <NUM> may be engaged with a top of the plug <NUM> to drill some of the excess and verify integrity of the plug <NUM>. Rotation may be halted and the BHA <NUM> may be raised to the formation 130a. The BHA <NUM> may be raised so that the inner RCW mill 201i is slightly above a top of the formation 130a and between couplings of the inner casing 119i. Rotation of the workstring <NUM> may resume and injection of the milling fluid 114f may be increased to or greater than the threshold flow rate, thereby causing a substantial pressure differential across the seat <NUM> and the blade piston <NUM>. The pistons <NUM>, 211a,b of the inner RCW mill 201i may then push the flow tube <NUM> upward and the arms 215r outward until an outer surface of the trailing portion cutters engage an inner surface of the inner casing string 119i. During extension of the inner RCW mill 201i, the other mills 201o, 202i,o, 203i,o may be restrained from extension by their respective shear screws <NUM> and milling fluid may be prevented from discharge through the blade pistons <NUM> by their respective seats <NUM>.

The inner RCW blade portions 217r may engage the inner casing 219i and begin to radially cut through the inner casing wall. Milling fluid may be circulated through the workstring <NUM> and up the workstring-inner casing annulus and a portion of the milling fluid may be diverted into the inner RCW pockets 207p through the blade piston passages 210p. The BHA <NUM> may be held longitudinally in place during the radial cut through operation. The workstring torque may be monitored to determine when the inner RCW mill 201i has radially cut through the inner casing 119i and started the window 160i as indicated by a decrease in torque. As shown, the window 160i may extend entirely around and through the inner casing 119i. As discussed above, the RCW blade portions 217r may be specifically configured to radially cut through the respective casings 119i,o. The arms 215r may extend until engagement with the arm stops 230i. Weight may then be set down on the inner RCW mill 201i. The inner RCW mill 201i may then longitudinally open the window 160i while the inner RCW pads (see pads <NUM> in <FIG>) of the body portions 216r may engage the inner surface of the inner casing 119i, thereby stabilizing the inner RCW mill. Longitudinal advancement of the inner RCW mill 201i may continue until the blade portions 217r of the inner RCW mill 201i are worn away. Again, torque may be monitored to determine when the blade portions 217r are exhausted.

<FIG> illustrate operation of the inner second stage 202i and third stage 203i section mills. Rotation of the workstring <NUM> may be halted. The second stage inner section mill 202i may then be aligned with the inner window 160i or may already be aligned with the inner window. The launcher <NUM> may be operated to deploy ball 120b. The ball 120b may travel through the deployment string <NUM> and into the BHA <NUM> until the ball engages the catcher seat <NUM>. Continued injection of the milling fluid 114f into the workstring <NUM> may increase pressure in the bore above the seated ball 120b until a first threshold pressure is reached. Exertion of the first threshold pressure on the second stage pistons 211a,b (may or may not include 211b) may exert sufficient force to fracture the inner second stage shear screws <NUM>, thereby allowing upward movement of the flow tube <NUM> until the ports 214a are opened and the arms extend and engage the arm stops 230i. The third stage section mill 203i and the outer mills 201o-203o may have a greater number of shear screws <NUM> so that the first threshold pressure is insufficient to operate them. Fracturing of the shear screws <NUM> at surface may be detected by a pressure decrease as the ports 214a open followed by a pressure increase as the arms <NUM> reach full extension and partially obstruct flow through the pockets 207p. Injection of fluid may continue until the bore pressure reaches a second threshold which is greater than the first threshold. The ball 150b may be squeezed through the seat <NUM> at the second threshold pressure and caught in the cage <NUM>.

Before resuming rotation, the BHA <NUM> may be lowered so that the second stage inner section mill 202i engages a lower end of the inner window 160i and weight may be set down on the second stage inner section mill to ensure that the arms <NUM> are fully extended. The workstring <NUM> may then be rotated. As with the inner RCW mill 201i, the pads (see pads <NUM> in <FIG>) may engage the inner surface of the inner casing 119i and serve to stabilize the section mill 202i. The second stage section mill 202i may be advanced and may mill the inner casing 119i while torque is monitored at surface to determine when the blade portions <NUM> have been exhausted. As discussed above, the exhausted inner RCW mill 201i may remain in the extended position to further stabilize the inner section mill 202i. Once the second stage inner section mill 202i has been exhausted, the larger ball 150a may be deployed and pumped through the deployment string <NUM> until the ball 150a lands against the seat <NUM>.

