Apparatus for generating hydraulic shock waves in a well

Apparatus for electrically generating hydraulic shock waves in a fluid bearing formation (14) of a bore hole (10) including a tool (16) having an upper electrode (42) and a lower electrode (46) forming a spark gap therebetween. The tool (16) has an upper end portion (40) comprised of a plurality of sections (40A-40E) threaded to each other in end to end relation and an armored electrical cable (18) having an outer metal sheath (34) is anchored to the uppermost section (40A). The tool (16) is particularly adapted for fitting within a casing having a four inch diameter and the electrodes (42,46) have diverging planar end surface portions (84) coated with tungsten carbide to provide wear resistant surfaces.

BACKGROUND OF THE INVENTION 
This invention relates to an apparatus for generating hydraulic shock waves 
in an oil, gas, or water bearing formation, and more particularly to such 
apparatus including a tool that may be lowered within a well from an 
electrical cable and provide an electrically generated shock wave for the 
recovery of fluids, such as crude oil, gas or water. 
Heretofore, various processes and apparatus have been provided for the 
recovery of crude oil by means of sonic wave generations. The generation 
of a sonic wave front at or near an oil bearing formation provides sonic 
energy projecting a substantial distance into the formation for 
stimulating the recovery of oil from the formation particularly at 
adjacent wells. Sonic waves may be produced from shock waves resulting 
from the release of electrical energy adjacent the oil bearing formation 
and it is desirable to release substantial electrical energy in order to 
generate the intensity of shock or pressure waves required for a 
substantial recovery of oil or gas. Some of the disadvantages associated 
with the use of sonic and ultrasonic energy at both high and low frequency 
ranges are (1) high frequency waves attenuate too quickly in the formation 
and (2) low frequency waves at high amplitudes release debris which tends 
to restrict flow around the wellbore. 
Electrical energy has been previously employed for the generation of shock 
waves for the recovery of oil. For example, U.S. Pat. No. 4,074,758 dated 
Feb. 21, 1978 shows a method and apparatus utilizing an electrical energy 
storage capacitor bank to provide electrical energy to a shock wave 
generator adjacent an oil bearing formation and having a pair of discharge 
electrodes forming a spark gap therebetween. An electrohydraulic shock 
wave is generated by liquid in the well from a shock wave generator 
lowered within the casing to a desired depth. 
U.S. Pat. No. 4,345,650 dated Aug. 24, 1982 also shows a method and 
apparatus utilizing electrical energy to provide a shock wave at the oil 
bearing formation. However, the '650 patent utilizes a tool having an 
electrical energy storage capacitor bank therein adjacent the electrodes 
in the borehole to provide sufficient electrical energy. A relatively 
complex downhole tool is required, however, in order to house the storage 
capacitor bank and associated equipment shown in this patent for 
generating the explosive spark across the electrodes. 
As well known, when electrical energy of a high voltage, such as 75000 
volts or higher, is required downhole, power surges occur in the system 
from electrical energy flowing both down into the borehole and up from the 
borehole. Such power surges oftentimes result in overloading of the system 
and actuation of overload safety devices. The '650 patent minimizes this 
problem by locating the storage capacitor bank downhole. 
SUMMARY OF PRESENT INVENTION 
The present invention is directed to an apparatus including a tool 
containing two electrodes and particularly designed for utilizing 
electrical energy from a storage capacitor bank at a surface location. The 
tool is designed for the flow of electrical energy to the electrodes as 
high as 70,000 volts for example, and has been found to function 
effectively with minimal power surges occurring in the associated system. 
Additionally, the tool is designed to have a diameter of around three and 
one-half (31/2) inches for easily fitting within a four (4) inch casing 
which is a common size of casing employed for wells. 
The tool is lowered within a borehole to a desired depth beneath a liquid 
fluid, such as crude oil, for firing an explosive charge of electrical 
energy into an adjacent oil bearing formation upon the generation of spark 
across the gap between the electrodes. The storage capacitor bank is 
located at a surface location and an electrical cable supporting the tool 
extends between the tool and capacitor bank for transmitting the 
electrical energy to the tool. 
The firing of the charge of electrical energy between the electrodes 
results in energy advancing from the electrodes is two forms, a shock wave 
and a hydraulic wave. The shock wave is transmitted through the formation 
at a relatively rapid rate and tends to separate crude oil from the 
formation. The hydraulic wave follows the shock wave with the advancing 
front of the hydraulic wave defining a sphere and tending to push 
outwardly any separated liquid, such as crude oil. Controls at the surface 
location are set for a desired voltage, amplitude, and frequency depending 
on such factors as the type of formation and viscosity of the liquid being 
removed. 
