Apparatus For Disengaging Well Tools From Borehole Walls

Abstract

In the several representative embodiments of the invention disclosed herein, outstanding too-disengaging members are uniquely arranged on a typical testing tool including a fluid-sampling member which is adapted for selective engagement with a borehole wall to obtain samples of connate fluids from earth formations adjacent to the borehole. These tool-disengaging members are cooperatively arranged to space the tool body away from the mudcaked borehole wall for minimizing the surface area contacting the wall so as to reduce the possibility of the tool becoming tightly stuck against the borehole wall by differential pressure as fluid samples are taken. Various arrangements of these unique tool-disengaging members are disclosed whereby these members are also cooperatively moved in relation to the borehole wall in response to selected movement of the tool body for readily disengaging the tool from the wall when it is to be moved away from its testing position.

Description

Those skilled in the art will readily appreciate that well logging and testing tools often become stuck in a borehole during the course of a typical operation. Perhaps the most common cause for these tools becoming stuck is where some portion of the tool body or an outstanding member thereon becomes at least partially embedded in the mudcake typically lining the borehole walls adjacent to a formation containing connate fluids. By way of explanation, so-called "differential sticking" typically occurs where the embedded portion or member of the tool becomes sealingly engaged against the mudcake so that the hydrostatic pressure of the well control fluids or so-called "mud" in the borehole which is imposed on the rear of the sealingly engaged member will press the forward face of this member laterally against the mudcake or formation. Thus, if the cross-sectional area of this member and the differential between the borehole hydrostatic pressure and the pressure of the formation fluids are of substantial magnitude, the sealingly engaged member will be held so tightly against the mudcake that an extreme vertical force will be required to release the tool.

This problem of differential sticking becomes particularly acute where the well tool is a cable-suspended fluid-sampling tool such as those disclosed in U.S. Pat. No. 3,011,554, U.S. Pat. No. 3,329,208 and U.S. Pat. No. 3,335,364 and which is adapted for obtaining samples of connate fluids from earth formations penetrated by the borehole. As seen in those patents, these sampling tools typically include one or more sealing pads with fluid-sampling ports or tubes which are cooperatively arranged on the forward face of an elongated tool body for selective placement into sealing engagement with a borehole wall for obtaining samples of connate fluids contained in the adjacent formations. For selectively moving the sealing members into sealing engagement with the borehole wall, one typical arrangement includes an extendible backup shoe which is mounted on the rear face of the tool body and adapted for laterally shifting the tool to set the fixed sealing pad into sealing engagement with a mudcaked borehole wall. Alternatively, the sealing pad may be adapted to be extended forwardly of the body so as to force the rear face of the tool body against the opposite wall of the borehole for maintaining the sealing pad firmly engaged. A third typical arrangement employs both an extendible backup shoe and an extendible sealing pad which are extended in opposite directions to anchor the tool in the borehole.

Regardless of which arrangement is employed, it will be appreciated that the members which contact with the borehole wall have substantial cross-sectional areas that are subject to becoming differentially stuck against the mudcake lining the borehole. For example, the extendible backup shoe typically used on fluid-sampling tools such as the one shown in U.S. Pat. No. 3,011,554 has a surface area in the order of sixty square inches. If a pressure differential of only 500-psi is acting on this backup shoe, it will be appreciated that if the entire front surface of the shoe is sealingly engaged the shoe will be urged against the borehole wall with a pressure force in the order of 30,000-pounds. Thus, since the coefficient of friction between the backup shoe and a typical layer of mudcake may be as high as 0.4 or 0.5, a pull in excess of the cable's strength might be required for releasing the tool.

Accordingly, it is an object of the present invention to provide new and improved apparatus for disengaging cable-suspended well tools which have become differentially stuck against a borehole wall without requiring the imposition of extreme upwardly directed forces which might exceed the safe tensile loading of the tool suspension cable.

This and other objects of the present invention are obtained by arranging one or more laterally outstanding tool-disengaging members having minimal wall-contacting surfaces to project outwardly from a tool body. Upon movement of these outstanding members into contact with a mudcaked borehole wall, the tool body will be spaced laterally away from the wall and only the minimum-area surfaces of the outstanding members will be subjected to becoming differentially stuck against the wall. Means are further provided for causing the tool-disengaging members to move relative to the borehole wall in response to vertical movement of the tool body for disengaging the tool from the borehole wall when it is subsequently moved to a different position in the borehole.

