Note: Descriptions are shown in the official language in which they were submitted.
2~ 22~42
TECHNIQUE FOR STIMULATING LONG INTERVALS
This application is a division of S.N. 2,030,103 filed
November 15, 1990.
This invention relates to a technique for stimulating a
subterranean formation and more particularly to a device which
employs a very long charge of propellant material which gener-
ates, during combustion, a large quantity of high pressure gases
to stimulate a thick subterranean formation or a smaller
quantity of high pressure gases to unplug perforations or a
slotted liner.
There are several techniques for stimulating subterranean
formations. The most common technique is "hydraulic fracturing"
in which a liquid is injected into a formation carrying a large
quantity of sand or other proppant. The liquid is pumped into
the formation so rapidly that a temporary fracture is created.
The proppant is deposited in the fracture and prevents it from
completely closing at the cessation of pumping. Hydraulic
fracturing works quite acceptably in a large variety of situa-
tions but indisputably has its disadvantages, foremost of which
is cost. Hydraulic fracturing often requires the well be killed
and the tubing pulled. In addition, hydraulic fracturing uses
pump trucks, proppant material and a carrier liquid, all of
which are more-or-less expensive depending on many factors.
Another technique for fracturing subterranean formations
includes the detonation of an explosive charge in the well bore
which fractures the formation by shattering or rubblizing. This
technique is somewhat less expensive than hydraulic fracturing
but has significant disadvantages. In its oldest form,
explosive fracturing of a well is accomplished by placing one or
more nitroglycerine charges in the well bore and then detonating
them. Considerable damage is often done to casing in the well
or considerable junk is left in the hole requiring significant
effort to clean up the well and repair the damage done.
Although more modern explosive fracturing techniques are
available, these also suffer from the same disadvantages. The
second disadvantage of explosive fracturing techniques involves
the obvious danger in handling, transporting and detonating the
2122842
explosive. Personnel of extensive training and experience are
required for this technique and such are not always available.
A third type of well fracturing technique involves the use
of a device incorporating a gas generating charge or propellant
which is typically lowered into a well on a wire line and
ignited to generate a substantial quantity of gaseous combustion
products at a pressure sufficient to break down the formation
adjacent the perforations. It is this type fracturing technique
that this invention most nearly relates. This type fracturing
differs from explosive fracturing in a number of respects: (1)
fracturing is caused by high pressure gaseous combustion
products moving through and possibly eroding the formation
rather than shock wave fracturing; and (2) the process is one of
combustion rather than explosion which has numerous ramifica-
tions. For example, an explosion propagates through the ex-
plosive material by, and at the rate of, the shock wave that
moves through the material. This causes explosive processes to
propagate much faster than combustion, generate much higher
pressures than combustion while the time for the reaction to be
completed is much shorter. Typical disclosures of gas generat-
ing fracturing devices are found in United States patents
3,422,760; 3,602,304; 3,618,521; 4,064,935; 4,081,031 and
4,823,876.
Present commercially available gas generation stimulation
tools include an elongate propellant charge, usually but not
necessarily in a perforated carrier, of a length to be easily
handled. Thus, presently available tools are 10-25' long. The
propellant in these tools is typically ignited by an electrical
signal transmitted through an insulated wire line to an assembly
including an aluminum ignition tube having gunpowder or other
ignition mixture therein. The electrical signal starts an
igniter which starts the gunpowder burning. The gunpowder burns
through the length of the ignition tube and starts the propel-
lant burning.
There is occasionally a requirement to stimulate a thick
subterranean formation which cannot be adequately stimulated by
the operation of one of the presently available tools. In such
circumstances, a wide variety of techniques have heretofore been
employed or proposed. Such techniques include, for example,
3 2122~2
dumping a large quantity of bulk propellant material into the
well, allowing it to settle to the bottom and then igniting it
by one method or another. It will be appreciated that there is
little one can do to control such a technique.
