Note: Descriptions are shown in the official language in which they were submitted.
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NON-HYDRAULIC FRACTURING AND COLD FOAM PROPPANT DELIVERY
SYSTEMS, METHODS, AND PROCESSES
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims priority to U.S. Application Serial No.
13/858,780, filed April
8,2013.
FIELD
[0002] The present disclosure relates to non-hydraulic fracturing and cold
foam proppant
delivery systems and methods for increasing the permeability of underground
hydrocarbon
formations, thereby increasing the ability to extract such hydrocarbons.
BACKGROUND
[0003] Hydrocarbon assets, such as oil and natural gas ("NG"), are often
found underground
in "tight" geological formations, such as sandstone or shale. These require
"unconventional"
drilling and completion techniques, including the "fracturing" (or "fracking")
of the geological
strata that contain the hydrocarbons to allow those hydrocarbons to be
released for, recovery,
treatment, storage and distribution. Existing fracturing methods are
hydraulic, i.e., they use liquids
=
for fi acturing and for delivering proppant to the fractures.
[0004] However, hydraulic fracturing and proppant delivery methods suffer
from a number of
significant disadvantages. The liquids that are presently used in standard
hydraulic fracturing¨
for example, chemically modified or treated water at ambient temperatures,
and/or cryogenic
liquid nitrogen resultin waste streams of contaminated liquid water or gaseous
methane
containing nitrogen. More particularly, using water or nitrogen results in
contamination (or
undesirable blending) of both the fracking fluids and the hydrocarbons, and
using nitrogen or liquid
carbon dioxide requires foaming agents.
[0005] The waste streams and contaminated mixtures need to be treated, and
the cost of fully
cleaning and properly disposing of the "spent" hydraulic fracturing fluid
substantially increases
the cost of hydraulic fracturing¨both in economic terms and environmental
terms. If that clean-
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up is not properly accomplished, the damage of hydraulic fracturing on the
environment may be
adverse, causing regulators and/or policy-makers to limit the use of hydraulic
fracturing in
response to concerns by the public at large, as is already the case in some
regions today. Hydraulic
fracturing also often results in significant methane emissions (with methane
being a much more
environmentally damaging greenhouse gas than CO2) and may require complex
apparatus for
mitigating such emissions.
[0006] Furthermore, some existing hydraulic fracturing technologies are
energy- and capital-
intensive. For example, use of liquid nitrogen requires the installation of a
plant for air separation
that uses deep refrigeration to liquefy ambient air, which is then broken down
to yield nitrogen.
Using nitrogen for fracking generally requires substantial energy input to
achieve the liquid states
of the nitrogen. Also, when nitrogen (or more precisely, liquid nitrogen) is
pumped to high
pressures, as required for the fracturing of deeper formations, a phase shift
occurs that shifts the
N2 from its liquid form to its gaseous state, and the delivery of proppant
under those conditions
becomes problematic.
[0007] Proppant often is delivered into fractured subterranean formations
by foams because
they tend to have lower rates of "leak off" than delivery by liquids, that is,
reduced loss of fracking
fluid from the fracturing. Most existing proppant delivery utilizes liquid CO2
or liquid nitrogen.
However, there are several drawbacks to those techniques such as the
transportation costs and
logistical complexity of importing the liquid CO2 or liquid nitrogen to the
well site, contamination
of the hydrocarbons by the liquid CO2 or liquid nitrogen, and the need for
water as the liquid base
for the foam. The use of water as the liquid base for the foam that is
energized by the pumped-to-
pressure vaporized CO', or N2, and which foam delivers the proppant, requires
many of the same
chemicals (cross linkers, slickening agents, anti-swelling compounds...) as
standard hydraulic
fracturing.
[0008] Accordingly, there is a need for an effective fracturing method that
does not use liquids.
There is also a need for a more energy-efficient fracturing process. There is
a further need for a
fracturing method that does not create contaminated waste streams requiring
difficult clean-up
measures. There is also a further need for a fracturing method that increases
the recovery of
hydrocarbons from underground formations by avoiding the use of water (which
hydrocarbons do
not interact well with).
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[0009] There is also a need for foam-based proppant delivery systems and
methods that do not
require liquid CO2, liquid nitrogen, or extensive volumes of water. Thus,
there is a need for non-
hydraulic fracturing and proppant delivery systems and methods which are less
energy-intensive,
do not require liquids for fracking and proppant delivery, do not
significantly add contamination
or waste to the fracking process, and have the potential to increase
hydrocarbon recovery.
SUMMARY
[0010] The embodiments of the present disclosure alleviate to a great
extent the disadvantages
of known fracturing and proppant delivery processes by providing non-hydraulic
fracturing and
proppant delivery systems, methods and processes using metacritical phase
natural gas (which may
be referred to hereinafter as "meta-NG") as a fracturing and proppant
transport medium. The
metacritical phase of a gas is that set of conditions where the gas is above
its critical pressure and
is colder than its critical temperature. The meta-NG, which is pumped to a
high pressure, is used
to create or extend fissures in subterranean formations and hold those
fissures open to release
hydrocarbons contained in those formations. The meta-NG may be pumped to a
high pressure,
warmed and used to deliver suitable proppant to the fissures in the
subterranean formations.
[0011] Embodiments of the present disclosure provide systems and methods of
energized gas
fracking by delivering proppant using foam wherein the liquid is a non-aqueous
fluid that may also
include a surfactant and to which proppant is added, which fluid is energized
into a foam by meta-
NG, and recovering the liquids that result from the collapsed foam, which
liquids return to the
surface with the released hydrocarbons. Disclosed systems and methods of
proppant delivery may
use meta-NG pumped to pressure at the well, where the meta-NG is produced at
the well or nearby,
and where the feed NG is not significantly different from the NG that is about
to be liberated. A
non-aqueous liquid, such as one of many alcohols, including but not limited to
ethanol, methanol,
or glycol, is used as the pumped-to-pressure liquid that is energized, or
foamed, in some
embodiments together with a surfactant, by the high-pressure meta-NG or
compressed natural gas
(CNG).
[0012] Use of a non-aqueous liquid, such as one of many alcohols, including
but not limited
to ethanol, methanol, or glycol, as the pumped-to-pressure liquid that
(together with a surfactant)
is energized (foamed) by the high-pressure meta-NG. The choice of meta-NG
versus compressed
natural gas (VNG") for foaming will depend on the desire to produce foam that
is more viscous
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with the colder meta-NG or less viscous with CNG. Foam viscosity is one of
many factors that
well-completion experts control in order to achieve deep proppant delivery
inside the fracture.
Higher viscosity helps create wider fractures and helps carry the proppant
deeper inside the
fractures. Additional factors that may determine if a particular well
completion effort would use
meta-NG or CNG may include the temperature tolerance of the well casing and
the foam delivery
piping/tubing.
