Abstract:
A method is for sealing a conduit in a sub terrain well bore, where a fluid is present in the well bore. The method comprises the steps of introducing flexible bodies at a position higher in the well bore than the intended position for the seal and being in fluid contact with the intended position for the seal, the flexible bodies having a density that is higher than the fluid in the well bore, allowing the flexible bodies to settle at the intended position for the sealing, and allowing the flexible bodies to consolidate to form a sealing plug in the conduit. Flexible bodies are for use in the method and a method is for introduction of the flexible bodies.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a method for providing a seal in a conduit in a subterrain borehole. More specifically, the present invention relates to sealing of a conduit, such as an annulus between a casing and the borehole, or between the outside of a pipe that is inserted into another pipe. The invention additionally relates to sealing elements to be used in formation of such a seal. 
       BACKGROUND ART 
       [0002]    Boreholes in subterrain formations, especially boreholes for production oil or natural gas are lined with a tubular casing, or outer pipe, that is inserted into the borehole and cemented both to fasten the casing and to prevent fluids from flowing in the annulus formed between the casing and the borehole. Additionally, concentric casing is arranged within the outer casing, the internal concentric casing parts isolating progressively deeper intervals of the borehole. The concentric production tubing and the different casing sections defines a number of annuli inside the borehole. The annuli will during the drilling operations of the subterrain oil or gas well serve the purpose of containing the wellbore pressures to define the wellbore pressure barriers to reliably seal and drill the well. 
         [0003]    Length sections of an annulus are isolated, or sealed from other length sections of an annulus by means of cement. 
         [0004]    Different technologies are used during completion or later intervention for remedial sealing, or reparation, of an oil or gas well. Cementing is used for to supporting the casing strings as well as sealing the annular gap. Failure of the cement sheet may be caused by conditions during well construction, such as lost circulation, un-centralized casing, insufficient pump rate, premature setting or other reasons. The cement can also be damaged in the productive life of the well such as temperature and pressure cycles, temperature degradation, well stimulation operations and techtonic rock displacement. Such cementing failures frequently result in gas leaks resulting in sustained casing pressure. Sustained Casing Pressure is elevated pressure in the annulus that will reoccur even after being bled off from the annulus. A thorough discussion of challenges and lack of remedial solutions can be found in a document prepared for the US regulatory body; MMS; “A REVIEW OF SUSTAINED CASING PRESSURE OCCURRING ON THE OCS” by Borgoyne et al. Remedial cementing often meets problems in getting access to the place to be repaired, and there is a high risk of a pipe introduced to inject cement getting stuck in the well due to the geometry of any gaps, such as annular gaps, to be cemented. If cement is not injected locally but dropped in the fluid in the wellbore to settle in the place to be cemented, the cement will be diluted by the fluid to a non-hardening mixture. Accordingly, cement may only be used if it is possible to inject the cement accurately at the desired position. Reliable bonding and sealing with cement also require high pumping rates, and this is difficult to achieve in small intervention strings that may be inserted in the annulus of a wellbore. 
         [0005]    An alternative method for repair of the cementing treatment involves pulling casing, with heavy rig operations which is expensive and complex. 
         [0006]    The present invention is directed to methods and means for providing sealing in oil or gas wells in an easier and less expensive way than the presently used methods. 
       SUMMARY OF INVENTION 
       [0007]    According to a first aspect, the present invention relates to a method for sealing a conduit in a subterrain well bore, where a fluid is present in the well bore, the method comprising the steps of
       introducing flexible bodies at a position higher in the well bore than the intended position for the seal and being in fluid contact with the intended position for the seal, the flexible bodies having a density that is higher than the fluid in the well bore,   allowing the flexible bodies to settle at the intended position for the sealing, and   allowing the flexible bodies to consolidate to form a sealing plug in the conduit.       