The present system and tool may also be employed for such purposes such as 
fracturing a formation, opening of plugged perforations in a screen or the 
like, and cleaning of downhole screens without removal from the borehole. 
The generally cylindrical tool body includes an upper body portion for 
housing the positive electrode and a lower body portion for housing the 
negative electrode. The upper body portion includes a plurality of 
sections threaded to each other in end to end relation. The electrodes are 
arranged in end-to-end relation and define a gap therebetween adjacent one 
side of the electrodes. The end surfaces of the electrodes adjacent the 
spark gap diverge outwardly from each other and are formed of a tungsten 
carbide surface for minimizing wear and corrosion resulting from the 
firing of the electrical charge. 
The upper body portion comprises four separate sections threaded to each 
other in end to end relation including (1) an upper anchor section for 
anchoring an armored electrical cable, (2) a ground section containing a 
plurality of ground wires arranged in an annular pattern, (3) an upper 
connector section for anchoring the ground wires and transmitting the 
negative electrical field, (4) and a lower connector section for a hand 
wrapped stress relief connection. A preassembled cage assembly for the two 
electrodes in formed from a positive electrode section and a negative 
electrode section which are connected to each other by a pair of spaced 
bars and provide mounting for the two electrodes. The cage assembly has 
female end connections which are easily connected to adjacent sections. 
A large diameter armored electrical cable capable of supporting a tensile 
load of 90,000 pounds extends between the storage capacitor bank at a 
surface location and the tool. The cable has an outer metal sheath secured 
to the anchor section of the upper body portion for anchoring the cable. A 
plurality of ground leads from the electrical cable are spaced about the 
cable in the ground section and have their ends connected to a steel ring 
for transmitting the negative electrical field to the lower electrode. 
It is an object of the present invention to provide an apparatus for 
generating electrical shock waves including a downhole tool having a pair 
of electrodes for firing an explosive charge of electrical energy into an 
adjacent fluid bearing formation from electrical energy supplied from a 
storage capacitor bank at a surface location thereby to provide a 
hydraulic shock wave. 
It is a further object of the invention to provide such a tool having a 
pair of electrodes arranged end to end to define the spark gap on one side 
of the electrodes with adjacent end surfaces diverging from the spark gap 
and formed of a wear and corrosion resistant surface. 
A further object of this invention is to provide a small diameter 
electrical tool for fitting within a four inch casing and assembled from a 
plurality of sections threaded to each other in end to end relation and 
including a preassembled cage assembly for the electrodes. 
Other objects, features, and advantages of this invention will become more 
apparent after referring to the following specification and drawings.

DESCRIPTION OF THE INVENTION 
Referring now to the drawings for a better understanding of this invention 
and more particularly to FIG. 1, a borehole is shown generally at 10 
extending from the surface 12 of an earth formation having an oil bearing 
stratum or layer at 14. A tool generally indicated at 16 and forming an 
important part of this invention is shown downhole adapted to provide a 
hydraulic shock wave from the discharge of explosive electrical energy 
through the fluid bearing layer 14 of a producing zone, such as oil or 
gas. 
An armored electrical cable 18 supported from a derrick 20 is connected to 
tool 16 and controlled from a storage reel 21 in a logging truck 22. Truck 
22 includes a source of electrical energy provided by generator 24 
connected to power control circuitry 26 and fire control circuitry 28 for 
tool 16. A suitable control panel is provided so that an operator in truck 
22 may control the operation of tool 16. Power control circuit 26 includes 
a capacitor bank 30 having a predetermined number of large magnitude 
capacitors 32 electrically connected to cable 18 and tool 16. Thus, the 
release of electrical energy from tool 16 is controlled by an operator 
from truck 22 for the producing of shock waves and resulting hydraulic 
waves from the oil for migration of the oil away from bore 10. 
