Patent Publication Number: US-9840884-B2

Title: Downhole seal element and related apparatuses

Description:
BACKGROUND 
     Disclosed herein is a downhole seal element, and related apparatuses. 
     In the art of downhole tools, i.e. devices intended to be conveyed into water, oil and gas wells and similar elongate boreholes extending into subterranean formations, various types of seal element are known. They are of particular utility when used to convey tools into wells that are normally filled with a pumped, circulating fluid the chemical composition of which will vary from one well to the next. 
     Such a seal element typically is made from a flexible, and commonly resiliently deformable, material and includes a collar that normally lies at an in-use downhole (leading) end of the seal. The collar is sealingly secured on the exterior of some part of a downhole toolstring, typically a mandrel, that almost invariably is circular. The seal element includes a skirt that extends in use uphole (rearwardly) from the collar. By reason of the flexible nature of the material of the element the skirt is capable of moving from a collapsed position lying close to the mandrel to an expanded position flared outwardly therefrom. 
     The mandrel is of a smaller diameter than e.g. drillpipe temporarily defining the inner wall of the well in which it is to be conveyed. The skirt when collapsed while being of greater diameter than the mandrel nonetheless also is of a lesser diameter than the inner wall defined by the drillpipe. 
     The skirt may be caused to move from its collapsed to its extended position by the application of fluid pressure inside the drillpipe, with the pressure gradient acting in the downhole direction. As a result it is possible to employ one or more seals to cause movement of a downhole tool along a length of drillpipe in a fluid-filled well, as long as (a) the seal element is mounted the correct way round on the tool and (b) the circulation of fluid is such as to apply fluid pressure to the skirt in a desired direction causing the skirt to expand so that its outer periphery seals against the drillpipe. Since at this time both the innermost part of the seal element represented by the collar and the outer periphery of the skirt define seals the pumping of fluid in the well causes the tool supporting the seal element to be conveyed in a desired (normally downhole) direction. 
     In some cases such conveying of a tool is adequate for the purpose of deploying it from a surface location to a subterranean location. Following the completion of the intended action of the tool it may be recovered to the surface location for example by paying out a cable that may attach to the uphole end of the deployed tool using a per se known fishing neck arrangement. The cable may then be wound in to the surface location in order to recover the tool. 
     Such an approach is often acceptable when the tool in use is essentially autonomous. In many situations however the use of an autonomous tool is not possible. 
     One example of a non-autonomous tool is a wireline logging tool. 
     A logging tool is an elongate, cylindrical device that is conveyed to a downhole (operational) location for the purpose of logging (i.e. recording, processing and/or analyzing) data about the subterranean formation. 
     Wireline is a form of armored cable that is capable of transmitting electrical and electronic signals from the logging tool to a surface location. Many designs of logging tool are conveyed to their downhole locations trailing a length of wireline behind them so that log data may be telemetered immediately to an uphole location and analyzed. 
     Wireline offers numerous advantages in many logging situations but it is characterized by having a comparatively high mass per unit length. In some situations wireline must be paid out over a length of several thousand or even tens of thousands of feet in order to let a tool reach the total depth of a well. This means that many hundreds of kilograms of wireline may lie in the well while logging takes place. 
     If the well extends vertically or steeply downwardly the mass of the wireline is not seen as a particular disadvantage because gravity tends to avoid the need to apply additional energy in order to deploy it. In other words in such wells the mass of the wireline tends to be no hindrance to tool deployment. 
     Many wells however are not of this character, and extend horizontally (for example sideways into a hillside) or at least include sections that are not vertical or steeply descending. In such situations a need arises to pump the logging tool along the well in the manner outlined above using seals as aforesaid. When pumping under these circumstances is required the mass of the wireline becomes a significant problem because much energy is then needed to move the logging tool (which itself may weigh more than a hundred kilograms) and the wireline. This additional energy normally takes the form of an increase in the pumping pressure of the fluid circulating in the well. The pumping pressure is controlled by a logging engineer stationed at the surface location. 
