Abstract:
A logging tool or sub ( 10 ) for downhole use comprises an elongate logging tool body ( 11 ); one or more moveable reaction members; and a locking member for the reaction members. At least one arm ( 12 ) at a first location is pivotably secured to the body ( 11 ) to be extensible therefrom and compressible towards the body ( 11 ); and at a second location spaced from the first location is pivotably secured to a locking member ( 18 ); and a stop member ( 22 ) fixed or fixable relative to the body ( 11 ). The locking member ( 18 ) defines a moveable end ( 19 ) remote from the second location and moveable relative to the arm ( 12 ) between at least a first position in which force tending to compress the arm ( 12 ) towards the body ( 11 ) causes the end ( 19 ) to move away from the stop member ( 22 ), and a second position in which force tending to compress the arm ( 12 ) towards the body ( 11 ) causes the stop member ( 22 ) to resist movement of the locking member ( 19 ) thereby preventing compression of the arm ( 12 ) towards the body ( 11 ). Alternatively, one or more cam and follower arrangement or rack and pinion arrangement can be used.

Description:
FIELD OF THE DISCLOSURE 
       [0001]    The invention relates to a downhole logging tool. 
       BACKGROUND OF THE DISCLOSURE 
       [0002]    As is well known, prospecting for minerals, hydrocarbons such as oil and gas, and other natural resources of commercial value is economically an extremely important activity. For various reasons those wishing to extract resources from below the surface of the ground or the floor of an ocean need to acquire as much information as possible about both the potential commercial worth of the natural resources in a geological formation and also any difficulties that may arise in extracting them to surface locations at which they may be used. 
         [0003]    Techniques of logging of subterranean formations have developed for the purpose of establishing, with as much accuracy as possible, information as outlined above both before extraction activities commence and also while they are taking place. 
         [0004]    During exploratory drilling operations, a hole is drilled from a surface location to a location underground near where the prospective resource is located. The resulting borehole may extend for several thousand or tens of thousands of meters from a surface location. 
         [0005]    Drill pipe is typically a hollow, thick-walled, steel piping used on drilling rigs to facilitate the drilling of a borehole/wellbore. Drill pipe consists of numerous pieces, sometimes called “stands”, screwed one to another. Each stand is approximately ten meters long. Usually a stand has external male threads at one end and female threads in the internal diameter of the other end. The male threads of one stand are engageable with the female threads of another stand, thereby allowing joining of the stands together. 
         [0006]    Normally while borehole drilling is carried out, a compound string of drillpipe stands is used in order to drive a rotatable drill bit mounted at the end of the pipe in contact with the geological formation being drilled. 
         [0007]    As the drill bit works its way down into the ground and the borehole length increases, the drill pipe is repeatedly extended by adding new stands to its upper end. As a result long lengths of drillpipe may be inserted into boreholes as they are formed. 
         [0008]    Broadly stated, logging involves inserting a logging tool, also known as a sonde, into a borehole or other feature penetrating a formation under investigation; and using the sonde to energize the material of the rock, etc., surrounding the borehole in some way. Such passage of the energy alters its character. The logging tool, that is capable of detecting energy, is intended then to receive emitted energy that has passed through the various components in the rock before being recorded by the logging tool. 
         [0009]    Typically the logging tool is formed as an elongate, rigid cylinder that may be e.g. one to five meters long. 
         [0010]    Wireline, as is well known in the art, is an armored cable that may be used for the purposes of lowering a logging tool into the borehole, or supporting the tool while it is being withdrawn upwardly along a borehole or well during logging. The logging tool is located at the end of the wireline. Logging measurements are in one known method taken by lowering the wireline including a logging tool attached as aforesaid to a prescribed depth and then raising it out of the well while operating the logging tool. Wireline is capable of electronically telemetering data from various types of logging tool from downhole to surface locations; and also of sending electronic commands to connected downhole equipment. In some situations however it is not possible or desirable to maintain the wireline connected to the logging tool following deployment of the latter. 
