WO1990000668A1

WO1990000668A1 - Well backsurging apparatus and method

Info

Publication number: WO1990000668A1
Application number: PCT/GB1989/000779
Authority: WO
Inventors: John L. Schneider; Hugo Miguel Barcia; Bruce Robinson
Original assignee: Phoenix Petroleum Services Ltd.
Priority date: 1988-07-12
Filing date: 1989-07-11
Publication date: 1990-01-25
Also published as: GB8816575D0

Abstract

A well backsurge tool for lowering down an oil or gas well at the end of a tubing or drill string comprises a body (10) having an upper end securable to the string and ports (22) in the wall thereof, a detachable cap (12) normally closing the ports (22) and the lower end of the body (10) and retained in place by releasable balls (13), themselves retained by a displaceable sleeve (30) sealed to the body by O-ring (32) and larger diameter O-ring (33) and located thereto by shear pin (31). Reduction of pressure in the string causes downward displacement of the sleeve (30), release of the balls (13), and consequent downward movement of the cap (12), assisted by spring (24). In alternative embodiments the sleeve (30) is mechanically displaced by dropping a bar or by breaking plugs to release a fluid from a hydraulic reservoir previously resisting displacement of the sleeve by differential well bore hydrostatic pressure. Sudden release of a higher well bore hydrostatic pressure in the region of the perforations to a lower hydrostatic pressure within the string, causes fluid flow from the reservoir to the wellbore and thus backsurges the perforations.

Description

WELL BACKSURGING APPARATUS AND METHOD

This invention relates to apparatus for, and a method of, backsurging wells and particularly, but not exclusively to backsurging oil wells or gas wells.

Frequently, a well, such as an oil well, is lined with a tubular steel casing through which perforations are formed to allow oil in the wellbore outside the casing to enter the casing and to rise or be pumped up production tubing for collection. After the perforations have been formed for some time, the rate of flow of oil through the ground outside the casing, and thence through the perforations, falls off, and the fluid in the wellbore exerts a greater or equal pressure on the perforations than the reservoir pressure; hence a positive or zero pressure differential will exist from the wellbore towards the reservoir across the perforations. In some instances, the flow of oil into the casing can be revived somewhat if the pressure in the well bore in the region of the perforations is suddenly reduced to cause the positive or zero pressure differential across the perforations to become. a- negative differential.

Other circumstances where it is necessary to backsurge the perforations arise after a well has been killed by applying hydrostatic pressure greater than the reservoir pressure and a length of the casing is then "worked over", for example to seal off a region through . which oil or sand is being introduced or which is suffering from a mechanical defect.

It is an object of the present invention to provide a tool whereby such a sudden pressure differential can be achieved. According to a first aspect of the present inven¬ tion, there is provided a well backsurge tool for lowering down a well at the end of a tubing or drill string, the tool comprising a generally annular body having an upper end securable to the tubing or drill string and one or more ports in the wall thereof, closure means receivable within the body so as to close the ports and the lower end of the body, and releasable retaining means for retain¬ ing the closure means within the body, the arrangement being such that, when the string is sealed to the casing above the perforations by a packer, displacement of the closure means will suddenly release a higher well bore hydrostatic pressure in the region of the perforations to a lower hydrostatic pressure within the tubing or drill string, causing fluid flow from the reservoir to the wellbore and thus backsurging the perforations.

Conveniently the closure means include a cap member closing the lower end of the sleeve and locked thereto by a mechanical lock which is released by the impact of the said body, allowing the cap member to move downwardly from the sleeve. The closure means may be resiliently loaded away from the sleeve to accelerate •its separation from the- sleeve and permit communication from the well bore to substantially the whole cross- sectional area of the tubing or drill string, the release of the higher to the lower pressure preferably also assist¬ ing the downward movement of the closure means.

