Note: Descriptions are shown in the official language in which they were submitted.
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DEVICE FOR STIMULATING A SUBTERRP~NEAN FORMATION
Background of the invention
This invention relates to an apparatus for stimulating a
subterranean formation by fracturing it to increase its perme-
ability. In the case of hydrocarbon producing wells, the increasein per~leability increases hydrocarbon flow from the formation
into the well casing. In the case of injection wells, increasing
the permeability of the formation allows greater ~uantities of
fluid to be injected into the formation at lower injection pres-
10 sures.
It is well known in the art to fracture hydrocarbon pro-
ducing formations by a variety of techni~ues. The most commonly
used technique is known as hydraulic fracturing in which a liquid
loaded with a particulate propping agent is injected into the
15 formation at a pressure adequate to break down the formation and
at a volume sufficient to carry a substantial quantity of propp-
ing agent into the formation. Although hydraulic fracturing is
widely practiced, it is rather expensive because of the pumping
e~uipment necessary and because of the quantity of fracturing
2~ liquid and propping agent needed.
Another known technique for fracturing a subterranean
formation includes the detonation of an explosive charge in the
well bore which fractures the formation by shattering or rubblizing.
This technique is some~hat less expensive than hydraulic fracturing
25 but has several significant disadvantages. In its oldest form,
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explosive fracturing of a well is accomplished by placing one
or more nitroglycerine charges in the well bore and then deto-
nating them. The ~irst disadvantage of explosive well fracturing
is that considerable damage is often done to casing in the well
or considerable junk is often left in the hole requiring expen-
sive and time consuming efforts to clean out the well and repair
the damage done. Although there are more modern explosive
fracturing techniques available, these also suffer from these
disadvantages.
The second disadvantage of explosive well fracturing tech-
niques involves the obvious danger in handling/ transporting and
detonating such explosives. Personnel of extensive training and
experience are required for explosive fracturing techniques and
such personnel are not always readily 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
20 perforations. In this type approach, the desired fracturing is
caused by the high pressure combustion products produced by the
propellant rather than shock wave fracturing as in the case of
the explosive techniques. It is this type fracturing technique
that this invention most nearly relates. Typical disclosures of
25 this type fracturing device are found in United States patents
3,174~545; 3,264,986; 3,313,234; 3,602,304; 3,618~521; 4,064,935
and 4,081,031.
The goal of all formation stimulating techniques is to
produce zones of higher permeability that extend for long distances
30 from the well bore. The great advantage of hydraulic fracturing
is that there is apparently no limit on the amount of fracturing
liquid or propping agent than can be injected into a well. This,
of course, allows fractures of great areal extent to be created.
The great disadvantage of gas generated fracturing is that the
35 amount of high pressure gas that can be generated is substantially
limited by several factors. First, in a cased well which is
perforated, high gas pressures can be generated only adjacent the
perforations or within a limited distance therefrom. If other-
wise there is a significant danger of stressing the casing beyond
40 its mechanical strength, thereby splitting or bursting the casing.
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This is a calamity since it will mean loss of the well or the
expenditure of substantial sums to save the well. Second, there
is a limit to the amount of propellent charge that can be placed
in the requisite vicinity of the perfoations because of the
limited internal diameter of the casing.
In the last analysis, there are only a limited number of
things that can be done to provide more effective gas fracturing.
Among the broad catagories of improvements are: (1) design the
tools in such a way as to allow longer propellent charges; and
10 (2~ utilize the necessarily limited quantity of high pressure
gas more efficiently. As will become more fully evident as this
description proceeds, this invention uses both of these techniques.
It will be appreciated that one cannot merely use longer
propellent charges since the generation of gas at too high a
15 pressure too far from the casing perforations will split the
casing. One of the characteristics of the prior art gas fracturing
devices is that the pressure produced, when plotted against time
is of generated upwardly convex configuration having an initial
buildup region where pressure increases at a substantial rate,
20 a fairly short region where pressure is generally stable and a
pressure decline region in which pressure subsides at a rapid rate.