         [0011]    Wireline drop-off is a conveyance system that allows for openhole data acquisition while tripping (i.e. the act of pulling the drill pipe out of the hole or replacing it in the hole). In this conveyance technique, a logging tool powered by a battery having a memory function is conveyed downhole by wireline through the drill pipe and hangs into the openhole on a no-go at the bottom of the drill pipe. 
         [0012]    When drilling has reached total depth (the planned end of the well measured by the length of pipe required to reach the bottom), the wireline is released into the drill pipe. Typically there is a landing collar in the internal wall of the drill pipe, located near the mouth of the final (i.e., most downhole) stand, which receives a landing ring located on and protruding outwardly from the tool. The engagement of the landing ring and collar secures the tool and pipe one to another. When this engagement has occurred, the wireline is removed from the well. 
         [0013]    The result of this sequence is that part of the logging tool protrudes beyond the end of the drill pipe and therefore is exposed in a way that permits logging of the formation. A further part of the logging tool remains inside the drill pipe and defines the described landing ring connection to the drill pipe. 
         [0014]    To withdraw the drill pipe, the stands at the surface are unscrewed one by one from each other to separate them as the drill pipe is pulled upwardly in discrete steps. As a result the drill pipe is gradually withdrawn from the borehole. A dropped-off logging tool therefore moves towards the surface with the pipe, taking records (well logs) of the formation along the way. 
         [0015]    Each time a drill pipe stand is to be removed from the upper end, the withdrawal operation is interrupted while unscrewing of the drill pipe takes place. 
         [0016]    The protruding nature of the landing ring on the tool may prevent it from entering drill pipes having a small internal diameter. 
         [0017]    Prior art wireline drop-off techniques only enable the tool to log the openhole beyond the landing ring into which the tool protrudes. This limits the length of the openhole where formation data could be acquired. It would be desirable to log openhole that is well beyond the end of the drillpipe. 
         [0018]    Sometimes it is desirable to re-log the formation to prove the accuracy and repeatability of the measurements, a technique that is commonplace in conventional wireline logging. 
         [0019]    Furthermore, should the drill pipe become stuck in the wellbore a common technique to free it is to rotate and reciprocate the drill pipe. Reciprocation, e.g., moving the pipe up and down, is not possible with logging tools hanging out of the end of the pipe. 
       SUMMARY OF THE DISCLOSURE 
       [0020]    A way to solve the above problems is to have moveable retractable arms which enable the tool to become engaged with a landing surface, if so desired; or allow the tool to go beyond a drill pipe landing ring, into openhole. Such arms do not extend beyond the external cylindrical logging tool body in the retracted position. Consequently the overall external diameter of the logging tool is smaller than in prior art tools having permanently protruding landing parts. This therefore allows the tool to enter and pass through drill pipes that have a small diameter and for the tools to be latched (or re-latched) within the drill pipe using a wireline to latch them at a second position that ensures they do not extend beyond the end of the pipe 
         [0021]    However, there is a tendency for such arms not to lock in their maximally extended position and hence collapse into a compressed (retracted) position under the influence of e.g. fluid pressure or other forces acting on the arms in the drill pipe. There is therefore a need to improve the stability of the arms in use. 
         [0022]    According to a first broad aspect of the invention, there is provided a logging tool or logging tool sub for downhole use comprising an elongate logging tool body; at least one reaction member that is moveable between a retracted position in which it protrudes transversely no further than the extent of the logging tool body and an extended position in which the reaction member protrudes transversely beyond the logging tool body; and a releasable locking mechanism for locking the reaction member in the extended position. 
         [0023]    The presence of retractable and extensible reaction members means that the logging tool or logging tool sub advantageously may selectively be caused to land on a landing component, such as a landing ring or shoulder of drillpipe; or pass freely by any such landing feature with the result that the logging tool or logging tool sub may travel beyond the end of drillpipe into open hole. The inclusion of a locking mechanism means the/or each reaction member may selectively be stabilized in use against forces tending to cause the reaction member(s) to retract. This is desirable because a well can kick due to random gas or oil pressure events forcing the logging tools back into the drill pipe. When the latching mechanism reverts to being closed the logging tools will disconnect from the drill pipe. 