The closure means may also include a displace¬ able member, such as an axially displaceable sleeve, which is displaceable after overcoming a predetermined resistance offered by, for example, shear means or on drainage of a hydraulic reservoir, to release the retaining means. In different embodiments of the invention the member may be displaced by mechanical impact to shear shear pins or to rupture a plug holding fluid in the reservoir to allow excess pressure on the outside of the tool to cause the member to move under the influence of a differential force exerted thereon. Alternatively the member may be arranged to move simply when the excess pressure is sufficient to overcome the resistance afforded by a shear pin.

According to a second aspect of the invention there is provided a method of backsurging a well including the steps of lowering down the well a tubing or drill string having a surge tool as claimed in any preceding claim secured to the lower end thereof and a packer secured around the string at a position above the tool until the surge tool is above a perforated zone, setting the packer to seal the annulus between the exterior of the string and the interior of the casing, adjusting the pressure exerted by fluid within the string to provide a lower hydrostatic pressure above the tool than the pressure within the casing below the packer, and then releasing the retaining means to establish fluid communi¬ cation between the lower end of the tubing or drill string whereby the well is backsurged by the sudden application to the wellbore side of the perforated zone of a pressure less than reservoir pressure.

Some embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which Fig. 1 is a section along a plane including the longitudinal axis of a well backsurge tool, according to the invention; Fig. 2 is a similar section and part elevation of a pressure-actuated backsurge tool; and Fig. 3 is a section similar to that of Fig. 1 of a mechanically-operated hydraulic backsurge tool. The same reference numerals are used to indicate the same or similar parts in the various embodiments.

In Fig. 1, a ported sleeve 10 is shown fastened at its upper end by a standard tapered thread 11 to the lower end of a tubing or drill string (unshown) . A cap member 12 is telescoped within the lower end of the sleeve 10 and retained therein by circumferentially-spaced balls 13 engaged in a groove 14 within the sleeve 10 and in radial holes in the tubular part 15 of the cap member 12. The balls 13 are retained in the locking position by a port isolation plug 16, which is telescoped and sealed by O-rings 16a_ and 16b_ within the sleeve 10 and tubular part 15, respectively.

Before operation of the backsurge tool, the plug 16 is located to the sleeve 10 by a shear pin 17. The plug 16 has a central axial bore 18. Drillings 20,21 connect the bore 18 to the upper end of the sleeve 10 and to the trapped space between the plug 16 and the cap member 12, respectively. Radial ports 22 connect the exterior of the sleeve 10 with an annular space 23 between the sleeve 10, the plug 16 and the cap member 12.

A pre-compressed helical spring 24 acts between the lower end of the sleeve 10 and a flange on the cap member 12, and is retained when in uncompressed condition by screws 25. A tie bolt 26 connects the plug 16 to the cap member 12, so as to allow a limited axial movement towards each other, from the position shown in the drawing. When it is desired to backsurge the well, the sleeve 10 of the backsurge tool is screwed onto the bottom end of the tubing or drill string, which is then lowered to the region of the casing that is to be backsurged. A packer of known kind is provided on the tubing or drill string, which seals the annulus. The packer must of course be located above all the perforations through the casing which are to be backsurged.

Backsurging of the well is achieved by establishing a level or density of fluid within the tubing or drill string to provide a lower pressure immediately above the tool than the pressure within the casing in the region of the perforations which are to be backsurged.

The backsurge tool is operated, when required, by dropping a bar down inside the tubing or drill string. The impact shears the pin 17 and pushes the plug 16 further into the cap member 12. Fluid in the previously trapped space between the cap member 12 and the lower end of the plug 16 escapes through the drillings 21, axial bore 18 and drillings 20. When a necked length 27 of the plug 16 reaches the balls 13 they move inwards, releasing the cap member 12 from the sleeve 10. The spring 24 then assists in ejecting the cap member 12 from the sleeve 10, the bolt 25 drawing the plug 16 along with the cap member 12.

Accordingly, the higher pressure fluid within the well bore below the packer is released to the lower pressure fluid within the tubing or drill string, first through the ports 22 after the O-rings 16a. have become disengaged from the internal surface of the sleeve 10, thus assisting the spring 24 to complete the ejection of the cap member 12; and then through the open bottom end of the sleeve 10 when ejection has been completed. The cap member 12, plug 16, and associated components including the spring 24 subsequently fall together to the bottom of the well.