     Furthermore the wireline and/or the tool may become snagged or impeded in some way, and at such times high pumping pressures again are employed in order to try and move the logging tool. 
     These factors create limits to the extent to which wireline logging tools can be pumped in wells. The limits arise either because the pumps used to circulate fluid in the wells are not capable of creating sufficiently high pressures or (more commonly) because existing seal elements when subjected to high pressures tend to fail by turning “inside out” with the result that their skirts cease to seal against the inner wall of drillpipe or casing in the well. When this happens the seal becomes useless for its intended purpose of pumping the tool; and indeed the seal may become torn or broken up such that it merely is debris inside the drillpipe. 
     In view of the foregoing there is a need for an improved design of seal arrangement that in particular is suitable for use when a heavy mass of wireline must be pumped along a well together with the logging tool. 
     SUMMARY 
     According to the invention in a first aspect there is provided a downhole seal element comprising a cup portion formed of or including a resiliently deformable material, the cup portion extending between on the one hand a nose part comprising an annulus intended for sealingly mounting the seal element on a mandrel and on the other hand a skirt, the seal element flaring in shape between the nose part and the skirt and the skirt including extending therefrom away from the nose part a plurality of elongate, flexible limbs that are spaced at intervals about the skirt. 
     Such an arrangement is as described below of particular advantage when it is required to seal a tool (or a mandrel attached to a tool) as aforesaid for pumping purposes when the tool or mandrel includes a so-called centralizer. 
     Preferably the cup portion in the vicinity of the nose part is of circular cross section. This suits the nose part for sealing attachment to a mandrel or similar structure forming part of a toolstring. 
     Also preferably the annulus includes one or more annular and/or radially extending reinforcements. These are advantageous because the material of the seal may become strained in the vicinity of the nose part, where the seal attaches to the mandrel. 
     Conveniently the cup portion in the vicinity of the skirt is of circular cross-section; and further conveniently the cup portion is of circular cross-section between the nose part and the skirt. 
     These features suit the seal for sealing inside drillpipe. 
     The principles of the seal of the invention as defined herein however are also suitable for sealing in a bore lined with a component other than drillpipe. Thus the seal in a modified form may be used for sealing against well casing. 
     Preferably the elongate, flexible limbs are spaced at equal intervals about the skirt. This aspect of the seal of the invention is particularly suitable when the mandrel is part of a centralizer having a plurality of evenly spaced bowsprings or similar centralizer features such as spring-loaded arms, as described in more detail below. 
     In a particularly preferred embodiment of the invention each elongate, flexible limb includes a flexible core of or including Aramid fibers. This feature confers great strength and toughness on the limbs, such that they are likely to survive incidents in which they become snagged or turned inside out in a downhole situation. 
     A preferred form of Aramid fiber is Kevlar®, although other fibers may be employed in the flexible cores of the limbs. 
     Constructionally advantageous features of the seal of the invention include that each elongate, flexible limb is joined to the skirt by way of a portion of increased width; and that each elongate flexible limb terminates in a free end that is squared off. These features have been found to confer good service life on the seal. 
     According to a second aspect of the invention there is provided an elongate downhole tool comprising a mandrel having sealingly secured about an outer periphery the annulus of a seal according to the invention as defined herein. 
     In one form of downhole tool according to the invention the seal is oriented to promote pumping of the downhole tool in a downhole direction. 
     This is the version of the invention expected to be most commonly embodied but it is equally possible within the scope of the invention to mount the seal on the outer periphery of the mandrel in an inverted orientation. This then would permit the seal to be used for pumping the tool in an uphole direction. 
     Preferably the mandrel terminates at one end in a plug that is secured inside the hollow interior so as to prevent fluid flow along the interior via the said end. The plug preferably is secured by one or more frangible retention members that fracture on fluid pressure acting on the seal reaching or exceeding a threshold value. 