[0013] Exemplary embodiments include a method of fracturing subterranean
formations,
comprising pumping meta-NG into a subterranean formation to create or extend
one or more
fissures in the formation. The meta-NG may be produced on site. Methods may
further comprise
maintaining or increasing pressure of the meta-NG in the formation by pumping
more meta-NG
into the fissures to hold the fissures open. In exemplary embodiments, a
proppant is delivered into
the subterranean formation by the meta-NG. The proppant may be lubricated and
delivered via
warm compressed natural gas ("CNG") at a high pressure or by foam at various
pressures and
temperatures.
[0014] In exemplary embodiments, the high-pressure warm CNG is produced by
pumping to
pressure and warming the meta-NG. Exemplary methods may further comprise
releasing the
pressure of the CNG such that the proppant alone holds the fissures open. In
exemplary
embodiments, the fissures are created and held open without use of water or
other liquids, and the
proppant is delivered without water or other liquids. Moreover, the fracturing
and proppant
delivery steps may be performed without chemical additives for mitigating
adverse effects of liquid
use.
[0015] Exemplary embodiments of a non-hydraulic fracturing process comprise
pumping
meta-NG into a subterranean formation to create or extend one or more fissures
in the formation
and delivering a proppant into the subterranean formation. The process may
further comprise
maintaining or increasing pressure of the meta-NG to hold the fissures open.
In exemplary
embodiments, the proppant is lubricated, and the proppant may be delivered via
warm CNG
produced by pumping to pressure and warming the meta-NG. By using exemplary
embodiments
of disclosed processes, the fissures are created and held open without use of
water or other liquids
and the proppant is delivered without water or other liquids.
[0016] Exemplary embodiments of a non-hydraulic fracturing system comprise
a meta-NG
supply, a cryogenic storage tank for storing the metacritical natural gas, at
least one positive
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displacement device (e.g., a pump or compressor), and a network of pipes
(which piping may
include well casing and/or cement). The cryogenic storage tank is fluidly
connected to the meta-
NG supply, and the positive displacement device is fluidly connected to the
cryogenic storage tank.
The network of pipes is fluidly connected to the at least one positive
displacement device and the
cryogenic storage tank, and at least one pipe extends into a subterranean
formation. In exemplary
embodiments, the meta-NG is supplied by an on-site natural gas plant
configured to convert natural
gas into meta-NG by an appropriate balance of compression and refrigeration.
As discussed
throughout this disclosure, the meta-NG can be produced from CNG returning
from the
subterranean formation to an aboveground NG plant.
[0017] Exemplary systems are arranged so the meta-NG flows through the
network of pipes
into the subterranean formation such that the meta-NG creates or extends one
or more fissures in
the formation. The at least one positive displacement device increases the
pressure of the meta-
NG to hold the fissures open. The systems may further comprise a proppant
housed in a storage
vessel, hoppers, and/or other devices that allow the proppant to enter the
meta-NG so the meta-
NG can deliver proppant to the fissures in the subterranean formation. In
exemplary embodiments,
warm high-pressure CNG flows through the network of pipes and the proppant is
delivered into
the fissures of the subterranean formation via the warm high-pressure CNG.
[0018] Exemplary embodiments also include methods of delivering a proppant
via cold foam,
comprising providing a non-aqueous liquid, adding a surfactant to the non-
aqueous liquid, adding
a proppant to the non-aqueous liquid to form a non-aqueous liquid, surfactant
and proppant stream,
pumping to pressure the non-aqueous liquid, surfactant and proppant stream,
using pressurized
natural gas to energize the non-aqueous liquid, surfactant and proppant
stream, and delivering the
energized non-aqueous liquid, surfactant and proppant stream into a
subterranean formation. The
pressurized natural gas may be metacritical phase natural gas, or meta-NG. The
proppant holds
open one or more fissures in the subterranean formation.
[0019] In exemplary embodiments, the energized non-aqueous liquid,
surfactant and proppant
stream is at a temperature between ambient temperature and about -140 F. The
non-aqueous
liquid may be methanol, and the methanol, surfactant and proppant stream
enters a foam state
when it is energized by the high pressure NG. In this context, the term
"energize" refers to the
introduction of a high-pressure gas stream into a liquid stream that contains
(among other things)
a surfactant, such that foam is produced. In exemplary embodiments, the foam
state of the
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methanol breaks in the subterranean formation such that the methanol becomes
liquid or vapor.
The liquid or vapor methanol may then dissolve in a hydrocarbon liberated from
the subterranean
formation forming a methanol-hydrocarbon solution, and the methanol-
hydrocarbon solution then
travels out of the subterranean formation. Exemplary embodiments further
comprise recovering
the methanol by directing metacritical phase natural gas in a first direction
and directing the
methanol-hydrocarbon solution in a second direction substantially opposite to
the first direction.
In such embodiments, the metacritical phase natural gas cools the methanol-
hydrocarbon solution
and the methanol in the methanol-hydrocarbon solution condenses out of
solution.
[0020] Exemplary embodiments include methods of recovering proppant
delivery liquid.
Such methods comprise first recovering a solution of proppant delivery liquid
dissolved in a
hydrocarbon liberated from a subterranean formation when the solution travels
out of the
subterranean formation. Then the metacritical phase natural gas is directed in
a first direction and
the proppant delivery liquid-hydrocarbon solution is directed in a second
direction substantially
opposite to the first direction. In such embodiments, the metacritical phase
natural gas cools the
proppant delivery liquid-hydrocarbon solution and the proppant delivery liquid
in the proppant
delivery liquid-hydrocarbon solution condenses out of solution. The proppant
delivery liquid may
be an alcohol and may be methanol in exemplary embodiments. In exemplary
embodiments, the
metacritical phase natural gas is produced on site.
[0021] An exemplary embodiment of a proppant delivery system comprises a
proppant supply,
a surfactant supply fluidly connected to the proppant supply, a non-aqueous
liquid supply fluidly
connected to the proppant supply and the surfactant supply, a foaming vessel
fluidly connected to
the proppant supply, the surfactant supply and the non-aqueous liquid supply,
a natural gas supply
fluidly connected to the foaming vessel, at least one positive displacement
device fluidly connected
to the foaming vessel, and a network of pipes fluidly connected to the at
least one positive
displacement device with at least one pipe extending into a subterranean
formation.
[0022] In exemplary embodiments, a proppant from the proppant supply and a
surfactant from
the surfactant supply are added to non-aqueous liquid from the non-aqueous
liquid supply to form
a non-aqueous liquid, surfactant and proppant stream. The at least one
positive displacement
device may pump to pressure the non-aqueous liquid and proppant stream. The
pressurized natural
gas then foams the non-aqueous liquid, surfactant and proppant stream. In
exemplary
embodiments, the energized non-aqueous liquid, surfactant and proppant stream
flows through the
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network of pipes into the subterranean formation such that the energized non-
aqueous liquid,
surfactant and proppant stream holds open one or more fissures in the
subterranean formation. In
exemplary embodiments, the non-aqueous liquid is methanol.
