US2821256A

US2821256A - Pack-off shoe

Info

Publication number: US2821256A
Application number: US418650A
Authority: US
Inventors: Harry B Boller
Original assignee: Schlumberger Well Surveying Corp
Current assignee: Schlumberger Well Surveying Corp
Priority date: 1954-03-25
Filing date: 1954-03-25
Publication date: 1958-01-28
Anticipated expiration: 1975-01-28

Description

Jan. 28, 3958 Y H. B. BOLLER 2,821,256

PACK-OFF SHOE Filed March 25, 1954 3 Sheets-Sheet 1 r y j' INVENTOR. a l HARRY e. BoLLER Fg@ u HIS ATTORNEY Jan. 28, 1

Filed March H. E. BOLLER PACK-OFF SHOE 3 Sheets-Sheet 2 FIGA INVENTOR.

HARRY B. aoLLER HIS ATTORNEY H. E. BOLLER Jamas, i958 PACK/OFF SHOE Filed March 25. 1954 3 Sheets-Shee 5 Y INVENTOR. HARRYB-BOLLER Maf/W HIS ATTORNEY risen-ore snor Harry B. Boiler, Alhambra, Calif., assigner, by mesne assignments, to Schlumberger Well Surveying Corporation, Houston, Tex., a corporation of Texas Application March 25, 1954, Serial No. 418,659

16 Claims. (Cl. 16d-lull) This invention relates to an improved pack-ott shoe for engaging the sidewall of a borehole and, more particularly, pertains to a pack-ott shoe ideally suited for association with sidewall formation-iluid-sampling apparatus.

One type of formation sampler heretofore employed is provided with a pack-oli shoe that is driven into engagement with the sidewall of a borehole at the depth of a formation to be sampled. The shoe is designed to seal a portion of the formation from the drilling fluid that usually iills the borehole, and thus uid from the formation may ilow via an entry port in the pack-off shoe to a sample-retaining chamber. After a desired amount of fluid is obtained, the sample-retaining chamber is closed, the shoe is retracted and the apparatus is withdrawn from the borehole so that the sample may be measured and analyzed.

While the operation of this formation sampler is generally satisfactory, in some instances the sidewall of the borehole under investigation may be so irregular that a good seal cannot be obtained. Consequently, the derived sample of formation fluid may be undesirably contaminated with drilling fluid.

It is, therefore, an object of the present invention to provide an improved pack-olf shoe for engaging the sidewall of a borehole.

Another object of the present invention is to provide an improved pack-off shoe for a fluid sampler featuring a better seal between the formation to be tested and the borehole iuid than heretofore possible.

A pack-olf shoe in accordance with the present invention comprises a support member movable between retracted and operative positions and a sheet-like sealing face of flexible material. The sealing face is provided with a fore portion and is connected at a plurality of peripheral points to the support member. the sealing face is arranged so that its fore portion is normally spaced from a reference plane defined by the support member in a direction toward the aforesaid operative position.

Thus, in association with a Huid sampler mechanism adapted to traverse a borehole, as the support member is displaced from a retracted toward an operative position, the sidewall of the borehole is engaged initially by the fore portion of the sealing face. Thereafter, drilling fluid is displaced from the space between the sidewall and the sealing face as additional portions thereof move toward and engage the sidewall, thereby to provide an effective seal.

The novel features of the present invention are set forth with particularity in the appended claims. The present invention, both as to its organization and manner of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings in which:

Fig. l is a perspective view of a complete sidewall tiuid sampler incorporating a pack-od shoe embodying ln addition, 'i

, ZZLZ Patented Jan. 28, 3.958

the present invention and illustrating one portion of an operating cycle for the mechanism;

Fig. 2 is a cross sectional view taken along line 2 2 of Fig. l and drawn to an enlarged scale;

Fig. 3 is a side view of a portion of the apparatus of Fig. l, drawn to the scale of Fig. 2, but illustrating an other portion of an operating cycle, certain parts thereof being shown cut-away and in section to reveal various interior details;