The novel features of the present invention are set forth with particularity in the appended claims. The invention, together with further objects and advantages thereof, may be best understood by way of the following description of exemplary apparatus employing the principles of the invention as illustrated in the accompanying drawings, in which:

FIG. 1 depicts a typical fluid-sampling tool on which is mounted two embodiments of new and improved tool-disengaging means of the present invention;

FIGS. 2 and 3 respectively show different views of one of the preferred embodiments of the tool-disengaging means illustrated in FIG. 1;

FIGS. 4 and 5 respectively illustrate different views of the other preferred embodiment of tool-disengaging means depicted in FIG. 1;

FIG. 6 is a cross-sectional plan view of the fluid-sampling tool depicted in FIG. 1 showing the tool-disengaging means respectively illustrated in FIGS. 2-5 when they are fully retracted in relation to the tool; and

FIG. 7 illustrates an alternative embodiment of tool-disengaging means also arranged in accordance with the principles of the present invention.

Turning now to FIG. 1, new and improved tool-disengaging means 10-13 are arranged on a fluid-sampling tool 14 which is shown suspended from a typical multi-conductor cable 15 in a borehole 16 containing a well control fluid and cooperatively engaged with a particular formation interval 17 for collecting one or more samples of producible fluids from that formation. The suspension cable 15 is spooled in the usual manner from a winch (not shown) at the earth's surface and is coupled to suitable control devices and recording devices (not shown) at the surface for controlling the tool as well as monitoring its operation during the course of a typical fluid-sampling operation.

It is believed unnecessary to describe the specific arrangements or operation of the several components of the tool 14 since their constructional details are of no consequence as far as the present invention is concerned and the arrangement of such tools is well described in each of the several aforementioned patents as well as in other patents showing fluid samplers of a similar nature. Accordingly, it is believed sufficient to say only that the sampling tool 14 is comprised of an elongated body 18 which is typically arranged as a number of tandemly connected sections. As is usual, the upper portion of the tool 14 includes selectively operable control means 19, such as the hydraulic system described in U.S. Pat. No. 3,011,554, for controlling the sequential operation of the tool. Among other things, the control means 19 are operable for selectively extending and retracting wall-engaging means such as a pair of similar extendible shoes 20 and 21 which are respectively arranged on the forward and rearward faces of the tool body 18 and adapted for extension in opposite directions against the borehole walls to anchor the tool 14 in the borehole 16 for obtaining fluid samples. Sample-admitting means 22, such as the sealing members and associated sampling devices shown generally at "9" in U.S. Pat. No. 3,147,807 or at "21" in U.S. Pat. No. 3,452,592, are mounted on the forward shoe 20; and the other shoe 21 is used to insure that the sealing pad 23 is sealingly engaged with the borehole wall. As fully explained in the several aforementioned patents, the sampling tool 14 is cooperatively arranged for obtaining fluid samples from the formation 17 which are confined within an enclosed sample chamber that is typically included in the lower section 24 of the tool body 18.

As illustrated in FIG. 1, the forward tool-disengaging means 10 and 11 of the present invention are preferably similar or identical and are respectively mounted on the forward face of the tool body 18 at convenient spaced intervals immediately above and below the forward shoe 20 carrying the sealing pad 23 of the sample-admitting means 22. The new and improved rearward tool-disengaging means 12 and 13 which are also preferably similar or identical are respectively faced rearwardly and arranged near the upper and lower ends of the outer face of the backup shoe so as to be cooperatively engaged with the borehole wall opposite from that engaged by the sample-admitting pad 23 upon extension of the shoes 20 and 21. The significance of the arrangements of the new and improved tool-disengaging means 10-13 of the present invention will subsequently be explained once the particular details of their respective preferred embodiments are considered.

Turning now to FIGS. 2 and 3 detailed views are shown of the tool-disengaging means 10 which, as previously noted, are preferably identical or similar to the tool-disengaging means 11. As illustrated, the tool-disengaging means 10 are preferably comprised of a support or base 25 secured to the tool body 18 and operatively carrying a T-shaped member 26 having a transversely oriented cross bar 27 on the outer end of a short upright member 28 which is disposed in an outwardly opening recess 29 in the base. As best seen in FIG. 2, the T-shaped member 26 is pivotally mounted on the outstanding base 25 by a lost-motion connection such as that provided by a transversely oriented pin 30 which is carried on the base and cooperatively received in an elongated slot 31 formed in the inwardly directed end of the upright member 28 so as to allow the T-shaped member to move vertically between longitudinally spaced positions. To normally retain the T-shaped member 26 in its illustrated upwardly inclined position in relation to the tool body 18, biasing means are provided such as one or more tension springs 32 which are disposed in grooves 33 in the base 25 and connected in tension between the base and the cross bar 27.

