Abstract:
An apparatus and method for drawing small milling debris into a combination milling and debris retrieval tool, during the performance of the milling operation. A milling tool has a fluid intake port near its lower end, where debris-laden fluid is drawn into the milling tool and subsequently into a separator section. The separator section has a debris deflection tube and a screen for separating the debris from the fluid. Fluid is drawn into the tool by either a set of eductor nozzles or a downhole motor and pump.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. Ser. No. 09/038,782, filed Mar. 11, 1998, now abandoned. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention is in the field of equipment used to mill away metal objects downhole in a well bore, and equipment used to remove from the well bore the cuttings resulting from this milling. 
     2. Background Information 
     When a metal object, such as a section of casing, a packer, or a lost tool, is to be removed from a well bore, the best method of removal is often to mill the object into small cuttings with a mill such as a pilot mill, a section mill, or a junk mill, and then to remove the cuttings from the well bore. Furthermore, a milling tool will often result in the removal of scale, cement, or formation debris from a hole. 
     It is important to remove the cuttings, or other debris, because other equipment subsequently used in the well bore may incorporate sealing surfaces or elastomers, which could be damaged by loose metal cuttings being left in the hole. Most commonly, the metal cuttings and other debris created by milling are removed from the well bore by circulating fluid down the inside of the workstring and out openings in the milling tool, then up the annulus to the surface of the well site. This “forward circulation” method usually leaves some cuttings or debris stuck to the side of the well casing or well bore surface, and these cuttings or debris can damage some of the tools which may subsequently be run into the hole. Also, safety devices such as blow-out preventers usually have numerous cavities and crevices in which the cuttings can become stuck, thereby detracting from the performance of the device or possibly even preventing its operation. Removal and clean-out of such safety devices can be extremely expensive, often costing a quarter of a million dollars or more in the case of a deep sea rig. Further, rapid flow of debris-laden fluid through the casing can even damage the casing surface. Nevertheless, in applications where a large amount of metal must be removed, it is usually necessary to mill at a relatively fast rate, such as 15 to 30 feet of casing per hour. These applications call for the generation of relatively large cuttings, and these cuttings must be removed by the aforementioned method of “forward circulation”, carrying the metal cuttings up to the well site surface via the annulus. 
     In some applications, such as preparation for the drilling of multiple lateral well bores from a central well bore, it is only necessary to remove a relatively short length of casing from the central bore, in the range of 5 to 30 feet. In these applications, the milling can be done at a relatively slow rate, generating a somewhat limited amount of relatively small cuttings. In these applications where a relatively small amount of relatively small cuttings are generated, it is possible to consider removal of the cuttings by trapping them within the bottom hole assembly, followed by pulling the bottom hole assembly after completion of the milling operation. The advantage of doing so is that the cuttings are prevented from becoming stuck in the well bore or in a blow-out preventer, so the risk of damage to equipment is avoided. 
     Some equipment, such as the Baker Oil Tools combination ball type Jet and junk basket, product number 130-97, rely upon reverse circulation to draw large pieces of junk into a downhole junk removal tool. This product has a series of movable fingers which are deflected by the junk brought into the basket, and which then catch the larger pieces of junk. An eductor jet induces flow into the bottom of the junk basket. This tool is typical, in that it is generally designed to catch larger pieces of junk which have been left in the hole. It is not effective at removing small debris, because it will generally allow small debris to pass back out through the basket. 
     Moreover, the ability of this tool to pick up debris is limited by the fluid flow rate which can be achieved through the workstring, from a pump at the well site. In applications where the tool must first pass through a restricted diameter bore, to subsequently operate in a larger diameter bore, the effectiveness of the tool is severely limited by the available fluid flow rate. Additionally, if circulation is stopped, small debris can settle behind the deflecting fingers, thus preventing them from opening all the way. Further, if this tool were to be run into a hole to remove small cuttings after a milling operation, the small cuttings would have settled to the bottom of the hole, making their removal more difficult. In fact, this tool is provided with coring blades for coring into the bottom of the hole, in order to pick up items which have settled to the bottom of the hole. 