[0023] Accordingly, it is seen that non-hydraulic fracturing systems,
methods, and processes
are provided. The disclosed non-hydraulic fracturing systems and methods do
not require liquids
for fracking and proppant delivery because they use metacritical phase natural
gas for fracking the
subterranean formation and CNG produced from the metacritical phase natural
gas as the proppant
delivery medium. The disclosed systems and methods do not add (or result in)
contamination or
waste to the fracking process and are less energy-intensive. These and other
features and
advantages will be appreciated from review of the following detailed
description, along with the
accompanying figures in which like reference numbers refer to like parts
throughout.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] The foregoing and other objects of the disclosure will be apparent
upon consideration
of the following detailed description, taken in conjunction with the
accompanying drawings, in
which:
[0025] FIG. 1 is a phase diagram of methane, which is an analog for the
phase diagram of
natural gas;
[0026] FIG. 2 is a box diagram of an embodiment of a non-hydraulic
fracturing system in
accordance with the present disclosure;
[0027] FIG. 3 is a box diagram of an embodiment of a proppant delivery
system in accordance
with the present disclosure; and
[0028] FIG. 4 is a box diagram of an embodiment of a proppant liquid
recovery system in
accordance with the present disclosure.
DETAILED DESCRIPTION
[0029] In the following paragraphs, embodiments will be described in detail
by way of
example with reference to the accompanying drawings, which are not drawn to
scale, and the
illustrated components are not necessarily drawn proportionately to one
another. Throughout this
description, the embodiments and examples shown should be considered as
exemplars, rather than
as limitations of the present disclosure. As used herein, the "present
disclosure" refers to any one
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of the embodiments described herein, and any equivalents. Furthermore,
reference to various
aspects of the disclosure throughout this document does not mean that all
claimed embodiments
or methods must include the referenced aspects.
[0030] In general,
embodiments of the present disclosure's systems and methods¨called
Vandor's Refrigerated Gas Extraction ("VRGE") ____________________ use
cryogenic non-liquid, metacritical phase
natural gas for non-hydraulic fracturing and/or as a delivery medium for
proppant in the non-
hydraulic fracturing process. Meta-NG, which is also sometimes referred to as
"cold compressed
natural gas" or "pumped liquid natural gas," is natural gas in the
metacritical phase. As shown in
FIG. 1, the metacritical phase of a fluid is found on a phase diagram above
the fluid's critical
pressure, colder than the fluid's critical temperature, but not within the
solid phase. That
metacritical phase is above the liquid phase, to the left of the supercritical
phase and to the right
of the solid phase on FIG. 1. As such, metacritical phase fluids are not true
liquids, but will behave
much like liquids, most importantly in that they can be pumped to a higher
pressure by liquid
pumps, including reciprocating pumps, and other such positive displacement
devices. The density
of metacritical phase fluids can be nearly as dense (and sometimes even more
dense) than the
density of the liquid phase of the fluid. Metacritical phase fluids do not
"boil" because they are
above the liquid phase, and they do not need to be "condensed" in order to
allow for pumping,
because they are dense enough (even as a non-liquid) to be "viewed" as liquids
by pumps.
[0031] As an
overview, embodiments of disclosed non-hydraulic fracturing systems and
methods send pumped-to-pressure cryogenic meta-NG down through a network of
pipes into a
subterranean formation to create or extend fissures in the formation. The meta-
NG is produced at
the well site from nearby pipeline gas, or from a nearby (previously
completed) natural gas well,
rather than being imported to the site as liquefied natural gas ("LNG") or
liquefied petroleum gas
("LPG"). Meta-NG can be pumped to any required pressure by various known
pumping devices,
delivering a high-enough fluid pressure and "thermal shock" to the
subterranean formation so as
to fracture the formation.
[0032] When the
formation "yields" (or fractures), as indicated by pressure monitoring
equipment aboveground, pressure is increased by the aboveground pumps, so as
to keep the
fissures open, and is followed by the insertion and delivery of the proppant
by warm CNG. That
delivery is possible because the meta-NG can be pumped to a high pressure and
then warmed to
produce a high-pressure CNG stream, which will carry the proppant into the
fissures formed or
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extended by previously sent-down meta-NG. The ideal proppant would be selected
by on-site
experts familiar with local conditions and the array of available proppants,
including sand or man-
made proppants such as ceramic balls. The proppant may be lubricated,
facilitating its transit
through the piping, and avoiding scouring of the piping.
[0033] The lubricated proppant is delivered by warm, high-pressure CNG,
rather than by meta-
NG. After delivery of the proppant, the pressure can be released, slightly
relaxing the formation,
but the fissures would be kept open by the proppant, allowing the natural gas
previously sent down,
and the natural gas within the formation to blend and rise as one stream to
the surface. During the
early stages of the fracking process, including before proppant delivery, such
returning NG would
be re-compressed and re-chilled to form more meta-NG, and then re-circulated
to advance the
fracking process. Thus, the NO that is used to produce the meta-NG will be a
mixture of the
previously sent down meta-NG and any NG released by the fractured formation.
[0034] The different NG streams in disclosed embodiments, which vary in
phase, temperature,
pressure, and function, are enumerated as follows. Meta-NG is referred to
herein by number 50a;
warm, high-pressure CNG is referred to herein by number 50b; CNG-proppant
stream is referred
to herein by number 50c. These three streams, at different times and for
different purposes as
described in detail herein, are sent down into the subterranean formation.
Return flow CNG
stream, which returns to the surface from the subterranean formation, is
referred to herein by
number 52.
[0035] Turning to FIG. 2, an exemplary embodiment of a non-hydraulic
fracturing system will
be described. Non-hydraulic fracturing system 10 comprises a sub-system 12
supplying meta-NG,
a cryogenic storage tank 14 for storing the meta-NG, and a network of pipes
20a-20g connecting
the above-ground equipment to the subterranean formation 18. The meta-NG
supply equipment
12 includes an array of production equipment, which may comprise different
combinations of
components such as a prime mover 22, which can be any suitable engine, a
compressor 24, a chiller
26, a gas dryer 28, one or more meta-NG heat exchangers 30, and a cryogenic
pump 32, and any
other components, including but not limited to valves, sensors, and expanders,
which together
make up a natural gas plant 34 that can produce dense-phase meta-NG. At least
one positive
displacement device is included in the equipment as well, i.e., the compressor
24 and the cryogenic
pump 32 serves as the positive displacement device to move the meta-NG through
the pipes 20b-
20c into a subterranean formation 18. It should be noted, however, that the
positive displacement
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device could be any device that causes a fluid to move, trapping a fixed
amount of it then forcing,
i.e., displacing, that trapped volume into a discharge pipe, including but not
limited to, positive
displacement pumps, such as reciprocating pumps, or compressors configured to
perform "pump"
work, such as screw compressors.