Fig. 4 is a cross sectional view taken along line 4-4 of Fig. 3;

Fig. 5 is a view similar to that represented in Fig. 3, however, it illustrates yet another portion of an operating cycle for the mechanism;

Figs. 6 and 8 are cross sectional views taken generally along line xx of Fig. l, but illustrating respective, alternative pack-off shoe constructions embodying the present invention, shown in retracted positions; and

Figs. 7 and 9 represent the pack-olf shoe arrangements of Figs. 6 and 8, respectively, in extended positions.

ln Fig. l of the drawings, the lluid sampler incorporating a pack-olf shoe embodying the present invention is shown disposed in a borehole l@ traversing earth formations ll, l2 and 13, and containing a drilling fluid ld, such as a water base or oil base mud. it is assumed that formation l2 is the one of interest from which a fluid sample is to be obtained.

The fluid sampler comprises upper and lower pressureresistant housing sections l5 and lo connected together in longitudinally spaced relation by a pair of side rails i7 and 18, which, as shown in Fig. 2, are transversely spaced from one another. The device l5, i6 is suspended in the borehole l@ by a cable ll) which, in connection with a winch (not shown) located at the surface of the earth, is employed to lower and to raise the device in the borehole in a customary manner.

In the vicinity of

side rails

17 and 18, there is disposed a normally-retracted pad or pack-oir shoe Ztl embodying the present invention. Shoe 2li is of generally oval shape considering its projection on a plane parallel to the longitudinal axis of the iuid sampler, and comprises a sealing face 2l constructed of a sheet of ilexible material such as rubber. It has a fore portion provided with a central insert 22 having upper and lower apertures 23 and Z4, the functions of which will be described in detail hereinafter. Sealing face 2l is connected at a plurality of peripheral points to a generally oval-shaped, horizontally curved support ring 25. More specifically, the upper and lower portions of support ring 25' are somewhat forward of the side portions thereof and the support ring is suitably curved in each of the sections that connect its upper and lower portions with its side portions. The side portions of support 25 define a reference plane which is parallel to the longitudinal axis for device l5, lo. ln the normally-retracted position of shoe 2li, the central portion of face 2l containing insert 22 is spaced from this reference plane in a direction away from the longitudinal axis of the device. ln other words, the distance along the surface of tace 2l between any pair of opposite points on ring is longer than the straight-line distance between such points. Support member 25 preferably is of channel-like cross section and receives the entire peripheral edge of face 21.

As best shown in Fig. 3, a horizontally-curved driving plate 26 is welded, or otherwise peripherally connected, to support ring 25. lt is provided with a central opening 27 through which a member to be later described, and a portion of insert 22 may pass and is connected to upper and lower driving pistons 23 and 29. Each of the pistons 2S and 29 is associated with one of hydraulic cylinders 3h and 31 of upper and

lower cylinder housings

32 and 33 which are positioned between and secured to

side rails

17 and 18. The cylinders and pistons just described are oriented so as to impart transverse movement to driving plate 26 between a retracted and an operative position relative to the longitudinal axis of

device

15, 16.

The

housings

32 and 33 are further provided with

hydraulic cylinders

34 and 35 which receive

pistons

36 and 37, respectively. The latter pistons are connected to a horizontally-curved back-up pad or shoe 3S arranged for movement in a direction opposite to that of shoe 2t?.

Each of a pair of tension springs 39 and 4l) is connected at its extremities to adjacent end portions of driving plate 26 and back-up shoe 38. They are arranged to bias the plate and the shoe toward one another and maintain them in normally-retracted positions shown in Figs. l and 2.

In order to permit the

shoes

20 and 33 to attain relatively close, normally-retracted positions. driving plate 26 is provided with upper and

lower openings

41 and 42 for receiving the rear portions of

cylinders

34 and 35. Shoe 3S is similarly provided with openings 43 and 44 for receiving cylinders 30 and 31.