         [0024]    In some preferred embodiments of the invention the logging tool or logging tool sub includes a plurality of reaction members disposed at mutually spaced locations about the logging tool body. Such an arrangement beneficially permits landing of the logging tool or logging tool sub by way of multiple points of contact with one or more landing features of drill pipe. Such an arrangement is stable and secure. 
         [0025]    Preferably the/or each reaction member is drivable at least between the retracted and extended positions by a cam and follower arrangement. In practical embodiments of the invention the/or each reaction member is also drivable in the reverse direction between the extended and retracted positions by way of the cam and follower arrangement. 
         [0026]    Also, preferably the/or at least one said reaction member is or includes such a cam follower. 
         [0027]    In one embodiment of the invention, the/or each said reaction member is drivable at least between the retracted and extended positions by a rack and pinion arrangement. 
         [0028]    Preferably, the or at least one said reaction member is or includes a rack of a said rack and pinion arrangement. 
         [0029]    The/or at least one said reaction member optionally may be or include a dog that is engageable with a latching part of a landing component of a section of drill pipe. 
         [0030]    Such arrangements are beneficially effective in assisting secure landing of the logging tool or sub. 
         [0031]    In another preferred embodiment of the invention, the/or at least one said reaction member is or includes at least one arm that at a first location is pivotably secured to the logging tool body so as to be extensible therefrom and compressible towards the logging tool body, and at a second location spaced from the first location is pivotably secured to a locking member forming part of the locking mechanism; and the locking mechanism further includes a stop member that is fixed or fixable relative to the logging tool body; the locking member defining a moveable end that is remote from the second location and is moveable relative to the arm between at least a first position in which force tending to compress the arm towards the logging tool body causes the moveable end to move away from the stop member, and a second position in which force tending to compress the arm towards the logging tool body causes the stop member to resist movement of the locking member thereby preventing compression of the arm towards the logging tool body. 
         [0032]    Having moveable retractable arms enables the logging tool to become engaged with a landing surface or allows the tool to go beyond a drill pipe landing ring, into openhole. It is possible to keep the tool in the desired depth by means of the arms that provide stability to the tool. Once the logging tool has reached a desired downhole depth, the apparatus of the invention provides a locking mechanism. This locking mechanism keeps the arms locked in their extended positions even when compressional forces acting on the arms urge them to close back towards the logging tool. The ability of the arms to be maintained in the extended position is therefore also independent of the strength of the driving mechanism of the logging tool system. 
         [0033]    Preferably, the logging tool includes a moveable member that is moveable towards and away from the stop member, the moveable end of the locking member being pivotably secured to the moveable member whereby when the locking member adopts the second position force tending to compress the arm towards the logging tool body urges the moveable member into engagement with the stop member. 
         [0034]    In a preferred embodiment of the invention, the logging tool body includes a hollow interior accommodating the stop member and the moveable member and the locking member extends via an aperture to interconnect the arm and the moveable member. 
         [0035]    Being housed inside the logging tool body, the stop member, the moveable member and the locking member are protected against the downhole environment. Also the exterior dimensions of the logging tool are kept as compact as possible. 
         [0036]    Preferably, in the first position, the locking member lies to a first side of a normal to a longitudinal axis of the logging tool body, and in the second position the locking member lies to a second, opposite side of the normal. 
         [0037]    It is preferred that the locking member is rigid whereby movement of the locking member between the first and second positions causes the arm to extend from the logging tool body to a maximal extent. 
         [0038]    Further, preferably the logging tool body includes a recess within which the arm is receivable when in a compressed position. 
         [0039]    In a preferred embodiment of the invention, the pivoting of the arm exposes a surface that is engageable with one or more landing surfaces of a further component. 
