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FIELD OF THE DISCLOSURE 
       [0001]    The present disclosure is related to the field of methods and apparatuses for clearing a wellbore, and in particular to methods and apparatuses for clearing a wellbore using milling and circulation, in combination with the use of dual coiled tubing or multi-string spoolable coiled tubing. 
       BACKGROUND 
       [0002]    Since recent developments in the fields of horizontal drilling and multistage fracturing many Exploration and Production (E&amp;P) operators have experienced difficulties utilizing current technologies to mill or drill the balls and ball seats out of the ball frac sleeves of an open hole ball-type frac liner system completed into a formation or reservoir. The restriction caused by these balls and ball seats prevent optimal productivity of the well and can prevent the E&amp;P companies from entering the liner of the wellbore. Recent developments indicate that a work-over or intervention is required to remove the restrictions (balls and seats), to investigate inflow (production log or production evaluate), to re-stimulate the reservoir, and/or remove blockages such as sand or formation material. 
         [0003]    Currently, the technology being used in these situations is typically conventional coiled tubing, water and nitrogen mixtures, and mud motors equipped with drill-bits or mills. These systems can increase the diameter of the liner by removing balls, seats, or other obstructions to achieve a maximum inner diameter of the liner. Current processes, however, create an over-balanced effect/position on the reservoir which in turn can lead to a loss of work-over fluids, such as into the formulation. A loss of work-over fluids can result in the undesired effect of frac proppant (sand) coming out of suspension and “sanding-in” tools and tubing so that they cannot be removed. Sanding-in can result in the entire loss of tools, expensive fishing requirements, and potentially the loss of production from the well which can no longer be accessed. This over-balanced effect can also lead to formation damage resulting in reduced inflow from the formation. The wellbore is often left debris still present and not cleared from the liner, including solids from the seats, frac proppant (sand) and formation fines. This limits the E&amp;P companies from operating the well at its maximum productivity and interferes with the gathering of valuable data that would facilitate optimal development of a given field. 
         [0004]    Mixture of water and gas, such as nitrogen, is often circulated downhole to reduce hydrostatic preserve and lift debris to surface. 
         [0005]    For E&amp;P companies who are presently doing these operations, the cost and supply of nitrogen can seriously impact the economics and overall outcome. Safety is also major concern for E&amp;P companies using current systems and the operations environment can be categorized as moderate to high risk. One reason for the safety concern is that the injection lines, coiled tubing, and return lines, containing the highly compressible nitrogen can be under extreme pressure. If a pressurized line or tubing is to part or break, the energy stored in the volume of the lines explosively discharges. This sudden release can cause the lines to whip uncontrollably until the energy has bled off. The uncontrolled movement of the lines can, in turn, contact and injure personnel and/or damage other equipment. The choice fluid, a liquid/gas mixture, typically used during current operations is low in density to maintain high velocity. However, in turn, it is also known to wash out the surface iron (coiled tubing reel), flow back vessel manifolds and connections. 
         [0006]    Accordingly, there is a need to provide apparatus and methods for clearing a wellbore that can overcome the short-comings of the prior art, such as unstable job economics, potential for formation and equipment damage, and unsafe work environments. 
       SUMMARY 
       [0007]    Methods and downhole tool are provided for clearing a wellbore during milling and fluid circulation within a wellbore. Obstructions such as balls, seats, bridge plugs, or formation material can be milled within a wellbore, including a liner in a wellbore. As a result, larger, unrestricted, diameters can be obtained within the wellbore. The cleared wellbore can allow for various remedial tools to be run into the liner/wellbore. The milled particles can be circulated to surface. The downhole tool can be deployed using a spoolable single or multi-conduit coiled tubing system and can be configured as a well intervention or work-over technology. In some embodiments, the downhole tool can be temporary or mobile. 
         [0008]    The downhole tool disclosed herein comprises an outer tubing connector and an inner tubing connector received in a bore of the outer tubing connector, for respectively coupling to an outer tubing string and an inner tubing string received in a bore of the outer tubing string. The annulus between the inner and outer tubing strings forms a driving flow path for introducing a driving fluid flow downhole to a mill, and the bore of the inner tubing string forms a circulation flow path, for introducing a circulation fluid flow downhole for circulation debris to the surface. A flow diverter firstly directs the driving fluid flow to the mill via one or more axially extending driving flow passages, and secondly directs the circulation flow path into a wellbore annulus for debris circulation to surface via a flow redirector and one or more radially extending circulation flow passages. 