The

cylinders

30, 31, 34 and 35 are uidly connected to a hydraulic actuator (not shown) which may be disposed in upper housing section 15. This actuator may, for example, be of the type described in the copending application of Lawrence C. Chambers, iiled April 7, i950 and assigned the serial number 154,468, now Patent 2,674,313, for utilizing the pressure of borehole iiuid 14 to drive

pistons

23, 29, 36 and 37 in opposition to the bias of springs 39 and 44). Of course, any well-known system for imparting movement to plate 26 and to shoe 38 may be employed, such as a motor-driven screw-type jack.

To penetrate a desired formation and thereby facilitate the flow of formation fluid, the device is provided with a sample gun 45 supported within a channel 46 of a gun housing 47. Housing 47 is bolted or otherwise secured to insert 22 with a spacer 48 disposed therebetween. it is thus movable with the insert in a horizontal direction out of and into the space defined by

side rails

17 and 18 and

piston housings

32 and 33. In addition, since opening 27 is larger than the rear portion of insert 22 and spacer 48, these members are movable with respect to driving member 26.

Gun 4S contains a bullet 49 and has a barrel 5t) aligned with port 24 in insert 22. A frangible plug or seal 51 normally blocks iiuid communication between channel 46 and openings 52 and 53 in

members

47 and 48 which are aligned with port 24.

Gun barrel 56 is provided with a plurality of radial openings 54 so that, as shown in Fig. 4, the interior of the barrel is in fluid communication with a channel 55 connecting channel 46 with a downwardly bent tube 56. The end of tube 56 is received by a iiexible tube 57 which extends along rail 17, but is free to move so as not to inhibit movement of gun housing 47. The latter tube is connected to a sample-receiving chamber (not shown) in lower housing section 16. This chamber may be of any well-known construction, or may be of the type described in the aforementioned Chambers application, for obtaining a uniform, representative sample of formation fluid.

inasmuch as the seal provided by face 21 may be so eifective that shoe 25 cannot be retracted after a fluid sample is obtained, gun housing 47 is provided with a channel S that receives a get-away gun 59. The gun contains a bullet 6d and its barrel 61 is aligned with port 23 of insert 22, as well as with openings 62 and 63 in members 47 and 4S. A frangible plug 64 normally blocks fluid communication between openings 53 and 62 and another frangible plug 65 in port 23 normally bloclis uid communication between the outer and inner surfaces of face 21 which may occur via port 23 and a connecting, vertically-extending channel 66. This type of release mechanism is disclosed and claimed in the copending application of Clyde C. Chivens 4 and Roger Q. Fields, filed March 25, 1954, bearing the Serial No. 418,644, and assigned to the same assignee as the present application.

The various control circuits for the hydraulic actuator (not shown) of housing 15 and for the

guns

45 and 59 are completed via insulated electrical conductors of cable 19. These circuits may be of any conventional arrange ment or as arranged in the Chambers application.

In operation,

housing

15, 16 is lowered in borehole 10 by means of cable 19 to a position wherein insert 22 is adjacent the formation to be tested; in this example, it is formation 12. Plugs 51 and 64 serve to prevent the entry of drilling fluid 14 into `gun barrels 5t) and61 in this initial portion of an operating cycle.

Hydraulic fluid pressure is then applied to

cylinders

30, 3l, 34 and 35 so that

pistons

28, 29, 36 and 37 are displaced to move driving plate 26 and back-up shoe 38 in opposition to the bias of

springs

39 and 40 toward the sidewall of the borehole. As is evident from Fig. 2, since the sealing member 21 retains its shape as it is moved laterally and because of the configuration ofshoe 20 the central, fore portion of sealing face 21 containing insert 22 will be the rst to engage the sidewall. VThereafter, drilling uid is displaced from the space between the sidewall and the sealing face as additional portions thereof move toward and engage the sidewall, while, at the same time, back-up shoe 3S engages the opposite section of the sidewall. Since the surface distance along sealing face 21 between any pair of opposite points on ring 26 is longer than the linear distance between such points, tangential forces are developed in the sealingface. That is, compressive or compactive lateral forces are developed in the sealing face which tend to reduce its surface area. Since the sealing face is made of rubber, forces transverse to the lateral forces are also developed and the face material tends to become thicker. Consequently, the outer surface of the sealing face is forcibly urged against the sidewall of borehole lil.

