Note: Descriptions are shown in the official language in which they were submitted.
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STIMULA~ING SUBTERRANEAN FORMATIONS IN THE OPEN HOLE
Background of the Invention
This invention relates to a technique for stimulating
subterranean formations, such as those containing hydrocarbons,
in the open hole, i.e. in a section of the well bore which is not
cased.
At the present time, almost all stimulations of hydrocarbon
producing wells are conducted in cased holes, i.e. where steel
casing has been cemented in the well bore and perforations
provided between the interior of the casing and the formation
desired to be stimulated. In most areas, this is eminently
sensible because the productive formation may be relatively thin
and relatively close to a water bearing formation. Conventional
completion techniques, including casing the well and perforating
the desired interval, act to direct the stimulating technique to
the vertical section of the hole where it is desired. Because of
the inability to control or direct the stimulating efforts,
attempted stimulation efforts in the open hole at this period of
development is unusual in most parts of the world.
There are, however, some parts of the world where oil field
completions are habitually in the open hole. For example, in
Kentucky, the surface hole is drilled with mud and surface pipe
set to close off surface water sands. Below the surface pipe,
the hole is typically drilled with air, either by using a cable
tool rig or an air rotary rig. If a productive section is
encountered, the well begins to produce through the baloee line
so that the amoun-t and type of production can be estimated. If
it is desired to complete a well in this section, the bit is
removed from the hole, and tubing is run into the well. A hook
wall packer is seated against the wall of the bore hole and the
well is produced up the tubing string in a somewhat unusual
manner which happens to be typical for this area. Such wells may
be acidized merely by pumping acid down the tubing in the
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expectation that the acid will enter the formation somewhere
between the bottom of the packer and the bottom of the hole.
Because wells completed in this fashion typically produce from
the bottom of the hole, there is adequate control over the
S location where the acid is injected.
At one time, perhaps ~ifty years ago, it was common to
stimulate wells in the open hole by shooting them with nitro-
glycerine. This technique has now been substantially wholly
displaced in mo~t petroleum producing areas by hydraulic fractur-
ing in cased holes.
As alluded to, there are a wide variety of formationstimulation techniques known and practiced in the oil field. Of
chemical techniques, acidizing is by far the most common. Of the
mechanical techniques, the early nitroglycerine shooting has been
displaced by hydraulic fracturing. Another mechanical technique
involves the generation of large quantities of relatively high
pressure gas in the bore hole. This gas generation type approach
liberates the gas at a location adjacent the formation to be
fractured. Because the evolved gases take the path of least
resistance, they flow into the formation desired to be stimu-
lated. The duration of gas evolution of prior art tools is
relatively short, mainly because the quantity of propellent
material that can be lowered into the well is quite limited
because of the necessarily small internal diameter of casing in
the well.
This invention relates to a development in a gas generation
type technique for fracturing or stimulating a subterranean
formation which is typically productive of oil or gas. In the
technique of this invention, the operation is conducted in the
open hole. Consequently, a much larger quantity of gas generat-
ing or propellent material may be employed because the bore hole
diameter is necessarily substantially larger than casing or
tubing through which the prior art gas generating type tools have
had to pass. As an example, a typical bore hole is drilled with
a bit 7 7/8" in diameter into which 4 1/2" O. D. casing is
cemented. The internal diameter of 4 1/2" casing is approximate-
ly 4 inches. Because of the necessary clearances that must be
allowed, the largest tool that can be run in a 4 1/2" cased hole
is not greater than about 3 1/2 inch. Because the amount of
propellent material that may be placed in a well is proportional
to the square o~ the diameter, it will be seen that considerably
more material may be placed in the open hole than can be placed
inside casing.