     In such an embodiment fracturing of the one or more frangible retention members creates a fluid communication path across the seal. 
     Preferably the fluid communication path results from removal (typically in a downhole direction) of the plug from the downhole tool on fracturing of the one or more frangible retention members, thereby opening an otherwise closed end of the mandrel to the exterior of the downhole tool. 
     However in other embodiments of the invention rupturing of the one or more frangible retention members might result in opening of a fluid flow path e.g. through activation of a valve. 
     As a consequence the tool is pumpable, through the action of the attached seal, while the pressure of pumping fluid remains below a value corresponding to the threshold; and in the event of the pressure exceeding the threshold the plug becomes removed (or the fluid communication path becomes opened in some other way, as outlined) such that the well may be circulated by way of fluid passing along the mandrel and exiting via a downhole aperture in the tool. Logging and drilling engineers will appreciate the benefit of being able to circulate the well in this fashion, either as an emergency measure in the event of the fluid pressure exceeding the threshold value unexpectedly; or because the logging engineer intends that circulation should commence following deployment of the tool to a chosen location. 
     Especially when it is required to convey e.g. a logging tool along a length of horizontally extending well it is strongly desirable to centralize the tool in the well since otherwise the tool may not accurately record log data. 
     Various forms of centralizer are known. The majority include an annular array of spring-loaded centralizer arms that extend from a mandrel in a circular array so as to support the tool on all sides relative to the drillpipe or other medium lining the well. 
     The seal of the invention is as mentioned highly suitable for use in conjunction with a centralizer of the general kind described. To this end the tool preferably includes one or more resiliently biased, protruding arms, especially one or more bowspring members interconnecting two parts of the tool that are spaced from one another along its length. 
     Further preferably the downhole seal element is located on the mandrel such that at least a pair of the elongate, flexible limbs is extensible to either side of a said resiliently biased arms. 
     The arrangement of the seal element advantageously permits sealing of the seal to drillpipe or another well lining medium notwithstanding that the arms of the centralizer in the case of other seal designs would prevent the cup portion of the seal from engaging the well wall all the way around its inner periphery. In other words the flexible arms of the seal element of the invention provide for interruptions in the cup portion that accommodate centralizer arms in a way that permits maintaining of a seal. 
     As referred to herein the sense of the resilient biasing of the arms or other centralizer features is to bias them to protrude from the mandrel to which they are secured. Such an arrangement is known in the design of centralizers. Usually the biasing of each arm, etc., is the same; but in some designs of centralizer this is not the case. All such centralizer designs are viable in the downhole tool of the invention. 
     It is advantageous for the downhole tool to comprise at least one mechanism having a reversible energy store and a valve controlled by one or more moveable actuation members, wherein (a) kinetic energy of the actuation member(s) is convertible to potential energy of the reversible energy store; (b) a first movement of at least one said actuation member results in opening of the valve; (c) potential energy in the reversible energy store is convertible to kinetic energy of the actuation member(s); (d) a second movement of at least one said actuation member results in closing of the valve; and (e) opening and closing of the valve causes a change in a pressure difference across the seal element. 
     When the downhole tool as defined above travels in a fluid-filled bore it may enter a reduced space such as an upset or landing ring, that gives rise to variations in the diameter of the inner wall of the drillpipe. This leads to a temporary increase in the fluid pressure acting across the seal element. This can result in bursting of the seal. The foregoing aspect of the invention may be arranged to travel ahead of the seal, creating a pressure relief path on opening of the valve and hence preventing bursting of the seal at the nose part. 
     To this end the or each actuation member is preferably engageable with an inner wall of a fluid-filled bore such that variations in the cross-section of the bore cause movement of the actuation member(s). 
     In one embodiment of the mechanism, the reversible energy store is a spring that can be adjusted for preload. Preferably, the spring acts between a collar that is secured on the mandrel and a moveable sleeve movement of which results in opening and closing of the valve. 