By reason of this constructional and operational-feature, when the sampler is in the portion of an operating cycle illustrated in Fig. 3, the sealing face 21 engages and conforms very closely to a relatively large section of the sidewall. It thus provides a very eective huid seal between the sidewall of borehole 10 and drilling uid 14.

Of course, movement of insert 22 is accompanied by displacement of gun housing 47 and after shoes 20 and 3S are firmly positioned, gun 45 is fired to discharge bullet 49. The bullet ruptures frangible plug 51, passes through

openings

52, 53 and 24 and .comes to rest at a` position in formation 12 designated in dash outline 49. If the gas or oil contained in formation 12 is in producible form, it is under some pressure and may thus ow into the opening in the formation created by the bullet and into the sample-receiving chamber (not shown) of housing section 16 via a path including port 24, openings 53 and 52, gun barrel S0, radial openings 54,

channels

46 and 55, and

tubes

56 and 57. The uid sample continues to flow into the sample chamber for an interval of time suicient to permit entry of a desired quantity. Thereafter, the sample chamber is closed and the device is ready to be brought to the surface.

However, before

housing

15, 16 may be raised, ,shoes 2li and 38 must be retracted. To this end, the hydraulic pressure on

pistons

28, 29, 36 and 37 is reduced approximately to atmospheric pressure and the hydrostatic pressure of the mud 14 acting on the pistons via the shoes together with the bias of springs 39 and 4t), displaces the shoes toward one another.

Because of the close conformity of sealing face 21 and the sidewall of the borehole, the sealing face may act as a suction cup. That is, this conformity plus the pressure of drilling uid 14 on the rear surface of face 21 is such that the

springs

39 and 40, although they retract driving plate 26 as shown in Fig. 5, are unable to draw the face from engagement with the sidewall. In this event, get-away gun S9 is fired. its bullet 6i? ruptures frangible plugs ed and 65 and opens a fluid path between the front and rear surfaces of the sealing face 2l via port 23 and channel 6o, as shown in Fig. 5. The pressure differential between these surfaces is thus equalized and shoe 2b may be retracted.

Accordingly, with both shoes Ztl and 3S retracted in the dual phase of operating cycle, the fluid sampler may be withdrawn from borehole lil. At the surface, the sample-receiving chamber (not shown) can be removed from housing section lo and the sample therein analyzed.

ln Fig. 6 there is shown another type of a pack-off shoe constructed in accordance with the present invention. Flexible face 2l is peripherally connected to a support ring in turn, connected to a modiied driving plate 2.6. The driving plate has a dat central portion 7G, rather than being curved, so that it may be driven close to a plate or backing member 7l disposed behind sealing face rl`he plate 7l is curved to correspond to the retracted curvature ot scaling face 2l and it is secured by a screw 72 passing through a central portion thereof to a modified central insert 22' which is somewhat longer than the insert 22 shown in Figs. l-3.

This embodiment of the invention operates in essentially the ame manner as the arrangement of Figs. 1 3. However, when bach-up shoe 38' and driving plate 25 are driven toward respective sections of the sidewall of borehole lb, as shown in 7, the driving plate is displaced a greater distance than in the embodiment of Figs. l-3. Greater tangential forces are thereby developed in sealing face 22, and these forces act to urge the sealing face toward the sidewall of the borehole. Thus, by providing means for mechanically producing tangential forces in the sealing face, an effective seal may be obtained with the sidewall or borehole lil.