     Another type of product, such as the combination of a Baker Oil Tools jet bushing, product number 130-96, and an internal boot basket, product number 130-21. uses a jet action to induce fluid flow into the tool laden with small debris. The internal boot basket creates a circuitous path for the fluid, causing the debris to drop out and get caught on internal plates. An internal screen is also provided to further strip debris from the fluid exiting the tool. The exiting fluid is drawn by the jet back into the annulus surrounding the tool. However, here as before, if this tool were to be run into a hole to remove small cuttings after a milling operation, the small cuttings would have settled to the bottom of the hole, making their removal more difficult. Furthermore, here again, the ability of this tool to pick up debris is limited by the fluid flow rate which can be achieved through the workstring. 
     Another known design is represented by the Baker Oil Tools Model M reverse circulating tool, which employs a packoff cup seal to close off the wellbore between fluid supply exit ports and return fluid exit ports. A reverse circulating flow is created by fluid supply exit ports introducing fluid into the annulus below the packoff cup seal, which causes fluid flow into the bottom of an attached milling or washover tool. This brings fluid laden with debris into the central bore of the reverse circulating tool, to be trapped within the body of the tool. The reverse circulating fluid exits the body of the tool through return fluid exit ports above the packoff cup seal and flows to the surface of the well site via the annulus. This tool relies upon the separation of the supply fluid and the return fluid, by use of the packoff cup seal between the fluid supply exit ports and return fluid exit ports. To avoid damage to this cup during rotation of the tool, the packoff cup seal must be built on a bearing assembly, adding significantly to the cost of the tool. Additionally, here as before, the ability of this tool to pick up debris is limited by the fluid flow rate which can be achieved through the workstring. 
     It is an object of the present invention to provide a tool which incorporates a milling tool, which will pick up and retain all of the small cutting debris generated by the milling tool, during milling operations, thereby avoiding the necessity to pick up small cuttings from the bottom of the hole. It is a further object of the present invention to provide a tool which will pick up and retain small cutting debris generated by a milling tool, without the need for a packoff cup seal or other device susceptible to damage by rotation. It is a still further object of the present invention to provide a tool which will generate a high reverse circulation flow rate at the bottom hole assembly, even when the flow rate available through the workstring, from a pump at the well site, is limited. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is a tool for separating small cutting debris from fluid flow at the bottom hole assembly, during operation of an incorporated milling tool, and for capturing the small debris within the housing of the separator tool. The separator tool relies on a plurality of supply fluid exit ports through the wall of the separator housing into the annulus. In one embodiment, the supply fluid is directed through the supply fluid exit ports by a plurality of high speed eductor jets. The eductor fluid is supplied to the eductor jets by pumping fluid from the surface of the well site through a workstring to which the separator tool is attached. The eductor jets pull a vacuum within the separator tool housing, thereby inducing “reverse circulation” flow of debris-laden fluid into the separator tool through a milling tool attached to the bottom of the separator tool. 
     The induced fluid flow is directed through a deflector tube to reduce the velocity of the fluid and to deflect debris which has been brought into the deflector tube, allowing the debris to drop into an annular area around the deflector tube. The stripped fluid exits the tool by flowing through a screen back to the eductor jets, and thence back down through the annulus toward the milling tool. Excess fluid pumped from the surface returns uphole to the surface through the annulus. 
     In an alternative embodiment, the bottomhole reverse circulation flow is created by a downhole mud motor which drives a downhole pump. The pump circulates bottomhole fluid through exit ports and down through the annulus to the area adjacent to the milling tool, where the bottomhole fluid enters the milling tool carrying small debris. The debris is separated from the fluid as described above. Drive fluid is pumped down to the mud motor through the workstring, by a pump at the surface. Drive fluid exiting the mud motor flows through exit ports in the tool housing, to return to the surface via the annulus. It is not necessary to separate the drive fluid from the bottomhole fluid, because the bottomhole fluid in the annulus is kept clean by the separator tool. This embodiment can create a bottomhole fluid circulation rate at least five times the circulation rate achievable through the workstring. 