Gas generating tools have been proposed and used to unplug
long slotted liners. It has been observed that long gas gener-
ators activated in wells to unplug liners often damage the
slotted liner. It has been noticed that this damage often
recurs at similar spaced intervals. It is believed that this
recurrent damage is caused by periodic pressure peaks of
sufficiently high magnitude to damage the well casing or liner
by splitting it. In practice, it has been observed that an
uncemented 5 1/2" slotted liner showed splits at intervals of 8-
9 feet over 40 feet after being subjected to propellant gases
generated by a 1/4" OD tool. This periodic damage can occur
with small OD gas generators as well as larger diameter tools.
It has now been determined that these periodic pressure pulses
in long tools can be overcome or eliminated if combustion is
interrupted or delayed at more-or-less frequent intervals.
The invention of the parent application provides a method
of treating a subterranean formation comprising lowering into
the well a tool comprising first, second and third discrete
propellant charges and igniting a first of the charges and then
igniting the second charge from the combustion products of the
first charge and then igniting the third charge from the combus-
tion products of the second charge at a time when the first
charge is still burning. The embodiment used to perform this
method comprises an apparatus for stimulating a subterranean
formation penetrated by a well bore, comprising a series of
elongate vertically spaced propellant charges for generating a
large quantity of high pressure gaseous combustion products, an
igniter for initiating combustion of a first of the propellant
charges including an ignition tube having a combustible material
therein, the ignition tube extending axially substantially
through the first charge and means for transmitting combustion
of the first charge to a second of the charges, including a
combustion transferring tube extending into the first charge and
extending into the second charge and having a combustible
material therein.
2122~ 12
The invention of this application provides a method of
cleaning an uncemented slotted liner suspended in a well bore
penetrating a subterranean formation, comprising lowering a
running liner having a plurality of openings therein into the
slotted liner, lowering a propellant charge inside the running
liner into the slotted liner, igniting the propellant charge and
producing a quantity of high pressure combustion products,
delivering the combustion products through the openings of the
running liner and throttling the pressure of the combustion
products, and delivering the combustion products through the
openings of the slotted liner.
The invention also provides apparatus for cleaning an
uncemented slotted liner in a well bore penetrating a subter-
ranean formation, comprising: a propellant charge having the
capability of delivering not more than about 1000 cubic inches
of gaseous combustion products measured at standard temperature
and pressure per linear foot of propellant charge; means for
igniting the propellant charge and producing less than about
1000 cubic inches of gaseous combustion products measured at
standard temperature and pressure per linear foot of propellant
charge; and means for periodically changing the rate of combus-
tion of the propellant charge to vary the pressure adjacent the
slotted liner.
Other features and advantages of this invention will become
more fully apparent as this invention proceeds, reference being
made to the accompanying drawings and appended claims.
IN THE DRAWINGS:
Figure 1 is a side view, partly in cross-section, of the
upper section of a tool of this invention;
Figure 2 is an enlarged cross-sectional view of the connec-
tion between adjacent sections of the tool of this invention;
Figure 3 is a schematic of a long tool of this invention;
Figure 4 is a partial enlarged cross-sectional view of
another embodiment of the tool of this invention;
Figure 5 is a side view, partly in cross-section of another
embodiment of this invention;
Figure 6 is an enlarged cross-sectional side view of the
device of Figure 5;
5 2122842
Figure 7 is a cross-sectional side view of another embodi-
ment of this invention;
Figure 8 is an enlarged cross-sectional view of the-connec-
tion between adjacent sections of the tool of Figure 7;
Figure 9 is a pressure-time diagram of the tool of Figures
7 and 8; and
Figure 10 is a cross-sectional view, similar to Figure 6,
of another embodiment of this invention.
Referring to Figure 1, there is illustrated a gas generat-
ing tool 10 lowered inside a well 12 which penetrates a
formation 14 to be fractured. The well 12 includes a bore hole
16 and a casing string 18 cemented in the bore hole 16 by a
cement sheath 20. A multiplicity of perforations 22 have been
formed between the formation 14 and the interior of the casing
string 18 as is customary in the art.