[0036] The cryogenic storage tank 14 is fluidly connected via one or more
pipes or other
conduits to the meta-NG supply equipment 12 so the produced meta-NG can be
stored for use. In
turn, one or more of the positive displacement devices (i.e., the compressor
24 and the cryogenic
pump 32) are fluidly connected to the cryogenic storage tank 14 and the meta-
NG supply
equipment 12. Finally, the network of pipes 20a-20f is in fluid connection
with the positive
displacement devices (i.e., the compressor 24 and the cryogenic pump 32) so
they can effectively
"pump" the meta-NG into the pipes. Although multiple configurations are
possible, in an
exemplary embodiment, positive displacement devices (compressor 24 and
cryogenic pump 32)
are connected to pipe 20b and/or pipe 20c.
[0037] The meta-NG supply equipment 12 can be deployed as a single unit
above a
subterranean formation holding natural gas (and/or oil or condensates), with
the well immediately
adjacent to natural gas plant 34 delivering meta-NG, and/or with another well
some distance away
acting as the "methane extraction vent" where any warmed NG would return to
the surface. That
second well would be connected back to the first well and to the meta-NG
supply equipment 12
by surface (or near surface) NG piping, completing a "loop." That loop, which
would contain
several pressure-release valves, would allow for pressure build up in the
subterranean formation,
and would allow for rapid pressure letdown by way of the integrated valves.
Such rapid pressure
letdown would result in cooling of the methane within the subterranean
fissures served by the
"loop" and would act to create dynamic stressing of the formation due to the
fluctuating pressure
and the rapid cooling of the NG within the system, which would potentially
release more
hydrocarbons from the formation.
[0038] A variation could have two (or more) meta-NG supply equipment 12
deployments
some distance apart, connected to the wide network of subterranean piping with
one or more
surface-mounted piping connections between meta-NG supply equipment 12
deployments,
allowing for a flexible regime of meta-NG injection from and warm CNG
injections from several
directions in a manner that would enhance the thermal shocking of the
underground formation,
and would offer several "paths of least resistance" for the liberated methane
to rise to the surface.
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[0039] At least some of the underground piping may have perforations 21 in
the horizontal
pipes that allow the meta-NQ 50a to enter the fissures 19 in the subterranean
formation 18. As
discussed in more detail below, a two-pipe design may be provided including a
first pipe and its
surrounding annulus as well as a pair of pipes separated by some distance. The
pair of pipes can
be connected at the surface, to each other, and with the meta-NG supply
equipment 12 at that
connecting point.
[0040] The piping below ground, and within the hydrocarbon-bearing
formation, is shown,
where pipe 20c is the vertical piping that delivers the meta-NG 50a for
fracking, and later the
CNG-proppant stream 50c. The perforated horizontal piping system 20d is shown
(not in scale)
at some depth below the surface. The vertical meta-NG piping may be
supplemented by vertical
riser pipe 20e, which allows for the meta-NG 50a (and later, liberated gas,
vaporized methane,
and/or the recovered hydrocarbons) to travel back to the surface, as discussed
in more detail herein,
without warming up the cryogenic piping, and allowing cryogenic methane to
flow down while
warmer, vaporized methane flows up. For the sake of clarity, it is shown some
distance from pipe
20b, at a remote end of pipe 20d. If that were the deployed configuration, the
aboveground portion
of pipe 20e would return to the meta-NG supply equipment 12.
[0041] Exemplary embodiments may employ a two-pipe design, shown in HG. 2.
In this
configuration, the downward flow of meta-NG 50a can occur at the same time as
the return flow
of warmed CNG 52, allowing for the rapid cool-down of the subterranean
formation 18 that is
being fractured. Exemplary embodiments of two-pipe designs include a first
above-ground length
of piping (here, pipes 20a and 20b), as well as a pair of subterranean
vertical pipes 20c, 20e
separated by some distance, where pipes 20c and perforated pipe 20d act as the
meta-NG 50a and
proppant 42 delivery system and pipe 20e (located, e.g., about 200-500 feet
away) is in
"communication" with the same formation and serves as the "riser" that allows
the returning meta-
NO (as warm return flow CNG 52) plus any NG liberated from the formation to
rise to the surface.
Pipes 20e and 20a can be connected at the surface, to each other, and with the
meta-NG supply
equipment 12 at that connecting point, thus allowing the return from pipe 20e
to be re-cooled and
pressurized for renewed send-down. In exemplary embodiments, pipe 20e is
fluidly connected to
the meta-NG supply equipment 12, which is fluidly connected to pipe 20b.
[0042] However, pipe 20e may well be located in the same well bore as pipe
20c. More likely,
in order to avoid excessive costs, pipe 20e may be an annulus around pipe 20c.
In other words, an
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arrangement of concentric pipes could be used in which the different forms of
NG described herein
could be sent down in different concentric pipes and/or the NG could return to
the surface in a
different concentric pipe than the NG being sent down to the subterranean
formation. Those with
expertise in natural gas recovery systems can make various decisions as to how
to organize the
vertical piping that links to the horizontal piping.
[0043] Exemplary embodiments further comprise a CNG system 36 for use in
the proppant
delivery process. CNG system 36 includes different combinations of components
such as a CNG
heat exchanger 38 to warm the highly pressurized meta-NG 50a into high-
pressure CNG 50b, as
well as valves and program logic controls. As discussed in more detail herein,
the heat source for
warming the pumped-to-pressure meta-NG into CNG can be waste heat 23 from the
prime mover
22. If more heat is needed than can be recovered from the waste stream of the
prime mover, then
a gas-fired heater (not shown) may be used to supplement the available waste
heat. A proppant
hopper 40 is also provided, which is fluidly connected to the CNG system 36 to
dispense proppant
42 into the high pressure CNG stream 50b exiting the CNG system 36. Although
depicted in FIG.
2 as separate boxes for the sake of clarity, all of the aboveground equipment,
including the meta-
NG supply equipment 12 and the CNG system 36, may be installed as a single
process without
distinction between the meta-NG and the CNG production. It should be noted
that FIG. 2
illustrates one possible set of relationships between the aboveground
equipment and the below-
ground vertical and horizontal piping. Those of skill in the art will likely
find several other
arrangements, which are contemplated by the present disclosure.
[0044] In operation, a preliminary step of producing the meta-NG is
performed by the meta-
NG supply equipment 12, and could be accomplished by any known methods or
systems for
compressing and chilling NG such that it is converted to meta-NG 50a.
Processes for producing
meta-NG comprise applying the appropriate temperature and pressure to NG, and
those pressure
and temperature parameters are described in more detail herein. One
significant advantage of
disclosed embodiments is that the fracturing medium can be produced at the
site of the
subterranean formation being exploited. More particularly, the meta-NG 50a can
be produced at
the well site from nearby pipeline gas or from a nearby natural gas well
(which may be "stranded"
or may be connected to a pipeline), rather than being imported to the site as
LNG or LPG. Both
the feed gas for fueling the prime mover 22 and the feed gas to be compressed
and chilled to meta-
NG would be obtained from a nearby NG well, a nearby completed oil well
producing "associated
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gas," a nearby pipeline, a single batch of LNG delivered to the site, or some
combination of NG
sources. However, after the initial start-up, much of the meta-NG 50a sent
down into the
subterranean formation 18 is produced from the targeted subterranean formation
or recycled from
the CNG returning to the surface via pipe 20e, obviating the need for further
delivery of NG or
LNG from off-site, and obviating the need for large on-site storage vessels.
