GB2162564A - Downhole tool - Google Patents

Abstract

A downhole jar tool (10) comprises an elongate tubular casing (11) internally configured with annular grooves (12A, 12B) axially spaced by a land (13). A mandrel (22) projects into the casing (11) and is provided with a groove (22E) for cooperation with a ball camming mechanism comprising balls (24) located by sleeve (18) extending between casing (11) and mandrel (22). At the end of casing (11) where mandrel (22) protrudes a top sub (21) is connected to provide an anvil (14) for abutment by a hammer mechanism (22C) forming part of mandrel (22) and sleeve (18) is resiliently biassed unidirectionally by a spindle (17) in abutment with sleeve (18) and cooperating with a main biassing spring (15) which in turn is seated effectively against the casing (11). Axial forces tending to withdraw the mandrel from the casing act against the resilient force of the biassing spring (15) and cause sufficient movement of the camming mechanism to release the mandrel (22) therefrom in a snap-action so that the hammer mechanism (22C) impacts against the anvil (14) to effect a mechanical jarring action on casing (11). <IMAGE>

Description

SPECIFICATION Downhole tool This invention relates to a down hole tool especially, but not exclusively, for oil wells and more particularly to a mechanically operable wire line jar, that is a tool for converting a straight or axial pull on a wire line into an axially directed impact.

Such mechanical jars are of particular utility as a downhole tool in the oil industry when used in conjunction with a fishing tool (that is a tool designed to retrieve or fish out of a well an item which has been iost in or which has become stuck in a well).

It is an object of the present invention to provide an improved form of downhole tool for oil wells.

According to the present invention there is provided a downhole tool in the form of a mechanical jar comprising an elongate tubular casing assembly having a circular bore and terminated by top and bottom subs, an axially movable elongate mandrel extending through the top sub and being in releasable mechanical connection with the casing assembly by means of a ball cage assembly, main spring biassing means being provided in the casing assembly between the end of the mandrel and the bottom sub and being in mechanical condition with said ball cage assembly.

The main spring biassing means may be in the form of a coil spring of rectangular section material or may be in the form of a Belleville-washer spring.

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, in which: Fig. 1 is a half sectional side view of a tool in accordance with the present invention, the view being split at lines A-A and B-B into three portions in the interests of clarity having regard to the substantial axial length of the tool; and Fig. 2 illustrates a modification to one portion of the Fig. 1 tool, the remaining portions being as shown in Fig. 1.

In Fig. 1 of the drawings the tool 10 comprises an elongate tubular casing 11 having a circular bore 11A which is internally threaded at its end 11B and also at its end 110. Adjacent end 1 C the bore of the casing 11 is configured to provide first and second annular grooves 12A, 1 2B which are axially spaced buy a land 13, An impact collar (or impact sub) 21 is secured to the casing 11 at its end 11 B by means of screw threads. Collar 21 incorporates an annularflange 14 protruding into the bore of the casing to form an abutment anvil for a purpose to be described.

Top sub 21 acts as a guide for elongate mandrel 22 having an end 22Awhich protrudes from casing 11 for the purpose of actuating the tool as will be explained. The mandrel extends axially through casing 11 to a termination end 22B adjacent which the mandrel is provided with an annular radially outwardly protruding flange 22C. Flange 22C forms a hammer for impact against the abutment anvil 14 of impact sub 21. Termination end 22B of mandel 22 is further provided with an annular groove 22E forming part of a camming mechanism to be described.