The gas generating tool 10 comprises a frame or carrier
section 24 connected to a cable head assembly 26 and receiving
a charge 28 of propellant material. An igniter 30 includes a
pair of wires 32 connected to a conductor cable or wire line 34.
The wire line 34 suspends the tool 10 in the well 12 and
delivers an electrical signal through the wires 32 to activate
the igniter 30 thereby initiating combustion of the propellant
change 28.
The carrier or frame 24 comprises an elongate rigid
metallic tubular member or housing 36, open at both ends, having
many laterally facing openings 38 arranged symmetrically along
the tubular member. The openings 38 comprise a series of
staggered openings spaced longitudinally along the tubular
member 36. Typically, the housing 36 has a wall thickness on
the order of 1/4 - 3/8". The carrier 24 is open to liquids in
the casing string 18. In addition, the openings 38 allow the
gaseous high pressure combustion products to escape from the
propellant charge 28. The cable head 26 may include a collar
locator 40 to facilitate positioning of the tool 10 at a desired
location, as is well known in the art.
The propellant charge 28 contains a fuel and an oxidizer.
The fuel is conveniently in a resin form polymerized into a
unit. Typically, the oxidizer components are water soluble. In
21228~2
this event, the resin polymer is preferably of a water insoluble
type so that the liquid in the well 12 does not attack the
propellant charge 28. In the alternative, the propellant charge
28 may be painted so it is not attacked by well fluids. Because
the propellant 28 is inside the tubular housing 36, there is no
danger of the charge 28 bowing and thereby becoming stuck inside
the casing 18 or tubing through which it may be run.
The igniter 30 may be conventional and includes a section
of thin wall aluminum ignition tube 42 having gun powder or
other fast burning material therein. When the igniter 30 is
energized through the wires 32, it combusts thereby raising the
temperature of the propellant 28 adjacent thereto. This causes
the propellant 28 to begin burning thereby liberating high
pressure gaseous combustion products through the openings 38.
These high pressure gases create a large bubble adjacent the
formation and begin to raise the liquid column in the casing 18.
The combustion gases pass through the perforations 22 into the
formation and erode enlarged passages therein. In modern prior
art tools, when the propellant 28 in the tool 10 finishes
burning, the pressure adjacent the tool 10 declines, the gaseous
bubble deflates, the liquid column falls back into the bottom of
the casing string and the stimulating technique is over. Those
skilled in the art will recognize the tool 10, as heretofore
described, to be typical of commercially available gas generat-
ing type fracturing tools.
In situations where very long intervals are desired to be
stimulated, the tool 10 must necessarily be very long. Because
a tool 10 several hundred feet long cannot realistically be
transported any significant distance to the well 12, the only
realistic option is to make the tool into segments of a
shippable length and assemble the segments at the well location.
This requires some technique to transfer combustion from one
propellant charge to the next.
To these ends, the tool 10 includes a plurality of the
carrier or frame sections 24 secured together by a connector 44.
As shown best in Figures 1 and 2, the lower end of the carrier
section 24 is modified to mesh with the connector 44. The
bottom end of the ignition tube 42 is embedded in the propellant
28 at a location offset relative to the tool axis 46. The
21228~2
bottom end of the propellant 28 provides a downwardly facing
blind opening 48. The bottom end of the carriers 24 provide one
or more bolt openings 50 as do the upper end of the lower
carriers 24.
The connector 44 includes a substantial metallic body 52
symmetric about a central plane having an upper end 54 received
in the lower end of the upper carrier 24 and a lower end 56
received in the upper end of the lower carrier 24. Threaded
fasteners 58 are inserted through upper and lower bolt openings
50 in the carriers 24 to engage interiorally threaded openings
60. A mechanical connection between the carrier sections 24 is
thus provided.