As best seen in FIG. 3, the outer face of the cross bar 27 is preferably champhered to provide a fairly sharp transverse edge, as at 34, extending along the outermost portions of the cross bar. Moreover, as best illustrated in FIG. 6, the extremities of the cross bar 27 are preferably formed in a slight curvature so as to leave only an outwardly directed contact surface 35 of minimal area on the mid-point of the cross bar for contacting the wall of the borehole 16. It should be noted from FIG. 6 that the contact surface 35 is spaced laterally ahead of the forward face of the tool body 18 a distance which is greater than the maximum lateral projection of the sealing pad 23 when the front shoe 20 is fully retracted. The significance of these several constructional details will subsequently be explained.

Turning now to FIGS. 4 and 5, detailed views are shown of the tool-disengaging means 12 which are preferably similar or identical to the tool-disengaging means 13. As shown in the drawings, the tool-disengaging means 12 include a yoke-shaped member 36 having its mid-portion complementally shaped to closely straddle the outer face of the rear shoe 21 and which is cooperatively coupled thereto for generally longitudinal movement by means such as a lost-motion connection as provided by an elongated slot, as at 37, in each end of the member that slidably receives a laterally outstanding pivot, as at 38, on each longitudinal edge of the backup shoe. To normally urge the yoke-shaped member 36 upwardly in relation to the shoe 21, biasing means are provided such as a pair of tension springs 39 and 40 arranged in recesses 41 and 42 on the shoe and coupled in tension between the rear shoe and the yoke-shaped member. As best seen in FIG. 5, the outward face of the transverse portion of the yoke-shaped member 36 is preferably shaped so as to define a rounded outer surface, as at 43, for contacting the wall of the borehole 16 when the backup shoe 21 is extended.

Accordingly, it will be appreciated from FIG. 1 that when the fluid-sampling tool 14 is positioned adjacent to the formation 17 and the shoes 20 and 21 are extended, the sealing pad 23 will be in contact with one wall of the borehole 16 and the tool-disengaging means 12 and 13 will be in contact with the opposite wall of the borehole. It will be realized, of course, that the rounded surface 43 of the yoke 36 and its counterpart 44 of the tool-disengaging means 13 will respectively present only a minimum surface area, as at 43, which will be subject to becoming differentially stuck. Thus, under less-severe situations, when a fluid sample has been obtained and the tool 14 is to be repositioned in the borehole 16 or returned to the surface, the shoes 20 and 21 are simply retracted.

As previously discussed, severe borehole conditions can nevertheless cause even the tool 14 as depicted in FIG. 1 to become differentially stuck for one or more reasons. For example, should either or both of the yokes 36 and 44 become differentially stuck, it will be recognized that only a strong upward pull on the cable 15 can free the tool 14 if retraction of the backup shoe 21 is unsuccessful in disengaging the yokes from the wall of the borehole 16. As previously pointed out, however, the magnitude of the upward forces which can be applied to the tool 14 by way of the cable 15 is significantly limited by the tensile strength of the cable.

Accordingly, of paramount importance to the present invention, the tool-disengaging means 12 and 13 are cooperatively arranged to move in relation to the wall of the borehole 16 in response to a moderate upward pull on the fluid-sampling tool 14. As best seen in FIG. 4, it will be recognized that once the backup shoe 21 is not being urged against the wall of the borehole 16, upward movement of the tool 14 is limited only by its weight since the yoke 36 (as well as the yoke 44) is free to pivot downwardly in relation to the tool body 18. Thus, upon upward movement of the tool 14, the yoke 36 (as well as the yoke 44) will pivot about the pins 38 to the position 45 shown by the dashed lines in FIG. 4. The pivotal movement of the yokes 36 and 44 will allow the tool 14 to begin moving easily so that its inertia is overcome before any restraint due to differential sticking of the yokes is imposed on the tool.

Moreover, by virtue of the pivoting of the yokes 36 and 44, there is a rolling action of their outer faces, as at 43, in relation to the wall of the borehole 16 which will bring a new portion of their surfaces into contact with the mudcake 46 so that there will be at least a disruption of any sealing engagement between the mudcake and the yokes. Those skilled in the art will, of course, recognize that there is a time element involved in establishing a seal between a member and the mudcake 46 so that as new surfaces on the yokes 36 and 44 roll into potentially sealing engagement with the mudcake, the continued upward movement of the tool 14 will ordinarily prevent reestablishment of a differentially stuck surface on the yokes.