    
    
     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: 
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     FIG. 1 is a longitudinal section of the upper end of a first embodiment of the tool according to the present invention; 
     FIG. 2 is a longitudinal section of the lower end of the first embodiment shown in FIG. 1; 
     FIG. 3 is a longitudinal section of the upper end of a second embodiment of the tool according to the present invention, incorporating a downhole motor and pump; and 
     FIG. 4 is a longitudinal section of the lower end of the second embodiment shown in FIG.  3 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As shown in FIGS. 1 and 2, a rotating tool  8  according to the present invention has a drive sub  10  at its upper end, a plurality of sections of wash pipe  12 ,  16 ,  18  connected to the drive sub  10 , a screen crossover  14  and a triple connection sub  20  connected to the wash pipe, and a milling tool  22  connected to the lower end of the triple connection sub  20 . The drive sub  10  is adapted to connect to a rotating workstring (not shown) or to a downhole motor (not shown) connected to a non-rotating workstring, such as coiled tubing, by means such as a threaded connection. The sections of wash pipe  12 ,  16 ,  18 , the screen crossover sub  14 , and the triple connection sub  20  serve as a separator housing. The uppermost wash pipe ejection port section  12 , which is threaded to the drive sub  10 , incorporates a plurality of supply fluid exit or ejection ports  24  penetrating the wall of the wash pipe section  12  at spaced intervals. The screen crossover sub  14 , which is threaded to the ejection port section  12 , serves to hold a tubular filter screen  32  in place below the ejection ports  24 , with the screen  32  extending downwardly toward the milling tool  22  at the lower end of the apparatus. A first wash pipe extension section  16  can be threaded to the screen crossover sub  14 , if necessitated by the length of the screen  32 . A second wash pipe extension section  18  is threaded to the first extension section  16 . The triple connection sub  20  is threaded to the lower end of the second extension section  18 . 
     The milling tool  22  is threaded to the lower end of the triple connection sub  20 . A plurality of blades  23  are positioned at intervals about the periphery of the milling tool  22  for milling metal items, such as casing or liner pipe, from the well bore. The lower end of the milling tool  22  can have a drift plate  25 , which has a diameter close to the inside diameter of the bore hole in which the milling tool  22  will be used. The drift plate  25  serves to prevent metal cuttings from falling down the bore hole. One or more intake slots or ports  26  are provided in the lower end of the milling tool  22  below the blades  23 . In applications where the stuck pipe is not concentrically positioned in the casing or well bore, it has been found that the drift plate  25  can break loose, so in such applications, a milling tool  22  without the drift plate  25  is used, and a single intake port is located at the bottom of the milling tool  22 , instead of a plurality of slots  26 . 
     Importantly, a debris deflector tube  28  is threaded into an interior thread in the triple connection sub  20 , extending upwardly from the triple connection sub  20  toward the screen  32 . A plurality of side ports  30  are provided through the wall of the deflector tube  28 . A deflector plate  31  is provided in the upper end of the deflector tube  28  to deflect any metal cuttings or other debris which might be carried by fluid flowing through the deflector tube  28 , and to separate the debris from the fluid. Alternatively, other means of separating the debris from the fluid can be used, such as deflection plates within the deflector tube  28  to create a spiral fluid flow, thereby separating the heavy debris from the fluid. 
     Another important feature of the deflector tube  28  is that its reduced diameter facilitates movement of the cuttings along with the fluid, up to the point of separation of the cuttings from the fluid for deposit in a holding area. In a representative example, the body of the tool might have a nominal diameter of 7⅝ inches, with the deflector tube  28  having a nominal diameter of 2⅜ inches. It has been found that a fluid flow velocity of approximately 120 feet per minute is required to keep the cuttings moving along with the fluid, depending upon the fluid formulation. This flow velocity can be achieved in the exemplary deflector tube  28  with a fluid flow rate of only about ½ barrel per minute. If a reverse circulation tool without the deflector tube  28  were employed, a fluid flow rate of about 6 barrels per minute would be required to keep the cuttings moving. Put another way, if a reverse circulation tool were not used, with forward circulation instead being relied upon to move the cuttings all the way to the surface via the annulus, a fluid flow rate of 4 to 10 barrels per minute, or even more, would be required. This means that use of the tool of the present invention allows the use of smaller pumps and motors at the well site surface, and use of cheaper formulations of fluid. 