The connector 44 also includes an axial passage 62 having
loosely received therein a thin walled aluminum combustion
transfer tube 64 having a sealed upper end 66, an ignition mix
68 and a partition 70 made of paper, fiberboard or the like
supporting the ignition mix 68 leaving the bottom of the tube 64
empty. The bottom of the tube 64 extends into a seal 72 located
in the upper end of the frame 24 above the top of the propellant
charge 28 thereof. The seal 72 may be of any suitable type to
prevent liquid entry into the lower end of the transfer tube 64
while allowing the passage of hot combustion products axially
through the tube 64. Preferably, the seal 72 comprises a
resilient annular plug 74 received in an enlarged diameter
section 76 of the ignition tube 42 placed axially in the
propellant charge 28 of the next subjacent tool section. The
tool 10 of this invention may comprise as many of the carrier
sections 24 as is necessary to span the distance between the
uppermost and lowermost perforations of the formation 14 to be
stimulated. The lowermost carrier section includes a bull plug
(not shown) at the lower end thereof, as is customary in the
art.
Assembly of the tool 10 should now be apparent. The
lowermost carrier section 24 is lowered into the well 12 and
supported by slips in the rotary table of the workover rig (not
shown) used to pull tubing and the like from the well 10. The
connector 44 is inserted into the top of the lower carrier 24
and bolts 58 inserted through openings 50 and threaded into the
passages 60. The transfer tube 64 is passed through the passage
21~28l2
62 so the lower end passes through the central opening of the
annular resilient seal 74. The upper carrier 24 is then lowered
onto the connector 44 so the transfer tube 64 extends into the
blind opening 48. With the upper carrier 24 received on the
upper end 54 of the connector 44, the bolts 58 are threaded into
the openings 60.
When the tool 10 is lowered into the well 12 and the
igniter 30 energized, combustion is started in the ignition tube
42 of the upper carrier 24. Combustion of the upper propellant
charge 28 begins along substantially the entire length of the
charge 28 and the charge 28 burns radially away from the axis of
the upper ignition tube 42. When the flame front reaches the
blind passage 48 at the lower end of the carrier 24, the upper
end of the transfer tube 62 melts or burns to ignite the
ignition mix 68. Hot combustion products from the ignition mix
68 and possibly from the propellant charge 28 in the upper
carrier 24 pass through the transfer tube 64 and through the
seal 72 into the ignition tube 42 of the next lower tool section
to ignite the next lower propellant charge 28.
As is apparent, the tools of this invention may be of any
desired length. One of the peculiarities of this invention is
shown best in Figure 3 where a tool 78 comprises an upper tool
section 80 ignited by an igniter 82 and having an ignition tube
84 extending substantially therethrough in combustion transferr-
ing relation with a transfer tube 86. The transfer tube 86
communicates with an ignition tube 88 of a second tool section
90 which, in turn, communicates with a transfer tube 92. The
transfer tube 92 connects to an ignition tube 94 of a third tool
section 96. Combustion products from the first tool section 80
and/or transfer tube 86 ignites the second tool section 90 and
combustion products from the second tool section 90 and/or
transfer tube 92 ignites the third tool section 96 at a time
when the first tool section 80 is still burning. Thus, the
pressure generated by the tool sections 80, g0, 96 can be added
because they are all burning at the same time.
In the embodiment of Figures 1-3, ignition of the tools 10,
78 proceeds rather rapidly because there are no delays or inter-
ruptions designed into the combustion train. This may be
desirable in many well situations. On the other hand, there are
21 22~42
situations where it is desirable to delay combustion of one or
more successive propellant charges.
One simple technique for introducing a delay into the
combustion transfer between successive propellant charges is
shown in Figure 4 where a tool section 98 includes a propellant
charge 100 having an ignition tube 102 axially spaced from the
end of an axial blind passage 104 having a transfer tube 106
therein. Because the axial dimension 108 is substantially
greater than the radial distance between the tube 42 and the
blind passage 48 in Figure 2, there is a delay approximately
equal to the distance 108 divided by the combustion rate of the
propellant charge 100. Looking at the embodiment of Figure 4 in
a slightly different perspective, combustion of the propellant
charge 100 occurs radially adjacent the length of the ignition
tube 102 and then turns to an axial burning mode through the
axial dimension 108.