In the pressure-operated surge tool shown in Fig. 2 a cap member 12 having a shock-resistant bull nose 12a_ is telescoped within the ported sleeve 10 and retained therein by balls 13, which are themselves retained in position by a shifting sleeve 30. Shear pins 31 locate the sleeve 30 in the position shown, and an O-ring 32 seals the upper end of the shifting sleeve 30 to the ported sleeve 10, while an O-ring 33 which defines an area greater than that defined by the O-ring 32, seals the lower end of the shifting sleeve 30 to the cap member

12. Due to this difference in cross-sectional areas, a greater pressure outside than inside the tool acting through the port 22 will cause a force to act on the shifting sleeve 30 in a downward direction.

When it is desired to backsurge a well the surge tool of Fig. 2 is run down the well in the condition shown, as previously described. When the fluid pressure within the tool is reduced, for example by gas lift or pumping, to achieve a desired underbalance with respect to the hydrostatic pressure exerted by the formation, the shearing limit of the shear pins 31 is exceeded, the sleeve 30 is forced downwards releasing the balls

13. The cap 12 is thus released and the well backsurged as previously described. The lowering of the pressure in the tubing string is thus utilised both to actuate the tool and to surge the well, dispensing with the need to drop a bar.

An atmospheric chamber may be incorporated in the tool to assist or replace the action of the spring 24 in forcing the cap 12 downwards. As shown in Fig. 3 a mechanically-operated hy¬ draulic backsurge tool comprises a generally annular body 40, which is securable to the lower end of a tubing or drill string; a releasable sleeve 41 telescoped within the lower end of the body 40 and having a pink 42 attached to its lower end, the lower end of the pin having a tapered thread 42a_ onto which is screwed a tail pipe (not shown) which is blanked off at its bottom end; and a shiftable sleeve 43 slidable and captive within the body 40 and sleeve 41. The sleeve 41 is retained by balls 13 as previously described. The upper O-rings 32 correspond to those of Fig. 2 but are duplicated; however the lower single corresponding ring 33 serves an additional function in isolating the upper end of an annular chamber 44 (formed by a neck in the shiftable sleeve 43 and the bore of the releasable sleeve 41, and which is filled with oil under pressure. The chamber 44 is sealed at its lower end by O-rings 45, and is fillable through a port closed by a plug 46; communication with the bore through the sleeve 43 is normally blocked by break plugs 47. A down¬ ward force on the sleeve 43 relative to the body 40 is exerted by a helical spring 48 which is located in an annular cavity vented by drillings 49.

To operate the backsurge tool of Fig. 3 a packer is set to seal the annulus and pressure is reduced in the tubing string relative to well-bore pressure outside the tool, as previously described. A bar is then dropped or lowered on a wireline to fracture the break plugs 47, allowing oil to leak out of the reservoir and the chamber 44 to be open to the pressure obtaining in the bore of the shiftable sleeve 43, which of course is equal to that obtaining the cavity receiving spring 48. Because the cross-sectional area defined by the O-ring 33 is greater than that defined by the O-rings 32, the well-bore pressure acting through the flow port 22 exerts a dif¬ ferential downward force on the sleeve 43. As this force is no longer resisted by the pressure of oil in chamber 14 the sleeve 43 is caused to move downwards, assisted by the spring 48. The balls 13 are thus dis¬ engaged as previously described, allowing the releasable sleeve 41; pin 42; tail pipe (not shown) ; and the shift¬ able sleeve 43 to fall downwards, assisted by the action of spring 24. The consequent venting of reservoir pres¬ sure to the bore of the body 40 through the flow port 22 not only backsurges the well but assists the work of the spring 24.

An atmospheric chamber may be incorporated to assist the action of the spring 24, and also if desired the spring 48.