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Rather than merely increasing the size of the prior art gas
generation type fracturing tools, which are run into a well on a
wire line, this invention contemplates the assembly of the tool
inside the well at a location adjacent where fracturing is to
occur. In the event the fracturing location is at the bottom of
the well bore, an ignitor is placed at the bottom of the hole,
either by dropping it into the well or by suspending it in the
hole from a wire line. The propellent charge, which is made up
of fuel and oxidizer, is delivered into the well either by
pumping it through a pipe string suspended in the well, by
pumping the material into the top of the well or the like. After
the propellent charge accumulates at the bottom of the hole, the
ignitor is actuated in any suitable manner to ignite the fuel of
the propellent charge. The fuel and oxidizer react to generate a
large quantity of high pressure gaseous combustion products which
rapidly flow into the formation to be fractured. Because of the
quite high gaseous velocities, sufficient fracturing occurs in
the formation to increase the productivity of the well.
One of the unusual aspects of this invention lies in the
nature of fluids which occur in the well bore. A well bore is
almost never completely free of liquids, usually liquid hydro-
carbons or water. This presents a very unusual aspect to the
practice of this invention because it is normally believed that
it is necessary to keep the fuel and oxidizer dry in order to
allo~ combustion. For example, the old bromide states "keep your
powder dry." Because water or liquid hydrocarbons will almost
always seep or flow into the bottom of the well, it will be very
difficult, if not impossible, to assemble the stimulating tool of
this invention in a well under completely dry conditions. It may
be thought that this would create insurmountable difficulties in
igniting the gas generating charge of this invention. Instead,
the presence of a liquid in the well bore provides a very
advantageous side effectO
Ignition of the fuel component of the gas generating charge
of this invention occurs by the use of a heat source which is
sufficient to raise the temperature of the fuel, adjacent the
ignitor, above its ignition temperature. Under most conditions,
this temperature is substantially above the boiling point of
water or liquid hydrocarbons present in the well, at least
adjacent the ignitor. When the ignitor creates an ignition
temperature, the liquid therearound vaporizes and allows the fuel
to reach its ignition temperature. Accordingly, fuel immediately
- adjacent the ignitor begins to combust thereby generating large
quantities of high pressure gaseous products used in the
fracturing technique. As the fuel immediately adjacent the
igniter begins to burn, a large quantity of heat is generated
thereby vapori~ing additional bore hole liquid in the vicinity
of the burning fuel particles. This necessarily raises the
temperature of the area surrounding the burning fuel particles
which acts to ignite additional fuel particles. It will
accordingly be seen that combustion of the fuel continues until
all of the fuel is consumed.
The presence of the liquid in the bore hole has an
important advantage because it reduces the rate of combustion
thereby lengthening the overall combustion time of the
propellent charge. This is quite desirable because it is known,
in the use of gas generation tools, that the longer the duration
of combustion, the greater the increase in the productivity of
the formation. Accordingly, the liquid in the bore hole acts
as a moderator of the rate of combustion.
Gas generating type stimulation tools are disclosed in
United States Patents 3,422,760; ~,064,935 and 4,081,031. As 20 will be evident, these patents disclose gas generating tools
which are lowered into a well on a wire line and which are
ignited while connected to the wire line.
In summary of the above, therefore, the present
invention may be generally considered as providing a method of
increasing the productivity of a subterranean formation
penetrated by an uncased bore hole of a well, there being a
quantity of formation liquid in the bore hole ad~acent the
formation, comprising: positioning a thermal igniter in the
uncased bore hole of the well; delivering a quantity of flowable
material into the liquid in the uncased bore hole and creating
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a charge of fuel and oxidizer around the igniter; actuating the
igniter to heat the fuel above its ignition temperature thereby
initiating combustion of the fuel in the uncased hole;
propagating a combustion front through the quantity of flowable
material by progressively raising the temperature through the
flowable material above its ignition temperature; and delivering
high pressure combustion products from the charge into the
formation.
Other features and advantages of this invention will
become more fully apparent as this description proceeds,
reference being made to the accompanying drawings and appended
claims.
