Abstract:
A shock absorbing tool for use in pipelines and oil wells, particularly in combination with perforation guns comprises a central mass supported on two springs between the end terminal members. The shock wave is absorbed in the free oscillation of the mass. The mass is constituted by a piston assembly. A steel rope permits the shock absorbing tool to be maneuvered within the bore hole without damage to the springs.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This application relates to a shock absorber apparatus for use as part of a tool string for use in bore holes and/or pipelines. This invention particularly though not exclusively relates to a shock absorbing tool for use with perforating guns in the oil well logging industry.  
           [0002]    Present day wells for extracting oil, gas or water, are mostly constructed by drilling in the ground and placing a tubular steel casing as a liner in the well. Once the casing is in place, the well may be completed using a perforating gun to form perforations in the casings through which surrounding reservoir fluid, comprising oil, gas and water, may flow inwardly. From this reservoir fluid, the oil, gas or water may then be extracted.  
           [0003]    A perforating gun is an instrument equipped with a number of powerful explosives or ‘shape charges’. These are about one inch (25 mm) in diameter, and when detonated produce a plasma which destroys both the steel pipeline casing and typically 1 to 2 feet (300-600 mm) of the surrounding rock. Reservoir fluid from a deposit surrounding the well can then flow through the perforated rock into the pipeline and be pumped away to the surface.  
           [0004]    It is desirable to deploy sensors such as temperature and pressure sensors alongside the perforating gun so that data about the ‘perforating’ explosion and the resulting fluid flow may be collected. In this way perforation techniques may be improved.  
           [0005]    The perforating gun and sensor tools may be deployed in the bore hole as or as part of a tool string, which is an array of one or more guns or tools connected end-to-end on a wireline or cable. The wireline may provide both electrical power and a possible mechanical drive means for the tool string in the bore hole.  
           [0006]    The blast from the shape charges of the perforating gun is powerful enough to rip through both the steel casing and enough of the surrounding rock to release the reservoir fluid. The blast therefore poses a significant risk of damage to the sensors that accompany the perforating gun into the bore hole. Such sensors often employ photo-multipliers, gamma ray detectors and circuit boards which could easily be damaged by the shock wave from the explosion. For this reason, it is desirable to separate the tool sensors and perforating gun from one another on the tool string by a shock absorbing tool. A good shock absorber is important as it means that the sensor tools may not only be protected from the shock wave but also may be placed closer to the perforating gun in order to collect data closer to the explosion site.  
           [0007]    A number of shock absorbers for use down-hole in an oil well are known and rely on a piston or plunger mechanism to absorb the energy from the shock. Typically the piston or plunger chamber is filled with a fluid which is forced through holes in the piston chamber as the piston is depressed, providing an increasing resistive force in opposition to the force which is depressing the plunger or piston. A problem with shock absorbers of this type is that they are not responsive enough to adequately dissipate the energy released from the explosion. This is because the blast from the shape charges lasts only an instant and produces a shock wave that contains both high and low frequencies. At high frequencies, the piston and fluid arrangement simply cannot react quickly enough and the shock wave may travel through it as if it were a solid. As a result the sensors may be damaged  
           [0008]    A prior art shock absorber tool for use in an oil well is described in European patent application 0,414,334. This shock absorber comprises a piston and a fluid filled piston chamber which has a number of openings to allow fluid to escape from the chamber into a surrounding space as the piston is compressed. As the piston moves into the piston chamber the hole area through which the fluid can escape is diminished resulting in an increasing resisting force from the fluid. An additional like shock absorber can be connected in series with it.  
           [0009]    European patent application 0,489,527 discloses a further shock absorbing tool which makes use of two telescopic casings disposed one inside the other and defining a space between them which is filled with a compressible oil. Disposed between the two casings is a metering clearance sleeve which defines two spaced sealed voids in this space and through which the oil is caused to pass in response to a shock wave.  
           [0010]    U.S. patent application Ser. No. 5,320,169 discloses a gauge carrier having shock absorber means. The shock absorber means comprises an upper set of belleville springs or disc springs mounted on a rod member and contained between an actuator head portion of the rod member and an annular disc support. A lower set of belleville springs is contained between the disc support and a lower connector member. The rod member is screwed into a threaded bore in the connector member.  