 
         [0011]    The term flexible body as used in the present description and claims is intended to include bodies that are conformable and that will adopt its three dimensional shape when exerted to pressure from forces from its surroundings. The flexible body may be an elastic body, which reverts to its original shape after removal of any forces causing deformation. 
         [0012]    According to a first embodiment, the flexible bodies are made from flexible polymer material swelling in water, brine or/and hydrocarbons, and wherein the consolidation step comprises allowing the flexible bodies to swell. 
         [0013]    The use of flexible bodies made of a swelling polymer material swelling in water, brine and/or hydrocarbons, will facilitate the formation of a sealing plug in the consolidation step, as the swelling of the flexible particles will cause bodies to exert forces onto each other so that any voids between the bodies are filled with the swelling and deformed flexible bodies. The swelling will both improve the sealing properties of the plug and improve the plug integrity. 
         [0014]    The term “hydrocarbons” as used herein, includes compounds normally in organic chemistry known as hydrocarbons, i.e. polymer mainly comprising hydrogen and carbon. 
         [0015]    According to one embodiment, the flexible polymer material is swelling in water and brine. 
         [0016]    According to another embodiment, the flexible polymer material is swelling in hydrocarbons. 
         [0017]    The fluid filling an oil or gas well may be a water based fluid, brine or a hydrocarbon based fluid, or a mixture of oil and water, in which solids may be suspended and salts dissolved. Depending on the nature of the fluid present in the position of the required plug, and the specifics of the individual operation, it may be required to use a polymer material swelling in specific fluids as the polymer used for preparation of the swelling flexible body. 
         [0018]    According to one embodiment, the flexible body is provided with an outer layer of a material having reduced permeability or that is substantially impermeable to the fluid in the well. 
         [0019]    An outer layer of a material having reduced permeability or that is substantially impermeable to the fluid in the well, will retard the onset of the swelling of a swelling polymer and may be used if required. 
         [0020]    The outer layer may be provided with perforations to allow a controlled contact surface between the swelling polymer material and the fluid in the well. 
         [0021]    Perforations in an outer layer of a material having reduced permeability, or that is substantially impermeable to the fluid in the well, will allow a limited and controllable contact between the polymer material and the fluid. This will again allow for a controllable onset or result in a retarded start of the swelling. 
         [0022]    According to one embodiment, weight bodies are injected into the well bore after the injection of the flexible bodies, and are allowed to settle at the top of the sealing plug formed by the flexible bodies. The weight bodies have preferably a density higher than the density of the flexible bodies. 
         [0023]    The weight bodies are filled at the top of a plug of flexible bodies as described above. The weight bodies ascertain that the flexible bodies are held in place and exert a packing force to the plug. Accordingly, the weight bodies causes the flexible bodies to be forced even harder toward each other and against their surroundings, which again improves the sealing properties of the plug. 
         [0024]    According to one embodiment, one or more of the flexible bodies include an element that makes the bodies identifiable with logging equipment. 
         [0025]    Elements allowing identification, or localization of the flexible bodies or weight bodies including such an element by means of logging equipment normally used for logging of oil or gas wells, make it possible to confirm that the flexible elements are settled in the correct position where a sealing plug is required. 
         [0026]    According to a second aspect, the present invention provides flexible bodies for sealing of a subterrain oil or gas well, the flexible bodies being produced from a natural or synthetic polymer material, wherein the flexible bodies are substantially spherical bodies having a diameter from about 2 to 16 mm. 
         [0027]    The polymer material may be a material that is swelling in water, brine and/or hydrocarbons. 
         [0028]    According to one embodiment, a salt is embedded in the polymer material. Salt embedded in the polymer material may increase the swelling properties of the material, and may even cause a polymer material that on its own is non-swelling in the fluid in question, to be swelling. 
         [0029]    According to one embodiment, the diameter of the flexible bodies is from about 3 to about 12 mm. 
         [0030]    According to one embodiment, the bodies comprise a weighting material. 