Referring to Figure 5, there is illustrated another embodi-
ment of this invention which is particularly adapted for use in
cleaning plugged slotted liners or plugged perforations. A
running liner 110 preferably comprises joints 112 of standard
oil field tubing, such as 2 7/8" OD tubing, having a multiplici-
ty of slots or openings 114 therein. Adjacent joints are
connected together by threaded couplings 116. The uppermost
joint 112 of the running liner 110, illustrated in Figure 5,
connects to a firing head container 118 having a retainer
housing 120 secured therein in any suitable manner, as by the
use of set screws 122. The retainer housing 120 includes an
axial passage 124 receiving an ignition assembly 126 secured
therein by set screws 128.
The details of the ignition assembly 126 are shown in
greater detail in Figure 8 as will be explained more completely
hereinafter. For present purposes, the igniter assembly 126
connects to an elongate small diameter gas generating tool 130
extending downwardly through the running liner 110. The running
liner 110 is assembled in the slips of the rig (not shown) and
run into the hole in a conventional manner. The tool 130 is
likewise assembled and run into the running liner 110 as it is
being run into the well. After a sufficient length of joints of
the liner 110 and tool 130 have been run into the well, the
21 228'12
firing head container 118 is attached to the coupling 116 and
the ignition assembly 126 is attached to the tool 130 and
secured in the retainer housing 120. A safety sleeve 132 is
removed from the ignition assembly 126 to expose a piston 134.
A coupling 116 is attached to the upper end of the container 118
and the assembly is run into well at the bottom of a tubing
string 136 to a location adjacent the slotted liner to be
unplugged.
The ignition assembly 126 can be activated in a variety of
ways. A sinker bar (not shown) suspended on a wire line can be
dropped into the tubing string 136 to strike the piston 134 and
initiate combustion of the igniter assembly 126. A weight (not
shown) may simply be dropped into the tubing string. In addi-
tion, the igniter assembly 126 can be started merely by pumping
into the tubing string 136 from the surface to raise the pres-
sure and hydraulically force the piston 134 downwardly.
A typical technique of completing a well is to cement
casing at or near the top of a hydrocarbon producing zone, drill
a bore hole horizontally or vertically into the producing and
then run a slotted liner into the well bore without cementing
the slotted liner in place. Over the years, openings in the
slotted liner tend to become plugged with asphaltenes, formation
fines and the like. It has been proposed and attempted in the
past to unplug such slotted liners with gas generating tools.
One of the peculiarities of gas generation tools is that they
tend to split uncemented slotted liners at more-or-less
repetitive intervals. It has been learned that such damage can
be minimized or prevented by using very small capacity tools and
then burning the propellant to produce a varying rate of gas
generation. It has been learned that this technique is
effective when the capacity of the tools is less than 1000 cubic
inches of gaseous combustion products, measured at standard
temperature and pressure, per linear foot of tool. Preferably,
the capacity of the tool is less than 500 standard cubic inches
of gaseous combustion products. Optimally, the tool produces
about 300 standard cubic inches of gas or less. With gas
volumes so small, the tool has to be submersed in liquid near
the plugged slotted liner. The gas does not itself unplug the
liner, it simply drives the well bore liquid through the plugged
21228~2
11
slotted liner to dislodge the asphaltene or fine plugs. The
optimal tool 130 is 1/4" OD having an ID of .183 inches. The
volume of this tool is about 3.78 cubic inches per 12' length
which is a preferred joint length. With typical propellants,
the optimal tool 130 produces about 3024 cubic inches of gaseous
combustion products, measured at standard temperatures and pres-
sures, per 12' joint or about 252 cubic inches of standard
gaseous combustion products per linear foot of tool. This is a
very small quantity of gas and, in the absence of liquid
surrounding it, the tool ignites and makes an unimpressive
"poof." With liquid surrounding the tool 130 in the bottom of
a well, the preferred tool generates substantial pressures,
which have been measured in the range of 500-5000 psig,
depending on how plugged a liner section was before treatment.
