US3662825A

US3662825A - Well tester apparatus

Info

Publication number: US3662825A
Application number: US42373A
Authority: US
Inventors: Benjamin P Nutter
Original assignee: Schlumberger Technology Corp
Current assignee: Schlumberger Technology Corp
Priority date: 1970-06-01
Filing date: 1970-06-01
Publication date: 1972-05-16
Anticipated expiration: 1989-05-16

Abstract

A drill stem tester assembly is disclosed including an inner member movable upwardly and downwardly within an outer member, valve means for opening and closing a test passage through said members in spaced longitudinal positions, a balance piston on said inner member having its upper and lower faces subjected respectively to the pressure of fluids within the inner member and in the well annulus, and a bias piston responsive to the hydrostatic head of fluids in the annulus to apply upward force to said inner member.

Description

United States Patent Nutter 51 May 16, 1972 [541 WELL TESTER APPARATUS 3,305,023 2/1967 Farley ..166/226 3,308,882 3/1967 Lebourg.. ..l66/l50 X m] Invent Emmi 381M116 3,494,419 2/1970 Mullins 166/152 x [73] Assignee: Schlumberger Technology Corporation,

New York, NY. Primary Examiner-David H. Brown AttorneyEmest R. Archambeau, Jr., David L. Moseley, Ed- [22] June 1970 ward M. Roney and William R. Sherman [21] Appl. No.: 42,373

[ ABSTRACT 52 US. 01.... ..166 152, 166/226 A stem tester assembly is dimmed including an inner 51 Int. (:1 ..E2lb 49/00 member upwardly and dwmwmdly within 58 Field 6: Search ..166/150 152 226 0.5 member valve means for p ng and closing a test passage through said members in spaced longitudinal positions, a [56] References Cited balance piston on said inner member having its upper and lower faces subjected respectively to the pressure of fluids UNlTED STATES PATENTS within the inner member and in the well annulus, and a bias piston responsive to the hydrostatic head of fluids in the annu- 3,384,180 5/1968 Perkins 166/152 x to apply upward force to Said inner 3,249,124 5/1966 Berryman. ....l66/226 X 3,358,755 12/1967 Chisholm ..166/152 X 9 Claims, 5 Drawing Figures E 1 78 :n 45 p 1: 3 M

s a r PATENTEnMAHsmz 3,662,825

' SHEET 1 UP 2 FIG? /0min P. Nutter INVENTOR ATTORNEY PATENTEMHsIm 3.662.825

SHEET 2 OF 2 Benjamin P Nutter INVENTOR [wax/ ATTORNEY WELL TESTER APPARATUS This invention relates generally to a drill stem tester for use in a well bore, and more specifically to a hydraulically balanced formation tester apparatus employing longitudinally slidable members that are manipulated by upward and downward movement of the pipe string at the surface.

To conduct a drill stem test, a packer and tester valve are lowered into a well bore on a pipe string and the packer is set to isolate the test interval. By manipulating the pipe string the tester valve is opened and closed to flow and shut-in the formations. Pressure recorders make a permanent record of flow and shut-in pressures to enable a subsequent evaluation of the formation. A tester valve that can be operated positively and safely, solely in response to upward and downward movement of the pipe string, is disclosed in my U.S. Pat. No. 3,308,887, issued Mar. 14, 1967 and assigned to the assignee of the I present invention. The tester valve includes a mandrel movable upwardly and downwardly within a housing, with a mandrel and housing defining a flow or test passage that is opened and closed in spaced longitudinal positions of the members.

Inasmuch as the valve is operated by longitudinal relative movement between parts, it is possible to observe the operation thereof by monitoring what are called free points" on the rig weight indicator at the surface. A free point is a weight value on the weight indicator that remains constant while the pipe string is being moved vertically, and provides an indication of telescoping motion of a sliding connection in the test tools. If the pipe string is being lifted, the weight value will remain constant until the joint has completely extended, whereupon the weight value will start to increase. Conversely, if the pipe string is being lowered, the weight value will remain constant until the joint has completely closed, whereupon the weight value will begin to decrease. It will readily be appreciated that the tester valve mentioned above enables the observance at the surface of free points as the valve is cycled, since the structure embodies a longitudinal sliding joint.