         [0031]    According to another embodiment, one or more of the bodies additionally comprise components that are identifiable by logging equipment. 
         [0032]    According to a third aspect, the present invention relates to a method for injection of flexible bodies as described above into a conduit in an oil or gas well, wherein the flexible bodies are filled into a container, the container is thereafter pressurized and connected to a valve opening connected to the conduit to be sealed, and wherein the container is opened to allow the flexible bodies provided in the container to flow into the wellbore by means of gravity, gas flow and/or vibration. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0033]      FIG. 1  is a length section through a simplified oil or gas well including a casing and a pipe that are cemented into the well, 
           [0034]      FIG. 2  illustrates the same length section as  FIG. 1 , where the present flexible bodies are used for remedial sealing, 
           [0035]      FIG. 3  corresponds to  FIG. 2 , where weight bodies are filled onto a remedial sealing; 
           [0036]      FIG. 4  is a cross section along A-A of  FIG. 2 ; and 
           [0037]      FIG. 5  is a length section through a casing hanger at the top of an oil or gas well. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0038]      FIG. 1  is a simplified presentation of an oil of gas well  1  in a subterrain formation  2 . An outer casing  3  which as a tubular member which is inserted into a well bore  4  and is fixed by means of a cement filling  5  produced by introduction of cement into the annulus formed between the inner wall of the well bore  4  and the outer wall of the casing  3 . 
         [0039]    A second casing  6  is concentrically arranged inside of the outer casing  3 . An annulus  7  is formed by the inner wall of the outer casing  3  and the outer wall of the second casing  6 . The second casing  6  extends further into the well bore  4  than the outer casing  3 , and the end thereof extending below the casing  3 , is cemented into the well bore  4  by means of a cemented filling  8  produced by circulating cement into the annulus between the outer walls of the second casing  6  and the inner walls of the well bore  4 . To form a seal to control the fluid access to the annulus  7 , the cement filling  8  is continued into the lower part of the annulus  7 . In the exemplary oil or gas well  1  of  FIG. 1 , an additional third casing  9  is inserted into the second casing  6  to form an annulus  10  between the inner wall of the second casing  6  and the outer wall of the third casing  9 . The third casing  9  is again cemented into the well bore  4  by means of a cement filling  11 , produced in the same manner as described above. The skilled man will know that most oil or gas wells comprise a set of concentric casing strings that extend to different positions in the well  1 , and which define a set of concentric conduits between the outer wall of one pipe and the inner wall of the next. Surface casing strings will extend to the top of the well bore and are connected to a casing hanger  20 .  FIG. 5  illustrates typical casing hanger  20  arranged at the top of an oil or gas well. The casing hanger  20  is connected to top or the outer casing  3 , and in the illustrated figure, the top of three concentrically arranged second, third and fourth casings  6 ,  9 ,  15 . The casing hanger  20  defines the top sealing of the casings  3 ,  6 ,  9 ,  15 . A valve top opening  21  allows for introduction or withdrawal of fluids and/or tools into the inner of the inner casing  15 , and casing hanger annulus openings  22 ,  22 ′ and  22 ″ allows for introduction or withdrawal of fluids and/or tools into the annuli  7 ,  10  as mentioned above, and an annulus  14  formed between the outer walls of the third casing  9  and the inner walls of the second casing  6 , respectively. 
         [0040]    Remedial sealing, such as remedial cementing, or any chemical sealing treatment, may be needed due to leakages imperfect well cementing as illustrated by the gap  12 , above, shallow gas bearing intervals, casing or tubing leaks, or later damages to the cemented seal and/or casing or other piping in the well, as discussed in the introductory part of the description. 
         [0041]    When using cement for remedial sealing in an oil of gas well, a tubular conduit may be inserted into the annulus in question and directed to a position as close as possible to the position to be sealed. Cement may be injected through the cementing tube and allowed to set. Similar technology may be used for injection of other chemical treatment for remedial sealing and repair of an oil or gas well. 