Referring now to Fig. 8, the embodiment of the invention there shown comprises a central support mounted to portion i6 of the principal housing. A channel-shaped member Si is xed to support Sil and, in turn, is rigidly secured to a curved plate 82. Hate S2 is generally similar to plate 7l of Figs. 6 and 7, but is coextensive with the entirety of sealing face 2l". It is secured by a screw 83 to an elongated central insert 22 and is provided with a support ring 25" about its periphery having a relatively deep channel Sdwhich receives the periphery of sealing face 2l. Thus, there is provided a support member having a curved portion 82 and terminating portions for the curved portion included in support ring 25". Channel 84 of the support ring receives opposite points of the periphery of sealing face 2l". In the space dened by the channel 34 and the edge of sealing face 2l", there is disposed a resilient, expansible tube S5 sealed at one end (not shown) and tluidly connected to the hydraulic actuator (not shown) which drives the pistons of back-up shoe 33". One of these pistons, designated 37, is shown in Jliigs. 8 and 9.

In operation, back-up shoe 33 is driven into engagement with a section of the sidewall of borehole lil and the entire assembly, including the principal housing, supports Si? and Sl, plate SZ and face 2l, thus is displaced toward the opposite section of the sidewall. Accordingly, insert 22 of the sealing face 2l" engages the sidewall of the borehole and at the same time hydraulic pressure is applied to expansible tube 8S. lt is thus apparent that as drilling fluid is displaced from the space between the sealing face and the sidewall of the borehole, transverse expansion of tube 85 causes the peripheral edge of the sealing face to move relative to channel S4 and transverse forces are developed. in other Words, in this embodiment of the invention, means are provided for selectively urging opposite points of the periphery out of the channel portion 84 of of sealing face El support ring 25". Because of such mechanically prod duced transverse forces in sealing face 2l", and due to the presence of plate S2, an eiective seal may be obtained with the sidewall of borehole ld.

From the foregoing description, it is evident that a pack-off shoe constructed in accordance with the present invention provides a much more effective uid seal than heretofore possible. Consequently, a fluid-sampling operation is greatly facilitated and the possibility of contaminating the sample with drilling lluid is minimized.

Although a pack-oil shoe embodying the present invention has been illustrated in the environment of a fluid sampler, obviously it may be used in any other application wherein a fluid seal or a close association with the sidewall of the borehole is required.

While 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 modiiications as fall within the true spirit and scope of this invention.

i claim:

l. A pack-oli shoe for use with a carrier adapted to be passed through a borehole comprising a support member selectively movable relative to the carrier between retracted and operative positions, and a sheet-like sealing element of flexible material connected substantially only at outer edge portions to said support member including a wall-engaging front surface and a rear surface, and including a substantially centrally located fore portion normally extending in a direction toward said operative position, said support having orice means permitting any fluid in the borehole to communicate with said rear surface of said sealing element.

2. A pack-oli shoe for a sidewall lluid sampler including a carrier adapted to be passed through a borehole, said shoe comprising a support member selectively movable relative to the carrier between retracted and operative positions, and a sheet-like sealing element of flexible material connected substantially only at outer edge portions to said support member including a Wall-engaging front surface and a rear surface, and including a substantially centrally located portion normally extending in a direction toward said operative position and having an opening, said support having orice means permitting any lluid in the borehole to communicate with said rear surface of said sealing element, and said packofr shoe further comprising a relative rigid member sealed to the periphery of said opening and movable with said centrally located portion, and means for selectively establishing a iluid communication path through said rigid member.

3. A pack-ofi shoe for use with a carrier adapted to be passed through a borehole comprising a support member movable relative to the carrier between retracted and operative positions, a sheet-like sealing element of flexible material including opposite edge portions connected to said support member including a wall-engaging front surface and a rear surface, and including a substantially centrally located fore portion normally extending in a direction toward said operative position, said support having oririce means permitting any fluid in the borehole to communicate with said rear surface of said sealing element, and a driving member mechanically connected to said support member and to the carrier and selectively operable to displace said support member between said retracted and said operative positions thereby to carry said sealing element toward the sidewall of a borehole.

