Abstract:
A method of cleaning a casing-lined borehole comprises the steps of: circulating fluid in the borehole to entrain material in the circulating fluid; separating the entrained material from the fluid within the borehole; and then removing the separated material from the borehole.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method and apparatus for cleaning boreholes. In particular, but not exclusively, the present invention relates to a method and apparatus for removing particulate debris from a casing-lined borehole in an onshore or offshore oil or gas well. 
     BACKGROUND OF THE INVENTION 
     It is known to create an onshore or offshore oil or gas well by drilling a borehole extending from the surface (at ground or seabed-level respectively), before installing a cylindrical, typically metal casing in the borehole, and cementing the casing into the borehole. The borehole may be “deviated” (extending at an angle from the vertical) and may feature branch or lateral boreholes which may themselves be lined and cemented. Such operations often lead to the inside wall of the casing becoming soiled with materials such as drilling mud residue (“mud-cake”), well fluid residue, and cement residue, which may hamper subsequent downhole operations, and the satisfactory withdrawal of well fluids. 
     In order to overcome problems associated with the build-up of such materials, it is necessary to physically remove these materials from the casing wall. Typically this is accomplished by inserting a rotating string having a drill bit and\or a dedicated casing scraper tool into the casing, running the drill bit and\or scraper to the bottom of the casing, and then working the drill bit and\or scraper up and down the casing. The residue materials are then circulated out of the well by pumping a cleaning fluid through the casing, which transports the materials to the surface. 
     However, it becomes increasingly difficult to circulate the materials out of the casing in extended reach and deviated wells. Therefore a number of devices have been developed to facilitate entrainment and removal of the residue materials, incorporating brushes and other agitators. However, these devices have been found to be unreliable or ineffective in removing the residue materials. 
     It is amongst the objects of the present invention to obviate or mitigate at least one of the foregoing disadvantages. 
     BRIEF SUMMARY OF THE INVENTION 
     According to a first aspect of the present invention, there is provided a method of cleaning a casing-lined borehole, the method comprising the steps of: 
     circulating fluid in the borehole to entrain material therein; 
     separating the entrained material from the fluid within the borehole; and 
     removing the separated material from the borehole. 
     According to a second aspect of the present invention, there is provided an apparatus for use in removing material from a borehole, the apparatus comprising: 
     circulating means for circulating fluid in a borehole to entrain material therein; and 
     separating means for separating the entrained material from the fluid within the borehole. 
     References to a casing-lined borehole refer to a borehole which has been lined with a suitable casing, liner, or any other suitable tubular lining member, as will be appreciated by persons skilled in the art. 
     Thus the present invention may allow a fluid to be circulated in a casing-lined borehole to entrain material in the borehole, typically material gathered in the end of the borehole or in the “low” side of an inclined or horizontal bore, by entraining the material in a carrier fluid, separating the material from the fluid and subsequently removing the separated material to the surface. 
     The material may be mud residue, such as mud-cake, well debris, or cement residue or the like, produced by the operations involved in creating a lined borehole. Further, the material may be sand or scale, which may build up in the bore during production. The material may have been adhered to the inner wall of the borehole, and may be dislodged from the borehole inner wall in the course of the cleaning operation. Preferably, the material is dislodged using a drill bit and\or a casing scraper coupled to a support string which also supports the apparatus. The string, for example a string of drill pipe, may be rotated from the surface, and may be run into the borehole to the region of the borehole to be cleaned before being moved axially in the borehole to dislodge material from the wall thereof. Alternatively, the apparatus may be run on wireline or coiled tubing. 
     The fluid may be a viscous mud, a cleaning fluid such as brine, and may contain any appropriate additives. The fluid may be pumped down the borehole from the surface, through a string bore or through an annulus between a string and an inner wall of the borehole, or may be recirculated within the borehole. 
     Preferably, the circulating means includes an impeller, preferably a screw, which is rotatable to facilitate circulation of fluid in the borehole. The impeller may be coupled to a supporting string, such that rotation of the string imparts rotation on the impeller. Alternatively, the impeller may rotate while the supporting string remains stationary. In other embodiments, for example where the apparatus is mounted on wireline or coiled tubing, rotation may be provided by electric motor or hydraulic motor. The circulating means may further include a pump located on surface or in a supporting string. 