         [0042]    According to the present invention, flexible bodies are introduced into the annulus in question. The flexible bodies may be inserted directly in through the relevant annulus opening  22 ,  22 ,  22 ″, or via an insertion tube inserted into the relevant annulus. An optional insertion tube may be directed to a position closer to the position to be sealed than the casing hanger  20 . 
         [0043]    As soon as the flexible bodies are inserted into the relevant annulus, the flexible bodies will sink through the fluid in the annulus due to their high density, until they meet a surface to settle on. As soon as the first flexible bodies has settled at a surface, the particles following will settle at the top of the first flexible bodies and will soon form an annular plug  13 . In the illustrated embodiment, the annular plug  13  has settled at the top of the annular cement column  11  for sealing the annular gap  12  between casing  9  and the inside of the bore hole. 
         [0044]    Due to the flexibility of the flexible bodies, the bodies will be deformed and thus fill in into the annulus better than non-flexible bodies. The plug  13  may be allowed to set for a predetermined period to stabilize the plug before the plug  13  is exposed for normal operating conditions. 
         [0045]    The plug may, however be further stabilized and the sealing improved as described below. As soon as the plug  13  of a predetermined size/volume has been formed, weight bodies preferably having higher density than the flexible bodies, may be filled at the top of the flexible bodies to build a weight plug  14  to compress the bulk volume of elastic bodies, keep the plug in place and thus improve the sealing of the plug formed. 
         [0046]    The weight bodies are preferably made of inelastic materials such as gravel, sand or metal, such as steel, zinc, lead or other metals that is compatible with the environment in the well. The weight bodies do, however, normally have higher density than the flexible bodies. The amount of weight bodies will depend on the specific well and the need for weight to stabilize and keep the plug  13  in place during operation of the well. 
         [0047]    An unconfined plug of flexible bodies may be destabilized by a high pressure from below. The annular extrusion gap is frequently more than 35 mm wide, and the lack of firm axial confinement of the elastic bodies may cause extrusion of the flexible bodies through the annular gap. 
         [0048]    An annular plug of flexible bodies confined by an annular plug of weight bodies will not mobilize because of the mechanical friction between the weight bodies and the annular confinement. Unlike the flexible bodies, the weight bodies will not be mobilized by fluids because the plug of weight bodies is permeable. A plug of weight bodies may further be stabilized with a small addition of swelling flexible bodies within the plug of weight bodies. 
         [0049]    According to one embodiment, the flexible bodies are bodies swelling in the fluid in the well. Swelling flexible bodies are designed to swell in the fluid in question, such as oil and/or water/brine, within a time period allowing the elastic bodies to form a plug  13  before the swelling is completed. Swelling flexible bodies expanding after settling and forming a plug will cause the plug  13  to form a more efficient seal than the non-swelling flexible bodies. When swelling after forming a plug, the swelling will cause the flexible bodies to consume the free pore space fluid and the flexible bodies will better conform to each other and a positive swelling pressure will be exerted onto the confinement of the annular space. The plug  13  formed by swelling flexible bodies will therefore have improved sealing properties over non-swelling flexible bodies, and the force exerted against the walls of the annulus will be improved and thus causing the plug  13  to be a more durable plug. The skilled person will understand that both swelling and non-swelling flexible plugs  13  may be improved by means of a weight plug  14  as described above. 
         [0050]    A plug may also be formed in other conduits than annular spaces as described above. The present method and means may thus be used also for sealing other conduits accessible for inserting the presently claimed elastic bodies. 
         [0051]    A skilled person will understand that placement of elastic bodies can be performed not only on the top of a cement column, but also the base of a borehole, a mechanical plug, the horizontal section of a borehole, a mechanical obstruction or any other suitable chiefly horizontal base in the borehole. 