4. A pack-oil shoe for use with a carrier adapted to be passed through a borehole comprising a ring-like support member selectively movable relative to the carrier between retracted and operative positions, and a sheetlike sealing element of flexible material including a Wallengaging front surface and a rear surface, including a periphery coextensive with and connected to said support member, and including a substantially centrally located fore portion normally extending in a direction toward said operative position, said support having orifice means permitting any fluid in the borehole to communicate with said rear surface of said sealing element.

5. A pack-off shoe for use with a carrier adapted to be passed through a borehole comprising a ring-like support member selectively movable relative to said carrier between retracted and operative positions, and a sheetlike sealing element of ilexible material including a wallengaging front surface and a rear surface, and including an edge coextensive with and connected to said support member, the distance along said front surface of said sealing element between any pair of opposite points on said ring-like support member being greater than the linear distance between such points thereby to provide a substantially centrally located fore portion for said sealing element spaced outwardly relative to a longitudinal axis for the carrier, said support member having orifice means permitting any fluid in the borehole to communicate with said rear surface of said sealing element.

6. A pack-oft shoe for use with a carrier adapted to be passed through a borehole traversing earth formations comprising a support member selectively movable relative to the carrier 'between retracted and operative positions, a sheet-like sealing element of flexible material connected substantially only at outer edge portions to said support member including a wall-engaging front surface and a rear surface, and including a substantially centrally located fore portion normally extending in a direction toward said operative position, said support having orifice means permitting lany fluid in the borehole to communicate with said rear surface of said sealing element, said pack-olf shoe further comprising formation penetrating means connected to said fore portion of said sealing element for simultaneous movement therewith and movable relative to said support member.

7. A pack-off shoe for use with 'a carrier adapted to be passed through a borehole traversing earth formations comprising a ring-like support member selectively movable relative to the carrier between retracted and operative positions, a sheet-like sealing element of flexible material including a periphery coextensive with and connected to said support member, including a wall-engaging front surface and a rear surface, and including a substantially centrally located fore portion normally extending in a direction toward said operative position and having an opening, said support having orice means permitting any fluid in the borehole to communicate with said rear surface of said sealing element, said pack-off shoeV further comprising a closure for said opening in said fore portion, and explosively-operable means connected to said closure for simultaneous movement therewith and movable relative to said support member, said explosively-operable means being operative to establish a uid passage through said closure and to penetrate an earth formation.

8. A pack-off shoe for use with a carrier adapted to be passed through a borehole comprising: a support member selectively movable relative to the carrier between retracted and operative positions and having orifice means; a sheet-like sealing element of flexible material connected to said support member including a wall-engaging front surface and including a rear surface communicable with any fluid in the borehole via said oriice means; and means for selectively producing laterally compressive forces in said sealing element thereby to urge at least a portion of said sealing element in a direction from said support member toward the sidewall of a borehole.

9. A pack-off shoe for use with a carrier adapted to be passed through a borehole comprising: a support member selectively movable'relative to the carrier between retracted and operative positions and having orifice means; a sheet-likesealing element of flexible material connected to Said support member substantially only at outer edge portions and including a wall-engaging front surface and including a rear surface communicable with any fluid in the borehole via said orifice means; a backing member having a central portion adjacent a corresponding portion of said rear surface of said sealing element and having curved portions extending from said central portion toward said support member; and means for selectively driving said support member toward said curved portions of said backing member to displace said sealing element toward the sidewall of a borehole, thereby to produce transverse forces in said sealing element and develop a buckling action therein which, due to the presence of said curved portion of said backing member, urges said front surface of said sealing element into sealing engagement with the sidewall of the borehole.

l0. A pack-off shoe for use with a carrier adapted to be passed through a borehole comprising: a support member selectively movable relative to the carrier between retracted and operative positions `and including a curved portion, a pair of terminating portions for said curved portion, and orifice means; a sheet-like sealing elementV of tlexible material coextensive with said curved portion of said support member and having opposite edge portions thereof movably connected to said terminating portions, and said sealing element including a wall-engaging front surface and a rear surface communicable with any uid in the borehole via said orifice means; means for selectively displacing said support member toward the sidewall of a borehole; and means for selectively displacing said opposite edge portions of said sealing element relative to said terminating portions of said support member thereby to develop transverse yforces in said sealing element.