Referring now to FIGS. 2A, 2B and 3, tool 16 is shown connected at its 
upper end to an armored cable 18. An armored cable 18 includes an outer 
metal sheath 34 formed of counter wound metal layers, an adjacent outer 
insulating layer or lines 36 of a plastic insulating material 
encapsulating a plurality of spaced ground leads 39, an inner insulating 
layer 37 adjacent layer 36, and a central copper conductor core 38 
surrounded by insulating layer 37. Tool 16 is of a generally cylindrical 
shape and has a maximum diameter less than around 33/4 inches for easily 
fitting within a casing of four (4) inches in diameter. Tool 16 includes 
an upper end body portion generally designated 40 for housing an upper 
positive electrode generally designated 42, and a lower end body portion 
generally designated 44 for housing a lower negative electrode generally 
designated 46. A pair of connecting ribs or bars 48 are secured between 
body portions 40 and 44 for connecting upper body portion 40 to lower body 
portion 44 and to transmit the negative electrical field to negative 
electrode 46. Electrodes 42 and 46 are of substantially the same shape. It 
is noted that a cage assembly 47 is preassembled from a pair of spaced 
electrode sections connected by the small diameter bars 48 to mount 
electrodes 42, 46 is a precise relation for generating a hydraulic shock 
wave of tool 16 about substantially the entire periphery as will be 
explained further. 
Upper end portion 40 comprises an outer mandrel body having five threaded 
tubular sections 40A, 40B, 40C, 40D and 40E connected to each other in 
end-to-end threaded relation Section 40A forms an anchor section, section 
40B forms a ground section, section 40C forms a connector section, section 
40D forms a stress relief section, and section 40E forms a positive 
electrode section. The counter wound metal sheath 34 is stripped from the 
remainder of cable 18 and turned up to fill the void annular space 50 
within uppermost anchor section 40A which has an upper tapered end 49. The 
turned up end portions of metal sheath 34 fits tightly within the annular 
space and bears against tapered end 49 for anchoring cable 18 thereat. 
Moisture proof seals or rings 54 formed of an elastomeric material are 
mounted about outer insulating lines 36 within ground section 40B for 
sealing thereabout. Ground leads 38 have their ends extending from 
encapsulating liner 36 and clamped by set screws between inner and outer 
conductor rings 57,58 mounted in the upper end of connector section 40C. 
If desired, a single conductor ring may be provided with spaced openings 
to receive the ends of leads 39 with set screws tightly clamping leads 39 
within the ring. 
Conductor 38 has its lower end secured by set screws 60 within a receiving 
socket in upper end portion 62 of upper electrode 42 prior to the 
connections of section 40D and subassembly 47, and prior to the connection 
of shank 66 of lower end portion 64 within a bore in upper end portion 62 
of upper electrode 42 by set screws 68. 
Inner insulating layer 37 in connector section 40C is formed of a 
semiconductor material such as black tape and extends to electrode 42. 
Electrode 42 includes an upper end portion 62 and a lower end portion 64. 
Insulation 37 from the lower end of section 40D to its lower end adjacent 
electrode 42 is hand wrapped with fiberglass tape 59 after the connection 
of conductor 38 to upper electrode portion 62. Next, an insulating sleeve 
70 preferably formed of polytetrafluoroethylene is slipped about upper end 
portion 62. Then, connector section 40D is slipped over sleeve 70 and 
threaded onto section 40C. Next, a split shear ring 72 is mounted about an 
annular groove 73 of insulating sleeve 70 and engages a shoulder on 
section 40D. In this position, subassembly 47 comprising sections 40E and 
44A connected by bars 48 is threaded onto section 40D with shoulder 75 of 
section 40E abutting shear ring 72. Thus, shear ring 72 accurately 
positions insulating sleeve 70 in addition to transferring shear loads 
from section 40E to section 40D through opposed shoulders on sections 40D, 
40E. Additionally, ring 58 may act as a stabilizer between abutting 
sections 40B and 40C. Positive electrode 42 may now be mounted by fitting 
shank 66 of lower electrode portion 64 within the receiving bore of upper 
electrode portion 62 and securing set screws 68. It is noted that shank 66 
should be bottomed in the bore to insure proper electrical transmission. 
While upper electrode 42 is shown formed of two portions primarily for 
ease of assembly, it is to be understood that upper electrode 42 could be 
formed of a single member if desired. Electrodes 42 and 46 are preferably 
formed of a silicon bronze material. 
