Downhole shock absorber

Information

  • Patent Grant
  • 6412614
  • Patent Number
    6,412,614
  • Date Filed
    Monday, September 20, 1999
    25 years ago
  • Date Issued
    Tuesday, July 2, 2002
    22 years ago
Abstract
A shock absorber for damping shock energy generated by downhole perforating guns or stimulation devices is described. The shock absorber includes a spring assembly including at least a first spring and a second spring, the first spring having a tension greater than the second spring; a damper assembly; and a housing retaining the spring assembly and the damper assembly and including ends adapted for connection into a casing perforation assembly. The provision of springs of increasing stiffness permits the force generated by the perforating gun to be absorbed gradually and smoothly.
Description




FIELD OF THE INVENTION




The invention relates generally to shock absorbers for the insertion in a drill, tubing, or wireline string to isolate downhole explosive apparatus from other downhole tools. In particular, the invention relates to a shock absorber for isolating the jarring effect of perforating guns and high-energy gas stimulation systems from delicate instrumentation or other downhole equipment that may be prone to mechanical damage from perforating gun detonation.




BACKGROUND OF THE INVENTION




During oil or gas well completion operations, it is necessary to perforate the casing to provide communication between the hydrocarbon bearing formations and the wellbore, so that the hydrocarbons may be produced to the surface. The casing perforation operation is carried out using explosive shaped charges, which blast holes in the casing and its surrounding cement sheath to access the hydrocarbon bearing formation.




It is becoming common to place instrumentation packages in close proximity to the perforating guns. These instruments measure downhole pressures to provide an indication of the influx rate, pressure, or temperature of the wellbore to give an indication of the success of the perforations and the production rate of the well. The firing of the perforating guns or stimulation devices produces large shock waves, which exert huge forces upon the instrumentation or other downhole equipment, often causing their failure. A shock absorber, when positioned between the perforating gun and the downhole equipment, considerably reduces the forces upon the equipment and expands their operating range and useful lifetime. Shock absorbers also permit the deployment of certain other tools that otherwise may not be operated in conjunction with perforating systems.




Some prior art shock absorbers rely on elastomeric elements, formed of rubber for example, to absorb shock waves. However, the effectiveness of an elastomeric element is limited due to the limited range of motion of the element and its inherent damping characteristics. A typical elastomeric element might be able to withstand 0.5″ of travel, which is generally ineffective for use with perforating systems. Other shock absorbers have used spring and damper arrangements including compressible oil. Compressible oil is very expensive and transmits large loads to the instrumentation due to the high loading required to begin compression of the fluid and springs. The need exists for a shock absorber that is more effective and less expensive than those already in use.




SUMMARY OF THE INVENTION




A shock absorber for damping shock energy generated by downhole perforating guns or stimulation devices has been invented and is disclosed herein. The invention provides an apparatus for conducting downhole measurements while perforating wherein the impulsive energy of the perforating guns is absorbed and, thereby, shielded from the downhole measurement tools.




In accordance with a broad aspect of the present invention, there is provided a shock absorber for absorbing the energy generated by a perforating gun, comprising a spring assembly including at least a first spring and a second spring, the first spring having a tension greater than the second spring; a damper assembly; and a housing retaining the spring assembly and the damper assembly and including ends adapted for connection into a casing perforation assembly.




The shock absorber according to the present invention includes ends formed for attaching into a casing perforation assembly, the assembly including a perforation gun and a downhole tool desired to be shielded from the force exerted by the perforation gun during detonation such as, for example, a gauge recorder. Preferably the ends of the shock absorber are formed for threaded engagement into the perforation assembly. The shock absorber housing preferably includes two telescopically disposed parts between which the spring assembly and the damper assembly act.




The spring assembly is the primary mechanism for absorbing and dissipating the shock loading from the perforating guns. The shock absorber is designed to be double acting, so that the shock impulse can be applied from either direction and be absorbed and dissipated, without the need for a set of springs to handle impulse from each direction. The shock absorber includes a plurality of springs preferably of various stiffnesses, so as to give multiple discrete spring rates. In one embodiment, a plurality of springs are used, each of which is progressively stiffer and serves to gently absorb the shock wave. In a preferred embodiment some of the springs can be removed or further springs can be added to change the preload of the spring assembly. Changing the preload of the spring assembly provides that the shock absorber can be adjusted with consideration as to the downhole tools that are to be hung from the shock absorber. In this embodiment, the preload of the shock absorber can be adjusted to be in the neutral position when the downhole tools such as, for example, a gauge recorder, are hung from the shock absorber. The selection of preload allows maximum stroke and energy absorption from the shock absorber in service. In the preferred embodiment, the spring rates are fairly low so that the shock absorber can absorb most of the shock while the tools hung below the absorber remain relatively stationary during the detonation of the guns.