IN THE DRAWINGS:
Figure 1 is a view, largely in section, of a well
extending into the earth having a formation which is to be
stimulated by the technique of this invention;
Figure 2 is a view similar to Figure 1 illustrating the
well and an assembly of components which are ready to be
activated to fracture the formation by the technique of this
invention;
Figure 3 is an enlarged cross-sectional view of the
well of Figure 2, taken substantially along line 3--3 as viewed
in the direction indicated by the arrows;
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Figure 4 is a graph illustrating the pressure-time relation-
ship utilizing a gas generating charge;
Figure 5 is a longitudinal cross-sectional view of a well
extending into the earth illustrating another embodiment of this
invention;
Figure 6 is a view similar to Figure 5 illustrating the well
and an assembly of components that are ready to be activated to
fracture the formation by the technique of this invention;
Figure 7 is a graph illustrating the pressure-time relation-
ship produced by the technique of Figures 5 and ~; and
Figure 8 is a longitudinal cross-sectional view of a well
and an assembly of components that are ready to be activated to
fracture the formation by the technique of this invention.
Referring to Figures 1 and 2, there is illustrated a well 10
extending into the earth 12. As is conventional, the well 10 has
been drilled by first drilling a surface hole 14 into which
surface casing 16 has been cemented by a cement sheath 18. The
well head, blow out preventer or other pressure control equipment
20 may be attached to the exposed end of the surface pipe 1~ as
will be appreciated by those skilled in the art.
Below the surface pipe 16, the well 10 has been drilled with
a bit to produce an uncased hole 22 which extends into or through
a hydrocarbon bearing formation 24 which is to be stimulated.
The formation 24 may be of the type which includes matrix
porosity, such as sandstone, siltstone, dolomite, dolomitized
limestone, weathered limestone, or the like. In addition, the
formation 24 may be of the type in which the mobile hydrocarbons
are contained in fractures. Such formations are exemplified by
the Austin ~halk of South Texas and the Devonian shale of
Kentucky and West Virginia.
If the well 10 were drilled to produce the formation 24, the
well would be bottomed in the formation 24. Instead, Figure 1
illustrates that the bore hole 22 extends substantially below the
bottom of the fo~mation 24. I~ is desirable, in the conduct of
this invention, that the bottom of the bore hole 22 be in or near
the bottom of the formation 24. To this end, a plug 2~ fills the
bottom of the bore hole 22 to a level adjacent the bottom of the
formation 24. The plug may be of any suitable material, such as
cement, sand or the like.
As mentioned previously, one of the difficulties encountered
in creating an open hole stimulating tool resides in the accumu-
lation of formation liquid 28 in the bottom of the bore hole 22.
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The ~ormation liquid 2~ is typicall~ water, hydrocarbons or a
mixture thereof. If the liquid 28 is hydrocarbons, or partially
hydrocarbons, it may serve as part or all of the fuel of the gas
generating assembly of this invention.
It may be desirable to determine the location of the top of
the liquid 28. This may be accomplished by conventional wire
line pressure measuring instruments which are commonly used in
the oil field to determine the bottom hole pressure of oil or gas
wells. Such pressure instruments are capable of measuring the
pressure in the bore hole 22 to within .1 psi. With instruments
of such sensitivity, it is easy to detect the top of the liquid
28.
For purposes of illustration, it is assumed that the gas-
liquid interface is at or near .he top of the formation 24 as
illustrated in Figure 1. An ignitor 30 is positioned in the bore
hole 22 adjacent the formation 24. The ignitor 30 may be lowered
into the well 10 on a wire line or may simply be dropped into the
well 10. In the absence of centralizers (not shown) the ignitor
will not be located in the exact center of the bore hole 22 but
will instead be somewhat inclined, as illustrated.
As will be more fully explained hereinafter, the ignitor 30
is basically a heat source which is sufficient to raise the
temperature of the fuel-oxidizer mixture immediately adjacent
thereto to a level above the ignition temperature of the fuel.
Because the pressure in the bore hole 22 is normally rather low,
the ignition temperature of the fuel is typically substantially
above the boiling point of the liquid 28.
In Figure 1, the ignitor 30 is illustrated as being somewhat
longer than the vertical extent of the formation 24. As will be
pointed out more fully hereinafter, the length of the ignitor 30
relative to the quantity of fuel-oxidizer has an effect on the
mechanics and rate of combustion of the fuel-oxidizer charge.