     In the first embodiment of the present invention, as shown in FIG. 1, a plurality of high speed supply fluid eductor nozzles  34  are provided in the wash pipe ejection port section  12 , with each eductor nozzle  34  being aligned with one of the ejection ports  24 , at a downward angle. As the tool  8  is rotated to mill away the metal item from the well bore with the milling tool  22 , fluid is pumped by a pump (not shown) at the surface of the well site down through the workstring (not shown). The fluid flows from the workstring through the drive sub  10 , and then through the eductor nozzles  34 . Since the eductor nozzles  34  have restricted flow paths, they create a high speed flow of fluid, which is then directed downwardly through the ejection ports  24 . As the high speed fluid flows out of the eductor nozzles  34  and through the ejection ports  24 , it creates an area of low pressure, or vacuum, in the vicinity of the eductor nozzles  34 , within the ejection port section  12  of the separator housing. 
     This area of low pressure or vacuum in the ejection port section  12  draws fluid up through the intake ports  26  of the milling tool  22 , through the deflector tube  28 , and through the screen  32 . The fluid thusly drawn upwardly then passes out through the ejection ports  24  to the annulus surrounding the separator housing, to flow downwardly toward the milling tool  22 . Excess fluid supplied via the workstring can also flow upwardly through the annulus toward the surface of the well site, to return to the pump. 
     As fluid flows past the milling tool blades  23 , it entrains small cuttings or debris generated as the blades mill away the casing or other metal item. This debris-laden fluid then enters the intake ports  26  at the lower end of the milling tool  22  and passes into the interior of the deflector tube  28  within the wash pipe extension section  18 . As the debris-laden fluid exits the side ports  30  in the deflector tube  28 , the debris, which is heavier than the fluid, tends to separate from the fluid and settle into an annular area  56  between the deflector tube  28  and the wash pipe extension section  18 . 
     The fluid, which may still contain very fine debris, then flows upwardly to contact the inlet side of the screen  32 . As the fluid flows through the screen  32 , the fine debris is removed by the screen  32 , remaining for the most part on the inlet side of the screen  32 . Fluid leaving the outlet side of the screen  32  then flows upwardly to the area of low pressure, or vacuum, in the vicinity of the eductor nozzles  34 . 
     In most applications, this eductor nozzle embodiment of the invention will create a sufficient flow velocity to entrain virtually all of the small debris generated by the milling tool  22 . In fact, it has been found that a 7⅝ inch tool according to the first embodiment creates a sufficient flushing action to remove the cutting debris from a milling operation within a 30 inch casing. However, in some applications, the flow rate which can be pumped downhole through the workstring may not be sufficient to entrain the milling debris. Such a situation arises when the fluid flow rate which can be created down the sides of the wash pipe is insufficient to entrain the milling debris as the fluid passes the blades  23 . In this type of application, it can become necessary to use the second embodiment of the tool of the present invention, which incorporates a downhole motor and pump as the source of pressurized fluid, as illustrated in FIGS. 3 and 4. 
     The separator apparatus  8 ′ shown in FIGS. 3 and 4 has many elements similar to the apparatus  8  shown in FIGS. 1 and 2. That is, a plurality of ejection ports  24  penetrate the wall of the wash pipe ejection port section  12  at spaced intervals. The screen crossover sub  14  holds a tubular filter screen  32  in place below the ejection ports  24 , with the screen  32  extending downwardly toward the milling tool  22  at the lower end of the apparatus. One or more wash pipe extension sections  18  are threaded to the screen crossover sub  14 . The triple connection sub  20  is threaded to the lower end of the extension section  18 . 