     Preferably, a tubular member or sleeve is provided, having an inlet for receiving fluid circulating in the borehole. The inlet may be normally closed, and may be opened by fluid pressure force, for example by fluid being pumped through a supporting string. One or more fluid jetting outlets may be provided above the inlet, to permit fluid to be jetted into the annulus above the inlet to create a barrier to carrier fluid flow. Radially extending flow deflectors, which may be in the form of blades, may also be provided above the inlet, to scrape or otherwise dislodge material from the inner wall of the borehole and into the fluid inlet. The flow deflectors may be normally retracted, and may be extended by fluid pressure. Preferably also, the impeller is located within the tubular member. The material may be separated from the fluid within the tubular member. The tubular member may include an outlet having a filter which retains the solid material, allowing the fluid to pass therethrough to return to the surface or to pass to a downhole pump for recirculation. The filter may be annular or cylindrical, or may be formed by forming the outlet of restricted area openings, such as slits. A plurality of such filters may be provided, for example the filters may define successively reducing flow passages. Preferably, the impeller is adapted to clean the filter, for example the impeller may be a screw and move across the face of the filter. This prevents build up of material on the filter, and minimises the possibility of the filter clogging. 
     The fluid may be pumped into the borehole through a string of pipes passing through the tubular member and having an outlet in the borehole. The fluid outlet may be at or towards the bottom of the string. The outlet may be provided in a drill bit. Alternatively, the fluid may be pumped into the borehole down an annulus formed between an outer wall of the tubular member and the wall of the borehole. In a yet further alternative, the tubular member may be provided on a wireline, slickline, or coil tubing assembly. 
     A venturi may be disposed within the tubular member to create a restriction to flow of fluid through the tubular member, to increase the fluid velocity and aid circulation of the fluid and entrainment of solid material therein. 
     One or both of the impeller and tubular member may be coupled to a support string. In one embodiment, a differential gear assembly is provided to couple the tubular member to the support string. Thus, when the string is rotated, which may also serve to dislodge material from the inner wall of the borehole, the tubular member may be counter-rotated. Alternatively, the tubular member may be fixed against rotation within the borehole such that relative rotation between the tubular member and the string may be provided when the string is rotated. 
     In a further alternative embodiment, the impeller may be coupled to the tubular member. The string may remain rotationally stationary, and the tubular member and screw may be rotated to provide relative rotation therebetween. Alternatively, the string may be rotated from the surface to counter-rotate the tubular member and screw via the differential gear assembly. 
     The material may be isolated within the borehole by providing a storage chamber within the tubular member. The storage chamber may be disposed in an annular or cylindrical cavity defined by inner walls of the member and at least partially defined by a filter. Thus, material separated from the fluid by the filter may be collected in the storage chamber, which material may be removed from the borehole by withdrawing the tubular member. Alternatively, the tubular member may have an upper inlet; fluid may be pumped into the borehole below the tubular member and circulated around the tubular member through an annulus defined between the outer wall of the tubular member and the wall of the borehole, thereby transporting entrained material up the annulus. This may create a venturi effect, such that fluid exiting the annulus above the tubular member decreases in velocity, causing the entrained material to come out of suspension with the fluid and fall into the tubular member. 
     Preferably also, the apparatus includes means for dislodging material from the borehole wall, which means may include a drill bit, casing scraper or end mill. Most preferably, the apparatus includes a body carrying a scraper defined on a flat of the body, the body including means for urging the scraper towards the borehole wall. Said urging means may be normally retracted, and may be extended by fluid pressure. The urging means may be in the form of one or more shoulders, circumferentially spaced from the scraper, and adapted to direct fluid towards the scrapers. The scraper may include one or more blades, with a fluid channel defined in front of each blade, such that fluid may pass upwardly through the channels. Preferably, at least two blades are provided, a leading blade defining a relatively aggressive cutting surface to dislodge and break up material, such as scale, from the borehole wall, and the following blade being less aggressive to clean the wall. 