For many
deployments, available nearby NO sources will obviate the need for any
"importing" of LNG.
[0045] In exemplary embodiments, the meta-NG 50a used for fracking is
produced by the
meta-NG supply equipment 12 and stored in a cryogenic, moderate-pressure
(e.g., approximately
700-800 psia) storage tank 14. The stored meta-NG is pumped to pressure with a
cryogenic liquid
pump 32, or equivalent positive displacement device. This pressure would be in
the range of about
4,000-12,000 psia for many subterranean formations, but could be greater than
that if the formation
is very deep. As is understood in the art of fracking, deeper formations
require higher pressure.
When high-pressure CNG is sent down, for purposes of thermal shocking and/or
proppant delivery,
the high-pressure (slightly warmed by the heat of pumping) meta-NG 50a is heat
exchanged with
ambient temperature, low-pressure feed gas with the meta-NG supply equipment
12, cooling that
feed gas and warming the outbound high-pressure meta-NG to, e.g., 30 F CNG.
That cooling of
the feed gas to the meta-NG supply helps reduce the work required to produce
more meta-NG 50a.
It should be noted that the meta-NG supply equipment 12 offers the flexibility
to produce meta-
NG at any temperature, e.g., colder than about -150 F, at a pressure of 700
psia (or greater)
allowing that non-liquid, metacritical phase of natural gas to be pumped to
any desired pressure
(e.g., up to about 12,000 psia) with cryogenic liquid pumps or equivalent
positive displacement
devices. That method avoids the need to use compressors to bring the cold
methane up to the high
pressure.
[0046] The meta-NG 50a exits meta-NG supply equipment 12 and is pumped to
pressure by
one or more of the positive displacement devices. For instance, cryogenic pump
32 could pump
the meta-NG 50a to sufficient pressure for send-down in the pipes 20b-20d,
which would typically
be greater than about 2,000 psia. More particularly, the "loop" of fracking
gas in pipes 20a-20f
can be varied as to the temperature and pressure of the downward flowing meta-
NG 50a and as to
the duration of that flow. With the meta-NG supply equipment 12 and positive
displacement
device producing an appropriate flow rate, the meta-NG 50a flows downward into
the ground and
toward the subterranean formation 18 via vertical pipe 20c.
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[0047] In an exemplary embodiment, the pumped-to-pressure meta-NG would be
sent down
to the geological formation at 2,800 psia or greater pressure at a temperature
range of
approximately between -170 F to -220 F and may lose a significant amount of
pressure within
the geological formation, falling to approximately 500 psia but forming some
LNG within the
fissures in the formation at conditions between approximately 500 psia at -158
F and 285 psia at
-197 F. In another exemplary embodiment, the pumped-to-pressure meta-NG would
be sent down
to the geological formation at 2,800 psia or greater pressure and within a
range of -160 F to -200
F, and would lose only a portion of its pressure within the geological
formation, falling to 700 psia
or greater pressure and warming somewhat, having given up a portion of its
refrigeration content
to the "thermal shocking" of the geological formation.
[0048] When the meta-NG 50a enters pipe 20c via valve 58 and flows from
above to below
ground, it causes the geology that surrounds the vertical pipe(s) to freeze in
a radial pattern, thereby
providing a frozen zone of insulation. For this reason, it is not necessary to
insulate the vertical
pipes. Once in the subterranean formation 18, the meta-NG 50a exits pipe 20d
through
perforations shown approximately as 21 and delivers high pressure and thermal
shock to the
formation 18. When the formation 18 fractures to create or extend fissures due
to the pressure and
shock, the positive displacement device above ground then increases the
pressure on the meta-NG
flow to the pressure required for that formation's fissures to remain open,
ready to accept the
proppant. As mentioned above, pressures in the range of about 4,000-12,000
psia are typical, but
the pressure will vary based on the formation and the depth of the hydrocarbon
bearing rock, with
very deep formations requiring higher pressures. In order to maintain the high
pressures that are
built up during the fracking process, control valves including on pipe 20e
would be set to "plug"
such pipe and not allow pressure reduction by way of escaping NG. As will be
clear to experts in
fracturing techniques, the pressure build-up can be achieved in stages,
including by isolating
portions of the well bore.
[0049] At this point, proppant 42 is delivered to the fissures 19 in the
subterranean formation
18. Any suitable proppant could be used, including but not limited to, sand,
ceramics, fly ash, or
other such hard and smooth materials that may be selected in the future. Man-
made ceramic balls
at various small scales provide a uniform, relatively hard and smooth
proppant. Moreover, ceramic
balls tend not to clump together and block fissures and will not absorb
lubricant added to the
proppant stream.
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= [0050] While sand is the standard proppant material used in water-
based (or N2-based)
hydraulic fracturing to keep the expanded fissures from re-collapsing and
closing, other grainy
materials, such as fly ash may not be suitable for water delivery (i.e.,
standard hydraulic fracturing)
because the combination of fly ash and water would cause a cement-like
compound that would
restrict the NG flow rate. It should be noted, however, that disclosed
embodiments, which use no
water, do not have that limitation. Thus, exemplary embodiments may use sand,
and other small-
scale, uniformly shaped, hard particles that "flow" when delivered in a
lubricated manner, as
proppants, which are substantially non-aqeuous through an appropriately
designed blower that is
integrated down-stream of the CNG equipment 36. In exemplary embodiments, the
proppant 42
is lubricated by any suitable non-toxic and low-cost natural or synthetic
fluid, including but not
limited to vegetable oils or biodiesel. The lubricant serves to move the
proppant 42 smoothly,
with low friction, through the piping and into the underground formation.
[0051] The lubricated proppant 42 is delivered by warm, high-pressure CNG
50b. The high-
pressure is achieved by the pumping of meta-NG. More particularly, the CNG
would be produced
by pumping the meta-NG 50a to a high pressure, sending it through pipe 20f to
a heat exchanger
38 in the CNG system 36 for warming via heat exchange with the NG stream, thus
cooling the
feed gas, and where the waste heat 23 from the prime mover 22 would
substantially warm the NG,
shifting it from a metacritical phase to a supercritical state, ultimately
warming the meta-NG to
CNG. The high-pressure CNG stream 50b exits the CNG system 36, and proppant
hopper 40
dispenses proppant 42 in a controlled manner, through valves 55 and 56, into
the high-pressure
CNG stream 50b. Proppant 42 meets the high-pressure CNG stream 50b in pipe
20a. The warm,
high-pressure CNG 50b will carry the lubricated proppant 42 much like air
carries sand in a sand
storm, but without the scouring effect of "sand blasting."