It will be recognized, however, that since the test valve is closed until the tools are properly positioned so that the pipe string provides a low pressure region with respect to the formations, there is a loss of buoyancy the instant the valve is opened. Moreover, the pressures of fluids that act on the tester may change during flow and shut-in periods of the test cycle. In some cases, the pressures act in opposition to the weight of the pipe string and have an influence on the free point values which are monitored at the surface. Particularly where there are other sliding joints in the test tool string, it is desirable to have a constant free point for the test valve so that a sequence of test tool operations can be readily recognized at the surface. I 7

It is accordingly a broader object of the present invention to provide a new and improved formation tester assembly that is operated by upward and downward motion, and is hydraulically balanced in such a manner that the loss of buoyancy when the valve is opened does not affect the free point. Moreover, for a formation test where liquid is recovered, the free point for the tester assembly is constant regardless of bottom hole or surface pressure conditions.

This and other objects of thepresent invention are attained through the provisions of a formation tester apparatus that includes an inner member that is sealingly slidable within an outer member between extended and contracted relative longitudinal positions, the members being adapted for connection in a pipe string extending upwardly to the top of the well bore. The members define a fluid passageway therethrough and have valve means for opening and closing the passageway in spaced longitudinal positions. In order to hydraulically balance the members, a piston on the inner member is sealingly slidable within a cylinder in the outer member with the upper face of the piston being subjected to the pressure of fluids within the other member, the lower face thereof being subject to the pressure of fluids in the well annulus surrounding said members. The piston is sized and arranged such that its transverse cross-sectional area is substantially equal to transverse area circumscribed by the seal that prevents fluid leakage between the member during telescoping movement. Thus arranged, when buoyancy force is lost as the valve means is opened, this force is supplanted immediately by an upward force on the piston of substantially equal magnitude so that the free point for the sliding joint embodied in the tester apparatus remains the same and can be readily differentiated at the surface from the free points of other sliding joints in the string of test tools. Moreover, the pressures of fluids within the members that act in opposition to the weight of the pipe string are balanced out and cancelled by an equal force acting downwardly on the piston. Accordingly, it will be recognized that the tester assembly is hydraulically balanced so that its free point will remain substantially constant regardless of bottom hole or surface pressure conditions.

The present invention has other objects and advantages that will become more readily apparent in connection with the following detailed description of the structure and operation of a preferred embodiment, taken in conjunction with the appended drawings in which:

FIG. 1 is a schematic view of drill stern testing equipment to test an offshore well from a floating vessel;

FIGS. 2A and 2B are longitudinal sectional views of a tester assembly in accordance with the principle of this invention, FIG. 2B fonning a lower continuation of FIG. 2A;

' FIG. 3 is a cross-section taken in line 3-3 of FIG. 2A; and

FIG. 4 is a schematic plan view of the channel system used in controlling the operating sequence of the tester assembly.

Referring initially to FIG. 1, a schematic representation of an environment wherein the present invention has particular utility is in offshore testing from a floating vessel 10. A well bore 11 intersects the particular earth formations to be tested and is usually lined witha steel casing. A wellhead assembly 12 is located at the sea floor together with a typical blowout preventer stack 13, and a riser 14 extends upwardly to the vessel 10. A string of drill stem testing tools including a packer 15 and a tester assembly 16 is connected to the lower end of a pipe string 17 that extends to the surface and is suspended in the derrick 18 located on the vessel. The packer 15 can be of the type shown in U.S. Pat. No. 3,399,729, McGill, dated Sept. 3, 1968 and functions to pack off and isolate the interval to be tested. The tester assembly 16 has a valve that is opened and closed by manipulation of the pipe string 17 to flow and shut-in the formations. Typical pressure recorders 19 make a permanent record of changes in pressure vs. time that can be subsequently analyzed to determine various characteristics of the formation. A sample of fluids is produced into the pipe string 17 and can be removed for subsequent analysis.

In order to accomodate changes in the total length of the pipe string 17 due to the influence of wave and tide action on the vessel 10, to pressure and temperature changes that may occur during a test, or to operation of the test tools, a slip joint 20 is incorporated in the pipe string 17 below the wellhead assembly 12. Also, a second slip joint 21 may be disposed between the tester assembly 16 and the packer 15. The details of construction and the

slip joints

20 and 21 form no part of the present invention and the particular details can be seen, for example, in U.S. application Ser. No. 42,374, Kisling et al., filed concurrently herewith and assigned to the assignee of this invention.