Lower end portion 44 of tool 16 which is connected to upper end portion 40 
by rods 48 includes threaded sections 44A and 44B connected in end-to-end 
relation. Lower section 44B has a socket or end bore 72 receiving the end 
shank 74 of negative electrode 46. Set screws 76 in section 44A secure 
lower electrode 46 in position within lower end portion 44 and permit 
horizontal and longitudinal adjustment of electrode 46. A slot shown in 
broken lines at 78 is provided in section 44A to permit lower electrode 46 
to be easily removed from end portion 44 without disassembly of sections 
44A and 44B. A spark gap shown at G in FIG. 2B between the opposed ends of 
electrodes 42 and 46 is set for arcing at a predetermined voltage for 
firing of a charge or electrical energy to produce a shock wave in oil 
bearing layer 14. Gap G for example may be 1/4 inch. The ends of sections 
40E and 44A adjacent electrodes 42, 46 are of a hexagonal shape in plan as 
shown in FIG. 3 and include grooves for receiving and easily securing bars 
48 while permitting the sections to easily fit within a four inch casing 
for which tool 16 is particularly designed. Normally, tool 16 would be 
utilized at a location within a liquid fluid to provide a hydraulic shock 
wave. 
The enlarged end portions 64 and 80 of respective upper and lower 
electrodes 42,46 are generally identical. Referring to FIGS. 3 and 4, the 
shape of enlarged end portion 64 of electrode 42 is shown as comprising a 
generally flat or planar end surface 81 having a flat surface tip 82 at 
one side of the electrode and diverging from tip 82 at an angle of around 
45.degree. with respect to the longitudinal axis of tool 16 which is in 
alignment with the longitudinal axis of shank 66. Planar end surface 81 
including surface tip 82 is of tungsten carbide to provide an abrasion 
resistant and corrosion resistant surface. End portion 64 has a thickness 
of around 5/8 inch and diverging end surfaces 81 on electrodes 42,46 
direct the shock wave to form a strong shock wave in the direction of end 
surfaces 81 although the shock wave is directed about substantially the 
entire perimeter of tool 16 except for the areas blocked by rods 48 which 
are 3/4 inch in diameter. 
An operator in truck 22 has suitable controls for determining the voltage 
amplitude and frequency for firing a charge of electrical energy across 
electrodes 42 and 46 into the oil bearing formation 14. The tool 16 of 
this invention provides a housing for electrical cable 18 and electrodes 
42,46 which may be easily assembled from a minimal number of separate 
members. A relative large copper conductor 38 around 3/8 inch in diameter 
is carried by cable 18 and is electrically connected to upper electrode 
42. Ground leads 39 carried by cable 18 are electrically connected to 
lower negative electrode 42 through ground ring 58, sections 40C, 40D, 
40E, and bars 48 to sections 44A and 44B. 
For assembly, the separate body sections 40A-40D are connected to each 
other in sequence beginning with anchoring of section 40A to armored cable 
18 by the stripping and turning up of outer metal sheath 34 within section 
40A and the tight gripping of the turned up metal sheath by tapered end 
49. Next, seal 52 is inserted about layer 36 within anchor section 40A and 
ground section 40B is threaded onto anchor section 40A with seals 54 and 
56 about layer 36. The end of ground wires 39 extend outwardly from layer 
36. 
Next, the ends of ground wires 39 are clamped between rings 57, 58 by set 
screws 59 and connector section 40C is then threaded onto ground section 
40B, with section 40C receiving rings 57,58. As indicated previously, 
ground wires 39 may be clamped within openings in a single ring if 
desired. Conductor 38 is then connected to upper electrode portion 62 by 
set screws 60. Next fiberglass tape 59 is hand wrapped about insulating 
sleeve 37 and an adjacent end portion of electrode 42. Insulating sleeve 
70 is fitted about electrode portion 62. Then, section 40D is threaded 
onto section 40C. Split ring 72 is then mounted and the previously 
assembled cage subassembly 47 is threaded onto section 40D. Next, 
electrode portion 64 is mounted by set screws 68 and lower end section 44B 
is threaded onto section 44A. Now, lower electrode 46 may be inserted and 
spark gap G determined by adjustment of set screws 76. It is to be 
understood that suitable O-ring seals not shown in some instances are 
provided as seals between all of the connected sections 40A-40E. 
As a non-limiting example, tool 16 may be of a diameter of three and 
one-half (31/2) inches and of a length of around eight (8) feet with upper 
tool portion 40 being around five (5) feet long and lower tool portion 44 
being around three (3) feet long. Cable 18 and the control panel with 
associated circuiting 26, 28 are adapted to carry around 70,000 volts with 
cable 18 capable of supporting a load of 90,000 pounds. 
While a preferred embodiment of the present invention has been illustrated 
in detail, it is apparent that modifications and adaptations of the 
preferred embodiment will occur to those skilled in the art. However, it 
is to be expressly understood that such modifications and adaptations are 
within the spirit and scope of the present invention as set forth in the 
following claims.