     The milling tool  22 , identical to the milling tool used in the first embodiment, is threaded to the lower end of the triple connection sub  20 . A debris deflector tube  28  is threaded into an interior thread in the triple connection sub  20 , extending upwardly from the triple connection sub  20  toward the screen  32 . Here as before, a plurality of side ports  30  are provided through the wall of the deflector tube  28 , and a deflector plate  31  or a series of deflector plates are provided in the deflector tube  28 . As FIG. 4 illustrates, a plurality of stabilizers  29  can be used in either embodiment to space the deflector tube  28  from the wash pipe. 
     The difference between the first embodiment and the second embodiment is that the second embodiment uses a downhole motor and downhole pump instead of eductor nozzles  34  to draw fluid upwardly through the tool. A drive sub  11  is connected to the workstring, and a motor housing section  13  of wash pipe is threaded to the lower end of the drive sub  11 . A bearing housing section  15  of wash pipe is threaded to the lower end of the motor housing section  13 . The motor housing section  13  houses a downhole motor  36 , such as a mud motor, well known in the art. The downhole motor  36  drives a ported sub  38 , which is housed in the bearing housing section  15 . A bearing block  52  in the bearing housing section  15  supports the ported sub  38 . The ported sub  38  drives a downhole pump  44 ,  46  in the ejection port section  12  of the wash pipe. 
     As the second embodiment of the tool  8 ′ is rotated to mill away the metal item from the well bore with the milling tool  22 , fluid is pumped by a pump (not shown) at the surface of the well site down through the workstring (not shown). The fluid flows from the workstring through the drive sub  11 , and then through the downhole motor  36 . Drive fluid exits the ported sub  38  via discharge ports  40 , and exits the separator housing via drive fluid exit ports  42 . Drive fluid supplied via the workstring flows upwardly through the annulus toward the surface of the well site, to return to the pump. An electric motor could be used instead of the mud motor, without departing from the spirit of the present invention. 
     The downhole motor  36  drives the downhole pump  44 ,  46  to draw bottomhole fluid into the inlet  48  of the downhole pump  44 ,  46 . The bottomhole fluid is then discharged from a plurality of pump discharge ports  50 , to exit the wash pipe ejection port section  12  via the ejection ports  24 . A downhole motor driven by a fluid flow of 200 gpm can achieve a ported sub speed of 400 rpm. Turning the downhole pump at 400 rpm can easily produce a bottomhole recirculation rate of 1000 gpm. This high speed flow of bottomhole fluid is directed downwardly along the annulus surrounding the separator housing. An internal seal or packing  54  can be used to separate the drive fluid flow through the drive fluid exit ports  42  from the bottomhole fluid flow through the ejection ports  24 . 
     As the downhole pump  44 ,  46  draws bottomhole fluid upwardly into the ejection port section  12  bottomhole fluid is drawn up through the intake ports  26  of the milling tool  22 , through the deflector tube  28 , and through the screen  32 . The bottomhole fluid thusly drawn upwardly then passes out through the pump  44 ,  46  and the ejection ports  24  to the annulus surrounding the separator housing, to flow downwardly toward the milling tool  22 . 
     As bottomhole fluid flows past the milling tool blades  23 , it entrains small cuttings or debris generated as the blades mill away the casing or other metal item. This debris-laden fluid then enters the intake ports  26  at the lower end of the milling tool  22  and passes into the interior of the deflector tube  28  within the wash pipe extension section  18 . As the debris-laden fluid exits the side ports  30  in the deflector tube  28 , the debris, which is heavier than the fluid, tends to separate from the fluid and settle into an annular area  56  between the deflector tube  28  and the wash pipe extension section  18 . 
     The fluid, which may still contain very fine debris, then flows upwardly to contact the inlet side of the screen  32 . As the fluid flows through the screen  32 , the fine debris is removed by the screen  32 , remaining for the most part on the inlet side of the screen  32 . Fluid leaving the outlet side of the screen  32  then flows upwardly to the inlet of the downhole pump. 
     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.