         [0040]    Conveniently, the engagement of the resulting exposed surface of the arm with a landing surface of a further component causes pressing of the exposed surface and the landing surface together so as to stabilize the arm in an extended position. 
         [0041]    Preferably, the logging tool includes a pivot pin through which the arm is pivotably secured to the logging tool body, extending inside the recess. 
         [0042]    Also, preferably there are provided three moveable arms each supported by a locking member, at equiangular spacings about a circumference of the logging tool body, which typically is cylindrical. 
         [0043]    The inventors have found it desirable to have three moveable landing extensions but the invention is not limited to this number. More or fewer or arrays of moveable landing extensions therefore may be employed, in regular or irregular patterns. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0044]    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: 
           [0045]      FIG. 1  is a transparent, three-dimensional side view of a logging tool according to the invention inserted into drill pipe and having arms in an extended un-locked position; 
           [0046]      FIG. 2  shows the  FIG. 1  arrangement wherein the arms are in a maximally extended locked position; 
           [0047]      FIGS. 3A to 3J  form a series of schematic drawings showing how movement of the locking member locks the arms in a maximally extended position. 
           [0048]      FIG. 4A  is a three-dimensional perspective view of a logging tool according to an embodiment of the invention having a cam and follower arrangement. 
           [0049]      FIGS. 4B and 4C  depict a sectional view of the logging tool of  FIG. 4A  dissected along the line a. 
           [0050]      FIG. 5A  is a three-dimensional perspective view of a logging tool according to an embodiment of the invention having a rack and pinion arrangement. 
           [0051]      FIGS. 5B and 5C  depict a sectional view of the logging tool of  FIG. 5A  dissected along the line R. 
       
    
    
     DETAILED DESCRIPTION 
       [0052]      FIG. 1  shows, in bold lines, a superimposed schematic representation similar to that depicted in  FIGS. 3A to 3J . However, the schematic lines of  FIGS. 3A to 3J  do not form part of the embodiment of  FIG. 1 . 
         [0053]    Referring to  FIGS. 1 and 2  of the drawings, there is shown a logging tool or logging tool sub  10  that as is commonly the case has an elongate, hollow cylindrical body  11 . Parts of the logging tool intended to energize a subterranean formation or receive logging signals from underground rock are for clarity not shown in the figures. These features may take a wide range of forms that are known to the person of skill in the art. 
         [0054]    The cylindrical body  11  of the logging tool  10  supports reaction members that in the illustrated embodiment are three pivotably deployable arms  12  secured on the exterior of the cylindrical body  11  in the manner described below. 
         [0055]    In the preferred embodiment of the invention shown in  FIGS. 1 and 2 , three arms  12  are provided equiangularly spaced about the external circumference of the logging tool. As a result of the orientation of the logging tool in the figures, only two of the arms  12  are visible. 
         [0056]    The arms  12  lie near to the in-use uphole end of the logging tool  10  so that a major part of the length of the tool  10  extends in a downhole direction from the circumference at which the arms  12  are secured. This feature is of benefit when deploying the tool  10  in a drop-off (or similar “tool hanging”) manner so that part of the tool protrudes beyond the open end of drill pipe in a borehole. 
         [0057]    However, other numbers and patterns of the arms  12  are possible within the scope of the invention. It is not essential that the arms  12  are equiangularly spaced about a circumference of the tool  10 , or that they are secured at a common circumference. Indeed various irregular patterns of the arms  12  are possible but the regular arrangement shown is preferred because (a) it permits even accommodation of forces when the arms  12  engage a drill pipe landing ring; and (b) landing of the tool  10  may be effected reliably and repeatably as a result. 