The tester assembly 16 in accordance with the principles of this invention is shown in FIGS 2A 2D as including an indexing and delay section 25, a valve and sampler section 26, a hydraulic balancing section 27 and a hydrostatic bias section 28. The details of the indexing and delay section 25 and the valve and sampler section 26 are disclosed in U.S. Pat. No. 3,308,887, Nutter, issued Mar. 14, 1967, wherein the mandrel 30 is telescopically disposed within a housing 31 for longitudinal movement between extended and contracted positions, relative rotation being prevented by splines 32. A swivel sleeve 33 is mounted on the housing 31 for rotation relative to both the mandrel 30 and the housing. The sleeve 33 has an index pin 34 that follows in a channel system 35 that is formed in the exterior of the mandrel. As the mandrel 30 is moved longitudinally with respect to the housing 31, the interaction of the channel system 35 with the index pin 34 causes the sleeve 33 to be indexed between various angular positions with respect to the mandrel 30. In one angular position, inwardly extending lugs 36 (FIG. 3) on the sleeve 33 are engaged with companion lugs 37 on the mandrel 30 to limit downward movement of the mandrel in an intermediate position. In the other angular positions, the

lugs

36 and 37 bypass one another so that the limit of downward movement occurs when the collar 38 on the mandrel 30 engages the top of the housing 31, and the limit of upward movement occurs when the mandrel lugs 37 shoulder against the thickened section 39 of the housing. Referring briefly to FIG. 4, when the mandrel 30 is extended the index pin 34 is at position A in the channel system; as the mandrel is lowered the index pin moves to position B and the

lugs

36 and 37 have bypassed one another; as the mandrel is raised the pin moves to position C and the lugs again bypass one another, however, when the mandrel is lowered once again the pin moves to position D. In this position, the sleeve 33 is located such that the lugs 37 on the mandrel 30 engage the sleeve lugs 36 and provide an intermediate position stop. Of course again raising the mandrel 30 will position the pin 34 in the position A so the cycle is complete. The purpose of the indexing system will be explained in detail herebelow in conjunction with the structure and operation of the valve and sampler section 26.

The delay section includes a metering sleeve 41 that works within a closed hydraulic chamber 42 formed in the housing 31. The metering sleeve 41 is biased upwardly against a shoulder 43 on the mandrel 30 by a spring 44 and is sized to provide a small clearance between its outside surface and the inner wall 45 of the chamber 42 The sleeve 41 further has grooves 46 or the like in its inner periphery to provide fluid passage spaces. The upper end surface of the metering sleeve 41 seats against the shoulder 43 during downward movement of the mandrel 30 so that hydraulic fluid contained in the chamber 42 must flow from below the sleeve to above it at a metered rate, thus providing a retarding or delaying action with respect to downward movement. However, the mandrel 30 can be moved freely upwardly because the fluid will push the sleeve 41 downwardly away from the shoulder 43 and against the action of the spring 44 so that fluid can bypass through the grooves 46 from above the sleeve to below it. The wall surface 47 at the lower section of the chamber 42 can be enlarged in diameter so that the metering action provided by the sleeve 41 is released as the mandrel nears the lower end portion of its travel.

The test valve and sampler section 26 of the tester assembly 16 comprises longitudinally spaced valve heads 49 and 50 having

sealing elements

51 and 52 that engage spaced valve seats 53 and 54 when the mandrel 30 is extended or partially contracted, and are free of the seats when the mandrel is fully telescoped within the housing 31. The annular space 55 in between the valve seats 53 and 54 provides a flow-through sample chamber, in that when the mandrel 30 is in its lowermost position fluids can flow upwardly from ports 56 located below the lower valve head 50 through the chamber 55 and through ports 57 above a barrier 58 into the bore 59 of the mandrel. However, when the mandrel 30 is moved upwardly, the

seals

51 and 52 engage the

seats

53 and 54 simultaneously to trap a sample of fluids in the chamber 55. It will be appreciated that the valve is opened only when the mandrel 30 is completely telescoped within the housing 31 and that when the mandrel is in a position corresponding to positions C or D in FIG. 3, the valve is closed to shut-in the formations.