           [0011]    Further shock absorbing devices are disclosed in U.S. Pat. No. 6,109,355 and U.S. Pat. No. 4,817,710 which use an elastically deformable body, and resilient contact pads, respectively.  
           [0012]    We have appreciated that known shock absorbers, such as those described in the prior art referred to above, cannot respond quickly enough to adequately absorb the energy in a shock wave, and consequently that sensor tools used in conjunction with such shock absorbers and perforating guns are still at risk of damage from detonation of the shape charges.  
         SUMMARY OF THE INVENTION  
         [0013]    The invention is set forth in claim  1  below to which reference should now be made. Advantageous features of the invention are set forth in the appendant claims.  
           [0014]    A preferred embodiment of the invention, described in more detail below, takes the form of a tool comprising two terminal housings between which a body or mass is supported on two stiff springs. The mass, springs and housings are disposed longitudinally and the mass may freely oscillate on the springs between the two housings in response to the disturbance of a shock wave. The mass of the tool string connected to either of the terminal housings of the tool is significantly greater than that of the mass itself, and as a result, each collision of the mass with the terminal housing transmits only a small amount of energy from the oscillating mass to the rest of the tool string. Thus, much of the energy from the shock wave is delayed from transmission along the tool string.  
           [0015]    While the mass oscillates, energy is also dissipated in the fluid surrounding the mass and spring system. In this way the frequencies of the explosion are separated out, some of the frequencies are delayed from transmission along the tool string, and some of the frequencies are lost to the surrounding fluid.  
           [0016]    Preferably, the mass or body comprises a piston suspended on the two springs to provide a further dampening effect. The effectiveness of the piston may be increased by using deformable plugs inside the piston to absorb energy as the piston is compressed or as it is stretched.  
           [0017]    Preferably, one of the terminal housings is also provided with a strong cable or wire rope which limits the maximum extension of the springs such that they are not extended beyond their yield point or elastic limit. This is advantageous when the shock absorbing tool, or the tool string on which it is mounted becomes stuck or jammed in the bore hole and a larger than normal force is required to free the tool or tool string. This force is borne by the cable or wire rope and not the springs where it might cause damage. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    The invention shall now be described in more detail by way of example and with particular reference to the drawings in which:  
         [0019]    [0019]FIG. 1 is a longitudinal sectional view through a shock absorbing tool according to a preferred embodiment of the invention;  
         [0020]    [0020]FIG. 2 is an enlarged view of the male end terminal housing of the shock absorbing tool shown in FIG. 1;  
         [0021]    [0021]FIG. 3 is an enlarged view of the mass or piston assembly of the shock absorbing tool shown in FIG. 1; and  
         [0022]    [0022]FIG. 4 is an enlarged view of the female end terminal housing of the shock absorbing tool shown in FIG. 1. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0023]    A shock absorbing tool  2  according to the preferred embodiment of the invention will now be described with reference to FIG. 1. The tool is intended for use as part of a tool string in a bore hole or pipeline and is therefore elongate in shape and is provided with a male end terminal housing  10  and a female end terminal housing  20  so that it may be mechanically and electrically connected to other neighbouring tools in the tool string. The male and female terminal housings  10  and  20  are cylindrical in shape, have a diameter which is narrow in comparison to their length and are disposed apart from each other on the same longitudinal axis, being orientated so that the parts of the terminal housing which connect to adjacent tools in the tool string face away from each other.  
         [0024]    In the preferred embodiment shown in FIG. 1, the diameter of the terminal housings is approximately 50 mm; the length is approximately 250 mm.  
         [0025]    Connected at one end to the inside end of male terminal housing  10  is a first spring  30  which extends along the longitudinal axis of the tool in the direction of the female end terminal housing  20  but the other end of which connects to one end of a piston assembly  40 . A second spring  50  is connected at one end to the inside end of female housing  20  and extends longitudinally in the direction of the inside end of male terminal housing  10 , connecting to the other end of piston assembly  40 . Thus, the piston assembly  40  is supported between the two terminal housings  10  and  20  by the springs  30  and  50  so that the direction of movement of the piston is substantially aligned with the longitudinal axis of the tool.  