         [0009]    The driving fluid may be a liquid such as drilling mud. The circulation fluid may be a gas such as nitrogen. 
         [0010]    In some embodiments, the downhole tool disclosed herein also comprises a bottom sub and a mill release sub intermediate the bottom sub and the mill for releasing the mill in emergency situations. The bottom sub comprises a piston received in a bore thereof. The piston is normally locked at an uphole, operation position by shear pins, and may be axially movable to a downhole, emergency release position. The piston is coupled to the flow redirector, which in these embodiments is also axially movable between at an uphole, operation position and a downhole, emergency release position. 
         [0011]    In emergency situations, such as when the mill is stuck in downhole debris, a ball may be dropped or pumped through the inner tubing string to block the circulation flow path of the movable flow redirector. Gas is then highly pressurized and applies a sufficient downhole force to the flow redirector and in turn the piston for shearing the shear pins and unlocking the piston. The piston, and a downhole tubular coupled thereto, is then actuated downhole to trigger the mill release sub for releasing the mill. 
         [0012]    The downhole tool disclosed herein reduces the risks related to pressurized nitrogen, avoids wash-out and allows the use of a small diameter inner tubing string for controlled nitrogen use, leading to significant cost saving. 
         [0013]    According to one aspect of this disclosure, there is provided a downhole apparatus for clearing a wellbore. The apparatus comprises: a first tubing forming a first flow path; a second tubing, the first tubing received in a bore of the second tubing, and forming a second flow path along the annulus formed therebetween; a flow diverter connecting distal ends of the first and second tubings; and a mill connected to a downhole end of the flow diverter; wherein the flow diverter comprises a driving flow path therethrough and in fluid communication with the mill, and a circulation flow path in fluid communication with an annulus of the wellbore. 
         [0014]    In some embodiments, the first flow path is the circulation flow path and the second flow path is the driving flow path. 
         [0015]    In some embodiments, the apparatus further comprises: a mill release sub coupled to and intermediate the flow diverter and the mill; and a piston intermediate the flow diverter and the mill release sub, the piston actuatable, by the circulation flow path through the flow diverter, between a first position for normal operation and a second position for triggering the mill release sub to release the mill. 
         [0016]    In some embodiments, the flow diverter further comprises a flow redirector movable between a third position for directing the circulation flow into the annulus of the wellbore and a fourth position for actuating the piston to telescope downhole for releasing the mill. 
         [0017]    In some embodiments, the flow redirector further comprises a ball seat for receiving a ball through the first tubing for actuating the flow redirector to move to the fourth position. 
         [0018]    In some embodiments, the flow redirector further comprises one or more one-way valves for only allowing fluid to flow downhole. 
         [0019]    In some embodiments, the piston further comprises a bore and one or more ports for directing driving fluid flow into the bore of the piston. 
         [0020]    In some embodiments, the piston further comprises one or more one-way valves for only allowing fluid to flow downhole. 
         [0021]    According to another aspect of this disclosure, there is provided a method of clearing obstructions in a wellbore. The method comprises: locating a mill in the wellbore about the obstructions; introducing a driving fluid flow along a driving flow path from surface to downhole for driving the mill; introducing a circulation fluid flow along a circulation flow path from surface into the wellbore annulus at a location in the wellbore above the mill; driving the mill using introduced driving fluid flow to mill the obstructions; and circulating milled obstructions to the surface via the wellbore annulus using the introduced circulation fluid flow; wherein at least a portion of one of the circulation and the driving flow paths is within the other one of the circulation and the driving flow paths. 