In order to close end 1 1C of casing 11 a bottom sub 20 is threadedly connected to end 110, sub 20 having a substantially tubular body 20Awhich is of essentially the same bore as casing 11. Within this tubular body there is located a spindle 17 which is externally threaded at its end 17A for the purpose of receiving an adjustable thrust nut 16 against which one end of a Belleville spring 15 bears. The other end of the Belleville spring 15 abuts a retainer nut 23 which is threadedly secured to bottom sub 20 adjacent end 1 1C of casing 11. Spindle 17 extends through nut 23 and terminates in an enlarged head 17B which is located within the bore of a ball-cage sleeve 18 extending between the end 22B of mandrel 22 and casing 11.Sleeve 18 itself has an inwardly protruding flange 18A against which head 17B abuts and the flange 18A is biassed away from sub 20 by means of a reset spring 19 which abuts nut 23, the latter having a skirt portion 23Awhich in part limits compression of spring 19 and in part prevents spring 19 jamming between nut 23 and spindle 17.The region of sleeve 18 annularly extending between land 13 and groove 22E is in the form of a ball cage 18B arranged to house, say, 6 or 8 individually pocketed ball bearings 24 such that the ball bearings 24 protrude both radially inwardly and outwardly from the sleeve member 18 but are prevented from falling radially inwardly by the cupshaped nature of each ball pocket In operation the tool 10 is connected by means of bottom sub 20 directly or indirectly to an item which is firmly secured or stuck within a well, this being achieved by way of a threaded socket 20A formed in bottom sub 20, and protruding mandrel end 22A is connected directly or indirectly to a wire line (i.e. a cable) extending to the top of the well for actuation by an operator.A straight or axial pulling force is exerted by the operator on the wire line which in tool 10 is translated into an engagement between the wall of mandrel groove 22E and the ball bearings 24 thereby urging sleeve member 18 in the direction of impact sub 21 against the spring loading imposed on sleeve member 18 by means of main spring 15. When the pulling force on the wire line exceeds the force exerted by main spring 15 sleeve member 18 commences movement in the direction of impact sub 21 as does mandrel end 22B.

Immediately on ball bearings 24 clearing the annular land 13 the ball bearings 24 are forced radially outwardly by mandrel end 22B so that the ball bearings 24 extend into annular groove 1 2B thereby substantially instantaneously releasing the compressive force imposed by main spring 15 from mechanical connection with mandrel 22 which accordingly is rapidly accelerated in the direction away from casing end 1 1C until such movement is arrested by the impact engagement between the flange 22C, abutting the flange 14. This shock loading effects a mechanical jarring action on casing 11 which is transmitted axially along casing 11 so as to be incident on the item or items connected to tool 10 via socket 20D.

Following this rapid movement of mandrel 22 if the wire line pulling force is maintained mandrel flange 22C, remains in abutment with flange 14 of impact sub 21 thereby transferring the straight or axial wire line pulling force directly to casing 11 and if the aforesaid jarring action has loosened the item to be retrieved from the well the item together with tool 10 can be hoisted to the surface. If howeverthe aforesaid jarring action has not effected a dislodgement of the stuck item the procedure can be repeated provided that tool 10 is first reset.

In order to reset tool 10 the pulling force on the wire line is removed thereby allowing mandrel flange 22C, to move away from impact sub 21 towards sleeve member 18. When mandrel end 22B enters sleeve member 18 the chamfer 22F abuts with and engages ball bearings 24 (which themselves engage land 13) causing sleeve member 18 to move against the reset spring 19 which is relatively easily compressible until such time as ball bearings 24 can move past land 13 and into groove 12A. This enables chamfer 22F of mandrel 22 to move axially past the ball bearings 24to align mandrel groove 22E with the ball cage 18B so that ball bearings 24 under the action of reset spring 19 are guided to lie between mandrel groove 22E and land 13. Tool 10 is now reset.

It will be appreciated that the tension imposed by main spring 15 is adjustable in accordance with the axial position of thrust nut 16. Adjustment of nut 16 is effected by removing top sub 21 and mandrel 22 and inserting a screwdriverforengagementwith a diametrical slot (not shown) in the end of spindle 17.

Bottom sub 20 is apertured at 20B in partto accommodate anti rotation lugs on nut 16 and to permit visual calibration of force setting against location of nut 16 within bottom sub 20. To prevent jamming of ball bearings 24 the edges of grooves 12A, 12B and 22E each extend non-perpendicularly to the longitudinal axis of the tool 10 so that these groovestogetherwith land 13 effectively form a camming mechanism for the ball bearings 24.