Injection of milling fluid 114f may continue until the bore pressure reaches a third threshold pressure which is greater than the second threshold pressure. Exertion of the third threshold pressure on the inner third stage pistons 211a,b (may or may not include 211b) may exert sufficient force to fracture the inner third stage shear screws <NUM>, thereby allowing upward movement of the flow tube <NUM> until the ports 214a are opened and the arms <NUM> extend and engage the arm stops 230i. The outer mills 201o-203o may have a greater number of shear screws <NUM> so that the third threshold pressure is insufficient to operate them. Injection of fluid may continue until the bore pressure reaches a fourth threshold which is greater than the third threshold to squeeze the ball 150a into the cage <NUM>. The third stage inner section mill 203i may be extended and milling of the inner casing 119i may continue while leaving the exhausted second stage inner section mill 202i in the extended position for stabilization.

<FIG> illustrates raising the BHA <NUM> in preparation for operation of the outer mills 201o-203o. <FIG> illustrate operation of the outer RCW mill 201o. <FIG> illustrate operation of the outer second stage 202o and third stage 203o section mills. Once the desired inner casing section has been milled, the BHA <NUM> may be raised until the outer RCW mill 201o is aligned near a top of the inner window 160i and between couplings of the outer casing 119o. The operation may be repeated with the outer mills 201o-203o (except that a ball (not shown, larger than 150a) may be used to operate the outer RCW mill 201o to form the outer window 160o). Additional balls (not shown), each larger than the last and larger than outer RCW mill ball, may be deployed to operate the outer section mills 202o, 203o, as discussed above for the inner section mills 202i, 203i. Once the outer casing section 119o has been milled, the workstring <NUM> may be retrieved from the wellbore <NUM>. As discussed above, arms 215r,s of the outer mills may (at least partially) retract upon contact with the inner casing 119i (upper surface of the inner window 160i). The arms of the inner mills may or may not retract as retraction of the inner mill arms may not be necessary to remove the BHA <NUM> from the wellbore.

<FIG> illustrates the wellbore <NUM> plugged and abandoned. Once the section of the casings 119i,o lining the formation 130a have been milled, a BHA (not shown) may be connected to the deployment string <NUM>. The BHA may include the bridge plug 110a, a setting tool, and a cementing shoe/collar. The BHA may be run into the wellbore <NUM> using the deployment string <NUM> to a depth proximately below a bottom of the formation 130a. The bridge plug 110a may be set using the setting tool by pressurizing the workstring. The setting tool may be released from the bridge plug 110a. Cement 105a may then be pumped through the workstring to displace wellbore fluid from the formation 130a. The workstring may then be removed from the wellbore <NUM> and the cement 105a allowed to cure, thereby forming the cement plug. Alternatively, the bridge plug setting and cementing may be performed in separate trips. A casing cutter (not shown) may then be connected to the workstring. The casing cutter may then be deployed a predetermined depth, such as one hundred feet, in the wellbore. The inner and outer casings may be cut at the predetermined depth and removed from the wellbore. The bridge plug <NUM> may be set proximately below the cut depth and the cement plug <NUM> may be pumped and allowed to cure. The wellbore <NUM> may then be abandoned.

Additionally, the BHA may further include a fourth stage inner and/or outer section mill to clean any remaining cement and/or debris. The fourth stage inner section mill may be operated after the third stage and before the outer mills and the fourth stage outer section mill may be operated after the third stage mill and before removing the BHA. The fourth stage mills may have slightly modified blade portions to ensure any remaining cement and/or debris is removed.