If one were to integrate the area underneath such a curve, the
value obtained would be representative of the quantity of gas
produced during combustion of the propellent charge. If one
25 increased the length of the propellent charge, the amplitude of
the pressure pulse would increase as would the Yolume of gas
generated.
In accordance with this invention, the fracturing device is
designed to produce a pressure-time response curve of considerably
30 different configuration. The propellent charge is designed to
produce an initial pressure pulse of substantially the same magni-
tude of the prior art followed by a first pressure decline region
in which the pressure subsides to an intermediate value which is
significant but which is substantially lower than the maximum
35 pressure pulse. Following the first pressure decline region is
a region of fairly constant pressure, by which is meant that the
pressure does not progressively increase or decrease at a rapid
rate although there may be some fluctuation because of combustion
irregularities. The duration of this region of intermediate
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pressure is substantially longer than the region of rapid pressure
decline in conventional gas fracturing devices. The region of
intermediate fairly constant pressure is followed by a region of
relatively rapid pressure decline to a value approaching the
hydrostatic pressure in the well adjacent the perforations.
This technique has several substantial advantages. First,
it is believed that the duration of fracture propagation is
increased. It is known in hydraulic fracturing that much higher
pressures are required to initiate a fracture than it is to
10 maintain or propagate that fracture. While not wishing to be held
to any particular theory of operation, it is believed that the
same is true with gas fracturing. Thus, it is believed that the
initial pressure pulse will initiate fracturing adjacent the well
bore while the extended period of intermediate pressures will
15 propagate those fractures to a greater extent than heretofore.
Second, propellent charges can be made longer without
endangering the well casing since the longer period of combustion
occurs at significantly lower pressures. Thus, the danger of
casing failure is significantly reduced even though the location
20 of combustion may be further from the perforations than previously
thought allowable.
Thus, the device and process of this invention improve the
efficiently of gas fracturing by both major techniques available,
both by increasing the efficiency of a finite quantity of gas
25 generated and by allowing the generation of a greater quantity of
gas than previously allowed.
It is accordingly an object of this invention to provide
an improved technique of stimulating a subterranean formation by
the gas generating technique.
Another object of this invention is to provide an improved
well gracturing technique of the gas generating type in which the
duration of pressure generation by the device is substantially
prolonged.
Another object of this invention is to provide an improved
35 well fracturing technique of the gas generating type which delivers
an initial pressure pulse of substantial magnitude followed by
gas generation at an intermediate pressure.
Other objects and advantages of this invention will become
fully apparent as this description proceeds, reference being made
40 to the accompanying drawings and appended claims.
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IN THE DRAWINGS:
Figure 1 is a longitudinal cross-sectional view of a hydro-
carbon producing well demonstrating placement of a fracturing
device of this invention adjacent the perforations prior to
ignition of the propellent charge thereof;
Figure 2 is a chart showing the pressure-tim~ relationship
of a prior art gas generating fracturing device and the pressure-
time relationship of this invention;
Figures 3-5 are longitudinal cross-sectional views of the
10 embodiment of Figure 1 illustrated in various stages of combustion;
Figure 6 is a chart showing the rate of combustion against
time of various embodiments of this invention;
Figure 7 is a longitudinal cross-sectional view of another
embodiment of this invention; and
Figure 8 is a longitudinal cross-sectional view, similar
to Figure 7, of another embodiment of this invention.
Referring to Figure 1, there is illustrated a well 10
comprising a bore hole 12 extending into the earth 14 and inter-
20 secting a formation 16 which is typically, although not univer-
sally, hydrocarbon bearing. As will be more fully pointed out
hereinafter, it is desired to improve the productivity of the
formation, which term is intended to include the ability of the
formation 16 to give up fluids contained therein or to accept
25 fluids injected thereinto.