         [0058]    The arms  12  are elongate, essentially rectangular members that extend parallel to the longitudinal axis of the logging tool  10 . As shown in  FIG. 1 , the arms  12  are received within respective essentially rectangular recesses  13  that are aligned in register with the arms  12  and are dimensioned so that the arms  12  are neatly receivable retracted inside in them. The depth of each recess  13  is such that when the arms  12  are in the retracted position they protrude outwardly no further than the material of the cylindrical body  11 , and in preferred embodiments of the invention lie flush with the exterior of the body  11 . In  FIG. 1 , however, the arms are illustrated in a slightly different position and are only partly retracted. 
         [0059]    At its in-use, uphole end each recess  13  includes secured therein a transversely extending pivot pin  14  that extends across the recess from one major side to the other, opposite side. 
         [0060]    Each pivot pin  14  perforates one of the arms  12  near its uphole end and retains it pivotably captive relative to the cylindrical body  11 . The dimensions of the parts are such that the arms  12  may pivot between a compressed position and an extended position. As a result of pivoting about the pivot pins  14 , the uphole ends of the arms  12  retract slightly into the associated recesses  13  and the downhole ends protrude noticeably beyond the exterior of the cylindrical body  11 . 
         [0061]    The in-use, most downhole ends of the arms  12  are formed as exposed surfaces  16  that are engageable with drill pipe landing rings of conventional designs. 
         [0062]    As illustrated, the pivot pins  14  are secured in chord bores  17  formed in the material of cylindrical body  11 . Each pivot pin  14  may be e.g., a press fit at either end in a pair of such chord bores  17 , so that the pivot pin  14  spans the recess  13  in which it is fixed from one major side to the other. However, other methods of securing the pins  14  are possible within the scope of the invention. 
         [0063]    Near the exposed surface  16  at one end of each arm  12 , there is pivotally connected one end of a moveable locking member  18 . The other end  19  of the locking member  18  is pivotally fixed to a moveable member  21 . 
         [0064]    In a preferred embodiment of the invention, the locking member  18  is made of rigid materials but the invention is not limited as such. 
         [0065]    Thus, it is possible for example for one or more of the locking members  18  to be resiliently deformable (e.g., through the incorporation of a spring-biased hinge mid-way between its ends). Such a locking member would resist forces tending to compress the arms  12  into the recesses  13  up to a limit determined by the spring force acting at the hinge. A compressive force exceeding the resilience of the locking member  18  then would cause the arms  12  to adopt the retracted position even when they are locked as described below. 
         [0066]    Within the hollow interior of the logging tool  10 , an elongate stop member  22  additionally extends along the longitudinal axis of the body  11 . Stop member  22  in the illustrated embodiment of the invention is formed as an elongate, fixed rod one free end of which is engageable by the moveable member  21 . In other embodiments, however, the stop member  22  may take other forms. 
         [0067]    Moveable member  21  lies inside, and is longitudinally moveable along, a hollow bore  27  extending inside the body  11  of the logging tool  10  parallel to its longitudinal axis. In the preferred embodiment shown the bore  27  is concentric with the cylinder that is the logging tool body  11 ; but in other embodiments of the invention this need not necessarily be the case, and e.g., an off-center moveable member may be used. 
         [0068]    The moveable member  21  preferably is essentially cylindrical as illustrated but this need not necessarily be the case, and other cross-sections are possible. In such cases the cross-section of the hollow bore  27  may be altered accordingly. 
         [0069]    The moveable member  21  as illustrated is slideably moveable inside the hollow bore  27 . In other embodiments of the invention, however, movement of the moveable member  21  may be effected through a phenomenon other than sliding. Furthermore, it is not strictly necessary that movement of the moveable member  21  is longitudinal relative to the logging tool body  11 . Thus, it is possible to envisage variants of the invention in which e.g. rotational movement of the moveable member  21  is possible. In such an arrangement, it may be desirable for the locking members  18  not to be permanently connected to the moveable member  21 . Examples of such variants are described below with reference to  FIGS. 4 and 5 . 
         [0070]    As a result of the pivotable connections at the ends of the locking members  18 , the latter are moveable as the arms  12  and moveable member  21  move. 
