Patent Publication Number: US-6705085-B1

Title: Downhole electric power generator

Description:
FIELD OF THE INVENTION 
     1. Background of the Invention 
     The present invention relates to a power generator for use in a wellbore formed in an earth formation. The purpose of such power generator is, for example, to provide electric power to electrical wellbore equipment, to charge a battery for powering such equipment, or to create an electric charge or discharge in or around the wellbore. However, application of a conventional power generator in a wellbores is impractical or impossible in view of the relatively small diameter of the wellbore, particularly in the deeper sections of the wellbore. Furthermore, the installation of temporary power cables in a wellbore is difficult and expensive. 
     It is an object of the invention to provide a suitable power generator for use in a wellbore formed in an earth formation. 
     In accordance with the invention there is provided a power generator for use in a wellbore formed in an earth formation, comprising an internal combustion engine having a cylinder and a piston defining a combustion chamber in the cylinder, the engine being arranged to induce a reciprocating movement to the piston relative to the cylinder upon combustion of a combustible gas mixture in the combustion chamber, and a linear electricity generator having a stator and a drive shaft, the generator being arranged to generate electricity upon a reciprocating movement of the drive shaft relative to the stator, wherein the piston is connected to the drive shaft so as to transmit said reciprocating movement of the piston to the drive shaft. 
     SUMMARY OF THE INVENTION 
     The power generator can have a relatively small diameter so that the generator fits in the wellbore, by virtue of the movement of the piston and the drive shaft being a reciprocating movement. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be further described in more detail and by way of example with reference to the accompanying drawings in which 
     FIG. 1 schematically shows an embodiment of the power generator according to the invention; 
     FIG. 2 schematically shows in inlet valve of the embodiment of FIG. 1; and 
     FIG. 3 schematically shows an exhaust of the embodiment of FIG.  1 . 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1 there is shown a power generator  1  for use in a wellbore (not shown) formed in an earth formation (not shown). The power generator  1  includes an internal combustion engine  4  and a linear electricity generator  6  having a common longitudinal axis coinciding with, or parallel to, the longitudinal axis of the wellbore. 
     The engine  4  comprises a housing  7  provided with a cylinder  8  and a piston  10  extending into the cylinder  8  and being movable relative to the cylinder  8  in longitudinal direction thereof. A drive rod  12  connected to the piston  10  extends in longitudinal direction to the linear electricity generator  6 . The cylinder  8  is at the end thereof opposite the drive rod  12  closed by an end wall  14 , thereby defining a combustion chamber  16  formed in the cylinder  8  between the piston  10  and the end wall  14 . A compression spring  17  biased at one end thereof against a circular plate  16  fixedly connected to the drive rod  12  and at the other end thereof against an annular shoulder  18  provided in the housing biases the piston  10  in the direction of the end wall  14 . The combustion chamber  16  is provided with a glow plug (not shown) connected to a battery (not shown) for temporarily heating the glow plug. 
     The linear electricity generator  6  includes a stator  22  having a plurality of stator coils  25  and a drive shaft  24  having a plurality of magnets  26  and extending into the stator, the linear electricity generator  6  being arranged to provide an electric potential at power connections  28 ,  30  upon a reciprocating movement of the drive shaft  24  in longitudinal direction relative to the stator  22 . The drive shaft  24  is fixedly connected to the drive rod  12  of the engine  4 . 
     Referring further to FIG. 2 there is shown an inlet valve  32  of the engine  4 . The inlet valve  32  is in fluid communication with an oxygen reservoir  34  via a conduit  36  and with a hydrogen reservoir  38  via a conduit  40 . The oxygen reservoir  34  contains a supply of oxygen at a selected pressure, and the hydrogen reservoir  38  contains a supply of hydrogen at a selected pressure. The inlet valve  32  includes a valve body  42  provided with a disc shaped chamber  44  having a valve seat surface  46  provided with a first opening  48  in fluid communication with the conduit  36 , a second opening  50  in fluid communication with the conduit  40 , and a third opening  52  in fluid communication with an inlet opening (not shown) provided in the wall of the cylinder  8  via a conduit  54 . The position of the inlet opening is such that the piston  10  covers the inlet opening during an initial stage of the combustion stroke, and uncovers the inlet opening during a final stage of the combustion stroke. A membrane  56  divides the disc shaped chamber  44  in a first zone  60  in fluid communication with the respective openings  48 ,  50 ,  52  and a second zone  62  in fluid communication with the combustion chamber  16  via a conduit  64 . The membrane  56  is flexible so as to allow the membrane to lay against the valve seat surface  46  if a fluid pressure in zone  62  exceeds a fluid pressure in zone  60 . 