         [0026]    As is clearly seen from FIG. 1, the terminal housings are each capable of independent movement in relation to each other. They are coupled together through the first and second springs, but are not joined by any rigid member. The springs keep the terminal housings separated from each other in the fluid in the borehole, and prevents them from touching during use.  
         [0027]    The male end terminal housing  10  of the shock absorbing tool  2  will next be described in more detail with reference to FIG. 2.  
         [0028]    Male end terminal housing  10  comprises a cylindrical casing  102  which tapers at one end to bear a male type mechanical connector  104 . This connector is so designed as to be accommodated within a female type mechanical connector of a neighbouring tool in the tool string to form a secure mechanical connection. Such a female type mechanical connector will be described later with reference to the female end terminal housing.  
         [0029]    An electrical socket  106  is disposed within mechanical connector  104  and is designed to receive a corresponding electrical jack disposed in a female end terminal connector to form a secure electrical connection between the shock absorber tool and the neighbouring tool in the tool string. A wire  108  passes from electric socket  106  through pressure barrier  110  to cable  60 . Pressure barrier  110  is housed in barrier housing  120  formed within the cylindrical casing and provides a pressure resistant barrier to shield the electric socket  106  and wire  108  from the high pressure ‘wet’ environment in the bore hole. Pressure barrier  110  may be accessed by an access panel  122 . Cable  60  runs from the pressure barrier  110  and carries wire  108  along the length of the shock absorber tool to the female end terminal housing  20 . As described below, wire  108  then connects to an electrical jack in the female end terminal housing in order to complete an electrical path through the shock absorbing tool. Such a path may be used for the transmission of electrical power to tools in the tool string, for the transmission of sensor data back from the sensor tools in the tool string, or even for powering detonating guns.  
         [0030]    Some slack in the electric cable  60  is accommodated within barrier housing  120  to provide leeway for extension of the springs  30  and  50 .  
         [0031]    The first spring  30  is received within a spring mounting  112  located in the end of the cylindrical casing opposite that which bears the mechanical connector  102 . Male end terminal housing  10  is also provided with a hollow longitudinal bore  114  which is accessible from the side via an access panel  116 . Access panel  116  is received within a recess in the cylindrical casing of the male end terminal so that it lies flush with the housing surface. A stranded steel rope  70  is attached to a sliding anchor or mount assembly  72  disposed in the hollow bore  114  and able to slide along the length of the bore. Rope  70  passes out of the male end terminal housing through an aperture  118 .  
         [0032]    The piston assembly  40  will next be described in detail with reference to FIG. 3.  
         [0033]    Piston assembly  40  comprises a cylindrical piston casing  402  and a plunger  404 . Plunger  404  has a piston rod  406  which fits inside piston casing  402  and a plunger head  408  which remains outside. Piston  418  is formed integral to piston rod  406  and is located within the piston casing  402 . Piston casing  402  has a cap  410  through which the piston rod passes via a central aperture.  
         [0034]    First spring  30  is received within spring mounting  412  disposed within the end of the piston casing opposite the cap  410 . Second spring  50  is similarly received in a spring mounting  414  disposed within plunger head  408 .  
         [0035]    The diameter of piston rod  406  is smaller than that of the interior  416  of the piston casing  402 . A central region of the piston rod has a ring or flange of greater diameter than the rest of the rod, which makes contact with the interior of the casing, and which forms piston  418 . A securing pin  420  passes through a slot-shaped aperture  422  in the piston casing and is received in the piston  418  to prevent rotation of the plunger with respect to the piston casing. Rubber plugs  424 , preferably made of Viton (RTM), are disposed in the annular recess formed where the diameter of the plunger is substantially less than that of the casing interior  416 , and bear against a transverse wall at one end and the cap  410  at the other so as to provide a resistive force which opposes movement of the piston and plunger in either longitudinal direction. Although rubber plugs are preferred, the annular recess may alternatively be filled with any open-cell polyurethane foam, or suitable fluid.  