Referring again to FIG. 28, an extension 61 of the mandrel 30 is provided with an annular piston 62 with the upper face of the piston exposed to the pressure of fluids within the lower bore 63 of the mandrel extension by ports 64. A section 65 of the housing 31 provides a cylinder with an inwardly extending flange 66 sealed against the mandrel extension 61 by a seal ring 67, and with the cylinder wall 68 sealed with respect to the piston 62 by a seal ring 69. The lower face of the piston 62 is exposed to the pressure of fluids in the annulus surrounding the housing section 65 by ports 71. The transverse cross-sectional area of the piston 62 is made substantially equal to the transverse area encompassed by the outer peripheral surface of the mandrel 30. Accordingly, forces due to fluid pressure below and inside the mandrel 30 tending to elevate it are balanced or cancelled by the same pressure acting downwardly on the piston 62. The purpose of this arrangement is to prevent the influence of fluid pressure changes that occur during a drill stem test from affecting the longitudinal relative positions of the mandrel 30 and the housing 31, and to eliminate changes in free point indications at the surface due to such pressure changes.

An additional extension 72 of the mandrel 30 has an annular piston 73 that is sealed within a cylinder section 74 of the housing 30 and encloses, together with an inwardly extending shoulder 75, an atmospheric chamber 76 above the piston. The lower face of the piston 73 is exposed to the hydrostatic head of fluids in the annulus surrounding the housing section 74 by ports 77, and of course the upper face of the shoulder 75 is subjected to the same pressures. A seal 78 prevents fluid leakage between the lower end of the mandrel 30 and the housing 31, and other seal rings 79 and 80 prevent leakage into the chamber 76. The purpose of this arrangement is to provide a bias force that will tend to elevate the mandrel 30 once it has been lowered. Upon lowering, a force clue to the difference between the hydrostatic pressure of well fluids in the annulus and the lesser pressure in the chamber 76 acts in an upward direction on the mandrel 30. Moreover, an identical force acts downwardly on the shoulder 75 and is transmitted by the housing 30 to the tools therebelow to aid in retaining a packer seat and the packer bypass closed. The presence of the bias force will ensure that the mandrel 30 will move upwardly during a sequence of test tool manipulations, independently of the movement of other sliding joints in the system. The lower end of the tester housing 31 has a typical threaded pin 81 to adapt it for connection to the tools therebelow.

In operation, the tester assembly 16 and the packer 15 are suspended on the lower end of the pipe string 17 and lowered into the well bore 11 to test depth. During lowering, the mandrel 30 is extended with respect to the housing 31 so that the valve assembly is in closed condition with respect to the flow passage. The pipe string 17 is either empty of fluids or can be provided with a water column to cushion the effects of fluid entry when the tester is opened. In either event, the bore of the pipe string 17 provides a low pressure region with respect to the pressure of fluids in the formation to be tested, so that formation fluids will flow upwardly and into the pipe string when the tester valve is opened.

By appropriate manipulation of the pipe string, the packer 15 is set to isolate the formation interval to be tested and the weight of the pipe string is applied thereto. As shown in the previously referenced McGill patent, the packer has a typical bypass passage that is closed as the packer is set. The weight of the pipe string 17 will cause the mandrel 30 of the tester assembly 16 to advance downwardly, such movement being retarded by the metering action of the metering sleeve 41. The purpose of the metering feature is to ensure that the packer 15 is set and its bypass is closed before the tester valve has an opportunity to open.

As the mandrel 30 moves downwardly the index pin 34 maintains an angular relationship of the index sleeve 33 such that the

lugs

36 and 37 bypass one another to enable the mandrel to move to its fully contracted position within the housing 31. When the valve seals 51 and 52 clear the

seats

53 and 54, a flow path is provided past the barrier 58 so that formation fluids can flow upwardly via the

ports

56 and 57 and into the pipe string 17. The buoyancy force equal to the hydrostatic head times the seal diameter of the mandrel 30 is suddenly lost, however the force is immediately supplanted by an equal force acting upwardly on the hydraulic balance piston 62. Ac-

cordingly, there is no change in the free point for the tester assembly 16. A bias force is also being applied upwardly on the mandrel 30 as the hydrostatic head of fluids acts upwardly on the bias piston 73. This force assists in returning the mandrel 30 upwardly as the tester assembly 16 is cycled and ensures the mandrel will move upwardly to extended position independently of the telescopingmotion of other sliding joints in the system, for example the slip joint 21 located between the tester assembly 16 and the packer 15.