     According to another aspect of the invention there is provided a downhole tool assembly comprising two or more downhole tools each according to the invention as defined herein secured together so that the hollow interiors of the respective tools are capable of communicating with one another when e.g. the plug of one of them is removed as described above. This arrangement beneficially means that two of the seal elements may be provided at spaced intervals along a toolstring, thereby minimizing the risk that during pumping the toolstring may become skewed relative to the drillpipe. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       There now follows a description of preferred embodiments of the invention, by way of non-limiting example, with reference being made to the accompanying drawings in which: 
         FIG. 1  is a perspective view of a seal element according to the invention, in an unstressed (non-use) condition; 
         FIG. 2  is a cross-section view of part of a downhole tool mandrel having mounted thereon a seal element according to the invention; 
         FIG. 3  is an elevational view of a downhole toolstring according to the invention; and 
         FIGS. 4 and 5  are cross-sectional views of a variant of the downhole tool mandrel of  FIG. 2  showing the construction and operation of a pressure relief mechanism. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to the drawings a seal element  10  made predominantly from a resiliently deformable, or at least flexible, material such as a synthetic or natural rubber compound comprises a cup portion  11 . 
     Cup portion  11  in the embodiment shown adopts essentially the form of a circular cross-section dome as illustrated and extends between a nose part  12  at one end of the seal element  10  and a hollow skirt  13  at the other end opposite the nose part end. 
     The nose part  12  includes a central annulus  14  defining a circular cross-section passage extending between the hollow interior of the skirt  13  and the exterior of the seal element  10  in the vicinity of the nose part  12 . 
     The annulus  14  is of smaller internal and external diameter than the exterior of the nose part  12 , and is retained and supported relative thereto by a number of reinforcements  16  described in more detail below. 
     By reason of its circular section dome shape the seal element  10  as illustrated flares in shape between the nose part  12  and the skirt  13 . 
     The skirt  13  as shown is circular and includes extending in a direction away from the nose part a series of (in the embodiment described) six elongate, flexible limbs  18 . 
     The limbs  18  are spaced at equal intervals about the periphery of the skirt  13 , for a purpose described in further detail below. In consequence a series of equally-sized, elongate gaps  19  exists between the adjacent pairs of limbs  18 . 
     Although six equally spaced limbs  18  are shown in the preferred embodiment illustrated, a different number of limbs  18  may be provided in other arrangements falling within the scope of the invention. Furthermore the spaces defining the gaps  19  between adjacent limbs  18  need not be as shown; and indeed need not necessarily be spaced equally in respect of all the limbs of the series extending from the skirt  13 . 
     As illustrated the reinforcements  16  located in the nose part of the seal element are formed integrally with the other parts of the element  10  as a series of three (in the embodiment shown, although other numbers are possible) ribs extending radially between the annulus  14  and an outer collar  21 . 
     The radially extending ribs defining the reinforcements  16  are shown in an equally spaced arrangement in  FIG. 1  but in other embodiments within the scope of the invention other numbers and patterns of the reinforcements are possible. Reinforcement furthermore may be included in other parts of the seal, as desired. 
     Collar  21  defines a recess  17  in a direction receding away from the nose part  12  such that the in-use downhole facing end of the seal element  10  may be sealingly attached to a further component secured on or formed integrally with a mandrel  22  described in more detail below. 
     Although not visible in  FIG. 1  each of the limbs  18  includes inside it one or more elongate Aramid fibers (especially Kevlar® fibers) extending from one end to the other of the limb, or at least over a sufficient length as to have a strengthening effect. 
     Such fibers are herein stated to define in each limb at least one core, but this does not necessarily mean that the fiber(s) necessarily must extend centrally inside each limb. Indeed off-center fiber locations are possible within the scope of the invention, as are arrangements in which the positioning of the fibers relative to the limb cross section varies (e.g. sinusoidally) along the length of the limb  18 ; and indeed the cross section of each limb  18  may not lend itself to centralized location of the aramid fibers. 