     In FIG. 3 is shown an exhaust  42  of the engine  4 , the exhaust including an outlet opening  70  formed in the wall of the cylinder  8 . For reference purposes the piston  10  is shown together with the direction of movement  71  of the piston  10  during a combustion stroke thereof. The position of the outlet opening  70  is such that the piston substantially covers the outlet opening  70  during the initial stage of the combustion stroke, and uncovers the outlet opening  70  during the final stage of the combustion stroke. The outlet opening  70  is in fluid communication with an expansion chamber  72  provided with a non-return valve  74  allowing combusted gas to flow from the expansion chamber  72  via the non-return valve  74  to the exterior of the engine  4  and preventing inflow of fluid from exterior the engine  4  into the expansion chamber  72 . The non-return valve  74  includes a passage  76  for combusted gas, which passage  76  is provided with a body of permeable material  78  including sintered steel. 
     During normal operation a stream of oxygen flows from the oxygen reservoir  34  via the conduit  36  into the first zone  60  of the chamber  44  and a stream of hydrogen flows from the hydrogen reservoir  38  via the conduit  40  into the first zone  60 . In said first zone the streams of oxygen and hydrogen mix to form a stream of combustible gas mixture which flows via the conduit  54  into the combustion chamber  16 . Ignition of the gas mixture is achieved by inducing the battery to provide an electric current to the glow plug. Upon ignition of the gas mixture, the piston  10  performs a combustion stroke in the direction of arrow  71  thereby compressing the spring  17  and moving the drive shaft  24  of the electricity generator  6  in longitudinal direction relative to the stator  22 . The piston  10  uncovers the inlet opening and the outlet opening  70  during the final stage of the combustion stroke, thus allowing the combusted gas to flow via the outlet opening  70  into the expansion chamber  72 . The combusted gas expands in the expansion chamber  72  and flows from there via the non-return valve  74  to the exterior of the power generator  1 , thereby passing through the body of permeable material  78 . The non-return valve  74  and the body of permeable material  78  prevent fluid outside the power generator from entering the expansion chamber  72 . 
     As the combusted gas flows out of the combustion chamber  16 , the pressure in the combustion chamber drops to a level below the pressure of oxygen in the oxygen reservoir  34  and hydrogen in the hydrogen reservoir  38 . As a result another stream of oxygen flows from the oxygen reservoir  34  via the conduit  36  into the first zone  60  of the chamber  44  and a stream of hydrogen flows from the hydrogen reservoir  38  via the conduit  40  into the first zone  60 . In said first zone the streams of oxygen and hydrogen mix to form a fresh stream of combustible gas mixture which flows via the conduit  54  and the inlet opening into the combustion chamber  16 . 
     Upon completion of the combustion stroke, the spring  17  induces the piston  10  to perform a compression stroke whereby the piston  10  compresses the combustible gas mixture in the combustion chamber  17 . During the compression stroke the pressure in the combustion chamber  16  rises to a level above the selected pressure of oxygen and hydrogen in the respective reservoirs  34 ,  38 . Consequently the membrane  54  is biased against the valve seat surface  46  thereby closing the openings  48 ,  50 ,  52 . Further inflow of combustible gas mixture into the combustion chamber  16  is thereby prevented. When the piston  10  arrives at the end of the compression stroke the pressure in the combustion chamber  17  is at a level causing the glow plug, which is still hot as a result of the previous combustion cycle, to ignite the combustible gas mixture thereby inducing the piston  10  to perform another combustion stroke. During the initial stage of the combustion stroke, the pressure in the combustion chamber  16  is even higher so that the openings  48 ,  50 ,  52  remain closed during such initial stage. 
     The engine then automatically performs a sequence of combustion cycles, each combustion cycle including a compression stroke followed by a combustion stroke of the piston  10 , as described above. The drive shaft  24  of the linear electricity generator  6  is thereby induced to perform a reciprocating movement, and as a result electric power is generated at power connections  28 ,  30 .