It should also be noted that by virtue of the lost-motion pivotal connection provided by the slots, as at 37, the yokes 36 and 44 will pivot downwardly in relation to the shoe 21 without swinging in an arc which would otherwise cause the yokes to project further outwardly from the backup shoe and possibly jam the tool 14 as it is passing through a reduced-diameter interval of the borehole 16. Thus, by virtue of the lost-motion connections of the yokes 36 and 44 on the backup shoe 21, the downward motion of the yokes is effectively along a longitudinal path in relation to the tool body 18 rather than along an arcuate path.

It will, of course, be recognized that once the tool 14 begins moving upwardly, the yokes 36 and 44 will be returned to their initial upwardly inclined positions by their respective biasing springs as at 39 and 40. This will, however, pose no significant problem since should either the yoke 36 or 44 subsequently encounter an obstruction in the borehole 16, the yokes will be free to swing downwardly without materially slowing the continued ascent of the tool 14.

Referring again to FIG. 1, it will be appreciated that retraction of the shoes 20 and 21 can also result in freeing of the backup shoe without necessarily releasing the forward face of the sealing pad 23 from the wall of the borehole 16. Should this occur as the shoes 20 and 21 are retracted, the backup shoe will be returned against the tool body 18 and the body will be shifted laterally to the right as viewed in FIG. 1 toward the front shoe. Ultimately, however, the tool-disengaging means 10 and 11 will come into contact with the wall of the borehole 16. Thus, since the forward face, as at 35, of each of the T-shaped members 26 and 47 is extended forwardly of the face of the sealing pad 23 when the shoe 20 is fully retracted, the T-shaped members will engage the wall of the borehole 16 so as to allow the pad to continue its rearward movement. Since the sealing pad 23 is customarily of an elastomeric material, it is, of course, free to yield under the retracting force. Once this occurs, the T-shaped members 26 and 47 will then be in contact with the wall of the borehole 16.

Accordingly, as previously explained with reference to the yokes 36 and 44, the tool-disengaging means 10 and 11 function to release the tool 14 as the T-shaped members 26 and 47 respectively pivot about their pivots as at 30. Hereagain, this pivotal action will present new surfaces in contact with the mudcake 46 to give the tool 14 time to begin moving before a good seal is re-established. Moreover, the lost-motion connections, as at 30 and 31, will allow the T-shaped members 26 and 47 to move longitudinally in relation to the tool body 18 rather than swing outwardly.

Turning now to FIG. 7, alternative tool-disengaging means 50 are depicted which can be successfully substituted for either the tool-disengaging means 10 (and 11) on the tool body 18 or the tool-disengaging means 12 (and 13) on the backup shoe 21. To illustrate the operation of the tool-disengaging means 50, the preferred embodiment shown in FIG. 7 is depicted as being detachably mounted on the backup shoe so as to overlay the yoke 36 and immobilize the tool-disengaging means 12 when the tool 14 is being operated in an elongated borehole having a diameter which is so large that the shoes 20 and 21 cannot be extended into contact with the borehole walls. It will, of course, be appreciated that tool-disengaging means (not shown) similar or identical to the tool-disengaging means 50 are also detachably mounted over the yoke 36.

Accordingly, as shown in FIG. 7, the tool-disengaging means 50 include a base 51 which is preferably detachably secured, as by screws 52, to the outer face of the backup shoe 21. To immobilize the yoke 36, the base 51 is mounted directly over the mid-portion of the yoke and, if necessary, a transverse groove 53 is provided in the base to accommodate the protruding portion of the yoke. A circular roller 54 is mounted in an outwardly directed recess 55 in the base 51 and journably coupled thereto as by a transversely oriented axle 56 mounted on the base. As illustrated, at least the outer rim of the roller 54 is arranged to project rearwardly or outwardly from the base 51 so that the roller and its counterpart of the other tool-disengaging means (not shown) on the shoe 21 will contact the borehole wall upon extension of the backup shoe.

Accordingly, when the tool 14 is provided with a pair of longitudinally spaced tool-disengaging means as at 50 on the backup shoe, extension of the shoes 20 and 21 will bring the roller 54 into contact with the borehole wall so as to space the outer face of the backup shoe away from the mudcake. The effective area of the roller 54 which can be in sealing contact with the borehole wall will, of course, be minimal so as to preclude differential sticking under many borehole conditions.