     As shown the limbs  18  furthermore include strengthening ribs  23  extending along their lengths, but these in some embodiments of the invention may be dispensed with or may adopt a variety of different lengths, shapes and cross-sections. 
     The effect of the foregoing features is to strengthen the limbs  18  and prevent them from being misplaced, damaged or torn off in use of the seal element  10 . 
     Such strengthening features do not need to extend all the way along each limb  18 . They may for example be interrupted at intervals or they may extend continuously over only e.g. a first part of the limb  18  measured from the skirt  13 . Furthermore it is not necessary that each limb  18  is of the same design as the next adjacent limb, although in the preferred embodiment shown this is desirable in order to accommodate a series of identical bowspring arms as described below. 
     As shown at its point of attachment to the skirt  13  each limb  18  extends laterally in a radiused shape in order to provide a smooth joint transition and in order to maximize the amount of material of the seal element in the attachment locations. This feature assists in preventing tearing off of the limbs  18 . 
     At the opposite, free end each limb  18  is squared off as shown, but in some other arrangements other limb end shapes (especially those that induce particular fluid flow characteristics) may be employed. 
     As mentioned the seal element  10  may be secured onto the exterior of a hollow metal (e.g. steel) mandrel  22  as best illustrated in  FIG. 2 . 
     This is a cross-sectional view of part of a downhole tool  24  according to the invention. 
     In  FIG. 2  the seal element is shown in a shape it adopts when there is no appreciable fluid pressure acting in a downhole direction. 
     As indicated the inner diameter of the annulus  14  is such that the annulus  14  is a sealing fit on the exterior of the mandrel  22 . In practice in assembly of the downhole tool  24  annulus  14  is slid on to an in-use downhole end of the mandrel  22  until it encounters a shoulder  26  that prevents further movement in the in-use uphole direction. A sealing collar assembly  27  is then slid along the mandrel  22  also in an uphole direction so that it becomes inserted into the recess  17 . 
     In the as-assembled condition the collar assembly  27  is impervious to fluid flow, and as illustrated includes a plug member  28 . 
     Sealing collar assembly  27  includes inner and outer rigid (e.g. metal) collar sleeves  29 ,  31  that are secured one to the other by at least one shear pin  32 . Shear pin  32  prevents relative axial movement between the inner and outer sleeves  29 ,  31 . 
     Inner sleeve  29  includes protruding radially inwardly therefrom a tang  33  that is received in an annular groove  34  extending around the outer periphery of the mandrel  22 . The tang  33  prevents axial movement of the inner collar sleeve  29  once it has been installed on the free end of the mandrel  22  from an in-use relatively downhole location. 
     Shear pin  32  is received in a radial bore  37  extending through the inner and outer collar sleeves  29  and  31  and extends radially inwardly beyond the inner surface of inner sleeve  29 . As a result it defines a free end that is received in a recess  36  formed in the outer surface of plug  28  that in turn is a sealing fit inside the inner collar sleeve  29 . 
     The sealing collar assembly  27  may be assembled by firstly sliding or pressing the inner sleeve  29  onto the free end of the mandrel  22 . Thereafter the tang  33  may be deformed so as to enter into groove  34 . This locks the inner collar sleeve onto the mandrel end. 
     The plug  28  is then slid or pressed inside the inner sleeve  29  and the outer sleeve  31  slid or pressed onto its exterior. As long as the bore  37  is in line along its length the shear pin  32  may be pressed or hammered into place linking the inner and outer sleeves  29 ,  31  and the plug  28  so as to prevent axial movement of these parts relative to one another or relative to the mandrel  22 . 
     One or more annular ring seals  39  of an elastomeric material may be received in grooves in the outer sleeve  31  as shown in order to provide fluid-tight seals between on the one hand the inner and outer sleeves  29 ,  31 ; and on the other hand the exterior of the outer sleeve  31  and the recess  17  of the seal element. 