[0052] The CNG-proppant stream 50c then flows downward through pipes 20b
and 20c and
travels through pipe 20d, exiting through perforations 21 to flow deep into
each of the smallest
fissures that have resulted from the fracturing process. It should be noted
that, because of the
delivery by warm high-pressure CNG, the lubricant (and the proppant) do not
need to tolerate
deeply chilled delivery conditions, and therefore the lubricant and proppant
do not need to be
completely non-aqueous. Also, advantageously, the delivery (by CNG) of warm
proppant to the
fissures of the subterranean formation does not cause the formation of ice
crystals or frozen
"clumps" of lubricated proppant. However, the high-pressure CNG 50b can be
cold enough to
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also deliver frozen pellets of acetylene; which upon warming will produce
localized explosions in
the subterranean, hydrocarbon-bearing formation. Such a step may be used to
enhance the fracking
process prior to proppant send down.
[0053] After delivery of the proppant 42, the pressure on the CNG-proppant
stream 50c can
be released, slightly relaxing the subterranean formation 18. However, the
fissures 19 in the
formation 18 are held open by the proppant 42, allowing the natural gas
previously sent down, and
the natural gas within the formation to blend and rise as one stream to the
surface via pipe 20e. It
should be noted that the protocols for optimal fracturing, proppant delivery
and NG recovery steps
may vary depending on the application, and including such factors as the depth
of the formation,
the length of the horizontal piping in the formation, the targeted
hydrocarbon(s), and the geology
of the formation.
[0054] During the fracking process, the meta-NG 50a is warmed by the
ambient heat of the
subterranean formation 18 and then travels up pipe 20e or the annular space
surrounding the pipe,
returning to the surface as warmed return flow CNG 52 still somewhat
pressurized. The returning
(upward flowing), warmed return flow CNG stream 52, which will eventually
consist almost
entirely of NG released from the geological formation, will initially be warm
when it arrives at the
surface, but will get cooler and cooler over time as a result of the meta-NG
supply equipment 12.
Thus, the energy input required by the natural gas plant 34 at the surface
will be less and less as
the fracking continues. The cycle of deeply-chilled meta-NG 50a being produced
at the surface
and returning as colder and colder return flow CNG 52 is repeated until the
fracking results in
freely flowing NG, which is accomplished without the need for a large-scale
inflow of LNG to the
site, and indeed without any liquids used for fracking, proppant delivery, or
for mitigating the
effects of such fracking liquids. After proppant delivery, the returning CNG
stream 52 may carry
some amount of proppant that did not stay trapped in the fissures. Those
particles would be filtered
out of the returning gas stream prior to transport to off-site customers. Such
transport to off-site
customers may be by pipeline or by LNG or CCNG tankers. ("CCNG" is the
equivalent of meta-
NG, above its critical pressure and colder than its critical temperature.) If
off-site delivery is in a
cryogenic form (LNG or CCNG), the on-site CCNG equipment 34 would continue to
operate even
beyond the fracking process.
[0055] As discussed above, where the meta-NG is pumped to 2,800 psia or
greater pressure at
a temperature range of approximately between -170 F to -220 F, the pressure
subsequently falls
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to about 500 psia, but the meta-NG forms some LNG within the fissures in the
formation at
conditions between approximately 500 psia at -158' F and 285 psia at -197 F.
That portion of the
sent-down meta-NG that did not immediately form LNG upon pressure drop would
return to the
surface for recycling to meta-NG, followed subsequently by the portion that
formed LNG, after
that LNG vaporizes by the heat in the formation.
[0056] The meta-NG which liquefied into LNG when the pressure was released
somewhat and
fell below the critical pressure of natural gas (approximately 700 psia) will,
as the slightly colder
liquid phase of LNG, seep into crevices, whereupon warming (by the surrounding
formation), or
because of an increase in pressure from the pumping equipment aboveground, the
LNG will re-
form as a high-pressure (metacritical) vapor, further expanding the fissures.
To the extent that
LNG forms in any fissure, that fissure will propagate because the liquid will
fill the smallest cracks
and then increase the pressure at those points as the natural gas shifts in
phase from liquid to gas,
due to heat gain from the adjacent geology. This phenomenon allows adjustment
of the pressures
in the formation across the critical pressure of NO, thus "flexing" the
formation and using the
phase shift of the NG (from metacritical fluid to liquid and back) as another
"tool" for extending
or widening the fissures in the formation. In other words, VRGE can thermally
shock the
formation and cause fatigue cracks by allowing the NO in the formation to move
back and forth
across phases.
[0057] Where the pumped-to-pressure meta-NG is sent down to the geological
formation at
2,800 psia or greater pressure and within a range of -160 F to -200 F, it
would lose only a portion
of its pressure within the geological formation, falling to 700 psia or
greater pressure and warming
somewhat, having given up a portion of its refrigeration content to the
"thermal shocking" of the
geological formation. The returning 700 psia CNG would no longer be meta-NG
(because it will
be warmer than the critical temperature of methane), but at 700 psia it will
be well-suited for re-
cooling into meta-NG, without the need to compress that returning stream. In
embodiments where
NG is used as a refrigerant, the compressors 24 in the meta-NG plant 34 will
only need to compress
the methane that acts as the refrigerant meta-NG supply process, without
needing to compress the
"feed gas" that becomes meta-NG, thus further reducing the energy input needed
to keep VRGE
functioning.
[0058] As mentioned above, warm high-pressure CNG 50b could be sent down to
the
subterranean formation 18 via pipes 20a-20c with or without proppant. In this
case, the
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refrigeration function of the surface-mounted meta-NG supply would be re-
directed to produce
high-pressure CNG 50b, (without excessive operating costs associated with
ordinary compression)
and allowing the equipment to send down warm high-pressure CNG 50b, shocking
the previously
chilled formation, warming it, and then allowing it to be shocked again when
meta-NG 50a
(produced by the same, now-redirected, refrigeration equipment) is sent down.
More particularly,
if thermal shocking of the subterranean formation 18 is deemed to be
effective, a high-pressure
warm CNG stream 50b would quickly follow a period of meta-NG 50a circulation,
and those steps
could be repeated any number of times.
[0059] The warm high-pressure CNG 50b would be circulated in the geological
formation,
raising the formation's temperature toward (and above) 600 F, followed
rapidly by the insertion
of meta-NG at approximately -200 F, which would yield a temperature delta of
approximately
800 F between the conditions in the formation and the meta-NG. These steps
can be repeated any
number of times. Once the subterranean formation 18 is sufficiently cold
(frozen) from the meta-
NG, a high-pressure warm CNG stream 50b can again be sent down 20c and 20d,
causing
significant thermal shock to the formation, which will result in fracturing,
causing new fissures 19
to propagate. After the fracturing is deemed complete, pipe 20a would deliver
proppant-loaded
CNG 50c at a pressure suitable to drive the proppant 42 into the previously
formed fissures 19. As
the pressure is released, the proppant 42 will remain in the fissures, holding
them open and
allowing the previously trapped NG and other hydrocarbons (and any warm, high-
pressure CNG
50b used by VRGE) to return to the surface in pipe 20e.
