A through-tubing perforating apparatus for use in improving wellbore production. A plurality of shaped charges are interconnected by rigid links that each have eyelets to receive the suspension wire. These links, in conjunction with a guide which initially engages and slides along the suspension wire, keep the shaped charges in a substantially uniform orientation generally perpendicular to the sides of the wellbore. Once the apparatus emerges from the production tubing, a releasable latch interconnecting an upper terminal connector to the supporting wire is actuated permitting the charges, under the influence of the rigid links, to slump under their own weight into contact with the casing walls. This provides maximum penetration into the formation upon actuation of the charges. The expended apparatus can be dropped into the bottom of the wellbore with the aid of a sinker bar.

BACKGROUND AND SUMMARY OF THE INVENTION 
The present invention is directed to apparatus for perforating a wellbore. 
More particularly, the present invention is directed to perforating 
apparatus that can be run in through the production tubing. 
When perforating a wellbore, the use of an underbalanced technique (i.e., 
without the use of high density muds to control, or overbalance, the 
pressure of the wellbore fluids) has certain advantages. For example, when 
perforating is completed, the rush of produced fluids into the wellbore 
will clean out the newly formed perforations. In an overbalanced method, 
the combination of high density drilling fluid and surface pressure not 
only risks fracturing the reservoir, thereby permitting the trapped fluids 
to escape, but can also block off the perforations. 
In the underbalanced perforating method, a packer and production tubing are 
run in the hole first to permit flow control of produced fluids. 
Accordingly, two types of underbalanced perforating apparatus are 
available: tubingconveyed perforating tools and through-tubing perforating 
tools. Through-tubing charges are necessarily small and, therefore, should 
be shaped to project maximum energy into the formation and efforts should 
be made to minimize dissipation of that energy. These efforts could 
include orienting the charges to fire laterally into the formation, not 
placing any superfluous structure between the charge and the formation, 
and placing the charge immediately proximate the casing wall. Some prior 
art perforating tools hold the through-tubing charges against one side of 
the borehole wall. However, such a perforating pattern known as 0.degree. 
phasing can reduce produced fluids by as much as 10% when compared to 
180.degree. phasing (alternate charges actuatable into opposite sides of 
the formation). 
The present tool allows 180.degree. phasing with maximum formation 
penetration because the charges are pressed firmly against opposite sides 
of the borehole walls. Individual shaped charges are interconnected by 
rigid linkage means. The linkages are of such a length that twice the 
length of the charges extending beyond the connecting points plus the 
length of the linkage exceeds the diameter of the borehole causing the 
charges to be pressed firmly into contact with the wellbore. Longer 
linkage lengths may be used to reduce charge density. The configuration of 
the perforating apparatus is such that the maximum formation penetration 
possible will be achieved by projecting the full force of the charge in a 
direction perpendicular to the wall of the wellbore. Additional charges 
can be easily added as circumstances warrant. The expended charges can be 
released from the wireline and permitted to fall into the bottom of the 
wellbore (i.e., the rathole). A sinker bar can be used to assist in 
conveying the perforating apparatus through the production tubing and in 
assuring that the expended device falls into the rathole. 
Other features, advantages and characteristics of the present invention 
will become apparent after a reading of the following detailed 
description.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
The perforating apparatus of the present invention is shown FIG. 1 
transiting production tubing 11 generally at 10. The lower end of 
production tubing 11 is secured in casing 13 using a conventional 
inflatable packer 15. By using packer 15, the lower end of tubing 11 is 
isolated in wellbore 17 and by the use of a blowout preventer stack and 
associated control valves (not shown) atop tubing 11, the influx of fluids 
into the wellbore following perforation can be satisfactorily controlled. 
Further, this initial influx will clean out the perforations of any loose 
sand, gravel, etc., created in the perforating process. 
Perforating apparatus 10 comprises a plurality of shaped charges 12 each 
having a pair of connecting points (or ears) 14. These connecting points 
14 divide the charge 12 laterally into a first length l.sub.1, and a 
second length l.sub.2. A substantially rigid linkage means 16 
interconnects connecting points 14 of two adjacent charges 12. Linkages 16 
each have a third length l.sub.3. Each linkage means 16 preferably has an 
eyelet 18 that slidably receives suspension wire means 20. While 
suspension wire 20 may be the wireline itself, some advantages regarding 
preassembly attach to the use of a separate wire that can be attached to 
the end of the wireline using swaging, threaded connectors, or the like. 
Eyelets 18 help maintain a substantially uniform orientation of charges 23 
to insure that the force of the explosion will be substantially 
perpendicular to the casing 13 (or wall of wellbore 17, if it is unlined). 
Although only three charges 12 are shown for ease of illustration, it will 
be understood that significantly more charges will be used for the typical 
perforating job. 
The sum of the length l.sub.3 of linkage means 16 and two charge lengths 
l.sub.1 (i.e., the length of charges 12 which laterally augment the 
linkage 16) is at least somewhat greater than the diameter of casing 13. 
This ensures that the charges 12 are pressed firmly against the walls of 
casing 13 as the charges 12 slump downwardly under their own weight after 
emerging from tubing 11, yet cannot slump into contact with one another. 
The length l.sub.3 of linkages 16 can be increased and controlled to 
produce the desired charge density for a particular wellbore perforation 
design. The combination of the lateral orientation of charges 12 and 
firmly pressing the charges against the sides of the wellbore, provides 
maximum penetration into the formation optimizing the lateral reach of the 
perforation operation. 