         [0071]    Furthermore, as noted, the moveable member  21  lies inside the interior of the logging tool body  11  whereas the pivotable connection of each locking member  18  and its associated arm  12  lies in a recess  13 , externally of the logging tool body  11 . 
         [0072]    In order to accommodate both the connection between each arm  12  and the moveable member  21  a respective, through-going perforation  23  is formed in the walls of each recess  13 . In the embodiment illustrated each perforation is in the form of an elongate slot that extends parallel to the center axis of the logging tool body  11 . In other arrangements however other shapes of the perforations are possible. 
         [0073]    Depending on the sizes and shapes of (in particular) the recesses  13 , the perforations  23  and the moveable member  21 , one or more parts of the moveable member  21  may at certain points in its range of movement protrude via the perforations  23  into the recesses. In the embodiment of the invention illustrated in  FIGS. 1 and 2 , however, the ends of moveable member  21  at which the locking members  18  attach are shaped, e.g. flattened, so as to avoid protrusion of the moveable member  21  via the perforations  23 . 
         [0074]    Regardless of whether the moveable member  21  protrudes as described, it is advantageous for each pivotable connection of a locking member  18  to the moveable member  21  optionally to lie as shown inside a respective further recess  24  formed in the surface of the moveable member  21 . This protects the pivot against damage and assists the member  21  to move freely inside the logging tool body  11 . 
         [0075]      FIGS. 3A to 3J  illustrate, by way of schematic drawings, the movement of the arms  12  when the moveable member  21  moves in direction X (i.e. from right to left). Such movement of the moveable member  21  may be effected in a variety of ways as will be known to the person of skill in the art. 
         [0076]      FIG. 3A  shows the locking member  18  initially forming an angle of 61° (in the preferred embodiment described, although this choice of starting position may be varied within the scope of the invention) with a normal  26  to the longitudinal axis of the logging tool on one side. The size of the angle is for illustration purposes and the angle formed, as depicted in  FIGS. 3A to 3J , can range from 0° to 90°. 
         [0077]    As the moveable member  21  moves to the left, the arm  12  pivots away from the logging tool  10  and the angle between the locking member  18  and the normal  26  decreases. 
         [0078]    When the position of the moveable member  21  is as illustrated in any of  FIGS. 3A to 3G , any force acting on the arms  12  as shown by arrow Y in  FIG. 3G , which tends to urge the arm  12  to close (towards the logging tool), causes the moveable member  21  to slide to the right. The logging tool  10  would, therefore, not be stable at such a time. 
         [0079]    When the angle has decreased to zero, as shown in  FIG. 3H  (which approximately corresponds to the  FIG. 2  component positions), this signifies that the arm  12  is at its maximum opening and will not pivot further. In this configuration any compressive forces acting on the arms  12  will act longitudinally along the locking members  18  with the result that the compression will be resisted. The moveable member  21  is however not in contact with the stop member  22  and can continue to move to the left. 
         [0080]    Movement of the moveable member  21  further to the left, after the arms  12  have reached their maximum opening, causes the locking member  18  to pass the normal  26  and lie on the opposite side of the normal  26  to its starting position. This configuration is shown in  FIG. 3I . At this time any compressive force acting on the arms  12  drives the moveable member  21  further to the left. 
         [0081]    Continued movement of the moveable member  21  in direction X finally brings it into contact with the stop member  22  which then inhibits further movement of the moveable member  21  to the left. In this position, any force that pushes on the arms  12  (e.g. Force Z in  FIG. 3J ) cannot cause the moveable members  18  to pass the normal  26  in the opposite direction. In other words, the arms  12  will not compress back towards the logging tool  10 , and neither will the locking member  18  move to the right. Instead, the moveable member  21  is pressed against the stop member  22  and the arms  12  are locked in place. 
         [0082]    As can be seen above, the engagement of the moveable member  21  and the locking members  18 , when the arms  12  are in the position as illustrated in  FIGS. 3H and 3J , provides an axial force that locks the arms  12 . The mechanism of the invention may be restored to a configuration permitting compression of the arms  12  into the recess  13  by driving the moveable member  21  to the right (as referenced in  FIGS. 3A to 3J ). 