    
    
     BRIEF DESCRIPTION OF TEE DRAWINGS 
     Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
     FIG. 1 is a view of apparatus for use in cleaning a casing-lined borehole in accordance with an embodiment of the present invention, shown in section above line A —A and below line B—B; 
     FIG. 2 is a view of apparatus for use in cleaning a casing lined borehole in accordance with an alternative embodiment of the present invention, shown in section below line C—C; 
     FIG. 3 is a perspective view of apparatus in accordance with a further embodiment of the present invention; 
     FIG. 4 is an enlarged view of milling and cutting portions of the apparatus of FIG. 3; 
     FIG. 5 is an end view of the apparatus of FIG. 3, showing the milling portion; 
     FIG. 6 is an enlarged sectional view of the cutter portion of the apparatus of FIG. 3; 
     FIG. 7 is an enlarged sectional view of a cutter is positioning element of the apparatus of FIG. 3; 
     FIG. 8 is an enlarged sectional view of a solids separation portion of the apparatus of FIG. 3; and 
     FIG. 9 is a diagrammatic view of apparatus in accordance with a still further embodiment of the present invention; and 
     FIGS. 10 and 11 are diagrammatic sectional views of a scraper blade of the apparatus of FIG. 9, shown in extended and retracted configurations respectively. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring firstly to FIG. 1, there is shown an apparatus for use in cleaning a borehole in which a casing has been installed and cemented, the apparatus indicated generally by reference numeral  12 . The apparatus  12  is coupled to a tubular drill string  14 , above a drill bit  16  and a casing scraper  18 . Mud residue (known as “mud-cake”), well debris and\or cement residue (not shown) have become adhered to the inner wall  20  of the casing  10  during preliminary well operations, and require removal as such residues may hamper further operations and the extraction of well fluids from the borehole. 
     The apparatus  12  comprises an annular metal sleeve  22 , a right-hand threaded screw impeller  24  located within the sleeve  22  and coupled to a pipe  15  which forms part of the string  14 , an annular venturi restriction  28  extending radially from the inner wall  26  of the sleeve  22  into a sleeve cavity  30 , a slotted screen filter  32  forming an upper portion  34  of the sleeve  22 , and a gear assembly  36 . The solid-walled lower end of the sleeve  22  is of slightly larger diameter than the slotted upper portion  34  of the sleeve  22 , to minimise erosion of the screen by fluid flowing upwardly in the annulus between the sleeve  22  and the bore wall  20 . 
     To facilitate manufacture and replacement, the screw impeller  24  is formed of 3 feet (approximately 1 meter) segments. Also, the screw impeller  24  tapers as it extends upwardly; this tapering profile, providing increasing clearance between the screw  24  and the inner wall of the sleeve  22 , facilitates compaction of material above the screw  24  without stalling the screw. 
     As will be described, in use the apparatus collects debris from the bore in the upper slotted portion  34  of the sleeve and, while the apparatus  12  is in use, the debris is retained in the sleeve  22  by the action of the screw impeller  24 . However, while the apparatus  12  is being tripped out, there may be no rotation of the impeller  24 , such that the debris may tend to move downwardly within the sleeve  22 . To prevent loss of such material, the open lower end of the sleeve  22  is provided with a diaphragm  39  which acts as a one-way valve, that is fluid and solids may flow upwardly into the sleeve  22 , but are prevented from dropping back out of the sleeve. 
     To prevent the sleeve  22  from rotating in the bore when the apparatus  12  is in use, the sleeve  22  carries four circumferentially spaced blocks  37  which act as brakes and are activated to extend and contact the casing wall  20  when the pipe  15  rotates. The blocks  37  are configured to retract when the pipe  15  is placed in tension. 
     The drill bit  16  has cutting teeth  38  which, as the drill string  14  is rotated in a clockwise direction (viewing in the direction of the arrows B) from the surface, remove portions of the residue adhered to the inner wall  20  of the casing  10 , the dislodged residue material falling to the bottom  40  of the borehole. Likewise, the casing scraper  18  includes left-hand threaded scraper blades  42  which extend helically around the outer surface  44  of the scraper  18 , and which protrude from the surface  44 . The scraper blades  42  remove residue material from the inner wall  20  of the casing  10  in a similar manner to the drill bit  16 , with the dislodged material likewise falling to the bottom  40  of the borehole. This residue material is then removed, as will be described in more detail below. 
     The differential gear assembly  36  comprises three bevelled gears (not shown), a first of which is coupled to the drill string  14 , a second of which is coupled to the sleeve  22  in face-to-face disposition with respect to the first gear, and the third of which is disposed perpendicular to the first and second gears and coupling them together. Thus, rotation of the drill string  14  and associated screw  24  from the surface imparts a counter-rotation on the sleeve  22 , in the opposite direction to the drill string  14 . 