         [0022]    In some embodiments, at least a portion of the circulation flow path is within the driving flow path. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]      FIG. 1  illustrates a portion of a tubing string having a downhole tool for milling and clearing debris from a wellbore; 
           [0024]      FIG. 2  is a schematic cross-sectional portion of the structure and the flow paths of the downhole tool of  FIG. 1 , according to one embodiment; 
           [0025]      FIG. 3  is a cross-sectional view of an example of a dual-tubing assembly of the downhole tool of  FIG. 1 ; 
           [0026]      FIG. 4  is a perspective view of a flow diverter of the dual-tubing assembly of  FIG. 3 ; 
           [0027]      FIG. 5  is a side view of the flow diverter of  FIG. 4 ; 
           [0028]      FIG. 6  is an end view of the flow diverter of  FIG. 4 ; 
           [0029]      FIG. 7  is a cross-sectional view of the flow diverter of  FIG. 4 ; 
           [0030]      FIG. 8  is a cross-sectional perspective view of the flow diverter of  FIG. 4 ; 
           [0031]      FIG. 9  is a cross-sectional view of a flow redirector of the dual-tubing assembly of  FIG. 3  for coupling with the flow diverter of  FIG. 4 , and absent the ball seat and flapper valves; 
           [0032]      FIG. 10  is a perspective view of a piston cap of the dual-tubing assembly of  FIG. 3  for mill release actuation; 
           [0033]      FIG. 11  is a side view of the piston cap of  FIG. 10 ; 
           [0034]      FIG. 12  is a cross-sectional view of the piston cap of  FIG. 10 ; 
           [0035]      FIG. 13  is a cross-sectional view of the dual-tubing assembly of  FIG. 3 , showing a driving flow path for introducing an incompressible, driving fluid flow from surface to a hydraulic motor to drive a mill to drill obstructions; 
           [0036]      FIG. 14  is a cross-sectional view of the dual-tubing assembly of  FIG. 3 , showing a circulation flow path for introducing a compressible gas flow from surface into a wellbore annulus for circulating debris to surface; 
           [0037]      FIG. 15  is a cross-sectional view of the dual-tubing assembly of  FIG. 3 , showing a ball being dropped into the dual-tubing assembly for blocking the ball seat and releasing the mill in an emergency situation; 
           [0038]      FIG. 16  is a cross-sectional view of the dual-tubing assembly of  FIG. 15 , showing that the piston of the dual-tubing assembly has been actuated downhole after the ball is dropped; 
           [0039]      FIG. 17  is a cross-sectional view of a portion of the dual-tubing assembly of the downhole tool of  FIG. 1  according to an alternative embodiment, showing the flow diverter and the bottom sub; and 
           [0040]      FIG. 18  is a schematic cross-sectional view of the structure of the downhole tool of  FIG. 1 , according to an alternative flow path embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0041]    Turning now to  FIG. 1 , a downhole tool  100  such as a bottom hole assembly (BHA) is shown, and the components of which are described from an uphole direction to a downhole direction. The downhole tool  100  is located at a distal end section of a tubing string  10 . As shown, the tubing string  10  comprises an outer tubing  102  and an inner tubing  104  received therein. The outer and inner tubing strings  102  and  104  are coupled to a dual-tubing assembly  106  and are in fluid communication therewith. 
         [0042]    The dual-tubing assembly  106  is in turn coupled to and in fluid communication with a hydraulic motor  112 , such as a mud motor, through intermediate subs, such as a milling release tool  108  and a tubing jar  110 . The hydraulic motor  112  drives a mill or drill-bits  114  at a downhole end of the tubing string  10  for milling or drilling obstructions such as balls, seats, bridge plugs, or formation material. 
         [0043]    Herein, the dual-tubing assembly  106  establishes a driving flow path for introducing a flow of driving fluid Fi, which may be an incompressible driving liquid such as drilling mud. The driving fluid Fi is provided from the surface to the hydraulic motor  112  for rotationally driving the mill  114 . The dual-tubing assembly  106  also establishes a circulation flow path for introducing a flow of circulation fluid Fg, which may be a compressible gas, such as nitrogen, from the surface and into the wellbore at a circulation fluid jet position  116  uphole of the mill  114 . The circulation fluid Fg is introduced to the downhole tool  100  to circulate milled obstructions and other debris to the surface. 
         [0044]    With reference to  FIG. 2 , the downhole tool  100  is located in a wellbore  120 . Driving and circulation flow paths  122  and  142  are shown for driving fluid Fi and circulation fluid Fg. For ease of illustration, the milling release tool  108  and the tubing jar  110  are not shown therein. 
         [0045]    As shown in the embodiment of  FIG. 2 , the driving flow path  122  is established from the surface through a tubing annulus  124  between the outer and inner tubing strings  102  and  104 , and through a channel  126  through the dual-tubing assembly  106  and intermediate subs, to the hydraulic motor  112 , introducing driving fluid Fi drive the mill  114 . 