         [0052]    The method and means according to the present invention may also replace cement in the initial phase of well construction. While cementing processes are time consuming, the invention can be utilized at once the casing has been installed. Drilling of the deeper wellbore section can be performed simultaneously with the zonal isolation. The invention may be used both together with unexpandable casing and expandable casing. The invention may also be used while drilling with casing. 
         [0053]    As mentioned above, the flexible body may be produced of any flexible material compatible with the environment in the oil or gas well. 
         [0054]    The dimension and shape of the flexible bodies are important parameters. With regard to shape, a spherical body will meet less resistance in falling through a fluid and less likely to plug a narrow annular space than a body of irregular shape. It is therefore preferred that the elastic body is substantially spherical, or ball-shaped. Additionally, the flexible bodies must have a dimension being small enough to allow the bodies to fall substantially freely in the conduit where it is inserted. In practice, the tubing of an oil or gas well is not exactly concentric, resulting in wider and more narrow clearance between the walls defining an annular conduit. To avoid that particles get stuck in unintended positions where the gap is narrower than elsewhere, it is preferred that the diameter flexible bodies are spherical bodies having a diameter of about ⅓ of the design cross section, or mean cross section, of the annular gap, or the conduit to be sealed. 
         [0055]    Bodies having a small diameter, i.e. bodies having a large surface area to volume ratio, will meet more resistance in falling through the fluid in the well, which again will reduce the speed of the bodies through the fluid, and thus increase the time for the bodies to fall to the intended position. To maintain a sufficient speed of falling through the fluid, it is preferred that the bodies have a diameter of more than 1/10 of said design cross section, or mean cross section of the annular gap, or the conduit to be sealed. 
         [0056]    The diameter of the flexible bodies is also of specific importance for swelling flexible bodies. The swelling time is both dependent on the characteristics of the material, and the diameter of the elastic body. For elastic bodies made of the same material, the time for swelling is substantially increased by increasing the volume of the body. To ascertain that a substantial, or sufficient, part of the swelling occurs after the particles have settled at the location to be sealed, a large diameter is preferred. Accordingly, it is often preferred that swelling particles have a diameter closer to the above identified maximum diameter of about ⅓ of the design cross section, or mean cross section, of the annular gap in the annulus in question. 
         [0057]    Small particles have the undesirable property of sticking to surfaces due to interfacial tension in the annular space compared to larger particles potentially leading small particles not to reach the intended depth. 
         [0058]    A typical annulus between pipes or the wellbore (design or mean gap) in an oil or gas well is 30 to 50 mm. Accordingly, a typical diameter of the flexible bodies is from about 3 to 16 mm, such as 6 to 14 mm, or from about 8 to about 10 mm. 
         [0059]    The flexible bodies are, as mentioned above made of any suitable flexible material that is compatible with the environment in an oil or gas well. The flexible material is preferably a synthetic or natural polymer, such as natural or modified cellulose, synthetic or natural rubbers, elastomers, etc. 
         [0060]    A presently preferred non-swelling polymer material is soft silicon rubber. A soft silicon rubber is both compatible with the environment in the subterrain well, and is sufficiently elastic or pliable to be deformed sufficiently to fill in the voids created between the particles, when set under pressure at a location to be sealed. 
         [0061]    Dependent on the well to be sealed and the nature of the fluids therein, the swelling material may be water swelling, oil swelling or swelling in both oil and water. Swelling elastic materials that are compatible with the environment in an oil or gas well are well known, and described in the prior art such as annular swellable packers, and may be purchased from different sources. The preferred swelling materials are polymer materials such as rubber materials which, swell in crude oil or oils that is used in Oil Based Drilling fluids in petroleum wells, such as ethylene propylene rubber (EPM and EPDM), ethylene-propylene-diene terpolymer rubber (EPT), butyl rubber (IIR), brominated butyl rubber (BIIR), chlorinated butyl rubber (CIIR), chlorinated polyethylene (CM/CPE), chloroprene rubber (CR), styrene butadiene copolymer rubber (SBR), sulphonated polyethylene (CSM), ethylene acrylate rubber (EAM/AEM), epichlorohydrin ethylene oxide copolymer (CO, ECO), silicone Rubbers (VMQ) and fluorsilicone rubber (FVMQ). Apart from swelling in hydrocarbons, such rubbers can with addition of salt also be brought to swell in water or brines. 
