Abstract:
An apparatus for borehole cleaning comprises a tubular conveyance extending from the surface into a borehole to a region to be cleaned, a motor mounted at the end of the tubular conveyance that in use is introduced into the borehole, a pump connected to the motor and having a nozzle, and a power cable extending trough the tubular conveyance from the surface to provide power to the motor, The pump is arranged such that, when positioned in the borehole and operated by the motor, the pump withdraws material from the borehole through the nozzle and pumps it into the tubular conveyance to the surface.

Description:
TECHNICAL FIELD 
     Embodiments of the invention relate to systems for borehole cleaning that allow removal of materials in a borehole preventing flow. In particular, embodiments of the invention relate to system for use in wells such as oil and gas wells. 
     BACKGROUND OF THE INVENTION 
     As oil and gas are extracted from producing wells, sand and heavy oils that have flowed through the perforations accumulate. These are too heavy to flow to the surface along with the usual fluids produced by the well at normal production rates, and tend to accumulate in low-lying areas as shown in  FIGS. 1 and 2 . Additionally, drilling muds used during the drilling process are generally heavier than the reservoir fluid, and tend to also segregate to low-lying areas of the well. Finally, proppant used during reservoir fracturing operations is not always completely removed. The accumulation problem in particularly severe where the trajectory of the well  10  is at or close to horizontal in the producing reservoir  12  and sumps  14  are present. Deposits  16  in these regions reduce the effective cross-section of the well  10  with a corresponding decrease in flow area  18  and therefore increase the pressure drop of the production fluids. In order to maintain or increase the production of such a well, it is necessary to remove this fill. Conventional methods of fill removal (or cleanout) involve high-pressure jetting through coiled tubing (CT) to mobilize the fill around the cleaning tool and sweep it to the surface by slowly pulling the CT up, the flow of jetting fluid and production fluid carrying the loosened fill to the surface. This high pressure mobilization jetting does increase the Bottom Hole Pressure (BHP) of the reservoir though, so it is only applicable to wells in formations that can sustain a full hydrostatic column (or foam column, in the case of foam clean-outs) and the increased pressure due to jetting. An example of one such technique is the PowerCLEAN service offered by Schlumberger. 
     In most well cleanout applications, the reservoir pressure is high enough, and the rock permeability low enough, to allow increase of pressure in the well while performing cleanout operations. In others, foam can be used to sweep the fill up. However there are still many wells that either cannot hold a foam column, or where foam use is restricted due to logistics reasons (e.g. procurement and disposal of N2 foam). In these situations, the only existing cleanout solution is a concentric coiled tubing (CCT) service to power a downhole jet pump. Using CCT implies a high use of power liquid to move the fill out and is limited in length by the weight of the coil-in-coil assembly. 
     Examples of CCT techniques can be found in U.S. Pat. No. 2,548,616, U.S. Pat. No. 5,033,545, U.S. Pat. No. 5,269,384, U.S. Pat. No. 5,375,669, U.S. Pat. No. 6,263,984, U.S. Pat. No. 6,015,015, U.S. Pat. No. 6,497,290, U.S. Pat. No. 6,640,897, U.S. Pat. No. 6,712,150, U.S. Pat. No. 5,503,014, and WO 2005085580 A. 
     Embodiments of the invention aim to provide an alternative to CCT techniques while also extending the depths at which clean-out operations can be performed. An embodiment of the invention is based on the use of a downhole pump that is powered by a cable running inside the tubing conveyance. 
     SUMMARY OF THE INVENTION 
     One aspect of the invention provides apparatus for borehole cleaning, comprising:
         a tubular conveyance for extending from the surface into a borehole to a region to be cleaned;   a motor mounted at the end of the tubular conveyance that in use is introduced into the borehole;   a pump connected to the motor and having a nozzle;   a power cable extending trough the tubular conveyance from the surface to provide power to the motor;
 
the pump being arranged such that, when positioned in the borehole and operated by the motor, the pump withdraws material from the borehole through the nozzle and pumps it into the tubular conveyance to the surface.
       

     A second motor and pump can be located in the tubular conveyance above the pump so as to provide extra lift to the material to be removed from the well. Additional ‘booster’ pumps can be added in this way up to the power limit of the wireline cable. 
     A gas supply line can extend at least part way along the inside of the tubular conveyance and be arranged to introduce gas into the material-laden flow in the tubular conveyance above the pump. 
     The apparatus can further comprise a filter between the nozzle and the pump to prevent large particulate material passing into the pump from the borehole. Preferably, the filter removes material of greater than 1 mm from the flow. 
     It is particularly preferred that the apparatus comprises means to move the nozzle when the pump is operated downhole. Movement of the nozzle can be used to further mobilize the fill and suspend it in the fluids in the well. The means can rotate and/or reciprocate the nozzle. A separate motor can be provided to enable this mobilizing movement. Alternatively, a mechanical connection to a rotor in the pump can be provided for this purpose. 
     Features can be provided on the outside and/or inside of the nozzle to accelerate the flow of material when operated downhole to aid in movement of solids. 