         [0083]    The person of skill in the art is aware of techniques for causing such movement of the moveable member  21 , which in turn draws the locking members  18  to pass the normal  26  in the opposite (i.e. return) direction. This brings the apparatus to one of the configurations illustrated in  FIGS. 3A to 3G . Receipt of the arms  12  into the recess  13  is then possible, either as a result of compressive forces acting on the arms  12  from the exterior of the logging tool  10  or because of further, powered movement of the moveable member  21  to the right in the schematic figures. 
         [0084]    Alternative embodiments of the invention include one or more cam and follower arrangements for effecting movement of one or more reaction members. In such an embodiment the reaction members, which do not have to be embodied as pivoting arms, may include the followers of the cam and follower arrangement. 
         [0085]    An embodiment of the invention having a cam and follower arrangement will now be described with reference to  FIGS. 4A to 4C . 
         [0086]    In  FIG. 4A , there is shown a logging tool or logging tool sub  30  that as is commonly the case has an elongate, hollow cylindrical body  31 . Reaction members  34  of the logging tool  31  as illustrated in  FIG. 4A  are in their extended positions. 
         [0087]    The cam and follower arrangement can be seen in  FIGS. 4B and 4C , after the logging tool  30  depicted in  FIG. 4A  is dissected along the cutting line a and a front portion of the tool  30  is removed. 
         [0088]    Inside the cylindrical body  31  there is a rotatable shaft  32  that has fixedly mounted thereon, and rotationally drives a triangular element  33  with truncated corners defining three lobes that are separated from one another by the sides of the generally triangular shape of the element  33 . The lobs and sides together define a continuous cam surface extending about the periphery of the triangular element  33 . The element  33  need not be triangular, however, and can be in other forms and shapes. 
         [0089]    In the illustrated embodiment, there are three reaction members  34 , the reaction members  34  being equiangularly spaced within the cylindrical body  31 . There are three corresponding perforations  36  in the cylindrical body  31  which allow the reaction members  34  as desired partially to protrude beyond the surface of the cylindrical body  31  and retract within it. The number of reaction members and perforations in the scope of the invention is not limited to three, but this is the preferred member as it provides for good stability of landing of the logging tool  30 . 
         [0090]    Each reaction member  34  has an inner end that contacts the triangular element  33 , and an outer end that when the reaction members are retracted as shown in  FIG. 4B  lies flush with and forms part of the surface of the cylindrical body  31 . When the reaction members  34  are in the retracted position as shown in  FIG. 4B , inner end of each reaction member  34  lies flush with the surface of the cylindrical body  31 ; and the bottom half of each reaction member  34  sits on a side of the triangular element  33 . 
         [0091]    When the shaft  32  rotates, the triangular element  33  rotates and the sliding motion of the triangular element  33  is converted into an outward linear force that pushes the reaction members  34  to protrude beyond the cylindrical body  31 . The reaction members as a result attain their extended positions as shown in  FIG. 4C . The inner end of each reaction member  34  no longer sits on a side edge of the triangular element  33 , and is instead abutted by a truncated corner lobe of the triangular element  33 . 
         [0092]    The reaction members  34  will remain in their extended positions, with their outer ends protruding out of the surface of the cylindrical body  31 , as long as each truncated corner lobe of the triangular element  33  continues to support the inner end of each reaction member  34 . The reaction members  34  are hence locked in place. 
         [0093]    The reaction members are spring-biased towards the retracted position of  FIG. 4B . As a result when the triangular cam element  33  rotates further the inner (follower) ends of the reaction members  34  bear against the sides of the triangular profile of the element  33 . In consequence the reaction members  34  are able to retract under the influence of the spring biasing. 
         [0094]    Various techniques for commanding movement of the rotation of the shaft  32  even when the logging tool/sub  30  is far downhole are known in the logging tool art. 