In all the above-described embodiments other releasable retaining means, such as dogs, may of course replace the balls 13.

Conventional surge tools incorporate a glass or ceramic disk which is broken by the impact of a dropped bar to release the reservoir pressure to the lower tubing string pressure. Such tools suffer from the danger of being fractured by being subjected to too many pressure tests, and this disadvantage is overcome by all three of the above-described embodiments. Moreover, in a deviated well or one in which the top of the disk is covered by fill a dropped bar may not have sufficient impact to break the disk. This difficulty is overcome by the embodiment of Fig. 2 which does not rely at all on mechanical impact for its operation; and by the embodi¬ ment of Fig. 3 which requires only a minor impact to shear the break plugs. The expedient which may be utilised in all embodiments of attaching a tail pipe (which could be, for example, 10 m long) to the releasable sleeve, as described with reference to Fig. 3, allows any fill to be harmlessly received and prevents it from interfering with the operation of the tool: additionally when drop¬ ping a bar on a wireline the tail pipe allows the end of the wireline to be received. The shock-resistant bull nose 12a. of Fig. 2 may usefully be employed in the embodiment of Fig. 1.

The body 40 desirably has a re-entry profile to allow upward passage of the wireline in subsequent operation.

Claims

CLAIMS :

1. A well backsurge tool for lowering down a well at the end of a tubing or drill string, the tool comprising a generally annular body having an upper end securable to the tubing or drill string and one or more ports in the wall thereof, closure means receivable within the body so as to close the ports and the lower end of the body, and releasable retaining means for retaining the closure means within the body, the arrangement being such that, when the string is sealed to the casing above the perforations by a packer, displacement of the closure means will suddenl .release a higher well bore hydrostatic pressure in the region of the perforations to a lower hydrostatic pressure within the tubing or drill string, causing fluid flow from the reservoir to the wellbore and thus backsurging the perforations.

2. A tool as claimed in claim 1, in which the closure means include cap means normally closing the lower end of the body and locked thereto by a mechanical lock.

3. A tool as claimed in claim 2, in which the closure means comprise a displaceable member which also forms part of the mechanical lock, the member acting in a first position to maintain the lock, and being displaceable against restraining means offering a predetermined resis¬ tance to a second position to release the lock.

4. A tool as claimed in claim 3, in which the member is arranged to be displaced by the mechanical impact of a dropped body.

5. A tool as claimed in claim 3, in which the member is arranged such that excess pressure outside the tool acts on the member to produce a differential force, which, when it overcomes the predetermined resistance offered by the restraining means acts to displace the member to said second position.

6. A tool as claimed in any one of claims 3 to 5, in which the restraining means are shear means.

7. A tool as claimed in claim 5, in which the re¬ straining means is a hydraulic reservoir having an outlet rupturable by mechanical impact.

8. A tool as claimed in any one of claims 3 to 7 , in which the member is resiliently loaded to assist its movement from said first to said second position.

9. A tool as claimed in any preceding claim, in which the cap means is resiliently loaded away from the body to accelerate its separation from the body and permit fluid communication from the well bore to substantially the whole area of the tubing or drill string.

10. A tool as claimed in any preceding claim, in which the closure means are arranged so as to be urged away from the body on release of said higher well bore hydrostatic pressure.

11. A tool as claimed in any preceding claim, in which the closure means comprise a tail pipe, and/or a shock-resistant bull nose.

12. A method of backsurging a well including the steps of lowering down the well a tubing or drill string having a surge tool as claimed in any preceding claim secured to the lower end thereof and a packer secured around the string at a position above the tool until the surge tool is above a perforated zone, setting the packer to seal the annulus between the exterior of the string and the interior of the casing, adjusting the pressure exerted by fluid within the string to provide a lower hydrostatic pressure above the tool than the pressure within the casing below the packer, and then releasing the retaining means to establish fluid communi¬ cation between the lower end of the tubing or drill string whereby the well is backsurged by the sudden application to the wellbore side of the perforated zone of a pressure less than reservoir pressure.