         [0046]    The inner tubing  104  terminates in the downhole tool  100  at the circulation fluid jet position  116 , uphole of the mill  114 , and is fluidly connected to the wellbore annulus  130  between the downhole tool  100  and the wall, liner or casing of the wellbore  120 , via one or more generally radial circulation passages  132 . A circulation flow path  142  is then established from the surface, through the bore  144  of the inner tubing  104 , the one or more circulation passages  132 , and the wellbore annulus  130  back to the surface. The circulation flow path  142  introduces the circulation fluid Fg to the wellbore annulus  130  and to the surface as a circulation flow Fc. 
         [0047]    The driving flow path  122  is fluidly separated from the circulation flow path  142 . 
         [0048]    With reference to  FIG. 3 , in one embodiment, the dual-tubing assembly  106  comprises, from an uphole direction to a downhole direction, a dual tubular structure  152 , a flow diverter  154  and a bottom sub  156 , mutually coupled to each other using suitable means such as threaded connections. 
         [0049]    The dual tubular structure  152  comprises an outer tubing connector  162  and an inner tubing connector  164  received therein. The inner tubing connector  164  has an outer diameter (OD) smaller than the inner diameter (ID) of the outer tubing connector  162  to form a tubing annulus  124  therebetween. 
         [0050]    In this embodiment, the outer tubing connector  162  is a tubular ported at its uphole end for sealably connecting to the outer tubing  102  and secured thereto using set screws. The outer tubing connector  162  also has inner female threading at its downhole end for mating matching, outer male threading at an uphole end of the flow diverter  154 , to couple the outer tubing connector  162  to the flow diverter  154 . 
         [0051]    Similarly, the inner tubing connector  164  is a tubular ported at its uphole end for sealably connecting to the inner tubing  104  and secured thereto using set screws. The inner tubing connector  164  also has outer male threading at its downhole end for mating matching, inner female threading at an uphole end of the flow diverter  154 , to couple the inner tubing connector  164  to the flow diverter  154 . 
         [0052]    As shown in  FIG. 3 , and also referring to  FIGS. 4 to 8 , the flow diverter  154  is a tubular for directing the driving fluid Fi downhole therethrough via one or more axially extending driving flow passages, and directing the circulation fluid Fg to the wellbore annulus  130  via one or more radially extending circulation flow passages. 
         [0053]    The flow diverter  154  has outer and inner threading at its uphole end for coupling to the outer and inner tubing connectors  162  and  164 , respectively. The flow diverter  154  also has outer male threading at its downhole end for coupling to matching, female threading of the bottom sub  156 . 
         [0054]    As shown in the embodiment of  FIGS. 4 to 8 , the flow diverter  154  comprises one or more circumferentially spaced driving fluid passages  176  in the body or wall  172  portion thereof, extending axially from the uphole end of the flow diverter  154  to the downhole end thereof. As will be described in more detail below, the one or more driving fluid passages  176  form part of the driving flow path  122 . 
         [0055]    The flow diverter  154  also comprises one or more circulation passages  132  extending generally radially outwardly from the bore  174  of the flow diverter  154  for fluidly connecting the bore  174  of the flow diverter  154  to the wellbore annulus  130 . In this embodiment, the one or more circulation passages  132  are preferably angled towards an uphole direction. The one or more circulation passages  132  are part of the circulation flow path  142 . The one or more circulation passages  132  are fluidly isolated from the one or more driving fluid passages  176  to separate the circulation flow path  142  from the driving flow path  122 . 
         [0056]    As shown in  FIGS. 7 and 8 , the flow diverter  154  further comprises an uphole-facing shoulder  180  extending radially inwardly from the inner surface thereof into the bore  174  for delimiting the downhole position of a flow redirector  182  of  FIG. 9 . 
         [0057]    Referring again to  FIG. 3 , the flow redirector  182  is received in the bore  174  of the flow diverter  154 , and is fluidly sealably, axially moveable between an uphole, operation position for directing circulation fluid flow from the inner tubing  164  to the wellbore annulus  130 , and a downhole, emergency mill release position for releasing the mill  114  in emergency situations. 