In order to shut-in the tester assembly to record pressure build-up data, the pipe string 17 is elevated. As the mandrel 30 moves upwardly, the index pin 34 causes the sleeve 33 to swivel to an angular position such that the

lugs

36 and 37 again bypass one another and the pin moves to position C, FIG. 4. The valve seals 51 and 52 engage the

seats

53 and 54 to close off the test passage. The mandrel 30 is then moved downwardly to its intermediate position when the index pin is at D. The angular position of the sleeve 33 is such that the

lugs

36 and 37 engage one another so that the weight of the pipe string 17 can be imposed on the housing 31 and the tools therebelow. The valve seals 51 and 52 are still engaging the

seats

53 and 54, however, and the formations are shut-in.

As the pressure of fluids builds up in the isolated zone of the well bore, the pressure acts upwardly on the seal area of the mandrel 30. However, pressure is also acting downwardly on the balance piston 62, which has an area substantially equal to the seal area of the mandrel. Thus these forces cancel one another and have no effect on the longitudinal position of the housing 31 or the mandrel 30, and do not affect the free point indication for the tester at the'surface. The bias force acting upwardly on the bias piston 73 is constant during the test, since it is dependent only upon the hydrostatic head of the fluids in the annulus.

To terminate the test, the pipe string 17 is elevated to extend the mandrel 30 with respect to the housing 31. Such lifting is continued after the free point indications for the tester assembly 16 and the lower slip joint 21 are seen to open the bypass of the packer l5 and equalize pressures. The packer is then unseated and the equipment is withdrawn from the borehole.

It will be appreciated that the tester assembly is hydraulically balanced so that it will have a constant free point during the vertical pipe manipulations that are applied at the surface. The free point of the tester is thus readily differentiated from the free points of other sliding joints in the system, to make surface control of the tools more positive and reliable. Since certain changes or modifications may be made in the disclosed embodiment without departing from the inventive concepts involved, the aim of the appended claims is to cover all such changes and modifications falling within the true spirit and scope of the present invention.

lclaim:

l. A well testing apparatus comprising: an inner member movable upwardly and downwardly between spaced longitudinal positions within an outer member, said members defining a fluid passageway, said passageway being provided in part by upper and lower bores in said inner member extending respectively above and below a transverse barrier section disposed intermediate the ends of said inner'member and in part by a flow path extending externally of said inner member between locations above and below said barrier section seal means for preventing fluid leakage between said members during upward and downward movement; valve means on said members responsive to upward and downward movement of said inner member for opening and closing said second mentioned part of said flow passage; and piston means on said inner member below said barrier section sealingly slidable within cylinder means in said outer member, the upper face of said piston means being subject at all relative positions of said inner and outer members to the pressure of fluids in said passageway below said valve means the lower face of said piston means being subject at all relative positions of said inner and outer members to the pressure of fluids in the well annulus surrounding said members, the transverse cross-sectional area of said piston means being substantially equal to the transverse area circumscribed by said seal means so that the pressures of fluids within said passageway below said valve means act with substantially equal force in opposite longitudinal directions and said inner member is balanced with respect to such pressures I 2. The well testing apparatus of claim 1 wherein said cylinder means is provided by an annular chamber formed between spaced shoulder sections of said outer member that are sealed with respect to equal diameter portions of said inner member, the respective faces of said piston means being subjected to fluid pressure by ports through the wall of said inner member above said piston means and through the wall of said cylinder means adjacent to the lower one of said spaced shoulder sections.

3. The well testing apparatus of claim 2 wherein said valve means includes axially spaced sealing elements that simultaneously engage axially spaced valve seat surfaces when the valve means is in closed position to trap a sample of fluids flowing through said passageway.

4. The well testing apparatus of claim 2 further including spline means for preventing relative rotation of said members during upward and downward movement.

5. A well testing apparatus comprising: an inner member movable upwardly and downwardly within an outer member, said members defining a fluid passageway;

seal means for preventing fluid leakage between said members;

valve means on said members for opening and closing said passageway in spaced longitudinal relative positions of said members;

first piston means on said inner member sealingly slidable within first cylinder means in said outer member, the upper face of said piston means being subject to the pressure of fluids in said passageway, the lower face of said piston being subject to the pressure of fluids in the well annulus surrounding said members, the transverse crosssectional area of said piston means being substantially equal to the transverse area circumscribed by said seal means; and

second piston means on said inner member sealingly slidable within second cylinder means in said outer member and enclosing a low pressure chamber above said second piston means, the lower face of said piston means being subject to the hydrostatic head of fluids in the well bore to provide an upwardly directed bias force on said mandrel.