The damper assembly has a plurality of oil filled chambers and a valve mechanism to regulate the flow of oil from a first chamber to a second chamber. The oil is preferably substantially non-compressible to reduce the cost of the shock absorber over a shock absorber requiring compressible oil. Preferably, the first chamber includes a piston that is moveable to reduce the volume of the first chamber in response to a loading, such as that applied by the detonation of a perforating gun. When the piston reduces the volume, oil will be forced from the first chamber into the second chamber. In one embodiment, the chamber is selected such that its volume will only be reduced by a load over a selected level. In one embodiment, the damper valve mechanism includes a metering mechanism to restrict the flow of oil from one chamber to another in response to the loading. The damper assembly can be hollow, having a bore therethrough for permitting passage of a member such as, for example, a conductor (i.e. wireline) or full tubing bore therethrough and, thereby, through the centre of the shock absorber.




It is an object of the present invention to provide a mechanism for protecting the very delicate instrumentation, such as pressure recording gauges, when the perforating guns are detonated. It is a further object of the invention to provide a shock absorber that is capable of very rapid displacement and subsequent shock absorption and dissipation.











BRIEF DESCRIPTION OF THE DRAWINGS




A further, detailed, description of the invention, briefly described above, will follow by reference to the following drawings of specific embodiments of the invention. These drawings depict only typical embodiments of the invention and are therefore not to be considered limiting of its scope. In the drawings:





FIGS. 1A

to


1


E are together a section through a shock absorber according to the present invention in a neutral or rest position.





FIG. 2

is a section through the shock absorber of

FIGS. 1A

to


1


E in a compressed position.





FIG. 3

is a section through the shock absorber of

FIGS. 1A

to


1


E in an extended position.











DETAILED DESCRIPTION OF THE DRAWINGS





FIGS. 1-3

show a shock absorber


10


according to the present invention. Shock absorber


10


includes a shock mandrel


12


, illustrated herein as the upper end, telescopically disposed and moveable within a tube formed from an upper cap


14


, a spring housing


16


and a bottom connector


17


. Shock absorber


10


is selected to be disposed in the perforation assembly between a perforation gun (not shown) and a downhole tool (not shown). The shock absorber acts to shield the downhole tool from a shock wave passing through the assembly and generated by the detonation of the gun. The shock absorber can be positioned above or below the perforation gun depending on the location of the downhole tool.




The upper end of the shock mandrel


12


has threads


18


for connection to a tubing string or wireline, while the connection to the perforation guns or other downhole equipment is made through threads


20


on bottom connector


17


. Upper cap


14


, spring housing


16


and bottom connector


17


are connected by threads


22


and


24


.




Upper cap


14


carries a wiper ring


26


in gland


27


and a wear ring


28


in gland


30


. Rings


26


,


28


form a seal between the upper cap and the mandrel and clean the outer surface of mandrel


12


as it slides in bore


32


of upper cap


14


.




Spring housing


16


surrounds a spring


34


and a plurality of Belleville springs


36


. Spring


34


is a compression-type coil spring and is retained in a spring chamber


38


between housing


16


and shock mandrel


12


. Spring


34


is biased between spring spacer


40




a


and spring spacer


40




b


. Spring spacer


40




a


abuts the lower end of upper cap


14


, and a shoulder


42


machined on shock mandrel


12


. Spring spacer


40




b


is free to move axially within spring chamber


38


. A spring sleeve


44


is disposed in chamber


38


to limit the extent to which spring spacer


40




b


can move toward spring spacer


40




a


and, thereby, to limit the compression of spring


34


. Belleville springs


36


act against the opposite side of spring spacer


40




b


. The Belleville springs are stacked in three different configurations, so as to give different spring rates and to absorb the shock waves progressively. Abutting the Belleville spring stack at their end opposite spacer


40




b


is another spring spacer


40




c


, which transfers the spring loads to shoulder


46


in spring housing


16


. Spring


34


is preferably selected to have a stiffness less than that of any of the Belleville washer configurations, such that when a force is applied to the shock absorber, spring


34


will compress first followed by the least stiff configuration of Belleville springs and then the next stiff configuration of Belleville springs and finally the most stiff configuration of Belleville springs (i.e. the stack of Belleville springs all oriented in the same direction, identified as


48


). The provision of springs of increasing stiffness permits the force to be absorbed gradually and smoothly.