         [0095]    Alternative embodiments of the invention include one or more rack and pinion arrangements for effecting movement of one or more reaction members. 
         [0096]    An embodiment of the invention having a rack and pinion arrangement will now be described with reference to  FIGS. 5A to 5C . 
         [0097]      FIG. 5A  shows a logging tool or logging tool sub  40  that as is commonly the case has an elongate, hollow cylindrical body  41 . The reaction members of the logging tool  41  as illustrated in  FIG. 5A  are in their extended positions. 
         [0098]    The rack and pinion arrangement can be seen in  FIGS. 5B and 5C , after the logging tool  40  depicted in  FIG. 5A  is dissected along the cutting linep and a front portion of the tool  40  is removed. 
         [0099]    Centrally mounted inside the cylindrical body  41  is a rotatable shaft  42  with a toothed outer periphery  42  defining a pinion. 
         [0100]    In the illustrated embodiment, there are two P-shaped reaction members  43 . The scope of the invention however includes more or fewer of reaction members  43  that could be of various shapes and forms. 
         [0101]    There are two perforations  44  in the cylindrical body which correspond to the positions of the reaction members  43  and allow them to protrude beyond the surface of the cylindrical body  41 . 
         [0102]    Each P-shaped reaction member  43  is made up of a curved upper portion  48  that lies flush with the surface of the cylindrical body  41  when the reaction member  43  is in a retracted position. Each reaction member  43  has a perpendicular straight limb  47  a side of which facing the shaft  42  comprises teeth  46 . The teeth  46  meshingly engage with the teeth of the rotatable shaft  42 . The aforesaid sides of the straight limbs  47  constitute rack members and lie facing one another on opposite sides of the shaft  42 . The arrangement of the reaction members means they are capable of protruding as described below on opposite sides of the cylindrical body  41 . 
         [0103]    As a result of engagement between the pinion teeth of shaft  42  and the rack teeth  46 , clockwise rotational motion of the shaft  42  is converted to linear motion of the reaction members  43  whereby in the illustrated embodiment as depicted in  FIG. 5C , the reaction members  43  move outwards in opposite directions away from each other. 
         [0104]      FIG. 5C  depicts the logging tool  40  when the reaction members  43  are in their extended positions. As long as the rack teeth  46  on the straight limb  47  engage with the teeth on the shaft (pinion)  42 , the reaction members  43  are locked in their extended positions. 
         [0105]    Rotation of the shaft in the anticlockwise direction causes retraction of the reaction members  43 . When this is effected, the upper portions  48  move until they lie flush with the outer surface of the cylindrical body  41 . In this position, the free end of each straight limb  47  abuts the underside of the upper portion  48  of the other reaction member  43 . As a result, retraction of the reaction members  43  is limited to the position shown, and the reaction members  43  do not become recessed relative to the cylindrical body  43 . 
         [0106]    The interior of the cylindrical body  43  is hollowed to permit such movement of the described components. 
         [0107]    As mentioned above, several methods for commanding movement of the rotation of the shaft  42  even when the logging tool/sub  40  is far downhole are known in the logging tool art. 
         [0108]    In the embodiment of  FIGS. 5A to 5C , the reaction members  43  are aligned with each other lengthwise along the cylindrical body. This need not necessarily be the case however; and it is possible for the reaction members to be longitudinally spaced from one another along the cylindrical body  41 . 
         [0109]    In the various embodiments described above, the reaction members, or at least their in-use free ends, may be constituted as dogs that may latch or otherwise engage with landing features. 
         [0110]    Numerous means may be employed, within the scope of the invention, for causing the reaction members to extend transversely with respect to the elongate dimension of the logging tool or sub. 
         [0111]    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. 
         [0112]    Preferences and options for a given aspect, feature or parameter of the invention should, unless the context indicates otherwise, be regarded as having been disclosed in combination with any and all preferences and options for all other aspects, features and parameters of the invention.