Alternatively, the inner 201i-203i and outer mills 201o-203o may be deployed in separate trips or the inner or outer mills may be run for a single casing milling operation. Alternatively, instead of a plug and abandon operation, any of the BHAs may be used to form a window for a sidetrack or directional drilling operation. Alternatively, instead of casing strings, any of the BHAs may be used to mill one or more liner strings.

<FIG> illustrates a casing recovery operation using one of the RCW mills 201i, according to another embodiment of the present invention. Instead of milling sections of the casing strings for plugs and leaving portions of the casing strings in the wellbore, the RCW mills may be used to remove the casing strings from the wellbore. A BHA <NUM> may be assembled and connected to the deployment string <NUM>. The BHA <NUM> may include the disconnect <NUM>, the inner RCW mill 201i, and the shoe <NUM>. Additionally, the BHA <NUM> may include one or more additional inner RCW mills (not shown) so that the additional mills may be activated when or if the initial RCW mill becomes exhausted.

The workstring may then be deployed into the wellbore <NUM> and operated to radially cut 165i through the inner casing string 119i at predetermined intervals, such as one hundred to one thousand feet. Once the radial cuts 165i have been made along the inner casing string 119i, the workstring may be removed from the wellbore <NUM>. A BHA (not shown) including an anchor may be connected to the deployment string <NUM> and deployed into the wellbore <NUM>. The anchor may be operated to grip the first section of the inner casing string 119i. The workstring and first casing string section may then be removed from the wellbore <NUM>. The workstring may then be redeployed to remove the second section of casing 119i. This operation may be repeated until the inner casing string 119i has been removed from the wellbore. Once the inner casing string 119i has been removed, the outer RCW mill 201o may be deployed and the outer casing string 119o may be radially cut at the selected intervals and the sections removed from the wellbore <NUM>.

<FIG> illustrate an abandonment operation using the milling system, according to another embodiment of the present invention. Instead of milling the entire casing string sections lining the formations 130a,h, a plurality of mini-sections 170i may be milled in the casing strings 119i,o. A BHA <NUM> may be assembled and connected to the deployment string <NUM>. The BHA <NUM> may include the disconnect <NUM>, the inner RCW mill 201i, one or more inner section mills 202i, 203i, the catcher <NUM>, and the shoe <NUM>. Additionally, the BHA <NUM> may include one or more additional inner RCW mills (not shown) so that the additional mills may be activated when or if the initial RCW mill becomes exhausted.

The workstring may then be deployed into the wellbore <NUM>. The inner RCW mill 201i may be operated to form and open the window for the inner section mills 202i, 203i. Instead of milling to exhaustion, the inner RCW mill 201i may then be retracted and moved to a location of the next mini-section 170i and operated to form and open the window for the section mills 202i, 203i. This operation may be repeated until windows corresponding to all of the mini-sections 170i have been formed and opened. The BHA <NUM> may then be moved to align the section mill 202i with a first one of the windows. The section mill 202i may then be operated to extend the window into a mini-section 170i. The section mill 202i may then be retracted and moved to the next window. This process may repeated until all of the mini-sections 170i are formed. The workstring may then be removed from the wellbore <NUM> and the cement plug <NUM> pumped and allowed to cure. The BHA <NUM> may then be deployed and a similar mini-section operation performed for the casings lining the formation 130a.

<FIG> illustrate section milling of a damaged and/or partially collapsed casing 319o or liner string, according to another embodiment of the present invention. In this embodiment, the formation <NUM> to be plugged is lined with a casing string 319o having a size corresponding to the outer casing string 119o and a collapsed section <NUM> above the formation <NUM> to be plugged. Due to the great extension capability of the outer section mills 201o-203o (discussed above), the casing 319o lining the formation <NUM> may be milled in spite of the collapsed portion <NUM>. A BHA <NUM> may be assembled and connected to the deployment string <NUM>. The BHA <NUM> may include the disconnect <NUM>, the outer RCW mill 201o, one or more outer section mills 202o, 203o, the catcher <NUM>, and the shoe <NUM>. The workstring may then be deployed into the wellbore <NUM> to the formation <NUM> through the casing string 319o (including the damaged portion <NUM>). The outer RCW mill 201o may be operated to form and open the window for the outer section mills 202o, 203o. The outer section mills 202o, 203o may then be operated to mill the section of casing 319o lining the formation <NUM>. The cement plug (not shown) may then be pumped and allowed to cure. The shear pins <NUM> and partition seat <NUM> may or may not be omitted from the outer RCW mill 201o in this alternative.