ll. A pack-olf shoe for use with a carrier adapted to be passed through a 'borehole comprising: a support member selectively movable relative to the carrier between retracted and operative positions and including a curved portion, a pair of channel portions terminating said curved portion, and orifice means; a sheet-like sealing element of ilexible material coextensive with said curved portion of said support member and having opposite edge portions thereof received by respective ones of said channel portions, and said sealing element further including a wall-engaging front surface and a rear surface communicable with any huid in the borehole via said orifice means; means for selectively displacing said support member toward the sidewall of a borehole; and means for selectively urging said opposite edge portions of said sealing element out of said channel portions of said support member thereby to develop transverse forces in said sealing element.

l2. A pack-olf shoe for use with a carrier adapted to be passed through a borehole comprising: a support member selectively movable relative to the carrier between retracted and operative positions and including a curved portion, a pair of channel portions terminating said curved portion and individually having an end wall, and orice means; a sheet-like sealing element of tiexible material coextensive with said curved portion of said support member and having opposite edge portions thereof received by respective ones of said channel portions, and said sealing element further including a wall-engaging front surface and a rear surface communicable with any uid in the borehole via said orifice means; an enpansible tube disposed between one of said edge portions of said sealing element and said end wall of the corresponding one of said channel portions; means for lselectively displacing said support member toward the sidewall of a borehole; and means for selectively applying hydraulic pressure to said expansible tube to effect expansion of said tube thereby tol urge said edges of said sealing lelement out of said channel portions of said support member and develop transverse forces in said sealing element.

13. A pack-off shoe for use with a carrier adapted to be passed through a borehole comprising a support member selectively movable relative to the carrier between retracted and operative positions, and a relatively thin sealing element of tiexible material connected substantially only at its edges to said support member and being of generally curved coniiguration to present a normally extended, substantially centrally located fore portion, and said sealing element further including a wall-engaging front surface and a rear surface, said support member having at least one opening whereby said rear surface of said sealing face may communicate with any fluid in the borehole.

14. A pack-off shoe for use with a carrier adapted to be passed through a borehole comprising a support member selectively movable relative to .the carrier between retracted and operative positions, and a sheet-like sealing element of resilient material :connected substantially only at its edges to said support member and being of generally curved configuration to present a normally extended, substantially centrally located, fore portion and said sealing element further including a wall-engaging front surface and a rear surface, said support member having at least one opening whereby said rear surface of said sealing element may communicate with any fluid in the borehole.

15. A pack-oli shoe for use with a carrier adapted to be passed through a borehole comprising a support member selectively movable relative to the carrier between retracted and operative positions, and a sheet-like sealing element of exible material of generally curved coniiguration having a normally extended, substantially centrally located, fore portion, and side portions extending, at least in part, in the direction of movement of said support member and having edges connected to said support member, and said sealing element further including a wallengaging front surface and a rear surface, said support member having at least one opening whereby said rear surface of said sealing element may communicate with any iiuid in the borehole.

16. A pack-off shoe for use with a carrier adapted to be passed through a borehole, :comprising a support mem ber selectively movable relative to the carrier between retracted and operative positions, and a sheet-like sealing element of llexible material connected substantially only at outer edge portions to said Isupport member including a wall-engaging front surface and a rear surface, and including a substantially centrally located fore portion normally extending in a direction toward said operative position, said pack-off shoe having passageway means therein to permit any iluid in the borehole to communi cate with said rear surface of said sealing element.

References Cited in the ille of this patent UNITED STATES PATENTS 2,545,306 Pollard Mar. 13, 1951 2,674,313 Chambers Apr. 6, 1954 2,688,369 Broyles Sept. 7, 1954