         [0036]    Electric cable  60  and rope  70  pass through the centre of the piston assembly via apertures in the piston casing  402  piston rod  406  and plunger head  408 .  
         [0037]    The female end terminal housing will next be described in detail with reference to FIG. 4.  
         [0038]    Female end terminal housing  20  comprises a cylindrical casing  202  with a mechanical connector, being a socket  204 , at one end to receive a male mechanical connector of a neighbouring tool in the tool string. It is otherwise generally similar to the male end terminal housing  10 . Disposed inside socket  204  is an electrical jack  206  designed to connect with the electrical socket of the male end terminal housing of the neighbouring tool in the tool string to form a secure electrical connection. Wire  208  is connected to electrical jack  206  and passes through pressure barrier  210  to cable  60 . It will be appreciated that wire  208  is equivalent to wire  108  in the male end terminal housing.  
         [0039]    Second spring  50  is received in the female end terminal housing in a spring mounting  212  disposed at the opposite end of the housing to the socket  204 . Stranded steel rope  70  passes from the male end of the tool, through piston assembly  40  and into the female end housing via an aperture  214 , and is held in place in a retaining groove  216  in the cylindrical casing  202  by a retaining pin  218 . The rope  70  which is on the opposite side of the retaining pin  218  to the first aperture  214  terminates in a second aperture  220  in the cylindrical casing of the female end housing.  
         [0040]    The operation of the shock absorbing tool will now be described in detail with respect to the drawings.  
         [0041]    The shock absorbing tool is preferably connected to the tool string on one side of a perforating gun by means of the mechanical connector  104  or socket  204  disposed on the end housings of the tool, so that any sensor tools or other sensitive apparatus may be isolated from the perforating gun. A second like shock absorber may be connected to the opposite side of the gun, or at any other position in the tool string as desired.  
         [0042]    Referring now to FIG. 1, the operation of the device will be described in the case where the shape charges are detonated on a perforating gun connected directly to the male end terminal housing  10  of the device. Detonation of the shape charges will produce a brief but powerful explosion, resulting in a shock wave of high and low frequencies travelling along the shock absorbing device from the male end housing  10  in the direction of the female end housing  20 . The movement of the shock wave will push the perforating gun and male end housing  10  against the central piston assembly  40 , thereby compressing first spring  30 . Spring  30  subsequently acts upon central piston assembly  40  causing it to move in the direction of the shock wave and thereby compressing second spring  50  against the female end housing  20 . In comparison to piston assembly  40 , however, female end housing  20  is not as free to move since it is connected to the rest of the tool string behind it. The mass of the piston assembly  40  is small in comparison with the rest of the tool string, and the collision of the piston assembly with the female end housing is for the most part elastic, due to spring  50 . Therefore the piston assembly  40  rebounds back from the female end housing so as to travel in the opposite direction to the shock wave with only a small amount of the energy of the piston assembly being transmitted to the female end housing  20 . Most of the energy is reflected back to travel back in the opposite direction.  
         [0043]    The rebounding piston assembly now travels in the direction of the male end terminal housing  10 , causing, as it does so, the first spring  30  to be compressed and the second spring  50  to be extended. As before, the mass of the male end housing and the tool string to which it is connected is much greater than the mass of the piston assembly and the piston assembly is reflected back towards the female end terminal housing with only a small percentage of the shock wave energy being transmitted to the male end housing. Thus, the piston assembly  40  oscillates backwards and forwards between the female end housing and the male end housing at a natural frequency dependent on the properties of the springs and the piston assembly. The frequencies of the shock wave which correspond to an integer multiple of the natural frequency of the oscillator are readily absorbed by the motion of the oscillating piston assembly and may dissipate gradually in the fluid surrounding the piston assembly and springs. The remaining frequencies from the explosion are transmitted along the tool string little by little each time the piston assembly collides with one of the terminal ends.  