<FIG> is an offset section of an arm of an outer RCW mill 401o, according to another embodiment of the present invention. The outer RCW mill 401o may be similar or identical to the outer RCW mill 201o except that a frangible fastener <NUM>, such as a shear pin or shear screw, has been added in each pocket 207p to facilitate retaining of the arms 215r in the retracted position. The frangible fasteners <NUM> may also be added to the section mills 202i,o, 203i,o and/or the inner RCW mill 201i.

<FIG> is an offset section of an arm of an outer RCW mill 451o, according to another embodiment of the present invention. The outer RCW mill 451o may be similar or identical to the outer RCW mill 201o except that pocket cover <NUM> has been added to each pocket 207p to prevent accumulation of cuttings within the pockets while the inner mills 201i-203i are milling. Accumulation of cuttings in the pockets 207p may obstruct extension of the arms. The cover <NUM> may be a foamed polymer, such as polyurethane, and may be sprayed in the pocket after the arms have been inserted into the pockets and the arm stops have been connected. An insert (not shown) may be inserted into each pocket before spraying to prevent entry of the foam into a space of the pocket below the arm. Alternatively, the cover <NUM> may be made from a high temperature hot melt adhesive, such as a thermoplastic (i.e., polyamide or polyester). As with the spray foam, the molten adhesive may be applied after the arms have been inserted into the pockets and the arm stops have been connected using a conventional manual hot melt glue gun or a gas driven hot melt glue gun. The covers <NUM> may be jettisoned when the arms are extended or quickly disintegrated during milling. Alternatively, the cover <NUM> may be a polymer molded to fit each arm and be inserted into the pocket after the arms but before the arm stops and have a lip extending underneath an edge of the pocket and underneath the arm stops for connection. The arm covers <NUM> may also be added to the section mills 202i,o, 203i,o and/or the inner RCW mill 201i.

<FIG> is an offset section of an arm of an outer RCW mill 501o, according to another embodiment of the present invention. <FIG> illustrates a debris barrier <NUM> of the mill. The outer RCW mill 501o may be similar or identical to the outer RCW mill 201o except that a debris barrier <NUM> has been added to each pocket 207p for each set of guide pins <NUM> to prevent accumulation of cuttings within the pockets of the outer RCW mill 501o while the outer mills are milling. Accumulation of cuttings in the pockets may obstruct retraction of the arms. Each debris barrier <NUM> may be a strip of material, such as a polymer, and may be fastened to the housing using the guide pins <NUM>. Each debris barrier <NUM> may have a recess formed in a surface thereof for accommodating a respective guide pin. The polymer may have lubricative properties, such as polytetrafluoroethylene (PTFE), so as not to obstruct movement of the arms. Each strip may be sized to have a width forming a line fit with the respective groove <NUM>, such as having an allowance of less than or equal to one, three-fourths, one-half, or one-fourth percent of the groove width (and greater than or equal to zero). Alternatively, each strip width may be sized to form an interference fit with the respective groove. Each strip may at least partially extend into the respective groove when the arms are in the extended position.

<FIG> is an offset section of an outer RCW mill 551o, according to another embodiment of the present invention. <FIG> illustrates a debris barrier <NUM> of the mill. The outer RCW mill 551o may be similar or identical to the outer RCW mill 201o except that a debris barrier <NUM> has been added to each pocket 207p to replace each set of the guide pins <NUM> and prevent accumulation of cuttings within the pockets of the outer RCW mill while the outer mills are milling. Accumulation of cuttings in the pockets may obstruct retraction of the arms. Each debris barrier <NUM> may be a strip of plain bearing material and may have rail portion for guiding the arms and a fastener portion for connection to the housing. The pin portions may be pressed or bonded into respective housing openings. The plain bearing material may be a metal or alloy, such as Babbitt metal, brass, bronze, or copper alloy (i.e., Beryllium copper). Alternatively, the debris barrier may be made from steel and the rail portion coated with the plain bearing material or PTFE. Each rail portion may be sized to have a width forming a line fit with the respective groove <NUM>, such as having an allowance of less than or equal to one, three-fourths, one-half, or one-fourth percent of the groove width (and greater than or equal to zero). Alternatively, each rail portion width may be sized to form an interference fit with the respective groove. Each rail portion may at least partially extend into the respective groove when the arms are in the extended position.