     The plug  28  closes off the otherwise open end  38  of the mandrel  22  such that under normal circumstances when the mandrel is inserted into a fluid-filled well no fluid flow via the interior of the mandrel in a downhole direction is possible. 
     In consequence with the seal element  10  attached as described to mandrel  22  that is inserted inside drillpipe any fluid pressure acting on the in-use uphole end of the seal element reacts against the material of the skirt  13  and the collar assembly  28  thereby tending to drive the seal element, and any component to which it is attached, in a downhole direction. 
     Such pressure causes the skirt  13  to flare outwardly and in the absence of other impediments seal about its annular periphery against the wall of the drillpipe, thereby giving rise to an effective seal arrangement. 
     In the event of fluid pressure inside the seal exceeding a threshold value for one of the reasons summarized above the shear pin  32  shears with the result that the plug  28  becomes free and is expelled from the downhole end of the tool. This opens the end  38  of the mandrel formerly closed by the plug  28 , with the result that circulation of the well via the drillpipe becomes possible. 
     The inner and outer sleeves  29 ,  31  at this time are retained captive on the mandrel  22  by reason of the remnant of the shear pin  32 , and the tang  33 , holding them against axial movement off the end of the mandrel. 
     The seal element of the invention includes further features, and in particular the limbs  18 , intended to enhance its use in conjunction with one or more centralizers. Use of the seal element in this way is shown in  FIG. 3 . 
     In  FIG. 3  a seal element  10 ′ is secured on a mandrel  22 ′ in the manner described above to define a downhole tool  44 ′. 
     In the arrangement illustrated the mandrel  22 ′ is the core member of a centralizer having secured on its outer periphery an annular series of resiliently deformable bowsprings  41 . 
     In the arrangement shown there are six bowsprings spaced at equal intervals on the exterior of the mandrel  22 ′, but only four of the bowsprings are visible in the view presented. 
     As is well known in centralizer design, the bowsprings  41  each are fixed to the mandrel  22 ′ at one end by way of a fixed, common collar  42  and are slideably secured at the other. The material of each bowspring  41  is resiliently deformable and may be for example a high Young&#39;s modulus steel. The result is an arrangement in which a series of leaf springs is presented on the exterior of the mandrel  22 ′. 
     The slideable connection of the bowsprings  41  is achieved by way of a common, slideable collar  43 . The arrangement overall is such that pressure on one of the bowpsrings caused e.g. by the mass of the toolstring pressing downwardly on the interior of horizontally extending drillpipe causes inward deformation of the bowspring in question. Since at the slideable and non-slideable collars  43  and  42  the bowsprings are joined together about the periphery of the mandrel  22 ′ the overall effect is to prevent the depressed bowspring from collapsing entirely, with the result that the tendency of the toolstring to lie on the lowermost part of the inside of the drillpipe is resisted. 
     In practice all the bowsprings are in contact with the drillpipe wall simultaneously such that the tool is maintained at a central position inside it. This is the preferred position of the tool while it is being pumped inside the drillpipe. 
     The bowsprings  41  however interrupt the available drillpipe wall for sealing by the skirt  13 . 
     In view of this the limbs  18  are provided in order to present sections of the seal element  10 ′ that lie interposed between the drillpipe wall regions obliterated by the bowsprings  41 . 
     The shapes and dimensions of the limbs  18  are such that in conjunction with the bowsprings  41 , the remainder of the seal element  10 ′ and the sealing collar assembly  27  an adequate seal is maintained to cause movement of the tool when a downhole pressure gradient is applied. During such a time the limbs  18  are protected against damage by the various strengthening features, such as the Aramid fiber cores and the ribs  23 , described herein. 
     One significant advantage of being able to locate the seal element  10 ′ inside the envelope defined by the bowsprings  41  is that the overall length of the tool does not have to be increased in order to accommodate the seal element  10 ′. 