     In the position shown in FIG. 1, the apparatus  12  has been run to the bottom  40  of the borehole, with the drill bit  16  and casing scraper  18  having dislodged the material adhered to the inner wall of the casing  10  as described above. To ensure complete removal of the residue material, the drill string  14  is raised and lowered along the length of the casing  10  before being returned to the bottom  40  as shown in FIG. 1. A viscous mud  46  is pumped down the inside of the drill string  14  from the surface, exiting the string  14  from the drill bit  16 , as shown by the arrows “D” in FIG.  1 . This fluid  46  entrains any residue material which has collected in the bottom  40  of the borehole, and the fluid then flows up the casing  10  to the scraper  18 . The scraper blades  42  are, as described above, raised from the outer surface  44  of the scraper  18 , creating a helical flow path for the fluid  46 , allowing the fluid to flow around the scraper  18  in the direction of the arrows shown. The fluid then continues up the casing  10 , entering the sleeve  22  via an annular sleeve inlet  48 . Suitable packing means (not shown) may be provided to seal the sleeve  22  in the casing  10 , or at least restrict flow between the sleeve and casing, whilst permitting the rotation of the sleeve  22 . 
     The right-hand threaded screw  24  draws the fluid  46  carrying the residue material through the sleeve  22 , as the screw  24  rotates in the same direction as the drill string  14 . The screw  24  is of outside diameter slightly smaller than the inside diameter of the sleeve  22 , to provide a close fit with the sleeve  22 , to prevent residue material from travelling down between the screw  24  and the inner wall  26  of the sleeve  22 . The venturi  28  increases the velocity of the fluid  46  exiting the portion of the sleeve  22  above the screw  24 , to faciliate circulation of the fluid  46 , and to assist in the removal of residue material from the screw  24 . The fluid  46  carrying the residue material then passes into the upper portion  34  of the sleeve  22 , and the fluid exits the apparatus  12  through the annular filters  32 , as shown by the arrows in FIG.  1 . The solid residue material separated from the fluid by the filters  32  is collected in the annular cavity  30  extending from the venturi  28  to the gear assembly  36 . When all of the residue material has been collected in the cavity  30 , or the cavity  30  has been filled, the apparatus  12  is retrieved to the surface, where the sleeve  22  is de-coupled from the drill string  14  for cleaning and removal of the residue material. 
     Referring now to FIG. 2, there is shown a bore cleaning apparatus indicated generally by reference numeral  50 . The apparatus  50  is mounted on an “electric” wireline of a type known in the art, enabling the apparatus  50  to be rapidly deployed or removed from the borehole. The apparatus  50  comprises a tubular metal sleeve  52  containing a bearing-mounted right-hand threaded screw  54  coupled to a tool string  55 , a venturi restriction  58  within the sleeve  52 , annular filters  60  in a lower portion of the sleeve  52 , and an electric motor  62  disposed within the sleeve  52 . The motor  62  is coupled to a power supply on the surface via the wireline  64 , and is coupled via a gear assembly  63  to the tool string  55 . The upper end of the sleeve  52  defines a number of apertures  66 , to allow fluid communication between the borehole and the sleeve interior. The tool string  55  is rotated by the electric motor  62  and rotation of the tool string  55  and the screw  54  draws fluid carrying entrained residue material from the borehole, through the apertures  66 , and into the sleeve  52 . The residue material and fluid travel down through the sleeve  52 , the fluid passing out of the sleeve through the filters  60 , where the residue material is separated from the fluid. The apparatus  50  is drawn up through the borehole simultaneously, to facilitate the flow of fluid and residue material through the sleeve  52 . 
     Various modifications may be made to the foregoing embodiments within the scope of the present invention. 
     For example, the fluid circulated through the borehole may be a cleaning fluid, or a viscous mud including a cleaning fluid or a cleaning additive. The fluid may be pumped down the annulus formed between a drill string and a sleeve and the inner wall of the casing, returning to the surface via the drill string. The sleeve may be fixedly sealed within the borehole via a suitable packer or the like. The screw may be coupled to the sleeve. The drill string may remain rotationally stationary, and the sleeve and screw may rotate around the string, driven by a suitable downhole motor. 
     A sleeve may be provided defining an upper inlet and with a closed lower end. Thus fluid pumped down a drill string and into the borehole below the sleeve may flow up through an annulus between the outer wall of the sleeve and the inner wall of the borehole, creating a venturi effect. The sleeve may define a chamber for collecting the solid material, which may fall out of suspension with the fluid when the fluid exits the annulus, adjacent the sleeve inlet location. 