         [0044]    The piston assembly is also free to move in a radial direction as well as a longitudinal direction, since it is supported between the two terminal end housings by means only of first and second springs  30  and  50 , allowing the oscillation of the piston assembly to absorb both longitudinal and radial frequency components from the shock wave.  
         [0045]    Although a simple mass suspended between the first and second spring  30  and  50  would form an adequate oscillator, and is contemplated within the scope of this invention, the preferred embodiment of the shock absorbing tool comprises a piston assembly  40  suspended between the two springs. A shock wave acting to accelerate the mass on the springs between the two terminal end housings must therefore also act to compress the piston in the piston casing or expand it. The use of deformable rubber plugs disposed in the piston casing between the casing interior  416  and the piston rod  406  are acted upon by the piston  418  whenever the plunger is compressed and whenever it expands and therefore provide a further dampening force which acts to absorb or dissipate the shock wave. The presence of the piston assembly is advantageous since it acts to absorb a broad range of frequencies in the shock wave, not just those that correspond to integer multiples of the natural frequency of the spring oscillator.  
         [0046]    The shock absorbing system described above, comprising two springs and a central floating mass or piston assembly, provides a number of advantages that prior art systems of single springs or single pistons do not. In particular single pistons suffer from an inability to react quickly enough to absorb the shock, with the piston assembly itself and any fluid in the piston assembly behaving almost like a solid for the transmission of the shock wave through the tool. In the present system there is no piston or plunger-like connection between the piston assembly and either of the terminal housings  10  and  20 . The sole mechanical connection is through the springs  20  and  50 . Consequently the preferred shock absorbing tool provides a free oscillator that can respond quickly to shocks. Furthermore, during its operation the oscillating element is subjected to less stress and pressure than known shock adsorbing tools, since having absorbed the shock it may re-transmit it in a controlled manner over time as it oscillates. Rather than absorbing the energy completely, the free oscillator acts to delay transmission of the energy along the tool string, and allows that transmission to be controlled.  
         [0047]    The overall attenuation to the shock wave provided by the preferred shock absorber is the product of two attenuations: one from the action of the first spring  30  on the mass or piston assembly  40 , and one from the second spring  50  onto a second mass, ie: the female end housing, if the shockwave is travelling from left to right. Each spring and mass acts as a low pass filter to the frequencies contained in the shock wave. Further spring and mass pairs may be added to the shock absorber to improve the attenuation of the shock wave as desired.  
         [0048]    The preferred shock absorbing tool also comprises a stranded steel rope  70  attached to the sliding mount  72  within hollow bore  114  of male end terminal housing  10 . The rope is fixed in place by the retaining pin  218  in the female end terminal housing  20 . The sliding mount assembly accommodates some increase in distance between female end terminal housing and male end terminal housing as springs  30  and  50  expand, but limits that increase so that the springs are not extended beyond their elastic limit.  
         [0049]    A tool string may be deployed in or retrieved from a bore hole by applying a longitudinal force to the wireline on which the tool string is suspended. Ordinarily the force that the wireline must support is of the order of 500 lbs. However, if the perforating guns, or indeed the tool string on which they were mounted, were to become jammed or stuck in the bore hole greater forces would be required to free them. These forces may range from 1000 lbs to 6000 lbs. Forces of such magnitude are likely to damage the first and second springs  30  and  50  of the shock absorbing tool by causing them to extend beyond their elastic limit or yield point. Thus, steel rope  70 , capable of bearing such forces, is provided, in order to protect the springs. The length of the rope is such that it will take up the force on the wireline before the springs extend too much and become strained.  
         [0050]    A stranded steel rope is preferred as it provides a poor helical shock path for transmission of the energy in the shock wave. Electric cable  60  and steel rope  70  pass along the inside of springs  30  and  50  via apertures provided in male end terminal housing, piston assembly and female end terminal housing.  
         [0051]    Although the preferred embodiment of the invention has been described for use with a perforating gun it will be appreciated that it may be used in other down hole or pipeline applications to absorb shocks caused in other ways.  
         [0052]    Many modifications may be made to the shock absorbing tool described and illustrated purely by way of example.