As previously pointed out, however, under severe borehole conditions, even the limited surface area of the roller 54 can become differentially stuck. If this should occur, it will be appreciated that once the control means 19 are operated to retract the shoes 20 and 21, the tool 14 will be free to move vertically in the borehole 16 and longitudinal movement of the body 18 will be effective for moving the roller 54 so as to shift a new surface thereof into contact with the mudcake 46. Hereagain, since a finite time is required before a seal is reestablished between the mudcake 46 and the roller 54, upward movement of the tool 14 can be accomplished with a minimum pull on the cable 15 so as to at least materially eliminate the risk of continued differential sticking of the roller.

Accordingly, it will be appreciated that the present invention has provided new and improved means for preventing a tool from becoming irretrievably stuck by differential sticking in a borehole. By arranging any of the several tool-disengaging means disclosed herein on a well tool which is subject to being differentially stuck in a borehole, the changes that the tool will become differentially stuck are greatly minimized since the effective areas in contact with the borehole wall are greatly reduced. Moreover, even should these greatly reduced contact surfaces become differentially stuck on a mudcaked borehole wall, the several tool-disengaging means disclosed herein are uniquely arranged to move longitudinally in relation to the borehole upon vertical movement of the tool and without imposing added restraint to continued movement of the tool. As a result, the tool can be safely disengaged from a mudcaked borehole wall without creating undue tensile forces in the suspension cable supporting the tool.

While only particular embodiments of the present invention have been shown and described, it is apparent that changes and modifications may be made without departing from this invention in its broader aspects; and, therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of this invention.

Claims

What is claimed is:

1. A well tool adapted for suspension in a borehole and comprising: a body member; means on said body member including a laterally extendible wall-engaging member adapted for selective movement between a retracted position adjacent to one side of said body member and an extended position spaced outwardly a selected lateral distance therefrom; and tool-disengaging means projecting laterally outwardly from one of said members for spacing said one member away from a borehole wall while said tool is stationary in a borehole, said tool-disengaging means being cooperatively arranged to move relative to said one member in a generally longitudinal plane in response to generally longitudinal movement of said tool after said wall-engaging member has been retracted for disengaging said tool from sticking engagement with a borehole wall.

2. The well tool of claim 1 wherein said tool-disengaging means include at least one movable member having an outstanding wall-engaging portion, means pivotally mounting said movable member along one side of said tool for generally longitudinal pivotal movement between spaced positions, and means normally biasing said movable member toward one of its said positions so that generally longitudinal movement of said tool in one direction will pivot said movable member in the opposite direction toward the other of its said positions for disengaging said outstanding wall-engaging portion from sticking engagement with a borehole wall.

3. The well tool of claim 2 wherein said means pivotally mounting said movable member include a lost-motion connection cooperatively arranged between said movable member and said tool for maintaining said outstanding wall-engaging portion at a substantially uniform lateral distance away from said body as said movable member pivots between its said positions.

4. The well tool of claim 1 wherein said tool-disengaging means include at least one substantially circular rotatable member, and means rotatably mounting said rotatable member along one side of said tool for rotation in a generally longitudinal plane so that generally longitudinal movement of said well tool will rotate said rotatable member for disengaging said rotatable member from sticking engagement with a borehole wall.

5. The well tool of claim 1 wherein said tool-disengaging means include first and second movable members respectively having outstanding wall-engaging portions, first and second means pivotally mounting said movable members respectively at longitudinally spaced locations along the outward side of said one member for pivotal movements between spaced upper and lower positions in said plane, and first and second means normally biasing said movable members respectively toward one of their said positions so that generally longitudinal movement of said tool in one direction will pivot said movable members in the opposite direction toward the other of their said positions for disengaging said wall-engaging portions from sticking engagement with a borehole wall.

6. The well tool of claim 5 wherein said first and second means pivotally mounting said movable members respectively include a lost-motion connection between said one member and said movable members cooperatively arranged for maintaining their said outstanding wall-engaging portions at a substantially uniform lateral spacing away from said one member as said movable members pivot between their said positions.

7. The well tool of claim 1 wherein said tool-disengaging means include first and second substantially circular rotatable members, and first and second means rotatably mounting said rotatable members at longitudinally spaced locations along one side of said one member for rotation in said plane so that generally longitudinal movement of said tool will rotate said rotatable members for disengaging said rotatable members from sticking engagement with a borehole wall.