     Reference is now made to FIGS. 3 to  8  of the drawings, which illustrate apparatus  70  in accordance with a further embodiment of the present invention. As will be described below, the apparatus  70  is utilised to dislodge debris from the wall of a bore, entrain the dislodged material in a stream of fluid, separate the material from the fluid, and retain the separated material within the apparatus  70 . 
     Reference is first made to FIG.  3 . The apparatus  70  is intended to be mounted on the end of a work string (not shown) or the like capable of transmitting drilling fluid and rotation from the surface, and thus the upper end of the apparatus  70  defines a conventional coupling for engagement with the end of the supporting string. The lower end face of the apparatus  70  defines a milling face  72 , and the side face of the apparatus  70  above the mill defines cutters  74 ,  76  (FIG. 4) for scraping and cleaning the bore wall. In addition, the side face of the apparatus  70  adjacent the cutters  74 ,  76  carries cutter positioning elements in the form of radially extendible shoulders  78 . As will be described, the shoulders  78  may be energised to locate the cutters  74 ,  76  adjacent the bore wall. Upwardly of the cutters  74 ,  76  and shoulders  78  are fluid inlets  80 , through which fluid is drawn by a screw impellor arrangement  82  (see FIG.  8 ). As will be described, and in a somewhat similar manner to the above-described embodiments, the screw  82  draws fluid and debris into an annular chamber having an external slotted screen wall  84 . Thus, solids entrained in the fluid are retained within the screen  84 , while the fluid is free to flow through the slotted screen  84  and up through the annulus to the surface. 
     Reference is now also made in particular to FIGS. 4 and 5 of the drawings, which show the milling face  72  defined by the lower end of the apparatus  70  and which carries aggressive cutting elements  86 . The face  72  also defines jetting nozzles  88  through which drilling fluid may pass from the hollow interior of the apparatus  70  to impinge on the surface being milled. 
     The cutters  74 ,  76  extend along the side wall of the lower end of the apparatus  70 , and details of the cutters are also shown in FIG. 6 of the drawings. As may be seen from this figure, the cutters are located on a “flat”  90  on the otherwise cylindrical sub  92  which forms the lower end of the apparatus. The first cutter  74  features an angled fixed blade  94  providing an aggressive cutting surface to break up mud cake, cement residue, scale and the like on the inner surface of the well bore casing, the second cutter  76  featuring a less aggressive fixed blade  96  which is intended to clean the casing wall. Both cutters  74 ,  76  define respective drilling fluid valleys  98 ,  99  along which the drilling fluid may flow, carrying debris produced by the action of the milling face  72 , and also carrying debris dislodged from the casing wall by the blades  94 ,  96 . 
     The three shoulders  78  are axially spaced along the sub  92  and are staggered around the sub circumference, and thus serve to direct drilling fluid towards the cutters  74 ,  76 . Further, when energised to extend radially from the sub  92 , the shoulders  78  tend to push the sub flat  90  towards the casing wall, and thus push the cutter blades  94 ,  96  into contact with the casing wall. A section of one of the shoulders  78  is shown in FIG. 7 of the drawings, and it will be seen that the shoulder  78  is mounted in an aperture  100  in the sub wall, and sits on a pair of mirror-image cammed pistons  102  which are normally pushed apart by a spring  104 , such that the shoulder  78  may assume a retracted position. The pistons  102  each have a face  106  in communication with the hollow interior of the sub  92  via passages  107  such that elevated drilling mud pressure within the apparatus  70  will tend to push the pistons  102  towards one another, and urge the shoulder  78  radially outwardly. 
     Reference is now made in particular to FIG. 8 of the drawings, this showing an enlarged sectional view of the solid separation portion of the apparatus  70 . This portion is located upwardly of the milling and cutting portion and includes the fluid inlets  80  which are defined in an outer sleeve  108  of slightly smaller diameter than the sub  92 . The sleeve  108  is mounted on an inner mandrel  110  and is rotatable relative thereto with bearings  112  being provided between the sleeve  108  and the mandrel  110  as appropriate. Furthermore, a drive cog  114  is provided between racks  116 ,  118  defined by the sleeve and mandrel  108  and  110 , which results in the sleeve  108  rotating in the opposite direction to the mandrel  110 . The resulting contra rotation of the screw  82 , which is mounted on the mandrel  110 , draws fluid in through the inlets  80  and carries the fluid to the separating portion. 