Spring spacer


40




c


also abuts against an end of a lower mandrel body


50


, which is rigidly attached to the lower end of the shock mandrel


12


by means of threads


54


. As such, lower mandrel body


50


moves with shock mandrel


12


. Spring


34


and Belleville springs


36


are therefore compressed between shock mandrel


12


, upper cap


14


, spring housing


16


and lower mandrel body


50


. Since spring


34


and Belleville springs


36


can be acted upon by any of shoulder


42


, upper cap


14


, shoulder


46


or lower mandrel body


50


, the shock absorber is double acting and functional against shock inputs from either end.




Holes


56


in the spring housing ensure that the mandrel can move freely within the housing. There will not be a trapped air or fluid pocket around the spring


34


that will impede its movement.




The shock absorber of the present invention also includes a damper assembly. In the illustrated embodiment shock mandrel


12


includes a mandrel plug


58


which acts as a retainer for a spring


60


. Mandrel plug


58


engages mandrel


12


through threads


62


and includes a hex


64


, which facilitates use of a hex key to install or remove plug


58


.




Shock mandrel


12


has a central axial bore


66


in which a balance piston


68


slides. Spring


60


acts between mandrel plug


58


and balance piston


68


to dampen the action of the balance piston and to bias it towards a selected neutral position. Balance piston


68


incorporates an oil filler plug


70


in a central passage


72


to allow for filling upper oil chamber


74


with oil. Mandrel plug


58


has a central passage


76


through which filler plug


70


can be accessed. Filler plug


70


also retains an O-ring


78


thereabout which provides a seal between the filler plug and central passage


72


.




Balance piston


68


also includes O-rings


80


in glands


82


,


83


that seal between the piston and bore


66


of shock mandrel


12


. A wear ring


86


in gland


88


functions to centralise balance piston


68


and to provide a low friction surface facilitating the sliding of the balance piston within the bore.




Extending into bore


66


of shock mandrel


12


is piston


90


which is connected, through piston rod


92


and piston retainer


94


, to move with the tube formed from upper cap


14


, spring housing


16


, and bottom connector


17


. In the illustrated embodiment, piston retainer


94


is secured in position by machined shoulder


96


and abuts the top land


98


of bottom connector


17


.




Piston


90


is attached and sealed to piston rod


92


by threads


104


and O-rings


106


disposed in glands


108


. Piston


90


seals to the bore


66


of shock mandrel


12


through O-rings


110


, and is centralised in the bore by a wear ring


112


. The wear ring also provides a low friction surface between mandrel


12


and piston


90


.




Piston


90


moves within bore


66


to effect the volume of upper oil chamber


74


and a lower oil chamber


114


on the opposite side of the piston. Piston


90


includes a valve mechanism


116


for permitting the flow of oil in a regulated manner between chamber


74


and chamber


114


. Valve mechanism


116


is disposed in a bore


118


through piston


90


. The valve mechanism includes a poppet case


120


and a balance spring


122


in bore


118


. Balance spring


122


forces the valve mechanism


116


into the poppet case. Therefore there is a reduced metering effect, due to the greater exposed area when the shock absorber is being compressed, than when it is being extended. This gives a greater damping effect when the shock absorber is being extended.




Chamber


114


is sealed by O-rings


110


as well as by O-rings


126


and


127


in glands


128


and


129


, respectively. O-rings


126


are carried by lower mandrel body


50


to provide a seal between it and piston rod


92


. Lower mandrel body


50


also contains a wear ring


130


to centralize body


50


within spring housing


16


. Holes


132


in the spring housing permit movement of lower mandrel body


50


within the spring housing and permit access to another filler plug


134


. Filler plug


134


is similar to filler plug


70


and has thereabout an O-ring


135


Filler plug


134


permits the shock absorber to be filled with oil and/or air to vent from the shock absorber if oil is being passed through another filler plug.