[0060] The warmed return-flow CNG 52 may be recycled by the natural gas
plant 34 for re-
refrigeration and compression, if needed, to become meta-NG 50a again. More
particularly, the
near-ambient high-pressure gas, now CNG 50b, is further warmed by recovered
heat of
compression that results in the meta-NG supply's 12 compression of the feed
gas, raising the
temperature of the warm high-pressure CNG 50b above 150 F. Further heating of
the warm, high-
pressure CNG 50b can be accomplished by waste heat recovered from the prime
mover 22 of the
meta-NG supply equipment 12 (an engine or gas turbine) or by the use of a NG-
fired heater or
other heat source. The meta-NG 50a produced and stored temporarily in a
cryogenic buffer
container, can be pumped to pressure, subjected to "cold recovery" (recovered
from meta-NG
before it is warmed), further heated to above about 600 F, and sent down to
the subterranean
18
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formation, under pressure, to raise the temperature of the formation, prior to
the thermal shocking
of the formation by high-pressure, cold (about -200 F) meta-NG.
[0061] The cold, pressurized meta-NG 50a in the subterranean formation 18
can be allowed to
"pressure drop" (by releasing a valve at the surface), which may cause pockets
of LNG formation.
No methane emission will occur during that pressure drop, because the low-
pressure NG would be
returned to the compressor in the meta-NG supply equipment 12 for
recompression and
refrigeration, yielding meta-NG. The cold content of the meta-NG 50a may be
recovered to allow
that refrigeration to produce more meta-NG. The new meta-NG 50a is then sent
down pipes 20b
and 20c for a second pass through the subterranean formation 18 to repeat the
cycle, which can be
further repeated any number of times. With each such cycle, the subterranean
formation 18 is
thermally shocked, and the meta-NG 50a will travel further into the expanding
fissures.
[0062] The cycle of cold send-down and warmer return can be repeated many
times, with the
only operating cost being the refrigeration produced by the meta-NG supply
equipment 12. Those
operating costs will be substantially lower than purchasing LNG from an off-
site (usually distant)
source and having that LNG delivered to the well site. More particularly, the
vast majority of
meta-NG produced can be used in the fracking process with a very small
percentage used as fuel
to run the meta-NG supply equipment 12. For example, of every hundred units of
natural gas
processed by the meta-NG supply equipment 12, about 80 to 95 units will be the
meta-NG
produced for the continued fracking and only about 5 to 20 units will be used
as fuel to run the
meta-NG supply equipment 12. As the fracking continues, the returning NG will
be colder and
colder with each cycle, allowing meta-NG supply equipment 12 to produce as
much as 95 units of
meta-NG for each 5 units of NG consumed as fuel to run the plant.
[0063] It is important to note that no liquid, even LNG, is sent directly
into the well or into the
subterranean formation in the disclosed systems and processes. Any moisture
that may be found
deep in the formation, near the horizontal pipe 20d, would freeze during the
meta-NG send-down
period, expanding as ice and helping to fracture the formation. Any such
moisture or any methane
hydrates in the formation would vaporize during the proppant delivery and
would rise to the
surface in pipe 20e, as a small portion of the return flow NG 52 arriving at
the meta-NG supply
equipment 12. In exemplary embodiments, that equipment may include dryers and
CO2 removal
systems, such as molecular sieves. Heavier hydrocarbons, such as propane,
ethane, butane and the
like can be separated (by refrigeration) in the meta-NG supply equipment 12,
and sent to market
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in liquid form, independently of the NG. It should also be noted that nothing
other than the original
NG used for fracking (and for proppant delivery) and the liberated
hydrocarbons (whether NG, oil
or condensates) returns to the surface.
[0064] That recovered/liberated NG stream 57 (and/or oil or condensate
streams) is a valuable
product recovered from embodiments of the fracturing processes and systems
described herein,
and, as such, is the major goal of VRGE. The NG's temperature and pressure can
be calibrated to
the mode by which it is taken from the VRGE deployment to market. For example,
if the site were
not near a NG pipeline, then NG stream 57 could be LNG (or meta-NG), suitable
for transport in
cryogenic vessels and delivered to such vessels via pipe 20g. However, if the
well is close to a
pipeline, then NG stream 57 can be NG at any appropriate pressure and
temperature and would be
delivered via pipe 20g. The colder the NG, the denser it will be at any given
pressure, and that
density is more sensitive to the temperature of the gas than to its pressure.
[0065] Once the subterranean formation 18 begins to release the formerly
trapped NG, the on-
site meta-NG supply equipment 12 can continue to provide a useful function. It
can produce LNG
(or meta-NG) from the recovered NG, allowing the recovered NG to he sent to
market (in tanker
trucks, trailers, rail cars or ships), even in the absence of a pipeline. If
the well is located at or near
a natural gas pipeline, the meta-NG supply equipment 12 can be used, beyond
its fracking and
proppant delivery role, to increase the density of the recovered NG stream by
compressing and
cooling it, thus allowing any given size pipeline to take away more natural
gas. In other words,
the equipment used for fracking and proppant delivery can be moved to a new
well site to continue
its fracking function or it can remain at its original location, enhancing the
density of the recovered
methane so that it can be taken to market more efficiently, while
simultaneously increasing the
capacity of the pipeline that carries it to market and also providing
refrigeration that can be utilized
to separate heavier hydrocarbons (propane, ethane, butane, etc.) from the NG
stream.
[0066] Turning now to FIG. 3, exemplary embodiments of proppant delivery
systems and
methods will be described. FIG. 3 generally illustrates Cold Fracturing, where
high-pressure, cold
foam is used to fracture the hydrocarbon formation and to deliver proppant
into the fractures. In
Cold Fracturing, meta-NC may be produced at or near the well site and is used
to foam a pumped
to pressure mixture of non-aqueous liquid, including but not limited to
methanol, ethanol, glycol,
or other non-aqueous liquid (with zero or some minority amount of water), plus
a surfactant, plus
a proppant, such that a viscous, high-pressure, cryogenic proppant-carrying,
foam is produced.
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The foam is delivered down the well bore to the formation to be fractured,
creating longer and
deeper fractures than warm hydraulic fracturing or warm foam fracturing, and
delivering the
proppant deeper into the fractures than other options. After some brief
period, the foam collapses
and the fractures close partially over the delivered proppant, such that the
proppant allows the
previously trapped hydrocarbons in the formation to flow to the surface at
warm temperatures due
to warming by the formation.
[0067] The collapsed foam's main components (NG and methanol) can travel
back to the
surface with the warm NG liberated from the formation, in some instances
carrying water vapor.