Because the jarring action is effected by the impact engagement between two flanges it is preferred that the mutually abutting faces of these flanges extend non-perpendicularly to the longitudinal axis of the tool 10 and for example these faces may be offset by 15 or thereabouts from the perpendicular. This arrangement provides an increased impact area.

Preferably this angle is arranged so that the external angle between the flanges and the body of mandrel is 105 .

When the tool is utilised in the environment of an oil well where substantial volumes of liquid are present this liquid tends to penetrate to the interior of tool 10 which in the interests of simplicity is devoid of seals preventing ingress of such liquid and during the rapid motion of mandrel 22 flange 22C, effectively acts as a piston, and to prevent fluid locking of tool 10 casing 11 is slotted as at 25 to permit rapid egress of liquid from the tool interior.

Aperture 20B functions similarily and bore 20C is provided in sub 20 for the same purpose.

In order to provide for the possibility of a breakage occurring in mandrel 22 at a point between protruding mandrel end 22A and flange 22C, which would thereby leave the remainder of tool 10 within the well the exterior of top sub 21 is provided with a fishing neck to meet API (American Petroleum Institute) standards. Also it is preferred that the protruding end of 22A of mandrel 22 is provided with a sub 30 externally configured with a fishing neck to API standard. However it will be appreciated that because of the absence of any ,mechanism radially between mandrel 22 and casing 11 the body of mandrel 22 is relatively large so that such breakages are minimised.

Although not shown it is preferred that a shock absorbing device is fitted between the tool 10 and the wire line so that the mechanical shock loading generated by tool 10 is uni-directionallytransmitted to the item or items connected to bottom sub 20. By way of example the shock absorber may simply comprise a column of Belleville washers suitably contained in a telescopic housing one end of which is secured to mandrel end 22A via sub 30 of tool 10 whilst the other end is connected to the wire line.

In a modification'illustrated in part in Fig. 2 the tool is substantially identical to tool 10 of Fig. 1 but the Belleville-washer spring 15 is replaced buy a rectangular-section coil spring 35.

Claims (5)

1. A downhole tool in the form of a mechanical jar comprising an elongate tubular casing assembly having a circular bore and terminated by top and bottom subs, an axially movable elongate mandrel extending through the top sub and being in releasable mechanical connection with the casing assembly by means of a ball cage assembly, main spring biassing means being provided in the casing assembly between the end of the mandrel and the bottom sub and being in mechanical connection with said ball cage assembly.

2. A downhole tool as claimed in claim 1,wherein the main spring biassing means is in the form of a coil spring of rectangular section material.

3. A downhole tool as claimed in claim 1,wherein the main spring biassing means is in the form of a Belleville-washer spring.

4. A downhole tool in the form of a mechanical jar comprising b housing having a circular bore provided with first and second axially spaced annular grooves in a generally central region of the housing, an impact collar secured to the housing at the end thereof which is open, an elongate axiallymovable mandrel extending through the impact collar from a protruding mandrel end to a termination end incorporating a mandrel groove adjacent said first and second annular grooves and an impact surface for abutment against said impact collar, a ball cage sleeve member extending between the mandrel termination end and the housing, said sleeve member comprising an annularly disposed ball cage incorporating ball bearings dimensioned to protrude radially inwardly and outwardly from the sleeve member and to locate in at least one of the annular grooves of the mandrel and housing according to the axial position of the mandrel, and spring biassing means located within the bore of the housing adjacent the other end thereof and interconnected with the sleeve member to determine an equilibrium position thereof and to impose a substantial resilient force against movement of the sleeve member unidirectionally towards the impact collar.

5. A downhole tool as claimed in claim 1 and substantially as hereinbefore described with reference to either of the embodiments illustrated in the accompanying drawing.