     The tubular conveyance is preferably coiled tubing. 
     Another aspect of the invention provides a method of cleaning a well using an apparatus as defined above, comprising:
         extending the tubular conveyance into the borehole so as to position the nozzle in a region to be cleaned;   operating the pump so as to draw fluid and solid material from the region and pump them to the surface through the tubular conveyance.       

     The method can further comprise injecting gas into the materials in the tubular conveyance to create foam of reduced density to assist pumping of the materials to the surface. 
     Preferably, the solid materials are agitated downhole to improve removal by the pump. 
     The nozzle can be rotated or reciprocated while operating the pump. 
     Alternately advancing and withdrawing the tubular conveyance over a limited distance can be used to reciprocate the downhole end of the conveyance in the region to be cleaned. 
     In one embodiment, the tubular conveyance is extended until the pump is located at the bottom of a region to be cleaned and progressively withdrawing the conveyance to move the pump upwards through the region as the pump is operated. In another, the tubular conveyance is extended until the pump is positioned at the top of a region to be cleaned and progressively advancing the conveyance to move the pump downwards through the region as the pump is operated. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a schematic view of a well in which the invention can be used; 
         FIG. 2  shows a cross-section through the well on line  2 - 2  of  FIG. 1 ; 
         FIG. 3  shows a schematic view of a system according to the invention deployed in a well; 
         FIG. 4  shows a detailed view of part of the system of  FIG. 3 ; 
         FIG. 5  shows an embodiment of the invention for handling larger particulate materials in the fill; 
         FIG. 6  shows an embodiment of the invention for fill mobilisation; 
         FIGS. 7-11  show embodiments of features that can be added to a nozzle to improve fill mobilisation; and 
         FIG. 12  shows an embodiment of the system comprising a flow-diverter. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 3  shows the use of a system according to an embodiment of the invention in a well of the type shown in  FIGS. 1 and 2 . The system includes a CT surface system  20  that reels a coiled tubing  22  into the well  24  through surface pressure control equipment  26 . An electrically powered motor  28  and pump  30  are located at the end of the CT  22 . A power cable  32  (see  FIG. 4 ) runs from the surface to the motor  28  through the CT  22  for protection. The CT  22  also acts as a conduit for fluid/fill mixture removal. The pump  30  is configured to flow in the ‘reverse’ sense, sucking from the lower end and moving the fluid and fill solids upwards through the pump  30  itself and through the CT  22  towards the surface. The CT insulates the well from any pressure increase caused by the pump as it pumps fluid to the surface so avoiding damage to the formation. This can be particularly important in Low BHP reservoirs that can easily be damaged by relatively small increases in wellbore pressure above the in-situ reservoir pressure. 
     The power required to overcome the vertical height (TVD) hydrostatic pressure can be relatively large compared to the power usually available for downhole tools powered via an electric cable (e.g. wireline tools). Since 3-9 kW of electrical power is typically available to power the pump with current wireline technology, for a flow rate of 10 gpm (considered as suitable for this type of application), flow can only be assured for the first few kilometers (depending on CT size and fluid/fill density and viscosity). Therefore, an additional boost may be required to move the fluid mixture to the surface where it can be disposed of or separated. 
     One method of boosting the hydraulic power is to add a second pump/motor combination  34  in series with the first pump of an embodiment of this invention; either right next to it, or further up along the CT  22 . Another method of using dual pumps to carry cuttings during drilling has been disclosed in GB2416550A. 
     Another preferred method of assisting the fluid to reach the surface is to run a pilot line  38  partway along the CT (to point  36 ) to inject N 2  gas via a nozzle  40 . The length of this pilot line  38  can be determined a priori by knowing the geometry of the well, as it is preferable to inject the gas into the CT above the horizontal section. This will decrease the hydrostatic weight of the fluid above the injection point as the gas moves up, and will also avoid the risk of creating a gas ‘plug’ if the gas were injected lower (if the gas is injected in the horizontal section  42 , then the pump  30  will need to push the fluid and the gas up the CT  22 , thus increasing the system losses and causing a ‘slug’ flow condition where gas and fluid ‘slugs’ alternatively flow to the surface; leading to a less efficient carrying capacity system). 
     In the case of the sand plug where it is not possible to force the end of the CT  22  through merely by pushing the CT from the surface, pump operation can start from the top of the plug and the CT  22  slowly be run-in-hole to pump a mixture of well fluid and sand. 
     A mechanical sand mobilization means may be beneficial under this situation so as to fluidize the sand and make it easier to flow through the pump and up the CT. These are described below in more detail in relation to  FIGS. 7-9 . 
     In the case of leftover drilling muds or heavy oils, then the apparatus can be run to the bottom of the well and pulled out of hole while the pump is operated. This will use the heavy fluids behind (i.e. above) the apparatus to act as a temporary dynamic seal and the removal of fluid from the lower part of the well can create a localized drawdown at that level. If the upper fluid is not viscous enough, then the drawdown will not materialize locally, but rather from a reduction of the well fluid level; in turn lowering the hydrostatic pressure over the entire well. If this is not desired, water or other appropriate fluid can be injected at the well head to compensate for the fluid removal through the CT. 