A string of casing 18 extends downwardly into the earth 14
to adjacent the formation 16 and preferably extends somewhat
therebelow. The casing string 18 is bonded to the wall of the
bore hole 12 by a cement sheath 20. A plurality of perforations
30 22 communicate between the interior of the casing string 18 and
the formation 16 to allow passage of fluids therebetween. Typically,
the casing 18 is at least partially filled with a completion fluid
24 such as brine, KCl water, lease crude or the like.
A tool 26 of this invention is illustrated as having been
35 run into the well 10 in any suitable fashion, as by the use of a
wire line 28. The tool 26 comprises, as major components, a main
propellent charge 30, means 32 for igniting the charge 30 and
means 34 for connecting the wire line 28 to the tool 26 and for
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sealing the ignitiny means 32 against liquid contamination from
the completion fluid 24.
A typical prior art gas generating fracturing device includes
a central ignition tube or other means to initiate and maintain
5 radial combustion of the propellent charge. Conventional frac-
turing devices of this type produce a pressure-time relationship
illustrated by the curve 36 in Figure 2 which includes an initial
pressure buildup region 38 where pressure increases at a substan-
tial rate, relatively short region 40 of relatively high constant
10 pressure, and a pressure decline region 42 where the pressure
declines at a substantial rate toward the hydrostatic pressure 44
in the well bore adjacent the tool. As mentioned previously,
integrating the area under the curve 36 produces a value which is
representative of the quantity of gas generated by the prior art
15 tool As will be evident~ the general shape of the curve 36 may
be designated as a radial burn curve. The duration of pressure
increase by conventional commercially available tools is on the
order of 40 milli-seconds.
The pressure-time response curve 46 of this invention is
20 of significantly different shape than the curve 36 and includes a
pressure buildup region 48 where pressure increases at a signifi-
cant rate to a maximum value approximating that of the high
pressure region 40 of the prior art. Almost immediately following
attainment of a maximum pressure, the pressure generated by the
25 device 26 of this invention passes through a first pressure decline
region 50 where the pressure falls off at a relatively rapid rate
toward an intermediate pressure in a region 52 of intermediate,
generally constant pressure. In the region 52, pressure is not
necessarily sustained at an exact constant value. What is meant
30 by the term generally constant pressure in reference to the region
52 is that the pressure does not progressively increase nor de-
crease at a relatively rapid rate for any appreciable length of
time. It will be understood, of course, that the instantaneous
pressure in the region 52 may be subject to random variations
35 caused by combustion irre~ularities in the propellent charge 30.
At the end of the region 52 of intermediate constant pressure,
the pressure in the well bore declines in a second region 54 of
rapidly declining pressure.
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As will be more fully apparent hereinafter, if the device
26 contains the same quantity of propellant as a prior art device,
both devices would produce the same quantity of high pressure
gaseous combustion products. In this event, the area under the
5 curve 36 would be the same as the area under the curve 46. It
is believed that the device of this invention would be more
effective to fracture the formation 16 because the duration of
the intermediate constant pressure region 52 allows propagation
of fractures to a significantly greater extent than the prior
10 art gas generating fracturing devices.
As will be more fully pointed out hereinafter, a device
operating in accordance with this invention has a substantial
advantage since the tool may be made longer to provide a greater
quantity of propellent material without significantly increasing
15 the risk of splitting the casing 18. Because the device 26
operates~ during the region 52 of intermediate constant pressure,
at a substantially lower pressure than the prior art devices,
it is possible to increase the length of the propellent charge
30 thereby increasing the quantity of propellent material available
20 for combustion and consequently increasing the quantity of high
pressure gaseous combustion products.
There are a wide variety of techniques that may be used to
design a gas generating fracturing device which produces a
pressure-time response curve as shown by the curve 46 in Figure
25 2. The necessary criteria, of course, is that the tool provide
a first combustion region which generates high pressure gaseous
combustion products at a relatively high rate and a second com-
bustion region which generates high pressure gaseous combustion
products at a relatively lower rate.