Referring to Figure 6, there is illustrated one technique
for varying the pressure generated by a gas generating tool of
this invention. The joints 138 of the tool 130 have therein a
charge 140 of propellant material and are connected by a
coupling 142. The propellant charges 140 contain a fuel and an
oxidizer and are preferably a relatively loosely packed gun
powder having a relatively high propagation rate, e.g. about
1200 feet per second which is slightly greater than the speed of
sound in air. When the igniter assembly 126 is energized, it
combusts thereby raising the temperature of the powder 140
adjacent thereto. This causes the propellant 140 to begin
burning thereby liberating high pressure gaseous combustion
products which split the upper joint 138 and escape into the
well and formation adjacent thereto.
The connector 142 conveniently includes an upper threaded
end 144 receiving the lower threaded end of the joint 112, a
lower threaded end 146 receiving the upper threaded end of the
next subjacent joint 112 and a compartment 148 having a restric-
ted lower end 150 including a compressed black powder element
152 therein. The powder element 152 has a propagation rate
substantially lower than 1200 feet per second and is in flame
transmitting relation with the propellant charges 140. To this
end, the powder element 152 abuts the propellant charges 140.
When the running liner 110 and the tool 130 are lowered
into a well and the igniter is energized, combustion is started
12 212~8~2
in the propellant charge 140. Combustion of the upper propel-
lant charge 140 occurs axially, or in a cigarette burning mode,
and burns along substantially the entire length of the charge
140 at the propagation rate of the powder thereof. When the
flame front reaches the bottom of the uppermost joint 138, the
compressed black powder element 152 is ignited. Because the
powder element 152 occupies substantially the entire cross
section of the compartment 148 and is a rigid material,
combustion cannot flare through the compartment 148 and must
proceed at the propagation rate of the powder element 152, which
is designed to be substantially slower than the rate of the
propellant charge 140, for example, 800 feet per second. Thus,
the pressure buildup in and adjacent the tool 130 increases
while the uppermost propellant charge 140 is being consumed and
either tails off or stabilizes as the powder element 152 is
burning. This creates a delay in the tool 130 and, in combina-
tion with its small gas capacity, substantially prevents
splitting of the uncemented liner 154 having slots 156 therein.
The running liner 110 has a number of advantages. First,
the running liner 110 accumulates debris from the tool 130 and
allows most of the debris generated by the tool 130 to be
removed from the well in a simple and expeditious manner.
Second, the slots or openings 114 in the running liner 110 act
as a throttle or choke to reduce the pressure applied to the
uncemented slotted liner 154 present in the horizontal, inclined
or vertical bore hole 158 of a well thereby preventing or
minimizing damage to the uncemented slotted liner 154.
At one time, it was common to complete wells in the open
hole, i.e. without casing cemented through the productive
formation. In these situations, a slotted liner was often run
into the well opposite the producing formation to prevent the
formation from collapsing. In such situations, the slotted
liner is hung from casing cemented above the pay zone. This
same technique is now becoming common in horizontal wells. In
a typical situation where a 5 1/2" O.D. slotted liner is in
place in a well, the running liner 110 is conveniently a string
of 2 7/8" O.D. tubing having conventional screw couplings 116.
Rather than running the liner 110 on the end of the tubing
string 136 with the tool 130 therein, the running liner 110 may
13 2 122842
first be run in the well in a conventional fashion on the bottom
of the tubing string 136 and then lowering the tool 130 on a
wire line through the tubing string 136 and running liner 110.
The running liner 110 acts to reduce the peak pressures
applied to the formation adjacent the slotted liner lS4 as shown
in Table I.