         [0062]    Cellulose based materials, both natural and modified, are candidates for swelling flexible bodies. 
         [0063]    The polymer material may alternatively be polymer materials which do not swell in crude oil, such as butadiene acrylonitrile copolymer (Nitrile Rubber, NBR), hydrogenated NBR(HNBR, HNS) such as ZETPOL®, TORNAC®, TERBAN®, NBR with reactive groups (X-NBR), fluoro rubbers (FKM), such as VITON®, FLUOREL®, perfluoro rubbers (FFKM) such as KALREZ®, CHEMRAZ® and Tetrafluorethylene/propylene (TFE/P), such as AFLAS®, which would not swell when exposed to oil field crudes. Most of these elastomers can be crosslinked by more than one crosslinking agent, for example sulphur or peroxide. 
         [0064]    Apart from the thermoset (non swelling and oil swelling) elastomer matrix materials quoted above, also blends of elastomers can be applied. Although an almost inexhaustible combination of thermoplastic and thermoset elastomers are feasible, the most preferred ones are the EPDM/polypropylene blends such as SARLINK®, Levaflex®, Santoprene®, NBR-polypropylene blends such as GEOLAST®, NBR/polyvinyl-chloride blends and NR/polypropylene blends. All of these have a tendency to swell in petroleum crudes, especially at downhole well temperatures. 
         [0065]    Examples of suitable materials which swell when in contact with water are: starch-polyacrylate acid graft copolymer, polyvinyl alcohol cyclic acid anhydride graft copolymer, isobutylene maleic anhydride, acrylic acid type polymers, vinylacetate-acrylate copolymer, polyethylene oxide polymers, carboxymethyl cellulose type polymers, starch-polyacrylonitrile graft copolymers and the like. Preferably one or more salt is embedded in the elastic polymer material to improve swelling, especially for swelling in water. Suitable the salt is one of the group of acetates (M-CH 3 COO), bicarbonates (M-HCO 3 ), carbonates (M-CO 3 ), formates (M-HCO 2 ), halides (Mx-Hy) (H=Cl, Br or I), hydrosulphides (M-HS), hydroxides (M-OH), imides (M-NH), nitrates (M-NO3), nitrides (M-N), nitrites (M-NO 2 ), phosphates (M-PO 4 ), sulphides (M-S) and sulphates (M-SO 4 ), wherein M is a metal selected from the group of metals of the periodic table. Also, other salts are can be appli e d wherein the cation is a  n on-metal like NH4Cl. However the preferr e d salts are NaCl and CaCl 2 —of these, CaCl 2  is most preferred in view of its divalent characteristic and because of its reduced tendency to leach out from the bas e  rubber due to reduced mobility of the relatively large Ca atom in the base rubber. 
         [0066]    To limit leaching out of the salt from the elastomer, suitably the swelleable particles include a hydrophilic polymer containing polar groups of either oxygen or nitrogen in the backbone or side groups of the polymer matrix material. These side groups can be partially or fully neutralised. Hydrophilic polymers of such type are, for example, alcohols, acrylates, methacrylates, acetates, aldehydes, ketones, sulfonates, anhydrides, maleic anhydrides, nitriles, acrylonitriles, amines, amides, oxides (polyethylene oxide), cellulose types including all derivatives of these types, all copolymers including one of the above all grafted variants. 
         [0067]    Preferably swelleable particles should be capable of swelling in water of salinity as high as 140 grams/litre sodium chloride, and containing considerable concentrations of bivalent ions, such as at least 40 grams/litre calcium chloride and 8 grams/litre magnesium chloride. 