8. The well tool of claim 1 wherein said one member is said wall-engaging member.

9. The well tool of claim 1 wherein said one member is said body member, and said tool-disengaging means project laterally outwardly from said one side of said body member a distance less than said selected lateral distance so that upon retraction of said wall-engaging member said tool-disengaging means will be engaged with a borehole wall to space said body member therefrom.

10. A well tool adapted for suspension in a borehole and comprising: a body; means on said body including a laterally extendible pad adapted for selective movements between a retracted position adjacent to one side of said body and an extended position spaced outwardly a selected lateral distance therefrom; and tool-disengaging means including first and second movable members respectively having outstanding wall-engaging surfaces projecting from said one body side a lesser distance than said selected distance, first and second pivot means respectively coupling said movable members to said body at longitudinally spaced locations along said one body side above and below said extendible pad for pivotal movements in a longitudinal plane between upwardly directed and downwardly directed positions, and first and second biasing means cooperatively arranged between said body and said first and second movable members respectively to normally bias said movable members toward one of their respective said positions for placing said wall-engaging surfaces into contact with a borehole wall upon retraction of said extendible pad to a still-lesser distance than said lesser distance and to thereafter yield in response to generally longitudinal movement of said body in one direction for allowing said movable members to pivot in the other direction toward the other of their respective said positions for disengaging said wall-engaging surfaces from sticking contact with a borehole wall.

11. The well tool of claim 10 wherein said first and second pivot means further include a lost-motion connection respectively coupling said movable members to said body so that said pivotal movements of said movable members will not carry said wall-engaging surfaces substantially beyond said lesser distance.

12. The well tool of claim 10 wherein said first and second movable members are each generally T-shaped with transverse portions thereof defining said wall-engaging surfaces and leg portions thereof coupling said transverse portions to said first and second pivot means respectively.

13. The well tool of claim 12 wherein said first and second pivot means further include a lost-motion connection respectively coupling said leg portions to said body so that said pivotal movements of said movable members will not shift said transverse portions substantially beyond said lesser distance.

14. A well tool adapted for suspension in a borehole and comprising: a body; means on said body including a laterally extendible pad adapted for selective movements between a retracted position adjacent to one side of said body and an extended position spaced outwardly a selected lateral distance therefrom; and tool-disengaging means including first and second movable members respectively mounted at longitudinally spaced locations on the outer face of said extendible pad for contacting a borehole wall upon extension of said extendible pad to at least minimize the contact between its said outward face with a borehole wall and cooperatively arranged to move relative to said extendible pad in response to generally longitudinal movement of said tool after said extendible pad has been retracted for disengaging said movable members from sticking engagement with a borehole wall.

15. A well tool adapted for suspension in a borehole and comprising: a body; means on said body including a laterally extendible pad adapted for selective movements between a retracted position adjacent to one side of said body and an extended position spaced outwardly a selected lateral distance therefrom; and tool-disengaging means including first and second movable members respectively mounted at longitudinally spaced locations on said one body side above and below said extendible pad and cooperatively arranged to move relative to said body in response to generally longitudinal movement of said tool after said extendible pad has been retracted to a position spaced from said one body side a distance less than said selected distance for disengaging said movable members from sticking engagement with a borehole wall.

16. A well tool adapted for suspension in a borehole and comprising: a body; means including first and second extendible pads mounted on opposite sides of said body and respectively adapted for selective movements between retracted positions adjacent to said body and extended positions spaced outwardly therefrom; first tool-disengaging means including a first pair of movable members having outstanding wall-engaging surfaces respectively mounted at vertically spaced locations on the outward face of said first extendible pad for contacting a borehole wall upon extension of said first extendible pad to at least minimize the contact between its said outward face with a borehole wall and cooperatively arranged to move relative to said first extendible pad in response to generally vertical movement of said tool after said first extendible pad has been retracted toward one of said body sides for disengaging said first movable members from sticking engagement with a borehole wall; and second tool-disengaging means including a second pair of movable members having outstanding wall-engaging surfaces respectively mounted at vertically spaced locations on the other of said body sides above and below said second extendible pad and cooperatively arranged to move relative to said body in response to generally vertical movement of said tool after said second extendible pad has been retracted to a position spaced from said other body side a distance less than the lateral projection of said second members for disengaging said second movable members from sticking engagement with a borehole wall.