         [0058]    As shown in  FIG. 9 , the flow redirector  182  is a cylinder having an open uphole end  184 , and a closed downhole end  188 . The flow redirector  182  also comprises a chamber  186  fluidly accessible from the open uphole end  184  and to one or more ports  194  on the ported side wall  192  thereof. The uphole end  184  is radially outwardly extended, forming a downhole-facing shoulder  190  for engaging the uphole-facing shoulder  180  to delimit the downhole movement and support the flow redirector  182  in the bore  174 . 
         [0059]    Referring back to  FIG. 3 , and described from uphole to downhole, the flow redirector  182  receives in the chamber  186  a ball seat  202  having a bore therethrough, and one or more (e.g., two shown in  FIG. 3 ) one-way circulation valves  204 , e.g., flapper valves, for only allowing a fluid flowing therethrough along a downhole direction and preventing fluid backflow. 
         [0060]    As shown in  FIG. 3 , the bottom sub  156  is a tubular supporting a piston  212  and a piston cap or crossover  214  axially slidably and fluidly sealably received in a bore  210  thereof. The piston cap  214  is coupled to an uphole end of the piston  212  using suitable means such as threading. As shown, the piston  212  and piston cap  214  are normally locked at an uphole, operation position by shear pins (not shown) through one or more shear pin holes  220  on the body of the bottom sub  156  into recesses  222  on the body of the piston  212 , and may be telescoped downhole in emergency situations for triggering the mill release sub  110  to release the mill  114 . 
         [0061]    The piston  212  is a tubular having a bore  216  for directing the driving fluid flow downhole. A downhole tubular portion  218  of the piston  212 , which may be a downhole tubular coupled to the piston  212 , has a reduced diameter, forming a downhole-facing shoulder for engaging an uphole facing should on the body of the bottom sub  156  to delimit the downhole position of the piston  212 . Correspondingly, the bore  210  adjacent the downhole portion  218  of the piston  212  then forms a downhole chamber  228 B for allowing the piston to axially move downhole and telescope out of the bottom sub  156 . One or more equalization ports  224  on the downhole portion  218  are used for fluid equalization during piston telescoping. 
         [0062]    In this embodiment, the piston  212  also receives in its bore  216  one or more (e.g., two shown in  FIG. 3 ) one-way driving valves  226 , e.g., flapper valves, for only allowing fluid flow downhole and preventing fluid backflow. 
         [0063]    As shown in  FIGS. 10 to 12 , the piston cap  214  is a tubular having a closed upper end  230 , a bore  232  and an open downhole end  234 . An uphole portion  236  has a reduced diameter, and is ported on the side wall  238  thereof to form one or more ports  240  extending from the side wall  238  axially into the bore  232 . 
         [0064]    Referring again to  FIG. 3 , after assembling and in an operation configuration, the flow redirector  182  is located at an uphole position in the bore  174  of the flow diverter  154 , adjacent the downhole end of the inner tubing connector  164  and is uphole delimited there to. The piston  212  and the piston cap  214  are also positioned at an uphole position in the bore  210  of the bottom sub  156 , adjacent and preferably in contact with the flow redirector  182 . One or more shear pins (not shown) are received in the shear pin holes  220  of the bottom sub  156 , extending radially inwardly into the recesses  222  of the piston  212  to lock the piston  212  in position. As the uphole portion  236  of the piston cap  214  has a reduced diameter, the bore  210  adjacent the uphole portion  236  of the piston cap  214  then forms an uphole chamber  228 A. 
         [0065]    With reference to  FIG. 13 , the driving flow path  122  is formed from the outer tubing  102  (see  FIG. 1 ) through the tubing annulus  124 , the one or more driving fluid passages  176 , the uphole chamber  228 A, the one or more ports  240  of the piston cap  214 , the one or more driving flapper valves  226 , the bore  216  of the piston  212 , and the bore of intermediate subs (not shown) to the hydraulic motor  112 . A liquid such as driving mud may be introduced from the surface via the driving flow path  122  to the hydraulic motor  112  for driving the mill  114 . 
         [0066]    As shown in  FIG. 14 , the circulation flow path  142  is formed from the inner tubing  104  (see  FIG. 1 ) through the inner tubing connector  164 , the ball seat  202 , the one or more circulation flapper valves  204 , the one or more ports  194  of the flow redirector  182 , the bore  174  of the flow diverter  154 , and the one or more circulation passages  132  to the wellbore annulus  130 . A gas may be introduced from the surface via the circulation flow path  142  into the wellbore annulus  130  for circulating debris to the surface. 