6. The well tester apparatus of claim 5 wherein said inner member has a barrier to block the through-bore thereof, said passageway extending in part externally of said inner member and between axially spaced lateral ports through the wall of said inner member, said valve means including valve elements disposed on said inner member between said axially spaced ports, the lowermost one of said ports being located above said first piston means.

7. The well packer apparatus of claim 6 wherein said valve elements comprise axially spaced valve heads having sealing elements engageable with spaced valve seats on said outer member, the annular cavity disposed between said inner and outer members and said valve seats providing a flow-through sample chamber in which a sample of fluids is trapped when said valve heads engage said seats.

8. A well tester apparatus comprising: a mandrel movable upwardly and downwardly within a housing, the upper end of said mandrel and the lower end of said housing being adapted for connection to a pipe string, said members defining a fluid passageway; seal means for preventing fluid leakage between said mandrel and said housing; valve means on said mandrel and said housing for closing said fluid passageway in an upper position of said mandrel with respect to said housing and for opening said fluid passageway in a lower position of said mandrel with respect to said housing; a first piston section on said mandrel below said valve means and sealingly slidable within a first cylinder means on said housing, the upper and lower transverse surfaces of said piston section being exposed respectively to the pressure of fluids inside said mandrel and to the pressure of fluids in the well bore externally of said housing, the effective pressure area of said piston section be substantially equal to the transverse area circumscribed by said seal means; and a second piston section on said mandrel below said first piston section and sealingly slidable within a second cylinder means on said housing, said mandrel, second piston section and said second cylinder means enclosing a low pressure chamber above said second piston section, the lower transverse surface of said second piston section being exposed to the pressure of fluids in the well annulus surrounding said housing.

9. The well tester apparatus of claim 8 wherein said flow passageway extends in part through an annular cavity formed between said mandrel and said housing and between axially spaced ports through the wall of said mandrel, said seal means being positioned between said mandrel and housing above the uppermost one of said ports, said first piston section being located on the mandrel below the lowermost one of said ports.

* i i i i

Claims

1. A well testing apparatus comprising: an inner member movable upwardly and downwardly between spaced longitudinal positions within an outer member, said members defining a fluid passageway, said passageway being provided in part by upper and lower bores in said inner member extending respectively above and below a transverse barrier section disposed intermediate the ends of said inner member and in part by a flow path extending externally of said inner member between locations above and below said barrier section ; seal means for preventing fluid leakage between said members during upward and downward movement; valve means on said members responsive to upward and downward movement of said inner member for opening and closing said second mentioned part of said flow passage; and piston means on said inner member below said barrier section sealingly slidable within cylinder means in said outer member, the upper face of said piston means being subject at all relative positions of said inner and outer members to the pressure of fluids in said passageway below said valve means , the lower face of said piston means being subject at all relative positions of said inner and outer members to the pressure of fluids in the well annulus surrounding said members, the transverse cross-sectional area of said piston means being substantially equal to the transverse area circumscribed by said seal means so that the pressures of fluids within said passageway below said valve means act with substantially equal force in opposite longitudinal directions and said inner member is balanced with respect to such pressures .

2. The well testing apparatus of claim 1 wherein said cylinder means is provided by an annular chamber formed between spaced shoulder sections of said outer member that are sealed with respect to equal diameter portions of said inner member, the respective faces of said piston means being subjected to fluid pressure by ports through the wall of said inner member above said piston means and through the wall of said cylinder means adjacent to the lower one of said spaced shoulder sections.

5. A well testing apparatus comprising: an inner member movable upwardly and downwardly within an outer member, said members defining a fluid passageway; seal means for preventing fluid leakage between said members; valve means on said members for opening and closing said passageway in spaced longitudinal relative positions of said members; first piston means on said inner member sealingly slidable within first cylinder means in said outer member, the upper face of said piston means being subject to the pressure of fluids in said passageway, the lower face of said piston being subject to the pressure of fluids in the well annulus surrounding said members, the transverse cross-sectional area of said piston means being substantially equal to the transverse area circumscribed by said seal means; and second piston means on said inner member sealingly slidable within second cylinder means in said outer member and enclosing a low pressure chamber above said second piston means, the lower face of said piston means being subject to the hydrostatic head of fluids in the well bore to provide an upwardly directed bias force on said mandrel.