Table I
Pressure in psi
inside running outside running
10dePth in feet liner 110 liner 110
965 4700 3500
1025 3500 3S00
995 3800 . 3500
810 3500 1$00
687 5800 3800
1905 5500 4700
855 6000 5200
1945 5800 3800
Referring to Figures 7-8, there are illustrated other
techniques which can be used in either large diameter tools or
small diameter tools for interrupting continuous combustion of
the propellant charge of a gas generating tool. A gas generat-
ing tool 160 comprises an upper frame or carrier section 162
connected to a cable head assembly (not shown) and receiving an
upper charge 164 of propellant material. An igniter or ignition
tube (not shown) is connected to a conductor cable or wire line
(not shown) for suspending the tool 160 and delivering an
electrical signal to activate the igniter thereby initiating
combustion of the upper propellant change 164. The upper charge
164 is supported by a partition 168 of any suitable material,
such as paper, aluminum or plastic and spaced by an air gap 170
from a second propellant charge 172. The second propellant
charge is supported by a partition 174 of any suitable material
and spaced by an air gap 176 from a third propellant charge 178
supported by a partition 180. As many propellant charges as are
desirable may be provided in the upper carrier 160.
The tool 160 includes a plurality of lower carrier sections
182 connected by couplings 184. The carriers 162, 182 comprise
2122~42
14
an elongate rigid metallic tubular member or housing 186, 188
open at both ends. The upper end of the upper carrier 162 is
closed by the cable head (not shown) while the lower end thereof
is closed by the coupling 184. The lower end of the lowermost
carrier 182 is closed by a bull plug (not shown). The carriers
162, 182 are thus sealed against entry of liquids from the well
bore but split during combustion to allow escape of gases.
The carrier 182 preferably includes an upper propellant
charge 190 and then a multiplicity of separate propellant
charges 192, air gaps 194 and partitions 196 analogous to the
arrangement of charges in the upper carrier 162. It will be
evident that the air gaps in the propellant train in the tool
160 cause the tool 160 to sputter rather than deliver a more-or-
less constant supply of high pressure gaseous combustion
products. This is particularly desirable when cleaning
uncemented slotted liners in order to avoid the more-or-less
regular splits and bulges noted in such slotted liners when
treated with conventional gas generators.
The action of the tool 160 when used to clean uncemented
slotted liners should be distinguished from the action of the
tool shown in U.S. Patent 3,422,760 when used to fracture a
formation. As shown in Figure 9, the time-pressure profile of
the tool of U.S. patent 3,422,760 includes a series of pressure
rise intervals 198 which are seen when each of the individual
charges are burning separated by a periods 200 of more-or-less
constant pressure when combustion is being transferred from one
charge to the next. Ultimately, pressure in the well bore
exceeds the formation breakdown pressure 202, the formation
fails and combustion gases move into the formation and the
pressure in the well bore declines in an interval 204. The
reason the pressure increases until the breakdown pressure is
reached is that the formation is rather impermeable and only a
small quantity of the gas moves into the formation until it
fails at the pressure 202.
In contrast, the time pressure profile of the tool 160 of
this invention exhibits a series of high relatively constant
pressure intervals 206 when one of the charges is burning,
followed by an abrupt loss of pressure and a relatively low
pressure interval 208. When the next successive charge begins
15 21228~2
burning, the next high pressure interval 210 is seen. The
succession of high and low pressure intervals proceeds until all
of the propellant charges in the tool 160 are expended. The
pressure in the well bore is rarely, if ever, over the formation
breakdown pressure. The reason for the different time pressure
profile of the tool of this invention is that the formations
completed with slotted liners are much more permeable and take
large quantities of gas during a pressure build up phase. Thus,
it is difficult to fracture these permeable formations and,
indeed, this is not the purpose of the tool 160. Instead, the
purpose of the pulsating pressure of the tool 160 is to dislodge
material from the slots of the uncemented slotted liner in the
well.
The tool 160 also includes another feature of interest.
The air gaps in the carriers 162, 182 produce a sputtering
discharge of combustion products from the tool 160. It is
sometimes desirable to introduce greater delays in the combus-
tion process. This may be accomplished during the transmission
of combustion from the upper carrier 162 to the next subjacent
carrier 182 through the coupling.