         [0067]    In an emergency situation such as when the mill  114  is stuck in the wellbore, the tubing string may be pulled uphole to release the mill  114 . If, however, it is determined that the uphole pulling force is insufficient to release the mill  114 , as shown in  FIG. 15 , a ball  302  may be dropped downhole and introduced through the inner tubing string  104 . The ball  302  seats on the ball seat  202 , and blocks the gas flow. Gas is then accumulated uphole of the ball  302  and becomes highly pressurized. The high pressure gas then applies a downhole force to the fluid face of the flow redirector  182 , which in turn applies the downhole force to the piston  212 . When the downhole force on piston  212  grows to a level sufficient for shearing the shear pins in the holes  220 , the piston  212  is released downhole. The downhole tubular portion  218  of the piston  212  then extends or telescopes out of the bottom sub  156 , as shown in  FIG. 16 , to trigger the milling release sub  108  of  FIG. 1 , to release the mill  114 . During the downhole movement of the piston  212 , any fluid in the downhole chamber  228 B is discharged into the bore  216  of the downhole tubular portion  218  via the equalization ports  224  to release the fluid in the downhole chamber  228 B and prevent hydraulic resistance of the downhole movement of the piston  212 . 
         [0068]    Those skilled in the art appreciate that alternative embodiments are readily available. For example, in an alternative embodiment, the flow redirector  182  needs not include any circulation flapper valves  204 . In another embodiment, the piston  212  needs not include any driving flapper valves  226 . 
         [0069]    In yet another embodiment as shown in  FIG. 17 , the flow redirector  182  is coupled to the piston cap  214  via suitable fastening means such as bolt  330 . 
         [0070]    In still another embodiment, the flow redirector  182  is axially and sealably locked in the flow diverter  154 . The bottom sub  156  does not comprise piston  212 , nor piston cap  214 . In this embodiment, other mill release methods and related downhole devices may be used for releasing the mill  114  in emergency situations. 
         [0071]    In above embodiments, using an inner tubing  104  for the circulation flow path  142  and using an outer tubing  102  for the driving flow path  122 , the inner tubing  104  may have a much smaller diameter than that of the outer tubing  102 . A larger annular cross sectional area of the driving flow path  122 , than that of the circulation flow path  142 , provides sufficient hydraulic power to drive the mill. Considering the long length of the outer and inner tubings  102  and  104 , the above embodiments thus provide an advantage of lower tubing cost and lower tubing weight. 
         [0072]    With reference to  FIG. 18 , in an alternative embodiment, however, the inner tubing may be used for the driving flow path, and the outer tubing may be used for the circulation path. It can be seen that an advantage of this embodiment is that the downhole tool  100  has a simpler structure, e.g., not requiring a flow diverter. However, the inner tubing  104  in this embodiment must have a sufficiently large diameter to provide sufficient hydraulic power to drive the mill  114 . Consequently, the cost of the larger-diameter, inner tubing  104  may exceed the costing saving of the simpler structure of the downhole tool  100 , giving rise to a higher total cost and greater tubing weight. 
         [0073]    In some alternative embodiments, the driving fluid, circulation fluid or both may liquid or gas, depending on the design. 
         [0074]    In some alternative embodiments, a vacuum, such as that disclosed in Applicant&#39;s PCT Publication No. WO/2014/161073, may be located in the wellbore annulus  130  for suctioning the debris to surface, enhancing the circulation performance. 
         [0075]    Although embodiments have been described above with reference to the accompanying drawings, those of skill in the art will appreciate that variations and modifications may be made without departing from the scope thereof as defined by the appended claims.

Summary:
A method and a downhole apparatus for clearing a wellbore are disclosed. The method locates a mill in the wellbore about the obstructions, introduces a driving fluid flow along a driving flow path from surface to the mill, and introduces a circulation fluid flow along a circulation flow path from surface into a wellbore annulus at a location in the wellbore above the mill. Then the mill is driven by the introduced driving fluid flow to mill the obstructions; and milled obstructions are circulated to the surface via the wellbore annulus using the introduced circulation fluid and gas flow. At least a portion of the circulation flow path is within the driving flow path.