The ignitor may be any suitable heat source which is
sufficient to raise the temperature of the mixture in the bore
hole above the ignition temperature of the fuel component
therein. Manifestly, the exterior of the ignitor must be such
that it is not attacked by the well bore liquid 28. To this end,
the ignitor 30 may comprise a thin wall aluminum tube filled with
a propellent charge. In the alternative, the ignitor 30 may
comprise a gas generating type fracturing tool presently commer-
cially available from Servo ~ynamics, Inc. of Corpus Christi,
Texas. Those skilled in the art will appreciate that numerous
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diEferent types of ignitors m;ght be employed such as phosphorous
materials, thermite grenades and the like.
The ignitor 30 may be actuated in any suitable manner.
Conveniently, the ignitor 30 may include a radio frequency
receiver and battery which act to actua'e the ignitor 30 upon
receiving a radio frequency signal delivered down the well bore
22~ In the alternative, the ignitor 30 may be of the time delay
type which ignites after the passage of a predetermined time.
After the ignitor 30 has been placed in the liquid 28, a
quantity of flowable material comprising fuel and oxidizer
components 32, 34 are delivered into the bottom of the well bore
22 into the liquid 28. The fuel 32 may be a liquid, a mixture of
liquids, a mass of solid particles, or mixtures thereof.
Likewise, the oxidizer 34 may be a liquid, a mixture of liquids,
a mass of solid particles or a mixture of liquids and solids.
The function of the ignitor 30 is to deliver sufficient heat
into the mixture surrounding it so as to vaporize part of the
liquid 28 adjacent the ignitor 30 and elevate the temperature of
fuel particles around the ignitor 30 to a temperature above the
fuel ignition temperature. Thus, the ignitor 30 merely starts
the combustion of the material in the bottom of the well 10. In
order to keep the combustion going, the fuel-oxidizer mixture
must be such as to keep the combustion process going. Specifi-
cally, there is a requirement for a substantial amount of heat in
order to continue to vaporize the liquid 28 in the vicinity of
the combustion front in order to raise the temperature of the
fuel components 32 above their ignition temperature. Of course,
if the liquid 28 is liquid hydrocarbons, it will vaporize and
then combust in a manner that is rather satisfactory.
If the fuel and oxidizer components 32, 3~ are liquid, they
may be delivered into the bottom of the bore hole 22 by pumping
them through a tubing string (not shown). If the fuel and
oxidizer components 32, 34 are solids, they may be slurried and
then pumped into the well bore or may merely be dumped into the
top of the well 10 through the equipment 20. In any event, the
fuel and oxidizer components 32, 34 accumulate in the bottom of
the bore hole 22 raising the level of the liquid 28 substantial-
ly O
After the assembly of Figure 2 has been completed, the
ignitor 30 is energized to produce a substantial amount of heat.
The heat evolved by the ignitor 30 is sufficient to raise the
temperature in a zone 38, concentric with the ignitor 30, to a
value greater th~n the ignition temperature of the fuel 32.
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Under normal pressures occurring in the well bore 22 prior to
ignition, this temperature is sufficient to vaporize the forma-
tion liquid 28 in the zone 38. Accordingly, the ignitor 30
initiates combustion of the fuel 32 in the zone 38.
Combustion in the first zone 38 has two effects. Eirst,
there is evolved a substantial quantity of hot, high pressure
gaseous combustion products which ultimately pass into the
formation 24 to increase the productivity thereof. Second, the
temperature in a zone 38 is raised sufficiently to heat a second
zone 40 above the ignition temperature of the fuel 32 therein and
to vaporize the formation liquid 28. It will accordingly be seen
that the combustion of the fuel-oxidizer charge in the well bore
22 proceeds, in a radial fashion, through a multiplicity of
increasingly larger diameter zones 3~, 40, 42, 44 and so on. It
will be appreciated, of course, that the pattern shown in Figure
3 is stylized and is a conceptual explanation of what is believed
to be occurring in the bore hole 22 during combustion of the
fuel-oxidizer charge. Applicant is, of course, not bound by any
theory of the combustion occurring during the practice of this
invention.