         [0068]    The temperature in an oil or gas well may also vary substantially. For an offshore oil or gas well, the temperature at the top of the well is about 4° C., whereas the temperature in the lower, or producing sections of the well may be up to 200° C. In some wells, the temperature may be as high as 250° C., a temperature that is often met in well bores for steam injection. The material for the flexible bodies has to be selected so that they have the requested properties in the fluid present in the well at the expected temperature. 
         [0069]    Most polymer matrixes discussed above have a density too low for the intended purpose, and a weighting material has to be added. Preferably, the flexible particles are produced with a core of a weighting material such as glass, sand, bauxite, ceramics or metal, such as lead, bismuth steel shot or the like. 
         [0070]    For swelling bodies, the transition from non-swollen to fully swollen state suitably takes place in a period ranging from a few hours to several days, depending on the material of the elastic and swelling particles, the fluid used to trigger swelling of the particles, the volume/diameter of the elastic particles in addition to the temperature in the well. 
         [0071]    The time for allowing the elastic particles from the point of injection to the intended location is dependent on the distance from the injection point to the intended sealing position, the diameter and the density of the elastic particles, and the density and viscosity of the fluid in the well. Preferably, swelling particles will sink from the injection point to the location to be sealed within 5 hours, more preferably within 2 hour, and most preferably in less than 1 hour. 
         [0072]    In selecting the correct material and density of the bodies, care has to be taken to select elastic particles that have a density sufficiently high, and a time from contact with the fluid in question until full swelling as occurred, that is long enough to allow a substantial part of the swelling to occur after the flexible particles have settled at the position to be sealed. It is within the skill of a skilled person in the relevant technical field to select among the different commercially available flexible polymers based on the common knowledge thereof, and to thus to specify the required specifications of the flexible particles to be used according to the present invention based on the information herein and readily available material. 
         [0073]    If needed the flexible particles may be provided with an outer layer of a material having a low permeability to the fluid in the well to slow the initial swelling to allow the elastic particles to settle before major part of swelling occurs. The outer layer may be produced by any kind of polymer or other material having low or no permeability to water and hydrocarbons present in the oil or gas well, such as e.g. fluorinated rubbers. 
         [0074]    If the flexible bodies are provided with an outer layer having no or low permeability or diffusion, the outer impermeable or less permeable layer, may be provided with perforations to allow a predetermined area of contact between the swelling material and the surrounding fluid in the well. A less permeable or impermeable layer having a predetermined contact area to the surrounding fluid makes it possible to control the swelling, and at least retard the onset of the substantial part of the swelling. Swelling of the flexible body will increase the volume thereof, which again will increase the area of the perforations. The initial swelling of a flexible body covered with a less permeable or impermeable layer will cause the layer to be thinner and will increase the area of any perforations in the outer layer, which again will results in accelerated swelling. 
         [0075]    Alternatively to an outer layer of less permeable or impermeable material provided with perforations, the impermeable or less permeable outer layer may be degradable or slowly soluble in the environment of the well, such as wax, resulting in a retarded onset of swelling. 
         [0076]    A pressure is often built up in the conduits of a well bore, such as in the annuli between the casings. In such cases it is necessary to insert the flexible bodies and the weight bodies into the conduit under pressure. The flexible bodies are then first filled into a container and the container is thereafter pressurized. Thereafter, the container is connected to a valve opening connected to the conduit to be sealed, before the container is opened to allow the flexible bodies provided in the container to flow into the wellbore. The bodies may be introduced by means of gravity. Gas flow and/or vibration may be used to increase the introduction rate of flexible bodies from the container into the conduit. 
         [0077]    The elastic bodies, or some of the elastic bodies, or the heavy bodies may additionally comprise at least one component that is identifiable by means of logging equipment to ascertain that the plug is in the correct position. Identifiable materials include radioactive sources, magnetic particles, steel particles etc.