One such technique is illustrated in Figure 6 and another
is shown in greater detail in Figure 7-8 where the coupling 184
includes a retainer housing 212 secured therein in any suitable
fashion, as by the use of set screws 214. An ignition assembly
126 is secured in the housing 212 in any suitable manner, as by
the use of set screws 216. The ignition assembly 126 is illus-
trated in greater detail in Figure 8 than in Figures 5 and 7 and
includes an elongate tubular body 218 in an axial passage 220 in
the retainer housing 212. An ignition tube 222 having an
ignition mix therein is received in an internally threaded lower
end of the body 218 and extends into combustion transmitting
relation to the propellant charge 190 in the subjacent carrier
or joint 182. An O-ring or other seal 224 seals the exterior of
the ignition tube 222 to the body 218.
The body 218 provides an axial passage 226 having the
piston 134 closely fit therein and sealed with an O-ring 228.
A shear pin 230 extends through the piston 134 and prevents
depression of the piston 134 until a predetermined force is
applied to the piston 134. The piston 134 includes a firing pin
21228~2
16
point 232 which contacts an impact primer 234 at the bottom of
the passage 226. The primer 234 ignites an ignition mixture 236
in a passage 238 communicating with the upper end of the
ignition tube 222 and thus ignites the upper propellant charge
190.
Operation of the tool 160 should now be apparent. The tool
160 is run into a well, either directly into the well or inside
a slotted running liner. When the igniter (not shown) is ener-
gized, it combusts thereby raising the temperature of the charge
164 adjacent thereto. This causes the propellant 164 to begin
burning thereby liberating high pressure gaseous combustion
products which split the tube 186 in the vicinity of the charge
164 and escapes into the well, pushing bore hole liquid adjacent
the tool 160 through any uncemented slotted liner therein and
then into the formation adjacent thereto.
Sometime before the end of combustion of the charge 164,
the partition 168 gives way allowing hot combustion products and
burning propellant pieces to travel downwardly in the tube 186
to ignite the next lower charge 172. During combustion of the
charge 164, pressure adjacent the tool 160 is at a relatively
high level shown by the interval 206 in Figure 9. As combustion
is being transferred to the next subjacent charge 172, pressure
declines in the interval 208. Thus, combustion of the propel-
lant charges in the carrier 162 causes a sputtering discharge of
high pressure gaseous combustion products that are well suited
to dislodge undesirable accumulations in the openings of an
uncemented slotted liner. When combustion of the upper carrier
162 is substantially over, high pressure combustion products
shear the pin 230 allowing the piston 134 to travel downwardly
in the passage 226 to impact the primer 234 and start combustion
of propellant charges in the next lower carrier 182.
It will be evident that the ignition mechanism in the
coupling 184 may be used to initiate combustion of a gas
generating stimulation tool rather than simply transmit
combustion. In this event, the shear pin 230 is selected to
fail at the imposition of a pressure in the well bore than can
be reached by simply pumping liquid into the well.
Referring to Figure 10, there is illustrated a simple
technique for varying the pressure of a gas generating tool 240.
21228~2
17
The tool 240 comprises upper and lower joints 242, 244 connected
by a coupling 246. The upper joint 242 includes a partition
wall 248 above the top of the coupling 246 and a granular
propellant charge 250 supported on the wall 248. The lower
joint 244 includes a granular propellant material 252 supported
by a partition wall (not shown). The coupling 246 conveniently
threadably connects the joints 242, 244 and provides a central
axial passage 254. The partition wall 248 and coupling 246
provide an air gap above the propellant charge 252. Thus,
ignition of the charge 250 causes a pressure buildup adjacent
the tool. When the charge 250 is nearly consumed, combustion of
the charge 252 begins with a definite pause in combustion and
consequent fall in pressure adjacent the tool 240.
Although this invention has been disclosed and described in
its preferred forms with a certain degree of particularity, it
is understood that the present disclosure of the preferred forms
is only by way of example and that numerous changes in the
details of operation and in the combination and arrangement of
parts may be resorted to without departing from the spirit and
scope of the invention as hereinafter claimed.