One of the simplest techniques is shown in the device 26
illustrated in Figure 1. The propellent charge 30 is designated
to include a first section or region 56 which combusts at a high
rate to produce high pressure gaseous combustion products. The
propellent charge 30 also includes a second region or section 58
35 which combusts at a lower rate to produce high pressure gaseous
combustion products.
The first region 56 is designed to burn in a radial direction
because of the provision of a central passage 60 comprising part
of the igniting means 32. The passage 60 extends into the pro-
40 pellent region 56 from one end thereof. The opposite end of the
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passage 60 is closed in any suitable fashion, as by spacing thebottom wall 62 away from a boundary 64 between the first and
second regions 56, 58.
As will be more fully apparent hereinafter, the centr~l
passage 60 acts to initiate and maintain combustion of the first
propellent region 56 in a radial direction which produces high
pressure combustion products at a rate which increases as com-
bustion progresses. As the first region 56 is consumed, the
second region 58 is ignited and combusts at a slower rate because
10 it is burning axially with respect to a central axis 65. The
terminology used to describe the mode of combustion of the second
region 58 is a cigarette burning mode. It will accordingly be
evident that the difference in the rate of combustion of the
regions 56, 58 is due to the geometry thereof while the composi-
15 tion of the regions 56, 58 is preferably the same. The reasonthat the composition of the regions 56, 58 is preferably the same
is that it makes manufacture considerably simplier since the
propellent charge 30 may be cast at one pour.
The igniting means 32 may be of any suitable type for
20 initiating combustion of the propellent region 56 in a radial
direction in response to an electrical impulse delivered through
the wire line 28. A suitable technique for igniting the pro-
pellent charge 30 is shown in U. S. Patents 1,374,545; 3,264,986;
3,313,234; 4,064,935 and 4,081,031. In this type igniting system,
25 the passage 60 is filled with a high velocity ignition material
66 on top of which is placed an electric bridge wire ignitor 68.
The connect-ing means 34 may be of any suitable design. One
technique that has proved quite satisfactory is to embed a loop
of wire 70 in the propellent charge 30 as it is being cast. As
30 the propellant solidifies, the wire loop 70 becomes an integral
part of the propellent charge 30. The wire loop 70 may then
according.ly act as a bail for the device 26. Consequently, the
wire line 28 is connected to the wire loop 70 in load transferring
relation, as by tying the wire line 28 to the loop 70, by the
35 provision of a hook on the wire line 28 receiving the loop 70
or by physically tying a knot in the wire line 28 with the wire
loop 70 inslde the knot. - The terminal:end.of.the wire.line 28 ..
i:s ~heniinserted in the upper end`of the passage 60.in energizing
relation with the ignitor 68 and sealed, as by the application
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of a sealant body 72 comprised of wax, ~uick drying adhesive,
silicon sealant or the like.
After the device 26 is lowered in the well 10 to a location
adjacent the perforations 22, an electrical impulse is delivered
5 down the wire line 28 to energize the ignitor 68 and ignite the
ignition material 66. The ignition material 66 burns quite
rapidly and acts to ignite the propellant in the region 56 along
the axial extent of the passage 60 thereby commencing burning of
the propellant in the region 56 in a radial direction as suggested
10 in Figure 3. Figure 4 illustrates the tool 26 at a somewhat later
stage of combustion illustrating the region 56 subs~antially
consumed. On the pressure-time curve of Figure 2, it will be
evident that pressure in the well bore is approaching its maximum
value.
