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CROSS REFERENCE TO RELATED APPLICATIONS 
     Not Applicable 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a method of improving the permeability of an earth formation zone surrounding a wellbore formed in the earth formation. In the practice of producing hydrocarbon fluid from an earth formation via a wellbore to a production facility at surface, a perforated casing or liner is generally installed in the wellbore. The hydrocarbon fluid flows via the pores of the formation towards the casing or liner and via the perforations thereof into the wellbore. 
     BRIEF SUMMARY OF THE INVENTION 
     A problem frequently encountered is that the permeability of the earth formation is relatively low resulting in reduced production capacity of the wellbore. One cause of such reduced permeability is the presence of formation illite in the pores-. Formation illite is a clay mineral which partially occupies the interstices between the rock particles. The presence of illite in the form of needles or platelets significantly reduces the ability of hydrocarbon fluid to flow through the pores. 
     It is an object of the invention to provide a method of improving the permeability of an earth formation zone surrounding a wellbore formed in the earth formation. 
     In accordance with the invention there is provided a method of improving the permeability of an earth formation zone surrounding a wellbore formed in the earth formation, the method comprising 
     pumping a selected liquid via the wellbore into said earth formation zone so as to create a body of liquid extending into the wellbore and into the pores of said zone; 
     lowering a shock wave generator into the body of liquid in the wellbore; and 
     inducing the shock wave generator to generate a shock wave in the body of liquid. 
     It is thereby achieved that the shock wave travels through the pores of the formation where the body of liquid is present and thereby destroys the illite particles present in the pores. 
     The invention will be described further in more detail and by way of example with reference to the accompanying drawings in which 
    
    
     BRIEF SUMMARY OF THE DRAWINGS 
     FIG. 1 schematically shows an embodiment of a wellbore used in applying the invention; 
     FIG. 2 schematically shows a device for use in the embodiment of FIG. 1; 
     FIG. 3 schematically shows a first alternative device for use in the embodiment of FIG. 1; and 
     FIG. 4 schematically shows a second alternative device for use in the embodiment of FIG.  1 . 
     In the drawings like reference numerals relate to like components. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1 there is shown a wellbore  1  formed in an earth formation  2  having a hydrocarbon fluid reservoir  3 , the wellbore being provided with a casing  4  fixed in the wellbore  1  by a layer of cement  6 . The casing  4  is provided with a plurality of perforations  8  at the level of the hydrocarbon fluid reservoir  3 . An upper packer  10  is arranged in the casing above the perforations  8 , and a lower packer  12  is arranged in the casing below the perforations  8 . An electric cable  14  extends from a control facility  16  at surface through the casing  4  and through an opening (not shown) provided in the upper packer  10  to a shock wave generator  18  arranged in the space  20  between the packers  10 ,  12 . The space  20  is filled with a body of brine  22  which extends via the perforations  8  into the hydrocarbon fluid reservoir  3  up to an interface  24  with the hydrocarbon fluid present in the hydrocarbon fluid reservoir  3 . 
     In FIG. 2 is shown in more detail the shock wave generator  18  including a tubular housing  24  formed of a first tubular part  26  and a second tubular part  28  connected to the first tubular part  26  by a screw connection  30  whereby a shear disc  32  is biased between the first and second tubular parts  26 ,  28 . The first tubular part is provided with an end cap  34  and a plurality of openings  36 . The second tubular part is closed by a plug assembly  38  screwed in the second tubular part by means of screw connection  40 . The plug assembly  38  is provided with a bore  42  in which an ignition device  44  connected to the electric cable  14 , is arranged. A charge of deflagrating material  46  is arranged in the second tubular part  28 , between the ignition device  44  and the shear disc  32 . 
     In FIG. 3 is shown a first alternative shock wave generator  47  which is substantially similar to the embodiment of FIG. 2, the difference being that the shear disc  32  forms a primary shear disc and that each opening  36  is provided with a secondary shear disc  48 . 
     In FIG. 4 is shown a second alternative shock wave generator  49  which is substantially similar to the embodiment of FIG. 2, except that the plug assembly, the ignition device and the deflagrating charge have been replaced by a piston assembly  50  including a cylinder  51  in the form of second tubular part  28  and a piston  52  arranged in the cylinder  51 . The piston  52  is movable relative to the cylinder  51  in the direction of the shear disc  32  so as to compress a body of gas  54  present between the piston  52  and the shear disc  32 . The piston assembly  50  furthermore includes a plug  55  screwed into the cylinder  51  and provided with a central bore  56  having an internal shoulder  58 . A spring assembly  60  is arranged between the piston  52  and the plug  54 , the spring assembly  60  being compressed by a threaded tie rod  62  at one end thereof connected to the piston  52  and at the other end thereof extending through the bore  56  and being retained at internal shoulder  58  by an explosive nut  64  connected to the electric cable  14 . 
     During normal operation brine is pumped into the wellbore, the brine flowing via the perforations  8  into the hydrocarbon fluid reservoir  3 . Pumping is stopped after a selected quantity of brine has flown into the hydrocarbon reservoir  3  so that the body of brine  22  is formed. Next the lower packer  12 , the shock wave generator  18 , the upper packer  10  and the electric cable  14  are installed in the wellbore  1 . 
     The shock wave generator  18  (shown in FIG. 2) is then activated by transmitting a selected electric signal through the cable  14 , which signal induces the charge of deflagrating material  46  to detonate. As a result the pressure in the second tubular part  28  rises to a level at which the shear disc  32  shears. Upon shearing of the shear disc  32 , a shock wave occurs in the first tubular part  26  which travels through the openings  36  into the part of the body of liquid  22  present in the wellbore  1 , and from there via the perforations  8  into the part of the body of liquid present in the hydrocarbon fluid reservoir  3 . As the shock wave travels through the pores of the earth formation, the illite particles present in the pores are destroyed by the shock wave. This effect is even enhanced by reflection of the shock wave at the interface  24 . 
     Normal operation using the first alternative shock wave generator  47  is similar to normal operation using the shock wave generator  18 , except that additionally the secondary shear discs  48  are sheared off upon the occurrence of the shock wave in the first tubular part  26 . 
     Normal operation using the second alternative shock wave generator  49  is similar to normal operation using the shock wave generator  18 , except that the pressure rise in the second tubular part is now created by transmitting a controlled electric signal through the cable  14  in order to detonate the explosive nut  64 . Upon detonation of the nut  64 , the tie rod  62  breaks thereby inducing the spring assembly  60  to move the piston  52  in the direction of the shear disc  32  and to compress the body of gas  54 . As a result the pressure in the second tubular part  28  rises to the level at which the shear disc  32  shears. 
     It will be appreciated that the shock wave generation characteristics of the embodiments of FIGS. 2,  3  and  4  are mutually different, therefore either of these embodiments can be selected in accordance with the required characteristics. 
     Any suitable water- and pressure proof deflagrating material can be selected for the charge of deflagrating material, for example RDX (1,3,5 Trinitro- 1,3,5 triazacyclohexane).

Summary:
A method for improving the permeability of an earth formation zone surrounding a wellbore formed in the earth formation utilizing hydraulic shockwaves. The selected zone is isolated and fluid is pumped downhole to fracture the earth formation, the fluid extending into the fractures. A shock wave is then created in the fracturing fluid to reduce the presence of illite clays in the formation interstices.