     In practice as shown in  FIG. 3  the tool  44 ′ may be assembled into a toolstring with another, similar tool  44 ″ in which the bowsprings  41 ″ of a further centralizer encircle a second seal  10 ″ mounted on a second mandrel  22 ″. The respective mandrels  22 ′,  22 ″ are secured end-on to one another in a per se known way such that their hollow interiors are capable of communicating with one another in the absence of the plug of at least one of the sealing collar assemblies. 
     Such an arrangement provides for centralizing of the toolstring at two axially spaced positions while also providing for pumped driving of the toolstring at two such locations as well. This arrangement ensures that the toolstring is conveyed without tilting relative to the drillpipe wall. 
     Although the centralizer shown has resiliently deformable bowsprings, another design of centralizer includes resiliently spring-loaded, outwardly protruding arms. 
     Such a centralizer presents a similar sealing problem as the bowspring centralizer described above. The seal element of the invention is suitable for providing a seal in the vicinity of such an arm-type centralizer, with the limbs  18  interposed between the arms in a similar manner to that in which they lie between the bowsprings  41  in order to seal against the drillpipe wall. 
     In addition to the arrangements described above it is possible within the scope of the invention to secure the seal element  10  on the mandrel  22  or a similar article in an orientation that is inverted compared to that shown in  FIGS. 2 and 3 . 
     When so configured the seal element  10  may be employed to permit pumping of the tool  24  in an uphole direction instead of the downhole pumping made possible by the  FIGS. 2 and 3  combination. 
     When the seal element is applied in this uphole pumping orientation it may be necessary to modify the downhole tool or other equipment associated with it. Such modification may be needed for example to ensure correct fluid flows and/or to ensure that the seal element  10  becomes free to be inflated by downhole fluid only when uphole tool pumping is required. 
     A further problem that may arise during use of a logging tool as described above, when passing through a restriction in drillpipe such as a landing ring or internal upset, is a short-lived increase in fluid pressure inside the seal element  10 . This is caused by squeezing of the fluid-filled skirt  13  as it passes through the restriction, and may lead to tearing or bursting of the seal element  10  such that it ceases to be functional. 
     In order to avoid this problem it is desirable to provide a means of temporary pressure relief or equalization that may accommodate sudden pressure increases as described. Such an arrangement is described with reference to  FIGS. 4 and 5 , which show a pressure relief mechanism  51  that operates to control pressure levels across the seal element  10 . 
     The pressure relief mechanism  51  consists of a reversible energy store (which is preferably a spring  49  although other energy store types, as would occur to the skilled worker, also are possible), actuation member(s)  46   a  and  46   b , on outer sleeve  47 , and a moveable, inner sleeve  48 . The pressure relief mechanism  51  is secured on the mandrel  22  at the nose part  12  of the seal element  10 . 
     In particular the circular outer sleeve  47  is sealingly received within the annulus  14  of the seal element  10 . Moveable inner sleeve  48  is slidingly mounted on the exterior of the mandrel  22  and is moveable longitudinally relative to both the mandrel  22  and the outer sleeve  47 . The inner and outer surfaces of the inner sleeve  48 , and/or the inner surface of outer sleeve  47  and the outer surface of mandrel  22 , may include sealing arrangements such as the per se known O-ring and groove combinations  53  visible in  FIGS. 4 and 5 . 
     The inner sleeve  48  is partially received within the hollow interior of the outer sleeve  47  such that the two sleeves overlap over parts of their respective lengths, with part of the inner sleeve  48  protruding on the mandrel  22  in a downhole direction externally of the outer sleeve  47 . 
     When as shown in  FIG. 4  the logging tool  24  is running in a length of drillpipe  54  of relatively large internal diameter the degree of overlap between the outer  47  and inner  48  sleeves is such as to close off one or more fluid flow passages  52  defined by apertures in outer sleeve  47  that at such a time are covered by the inner sleeve  48  such that the full pump pressure difference acts across the seal element  10  in order to drive the tool  24  in a downhole direction. 