When the fuel and oxidizer combusts, a large quantity of
gaseous combustion products evolve. In order to fracture or
stimulate the formation 24, a substantial portion of these
gaseous products must be directed into the formation 24. It will
be fruitless, of course, to allow all of these products to move
upwardly through the well bore 22. In the prior art, gas
generating tools are ignited under a s~bstantial liquid column,
e.g. 1000-2000 feet of water, lease crude, diesel or the like.
Because the combustion time of prior art tools is quite short,
for example less than three seconds, the inertia of the liquid
cushion is sufficient to require substantially all of the evolved
gaseous products to pass into the formation being stimulated.
In the technique of this invention, the elapsed combustion
time is substantially longer than in prior art tools for two
reasons. First, because of the geometry of the bore hole 22, a
larger quantity of fuel and oxidizer may be delivered into the
bottom of the hole as compared to what can be run on a wire line
inside a cased hole or through tubing. Second, because of the
moderating effect of the liquid 28 on the combustion process, the
rate of combustion is somewhat slower. For these reasons, it is
necessary to provide some type of cushion or buffer on top of the
fuel-oxidizer mixture. The simplest technique is merely to dump
a quantity of sand 36 into the well. Even though the sand 36 is
_ 9 _
permeable, a large proportion of the gases evolved from the fuel
and oxidizer are delivered into the Eormation 24, presumably
because the elapsed time or combustion is so short that the
longer path through the sand 36 is not the path of least resis-
tance. In the alternative, a water cushion is acceptable as inthe prior art.
As mentioned previously, one of the side effects of the
formation liquid 28 is a moderation of the combustion rate during
the consumption of the fuel and oxidizer 32, 34. The presence of
the liquid 28 must reduce the combustion rate because some of the
heat generated goes to vaporizing the liquid 28 rather than
elevating the temperature of the fuel particles 32 in the
approaching zone of combustion. This may be seen in Figure 4
where the curve 46 illustrates the pressure-time relationship
during a typical fracturing operation using a gas generating
charge of predetermined size. The curve 48 illustrates the
pressure-time relationship using the same size charge during the
technique of this invention. In this invention, the combustion
rate is slower due to vaporization of the li~uid 28. In addi-
tion, a greater quantity of gas is produced from the same sizedcharge because of the steam or vapor produced by the liquid 28.
Referring to Figure 5, there is illustrated a somewhat more
sophisticated embodiment of the technique of this invention. In
Figure 5, a well 50 is illustrated as penetrating the earth 52
and includes an uncased bore hole 54 extending into a hydrocarbon
bearing formation 56 which is to be stimulated. ~n ignitor 58 is
placed in the bottom of the bore hole 54. It will be seen that
the vertical height of the formation 56 is substantially greater
than the vertical height of the ignitor 58. A quantity of
formation liquid typically resides in the bottom of the bore hole
54.
As shown in Figure 6, a quantity of fuel and oxidizer
components 62, 64 are delivered into the bottom of the bore hole
54. A suitable tamping material 66, such as sand or water, may
be placed on top of the fuel-oxidizer mixture.
When the ignitor 58 is energized, an initial combustion zone
68 is created which experiences radial combustion, i.e. the
combustion front progresses radially away form the axis of the
ignitor 58. As shown in Figure 7, this creates a relatively
sharp rise in pressure until a maximum value 70 is achieved at
about the time that the radial combustion process ends. After
the fuel and oxidizer in the combustion zone 68 is consumed, a
zone 72 begins to combust in an axial direction. Because the
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rate of combustion in the axial direction is slower than in the
radial direction, the pressure generated subsides. The axial or
cigarette burn in the combustion zone 72 causes a relatively long
relatively high pressure period 74, lower than the maximum
5 pressure value 70. As the fuel-oxidizer mixture is exhausted,
the pressure adjacent the formation 56 declines along a line 76.