<FIG> illustrate guides 608a,b for the mills, according to other embodiments of the present invention. Instead of the hollow guide pins <NUM>, the solid guide pin 608a may be used. The guide pin 608a may have a round head. Instead of the hollow guide pins <NUM>, the solid guide pin 608b may be used. The guide pin 608b may have a flat head. Additionally, each guide pin 608b may be coated <NUM> with the plain bearing material or PTFE to provide a line fit or interference fit as discussed above to obstruct or prevent cuttings from entering the pockets and obstructing retraction of the arms.

In another embodiment (not shown) discussed and illustrated at Figures 1A, <FIG>, <NUM>-3D, and <NUM> of the '<NUM> provisional, each of the mills may include a control module and the BHA may further include a telemetry sub for receiving instruction signals from the surface, thereby obviating the shear screws <NUM>. The inner RCW mill may or may not have a control module. Each control module may include a hydraulic or mechanical lock for restraining movement of the flow tube until the control module receives the instruction signal for releasing the flow tube from surface. The telemetry sub may include a receiver for receiving the instruction signal from surface and a relay for transmitting the instruction signal to the individual control modules. The instruction signal may sent by modulating rotation of the workstring, modulating injection rate of the milling fluid, modulating pressure of the milling fluid (mud pulse), electromagnetic telemetry, transverse electromagnetic telemetry, radio frequency identification (RFID) tag, or conductors extending along the deployment string. The telemetry sub may further include a transmitter for transmitting acknowledgment of the instruction signal, such as a mud pulser, electromagnetic or transverse electromagnetic transmitter, or RFID tag launcher. Each control module may further include a position sensor operable to monitor movement of the flow tube and the control module may transmit measurements of the position sensor to the telemetry sub for relay to the surface.

Claim 1:
A bottom hole assembly (<NUM>) for milling a plurality of tubulars (<NUM>) disposed inside one another in a wellbore, the bottom hole assembly comprising:
a plurality of arms (<NUM>);
a plurality of arm stops (<NUM>) including at least first and second sets of the arm stops (<NUM>); and
a plurality of mills (<NUM>-<NUM>) disposed on the bottom hole assembly for use in the wellbore, each of the mills (<NUM>-<NUM>) being configured with the arms (<NUM>) and the arm stops (<NUM>) and comprising:
a tubular housing (<NUM>) including a longitudinal axis;
a plurality of the arm stops (<NUM>) coupled to the tubular housing (<NUM>); and
a plurality of the arms (<NUM>) coupled to the tubular housing (<NUM>), each of the arms (<NUM>) including a body portion (<NUM>) and a blade portion (<NUM>), the blade portion (<NUM>) extending from an outer surface of the body portion (<NUM>),
each of the arms (<NUM>) being movable laterally and longitudinally along an eccentric extension path between a retracted position and an extended position relative to the tubular housing (<NUM>) while an arm length is maintained substantially parallel to the longitudinal axis, the extended position defined by abutment of each of the arms (<NUM>) against a corresponding one of the arm stops (<NUM>) and being adjustable to vary a lateral blade sweep dimension of the mill,
wherein at least one of the mills (<NUM>-<NUM>) is configured with the first set of the arms (<NUM>) stops and has the lateral blade sweep dimension adjusted to mill at least one of the tubulars (<NUM>); and
wherein at least another one of the mills (<NUM>-<NUM>) is configured with the second set of the arms (<NUM>) stops and has the lateral blade sweep dimension adjusted to mill at least another one of the tubulars (<NUM>).