It will be appreciated that there is some combustion of the
propellent material in an axial direction as may be seen from a
comparison of Figures 3 and 4. It will likewise be evident that
the rate of conswnption of the propellent material is increasing
as combustion proceeds from the state illustrated in Figure 3 to
20 the stage illustrated in Figure 4 ~ince the surface exposed to
combustion is increasing as the circumference of the consumed
material increases. The maximum rate of combustion of the tool
26 occurs near the configuration illustrated in Figure 4 where
the first region 56 is substantially consumed. It will be seen
25 that the next stage of combustion, illustrated in Figure 5, is
of a substantially wholly axial combustion mode where the rate
of combustion products generation remains substantially constant
as the flame front progresses downwardly through the second region
58. Consequently, it will be seen that the rate of combustion
30 of the propellent material in the device illustrated in Figure
is shown by a curve 74 in Figure 6. It will, of course, be
evident that the shape of the curve 74 is analogous to the shape
of the curve 46 in Figure 2.
As mentioned previously, it is evident that the desired
35 shape of the curve 46 may be accomplished in a variety of techni-
ques. Another simple techni~ue is illustrated in Figure 7 where
a tool 76 includes regions 78, 80 of different composition which
combust at significantly different rates. The first region 78,
which produces high pressure combustion products at a rapid rate,
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is in combustion transferring relation to a second region 80
which exhibits a lower rate of generating combustion products.
Igniting means 82 is provided for igniting the first region 78
in an axial or cigarette burn mode. Although many propellent
compositions are suitable for the regions 78, 80, the following
are suggested:
1) An example of the fast propellant is granular
black powder having a composition of 72% potassium nitrate, 15%
charcoal and 13~ sulfur. This composition should have air spaces
10 between the granules and be protected from fluid entry by adding
water and oil, as by the provision of a metallic enclosure 83
which splits or bursts during combustion of the black powder to
emit the pressure combustion products.
2) An example of the slow propellant is smokeless
15 powder which consists essentially of nitro cellulose. The second
region 80 is accordingly in a solid cylindrical form without any
perforations and consists of a single grain of nitro cellulose.
This particular slow propellant need not be enclosed in the water
proof container 83 except for the end face from which cigarette
20 burning is to start and progress.
In operation, the device 76 is ignited by an electrical
impulse delivered through the wire line 84 to energize the
igniting means 82 to ignite the first propellent region 78 which
burns at a first combustion rate. As the first propellant region
25 78 is consumed, it in turn ignites the second propellent region
80. The pressure curve generated in the well bore by the device
76 is substantially the same as that of the device 26 although
the rate of combustion curve 85 is of somewhat different con-
figuration.
Another simple technique for generating a pressure-time
response curve 46 of the type desired is illustrated in Figure 8
where a tool 86 includes a first region 88 which produces high
pressure combustion products at a rapid rate and a second region
90 which exhibits a lower rate of combustion~ Means 92 is pro-
35 Yided for igniting the first and second regions 88, 90 in a
radial mode. The composition of the regions 88, 90 may be of
any suitable type, for example granular black powder and nitro-
cellulose as in the embodiment of Figure 7. The igniting means
40 92 may be of any suitable type and is illustrated as comprising
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11 --
an elongate passage 94 which extends through the first and second
regions 90 which is filled with an ignition material 96 with a
bridge wire ignitor 98 connected to a wire line 100.
In the device of Figure 8, the first and second regions
88, 90 are simultaneously ignited and generate combustion products
at a rate which is the sum of the generation rates of the regions
88, 90. Since the region 88 combusts at a more rapid rate, ~he
propellent material of the region 88 is consumed prior to that
of the region 90. Accordingly, when the first propellent region
10 88 is consumed, the pressure generated by the device declines.
The rate of combustion of the device 86 is illustrated by the
curve 102 in Figure 6. The combustion rate is initially fairly
rapid and reaches a peak near the depletion of the first region
88. Following the maximum combustion rate, there is a transition
15 period in which the rate of combustion falls off until the rate
of combustion is dictated wholly by the consumption of the
material in the propellent region 90.
Although the invention has been described in its preferred
forms with a certain degree of particularity, it is understood
20 that the present disclosure is only by way of example and that
numerous changes in the details of construction and in the com-
bination and arrangement of parts may be resorted to without
departing from the spirit and scope of the invention as herein-
after claimed.