One of the peculiarities of the embodiment of Figures 5 and
S is that the propellent column located above the thermal ignitor
58 acts as stemming. This portion of the propellent column
10 serves as its own tamp until it has nearly been consumed in a
vertical combustion direction. This process may take several
seconds during which the gases generated are compelled to
penetrate the formation selected for stimulation. A conventional
tamp 66 of sand or liquid above the propellent may not be
15 required.
Referring to Figure 8, there is illustrated another embodi-
ment of the technique of this invention and is a variation of the
embodiment of Figures 5 and 6. In Figure 8, a well 78 is
illustrated as penetrating the earth 80 and includes an uncased
20 bore hole 82 extending into a hydrocarbon bearing formation 84
- which is to be stimulated. A quantity of formation liquid 86 may
have accumulated in the bore hole 82. A quantity of fuel and
oxidizer components 88, 90 have been delivered into the bore hole
82. It will be seen that the top 92 of the fuel-oxidizer mixture
25 extends above the liquid level 94. An ignitor 96 is then placed
in the bore hole 82 resting on the upper surface 92 of the fuel-
oxidizer mixture.
A second quantity 98 of fuel-oxidizer mixture is then
delivered into the bore hole 82 followed by a charge of tamping
30 material 100, such as sand or water.
When the ignitor 96 is energized, there is created a first
combustion zone 102 in which the combustion front propagates in a
radial direction away from the axis of the ignitor 96. This
causes a fairly rapid increase in pressure adjacent the formation
35 84 to produce a pressure-time curve similar to that shown in
Figure 7.
At about the time the fuel-oxidizer mixture is exhausted in
the zone 102, propagation of the combustion front becomes axial
in the combustion zone 104. Because combustion in the zone 104
40 is axial or similar to a cigarette burning, the combustion rate
falls off to a slower rate. For a more complete description of
the effect of the change of a radial combustion pattern to an
axial combustion pattern, attention is directed to the disclosure
of my U.S. Patent 4,530,396, entitled DEVICE FOR STIMULATING A
SUBTERRANEAN FORMATION, issued July 23 " 985.
In the description of the previous embodiments, a
number of assumptions have been made which may not be apparent
and which warrant further amplification. If a dry granular
propellent were placed in a well bore having no liquid therein,
the hot combustion gases generated by burning the propellent are
able to migrate freely and at high velocity through the gas
filled interstices between particles. This conceivably could
cause excessive pressure and under some conditions progress to
near detonation. On the other hand, if adequate liquid is
provided to fill the interstices, gas propagation between
particles of propellent is blocked and the reaction and heat
transfer is restricted to the thermal conductivity of the
blocking fluid-granule mix. In the embodiments discussed in
detail, the combustion process has been described as vaporizing
the liquid. It is not essential that the liquids be vaporized.
Indeed, under high enough hydrostatic heads, the vapor phase may
not be reached. It will be evident to those skilled in the art
that the temperature of the liquid, e.g. oil, water, mixtures
- thereof or some other liquid, may reach fuel ignition
temperatures without boiling.
In order to achieve ignition, those skilled in the art
will realize that a granular propellent must be heated above the
ignition temperature of the fuel provided that oxygen is
available at that temperature. In the event the oxidizer
contains chemically bound oxygen, the temperature must be raised
above the ignition temperature of the fuel as well as the
temperature at which the oxidizer gives off oxygen. In the
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event a monopropellant is used, which contains the oxygen-fuel
combination in the same molecule, the temperature has to be
raised above the thermodynamically unstable bond threshold.
As suggested previously, mixtures of oxidizer and fuel
or monopropellants may be used in pelletized form and conveyed
in this form into the well.
This invention is not to be confused with bore hole
heaters or the like in which a small amount of fuel is consumed
to heat the bore hole for a variety of reasons, for example
paraffin removal. The fracturing temperature of this invention
is easy to distinguish from bore hole heaters because the
technique of this invention uses considerably more than a few
pounds of fuel and oxidizer and generates in excess of 1000 psi
on combustion in order to be effective to fracture a formation.
Although this invention has been describ~d in its
preferred forms with a certain degree of particularity, numerous
changes in the details of construction and in the combination
and arrangement of parts may be resorted to.
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