     The actuation member has two rigid arms  46   a  and  46   b  forming a pantograph-like arrangement, wherein one arm  46   a  of the pantograph is pivotably secured one end to the end of a moveable inner sleeve  48  that protrudes beyond outer sleeve  47 . The opposite end of arm  46   a  is pivotably secured to an end of arm  46   b . The other end of arm  46   b  is pivotably secured to the exterior of outer sleeve  47  with the result that the joint between the arms  46   a  and  46   b  defines an elbow  58  that may be caused to engage the inner wall of the drillpipe  54 . 
     The spring  49  encircles the mandrel  24  and acts between the downhole end of inner sleeve  48  and an anchor plate  56  that is rigidly secured to the mandrel  24  downhole of the sleeve  48 . 
     Anchor plate  56  is optionally secured on a screw-mounted collar  57  the position of which relative to the seal element  10  may be adjusted in order to tune the preload of the spring  49 . The force exerted by the spring  49  maintains the elbow  58  in contact with the drillpipe inner wall  59 . The spring force is chosen such that the pump pressure does not under normal circumstances cause the inner sleeve  48  to move downhole relative to the outer sleeve  47 . 
     The above arrangement of the pressure relief mechanism however causes any compressional force on the actuation member to translate as longitudinal movement of the moveable sleeve. 
     When the downhole tool is travelling “normally” as illustrated in  FIG. 4 , and has not entered a reduced diameter part of the drillpipe, as noted the spring force prevents any longitudinal movement of the moveable sleeve  48  along the mandrel  22 . 
     However when the downhole tool enters a reduced inner diameter section of the drillpipe, such as the internal upset  61  visible in  FIGS. 4 and 5 , the elbow  58  of the pantograph becomes compressed as shown in  FIG. 5 . The compressive force overcomes the force exerted by spring  49 , with the result that actuation arm  46   a  moves in a direction away from the seal element  10  and as it does so, moveable sleeve  48  which is secured at one end to arm  46   a  also moves in the same direction. This results in opening of the fluid flow passage  52 . 
     The temporary increase in the fluid pressure acting across the seal, due to the tool entering a reduced space, is thereby relieved when the fluid flow passage  52  opens. 
     The kinetic energy from the movement of arm  46   a  and moveable sleeve  48  translates into potential energy of the spring  49  as the spring compresses. 
     When the tool  24  leaves the reduced-diameter section of the drillpipe, the restoring potential energy in the spring  49  causes the actuation arm  46   a  and the moveable sleeve  48  to move along the mandrel  22  towards the seal element (in the direction of the restoring spring force). The conversion of potential energy in the spring to kinetic energy of the actuation member closes the fluid flow passage  52  with the result that the full pump fluid pressure difference is again able to develop across the seal element  10 . 
     As is apparent from the foregoing and from  FIGS. 4 and 5  the arrangement thereof amounts to a valve that opens temporarily, in order to prevent the seal element bursting problem described above, before subsequently closing again. In the illustrated variant of the invention therefore the plug  28  of e.g.  FIG. 2  is replaced by the mechanism described. 
     As is also apparent from  FIGS. 4 and 5  in practical versions of the variant at least two pairs of the actuation arms  46   a ,  46   b  are provided connected in a similar manner on opposite sides of the mandrel. This ensures that any compressional force transmitted to the spring is centered in at least the plane of the actuation arms  46   a ,  46   b  thereby reducing the risk of binding of the inner sleeve  48  onto the mandrel. In other embodiments of the invention it may be possible to provide e.g. three or four pairs of the actuation arms at equi-spaced intervals around the circumstance of the inner sleeve  48 . Such arrangements while more complex than those illustrated nonetheless lie within the scope of the invention as claimed. 
     The listing or discussion of an apparently prior-published document in this specification should not necessarily be taken as an acknowledgement that the document is part of the state of the art or is common general knowledge.