     During a clean-out operation, the apparatus may encounter pebbles and larger particles that have gravitated to the low side of the well and are mixed with the fill. One particular embodiment of the invention for use in such circumstances is shown in  FIG. 5 . In this embodiment, the pump motor  50  is connected to the rotor  52  of a Moineau-type pump having an elastomer stator  54 . A junk basket  56  is added between the pump nozzle  58  and the pump  52 ,  54  and can be used to retain the larger particles that might otherwise harm the pump or that cannot be effectively transported to the surface while still allowing through the finer fill encountered in the well. A typical pass-through particle diameter can be &lt;1 mm. 
     The effectiveness of the mobilization of the fill can be greatly enhanced if a mechanical mixing of some sort takes place. There are various techniques that can be used for creating mechanical mobilization of fine particles and sand. 
     The surface injector can be used to stroke the CT backwards and forwards over a predetermined length (e.g. 1 foot (300 mm) as the nozzle is moved through the fill.) 
     A second motor can be provided at the tip of the pump tool to rotate the pump nozzle as is shown in  FIG. 6 . In this embodiment, an electric motor  60  is positioned in the CT  62  near the nozzle  64 . The nozzle  64  is provided with a wire brush or mill  66 . Operation of the motor  60  rotates the nozzle  64  and brush or mill  66  to mobilise the fill. 
     Features can be added at or around the pump nozzle to mechanically agitate the fill as the nozzle is rotated.  FIGS. 7-11  show examples of such features. In  FIG. 7 , blades/scallops and threads  70  are formed around the outside and inside respectively of the nozzle  72 , serving to accelerate the fill as the nozzle  72  is rotated. In  FIG. 8 , hard buttons  74  are provided around the nozzle  72  for the same purpose. In  FIG. 9 , a brush  76  is connected to the pump rotor (not shown) and projects through the nozzle  72  into the fill. As the rotor rotates, the brush  76  rotates to mobilise the fill. These and/or other features can be used alone or in combination to improve mobilisation of the fill. 
     In another embodiment, pump hydraulic power can be used to create a slow reciprocating motion of the nozzle (via a low power turbine for example), that can assist mobilisation of the fill using features such as those described above. 
       FIG. 10  shows and embodiment in which the nozzle  80  (carrying a brush/mill  82  similar to that shown in  FIG. 6 ) is linked to the lower part of the pump rotor  84  (for a Moineau-type positive displacement pump, for example) that is driven by an electric motor  86 . In use, some of the power of the motor  86  is used to rotate the nozzle and brush/mill  82 , the remaining power being used to pump the mobilised fill. 
       FIG. 11  shows one particular mechanism for converting the rotational motion of a pump rotor into reciprocating motion at the nozzle. In this embodiment, a Moineau-type pump having a rotor  90  and a stator  92 , the rotor  90  is driven by a motor  94 . The stator housing is extended at its lower end and carries a nozzle  96  mounted so as to be able to slide therein. Keys  98  positioned between the stator  92  and nozzle  96  prevent relative rotation of the stator and nozzle while allowing relative axial sliding. A rotatable J-slot holder  100  is positioned inside the nozzle  96  and connected to the pump rotor  90  by means of a drive shaft  102 . A J-slot  104  is provided in the outer surface of the holder  100 . A peg  106  projects from the inner surface of the nozzle  96  so as to engage in the J-slot  104 . As the holder  100  rotates with the rotor  90 , the peg  106  is forced to follow the path of the J-slot  104 , in turn causing the nozzle to move axially with respect to the stator  92  (the J-slot  104  and peg  106  act in the manner of a cam and cam follower to convert rotary motion into reciprocating motion). Depending on the particular design of the rotor and stator, various mechanisms can be used to provide the rotary drive to the nozzle. These can include simple drive shafts, shafts connected by universal joints, mutation disks and other such devices. 
     The limitation of how far the pump could be pushed in the well is usually the helical lockup of the CT in a deviated well. One way of circumventing this limitation is to combine an electric borehole tractor to pull the pump to depth, and then disengage and deactivate it to allow pumping while pulling the CT and pump back towards the surface. Hydraulic tractors can also be used when flowing in ‘standard’ (i.e. down the CT) circulation. However, their flow requirements can tend to increase BHP, which may be undesirable in very low pressure reservoir conditions. 
     The pump can also contain a flow-diverter  110  above it, best seen in  FIG. 12 , that may be commanded from the surface via optical or electrical means, that would allow opening ports to the annulus to flow through the CT in cases when well control or CT cleaning is required. Once the CT has been cleaned, or any obstructions have been removed, the flow-diverter can close the ports and normal ‘reverse’ circulation can resume. 
     Even without the flow-diverter described above, flowing in the ‘standard’ direction from the surface can also be used to clean the filter of accumulated pebbles by ejecting them further up the wellbore and then moving the tool back down to the fill and proceeding with the clean-out operation. 
     It will be appreciated that the various techniques described above can be combined to give the described advantages. Other changes can be made while staying within the scope of the invention.