     The fluid inlets  80  are normally closed by a sleeve  120  mounted on the mandrel  110  and which is urged to close the inlets  80  by a spring  122 . The sleeve  120  however defines a piston face  124 , in communication with the mandrel throughbore via a passage  126 , such that elevated drilling fluid pressure within the mandrel bore causes the sleeve  120  to retract and open the inlets  80 . Further passageways  128  are provided above the inlets  80 , the passageways  128  leading to jets  130  which, in use, create a fluid barrier in the annulus around the sleeve  108 , such that fluid and debris flowing up the annulus are directed into the inlets  80 . 
     In use, the apparatus  70  is run in to the well bore with minimum rotation and drilling mud circulation. On reaching the bottom of the bore or the desired work area, the flowrate and pressure of drilling mud pumped into the string is increased, energising the shoulders  78 , and pushing the cutters  74 ,  76  into contact with the casing wall. The string is also rotated. The drilling fluid exits through the jetting nozzles  88  in the milling face  72  and then passes upwardly through the valleys  98 ,  99  defined by the cutters  74 ,  76 , carrying away the debris displaced by the blades  94 ,  96 . Above the cutters  74 ,  76 , the increasing pressure of drilling fluid will have caused the drilling sleeve  120  to retract and open the fluid inlets  80 , and the flow of drilling fluid through the jets  130  above the inlets  80  creates a barrier such that the fluid flowing up the annulus is directed through the inlets  80 . The fluid, and any debris entrained therein, is then drawn upwardly through the sleeve  108  by the screw  82 , in a similar manner to the first embodiment, the fluid then passing through the slots in the screen wall  84 , leaving the debris trapped within the sleeve  108  above the screw  82 . 
     When the “clean up” operation is completed, the rate of circulation of drilling fluid is reduced, such that the sleeve  120  closes the inlets  80 , trapping any retained debris between the sleeve  108 , screen  84 , and the mandrel  110 . The apparatus  70  may then be withdrawn from the well bore. 
     Reference is now made to FIG. 9 of the drawings, which illustrates an apparatus  140  in accordance with a still further embodiment of the present invention. The apparatus  140  has a substantially similar screw and screen arrangement to the embodiments described above, and these features will therefore not be described again in any detail. However, the lower portion of the apparatus  140  differs somewhat from those embodiments described above. 
     The apparatus  140  features a mandrel  142  which is rotatable with a supporting workstring (not shown) and provides mounting for a drill bit  144 . Rotatably mounted on the mandrel  142 , rearwardly of the bit  144 , is a sleeve  146  which, in use, does not rotate relative to the well bore wall. The sleeve  146  has a tapered leading end which defines a number of fluid inlets  148  which open into an annulus between the sleeve  146  and the mandrel  142 . As with the above-described embodiment, passageways extend through the mandrel  142  and the sleeve  146  such that, in use, jets of drilling fluid  150  exit the sleeve  146  above the inlets  148 , creating a fluid barrier. 
     The sleeve  146  further carries three blades  152  which, in a similar manner to the shoulders  78  described above, are energisable by internal fluid pressure to extend outwardly from the sleeve  146  into contact with the casing wall. One of the blades  152  is illustrated diagrammatically in FIGS. 10 and 11 of the drawings, shown in retracted and extended configuration respectively. Directly in front of each blade is an aperture  154  opening into the annulus between the mandrel  142  and the sleeve  146 . The passage of fluid through this annulus, from the fluid inlets  148 , creates vortices which draw debris and fluid dislodged from the casing wall by the blades  152  in through the apertures  154 . 
     In use, the apparatus  140  is run into a bore to be cleaned mounted on an appropriate workstring. The string is rotated and thus rotates the mandrel  142  and the drill bit  144 . Drilling fluid is pumped through the string and exits the jetting nozzles in the drill bit  144 . This fluid then passes upwardly around the drill bit  144  and into the fluid inlets  148 , with the jets  150  creating a fluid barrier as described above. The elevated drilling fluid pressure within the apparatus  140  will have energised the blades  152 , which stabilise and rotationally lock the sleeve  146  in the bore and prevent the sleeve  146  from rotating with the mandrel  142 . Reciprocal movement of the apparatus causes the blades  152  to knock debris from the casing wall, and this debris is drawn into the blade apertures  154  by the fluid flowing upwardly between the S mandrel  142  and the sleeve  146 . The drilling fluid, and entrained debris, then passes through a screw chamber  156  and a separator (not shown) in a similar manner to the above-described embodiments. The screw chamber  156  is coupled to the sleeve  146 , and thus does not rotate, while the screw within the chamber  156  rotates with the work string and mandrel  142 .