Termination connector means 22 and 24 attach the uppermost and lowermost 
shaped charges 12 to wireline 20, respectively. Lower termination 
connector means 24 is fixed to wireline 20 as by a ring 25. A small 
explosive charge 26 can be attached to the wireline just above its 
connection to lower termination connector means 24. This charge can be 
actuated by the same Primacord high-speed fuse 28 that is used to detonate 
perforating charges 12. The Primacord 28 is set off by an electrical 
signal transmitted through wireline 20 to its place of connection thereto 
on charge 26. Upper termination connector 22 may have an eyelet 30 
identical to those in linkage means 16. This eyelet 30 may slide freely 
along wireline 20. More preferably, however, eyelet 30 may be releasably 
attached to wireline 20 at 32 by latch means such as a shear pin or solder 
joint, or the like, so that the charges 12 cannot prematurely slump under 
their own weight and become lodged in tubing 11. Further, in this regard, 
sinker bar 34 can be attached to eyelet 30 of lower termination connector 
24 by ring 25 to ensure (a) smooth transition through production tubing 11 
and, (b) that the expended perforating tool 10 will fall into the bottom 
of wellbore 17. Latch means at 32 may be disengaged by a doughnut shaped 
weight 36 dropped down wireline 20. 
One preferred embodiment depicting connection of charges 12 to the 
suspension wire 20 is shown in FIG. 4. Connecting ears 14 may be secured 
to charge 12 by a band which encirles it. The detonator cap 38 receives 
Primacord 28 and is also equipped with a slide guide 40 which slidingly 
engages suspension wire 20. Guides 40 keep charges 12 properly oriented 
with respect to wire 20 (and, hence, to casing 13) until the perforation 
tool 10 emerges from the bottom of tubing 11. 
In use, the wellbore perforating apparatus 10 will be made up on the end of 
a wireline or upon a separate suspension wire means 20 that can 
subsequently be attached to the end of a wireline by conventional methods. 
The number of charges 12 will be selected to provide the desired number of 
perforations in casing 13. The length of linkage means 16 will be chosen 
to provide the desired charge density. For all cases, the length l.sub.3 
exceed the diameter of casing 13 less twice the charge length l.sub.1 
outside (i.e., away from the center of the wellbore 17) attachment points 
14. The ends of linkage means 16 may be connected to attachment points 14 
by rivets, a bend around loop, or the like, so as to permit relative 
pivotal motion in a single plane which bisects the wellbore 17. Linkage 
means 16 are preferably formed with eyelets 18 which sliding receive 
suspension wire means 20 so as to maintain the charges substantially 
orthogonal to the suspension wire means 20 and, therefore to wellbore 
casing 13. Termination connector means 22 and 24 are secured to the 
uppermost and lowermost attachment points 14 of the uppermost and 
lowermost charges 12, respectively. The opposite end of upper termination 
connector means 22 is threaded onto wire 20 and then fixed thereto by 
soldering, shear pins, or the like, such that the apparatus is stretched 
to provide a narrow profile that will readily slide through tubing 11. The 
eyelet 30 of lower termination connector means 24 is fixedly attached to 
suspension wire 20 and sinker bar 34 by a ring 25. Severing charge 26 is 
secured to wire 20 and the detonators of all charges 12 and 26 are 
interconnected by Primacord fuse 28. 
Prior to lowering the perforating apparatus into wellbore 17, production 
tubing 11 will be run in with a conventional inflatable packer 15 to 
secure the lower end of tubing 11 in casing 13. The through-tubing 
perforating apparatus 10 of the present invention will be lowered into 
position using conventional wireline techniques. After the apparatus 10 
emerges from tubing 11 and is positioned generally into the desired 
location, doughnut 36 can be dropped through tubing 11 impacting upper 
termination connector means 22 severing the solder joint or the like at 
32. This allows charges 12 to slump under their own weight into secure 
contact with the interior of casing 13. An electrical impulse is 
transmitted from the surface through wireline 20 initially to severing 
charge 26 then to the other charges 12 through Primacord 28. As a 
practical matter, however, the signal is transmitted so rapidly through 
the Primacord 28 as to effectively simultaneously detonate all explosive 
charges 12 with severing charge 26. 
Because of the intimate contact between charges 12 and casing 13 and the 
fact that the charges are positioned substantially orthogonally to casing 
13, maximum penetration into the formation is achieved. Once the wire 20 
is severed by charge 26, sinker bar 34 pulls the expended charges 12 and 
associated linkage means and hardware into the bottom of wellbore 17 
(where doughnut 36 follows). 
The present perforating apparatus provides a simple, inexpensive 
alternative that is flexible in its utility (i.e., as many charges as 
desired may be added simply and easily). Very little structure is actually 
dropped into the rathole and, what is, can easily collapse into a 
compacted heap. 
Various changes, alternatives and modifications will become apparent to a 
person of ordinary skill in the art following a reading of the foregoing 
specification. For example, although a mechanical form of release is shown 
for the attachment at 32, another explosive charge could be employed and a 
separate isolated wire used to send an electrical signal from the surface 
to break the connection. It is intended that all such changes, 
alternative, and modifications as fall within the scope of the appended 
claims be considered part of the present invention.