Extending through piston


90


and piston rod


92


is a passage


136


through which oil can be passed to fill chambers


74


and


114


in preparation for use of the shock absorber. A filler plug


138


, similar to filler plug


70


, and O-ring


139


is disposed in passage


136


.




The lower end of shock mandrel


12


incorporates a wear ring


140


in gland


142


to guide and centralise piston rod


92


.




The shock absorber acts to absorb forces applied at either end thereof to prevent the force from being transmitted therealong. As an example, when a force is applied against shock mandrel


12


which would tend to drive it into the bore formed from upper cap


14


and spring housing


16


, shoulder


42


is driven against spring spacer


40




a


. This causes spacer


40




a


to be driven toward spring spacer


40




b


. Since spring


34


has a lesser tension than any of the configurations of Belleville springs


36


, spring


34


is compressed between spacers


40




a


and


40




b


. This compression will continue until the force is absorbed or until the compression is limited by spring sleeve


44


. If there is force in excess of the absorption capacity of spring


34


, this force will then be applied through spring spacer


40




b


to the Belleville springs. The Belleville spring configurations will then compress in turn, as necessary depending on their stiffness, to absorb the force. The force will be absorbed gradually through the coil and Belleville spring configurations, in order as determined by their degree of stiffness. The spring with the greatest stiffness (i.e. a stack of Belleville springs oriented in the same direction) will compress last.




As the springs are being compressed, shock mandrel


12


moves axially into spring housing


16


. As a result, piston


90


is advanced into bore


66


toward mandrel plug


58


. As piston


90


advances, balance piston


68


is driven against the tension in spring


60


. Simultaneously, the volume of chamber


114


increases, thus drawing oil through the valve mechanism


116


and thereby reducing the volume of chamber


74


. The spring


60


also assists in forcing oil through valve mechanism


116


by applying force to balance piston


68


as the spring


60


compresses. During advancement of the piston


90


into bore


66


toward mandrel plug


58


, the valve mechanism is forced away from the poppet case to minimise the metering effect. When the force is fully absorbed by the springs, or when the shock absorber is fully compressed the spring


34


and the Belleville springs


36


will begin to recover and force the shock absorber to approach the neutral position. Since the valve mechanism serves to regulate the flow of oil back into chamber


74


, the recovery of the springs will be controlled and slowed and any oscillations will be damped. As will be appreciated, to properly operate the oil is preferably substantially non-compressible.




Depending upon the intensity of shock energy applied to the shock absorber, the shock absorber may cycle through the compressed and extended positions several times to dissipate the shock energy and reach the neutral position.




In preliminary testing, the shock absorber was able to reduce the acceleration forces exerted upon an instrument pack by a factor of


20


, when compared to the acceleration forces generated through the same casing perforation assembly without a shock absorber.




While the invention has been described or shown in only some of its forms, it should be apparent to those skilled in the art that it is not so limited, but is susceptible to various changes without departing from the scope of the invention.



Claims
  • 1. A shock absorber for absorbing the energy generated by a downhole shock generating device, comprising: a housing tube; a shock mandrel telescopically disposed within the housing tube; a spring assembly disposed between the housing tube and the shock mandrel to resist axial movement of the shock mandrel within the housing tube, the spring assembly including at least a first spring and a second spring, the first spring having a stiffness greater than the second spring, the first and second springs being contained between first and second shoulders on the shock mandrel and between first and second shoulders on the housing tube and compressible by any one of the shoulders; a damper assembly to dampen axial movement of the shock mandrel within the housing tube, the damper assembly includes a valve permitting two way flow of fluid therethrough but providing a greater dampening effect against axial movement of the shock mandrel out of the housing tube than that against axial movement of the shock mandrel into the housing tube; an end of the shock mandrel extending at the first end of the shock absorber and adapted for connection into a downhole assembly; and an end of the housing tube extending at the opposite end of the shock absorber and adapted for connection into the downhole assembly.
  • 2. The shock absorber of claim 1, wherein the valve is a poppet valve.
  • 3. The shock absorber of claim 1, wherein the damper assembly includes a balance piston open on one side thereof to fluid pressure external to the shock absorber.
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Entry
Owen Oil Tools Inc 1997 Catalog 3½ Vertical Shock Absorber Owen Oil Tools Inc, Ft Worth TX 1997.
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