The returning warm NG, with its methanol and water vapor content is sent
through a condensation
system, with on-site meta-NG providing the necessary refrigeration, such that
methanol (and
water) are knocked out of the NG, and those recovered liquids can be reused
for subsequent
fracking stages or subsequent well completions. The delivery of cryogenic foam
to the formation
may be facilitated by an appropriately placed temporary liner or tubing that
tolerates the cryogenic
foam but protects the well casing. The liner/tubing may be removed after well
completion and
reused at the next well.
[0068] It should be noted that all the steps outlined above need not be
undertaken at deeply
chilled conditions. Rather, the present disclosure establishes a range of
conditions that can yield
substantial benefits when compared to standard hydraulic fracturing, or
fracturing with warm,
water-based foams, or with warm methanol. For example, the methanol foam may
be produced at
any temperature, using not only meta-NG (for cold foam), but also CNG for
ambient temperature
foam. When meta-NG is used to produce cold foam, the degree of refrigeration
can be controlled.
For example, one embodiment is to produce foam that is no colder than the
tolerance of an unlined
well casing, e.g., about 20 F, or possibly down toward negative (F)
temperatures. Another
determinant of foam temperature will be the balance sought between the desire
for high-viscosity
foam at the formation and free-flowing foam with less friction losses during
its trip down the
casing.
[0069] With reference to FIG. 3, proppant delivery system 110 includes
proppant supply or
hopper 40, non-aqueous liquid supply 41, foaming vessel 49, and natural gas
supply 12, which
may be meta-NG production equipment as described in more detail above.
Surfactant supply 51
may be provided as well, and the various components of the proppant delivery
supply system 110
are in fluid communication with each other via pipes and valves. The feed gas
to NG supply 12 is
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shown as stream 37, derived from a nearby natural gas source 39, such as, but
not limited to, a
natural gas pipeline, or previously completed well that produces natural gas.
[0070] In exemplary embodiments, the non-aqueous liquid 43 is an alcohol,
and particularly
methanol. The selection of methanol (or any other equivalent alcohol including
but not limited to
ethanol or glycol) as the liquid used to produce the foam offers several
benefits. First, unlike
water, methanol does not swell the formation clays or cause other harm, and
does not require
chemical additives to mitigate the harm caused by water. By using methanol,
the foam's viscosity
can be adjusted without requiring chemical additives other than about 1% (by
volume) of
surfactant. Methanol is readily available in large quantities at costs that,
while higher than water,
are not excessive relative to its benefits.
[0071] Methanol is readily recoverable from the liberated hydrocarbons that
flow to the
surface after well completion, allowing most of the methanol used in the
foaming of one stage of
fracking to be re-used in the next stage or at the next well. Up to about to
about 90% of the
methanol used to produce the foam can be recycled, which serves to
substantially reduce the
"importing" of methanol to the well. Only make-up methanol need be delivered
to the well on a
regular basis, substantially reducing the costs of the liquid used to produce
the foam. The methanol
can be removed/recovered and recycled from the liberated NG by applying
moderate-grade
refrigeration to the methanol-containing NG stream (which returns to the
surface warm), where
the vaporized methanol will drop out as a liquid. That refrigeration is
inherent and cost-effectively
available in the meta-NG produced/used at the wellhead.
[0072] Any remaining methanol in the hydrocarbon that is sent to market is
not a consequential
contaminant. Methanol will not leave a residue on the fractured formation, as
some other water-
based fluids do, and which residue can clog the formation, restricting
hydrocarbon flow. Methanol
can also prevent corrosion in metal pipes, reduce fluid friction, thus
lowering the fracturing fluid
pumping pressure required, and improve the removal of formation water by
reducing the capillary
forces that inhibit the water from flowing into the well casing.
[0073] Returning to FIG. 3, the methanol stream 43 flows out of the non-
aqueous liquid supply
41, and proppant 42 is added to it, forming non-aqueous liquid, surfactant and
proppant stream 45
(without water or containing up to approximately 25% water). The proppant flow
to stream 45 is
controlled from a hopper 40, which is periodically refilled by proppant
deliveries to the hopper.
Surfactant stream 44 is supplied from surfactant supply 51, which may be a
surfactant vessel
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(valves and make up point not shown). In exemplary embodiments, surfactant
stream 44 is a very
small fraction, e.g., less than about 2%, liquid volume of the other streams,
although different
proportions of surfactant may be used as needed.
[0074] The output of NG supply 12 is meta-NG stream 50a, which may be
produced as
described above. In exemplary embodiments, the purpose of meta-NG stream 50a
is to "energize"
a liquid (to produce foam 47) in foaming vessel 49 where meta-NG stream 50a is
introduced to
high-pressure methanol-surfactant-proppant stream 46. That high-pressure
stream achieves its
pressure by pump 35, which pumps methanol-proppant stream 45 to pressure. It
should be noted
that the proppant, liquid, and additives may be mixed together in a "blender"
(not shown) that then
delivers it to the pump 35. High pressure methanol-surfactant-proppant stream
46 and meta-NG
stream 50a meet in foaming vessel 49 where the meta-NG 50a energizes the high
pressure
methanol-surfactant-proppant stream 46, thereby producing energized methanol-
surfactant-
proppant, or foam 47.
[0075] The foam 47 then flows through pipe 20c, is delivered to horizontal
perforated pipe
20d, and on to fissures/fractures 19 in subterranean formation 18, extending
and enhancing
fractures and depositing proppant within those fractures. The methanol foam 47
will break after
some period (e.g., about one hour) within the fractured formation, freeing the
methanol from its
foam-state to a liquid or vapor state depending on down-hole temperatures and
pressures, which
in turn depend on the depth of the formation. The methanol is soluble in
hydrocarbons and will
therefore dissolve in the recovered hydrocarbons to form proppant delivery
liquid-hydrocarbon
solution 3, discussed below with reference to FIG. 4.
[0076] Because meta-NG stream 50a is deeply refrigerated (as cold as about -
150 F), and
even though high-pressure methanol-surfactant-proppant stream 46 is ambient,
foam 47 will also
be cold (e.g., as cold as about -100 F), but not colder than the
approximately -144 F freezing
point of methanol. The final temperature of foam 47 will be determined by the
ratio of meta-NG
stream 50a to high pressure methanol-surfactant-proppant stream 46 and the
extent to which meta-
NO stream 50a is entirely meta-NG or is blended with warmer CNG. Thus, the
skilled operator of
the fracturing process can easily select a wide range of possible foam
temperatures from ambient
down to about -144 F.
[0077] The ratio of methanol-surfactant-proppant stream 45 to meta-NG 50a
could be adjusted
by the skilled well-completion entity that would deploy the disclosed systems
and methods. That
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ratio would likely range from a 60/40 to 25/75 (high-pressure methanol-
surfactant-proppant stream
46 to meta-NG stream 50a), depending on the desired foam quality. The quality
may be extended
up to 90%. The skilled artisan will understand that the "quality" of the foam
is the relationship of
the gas to liquid ratio.
[0078] In exemplary embodiments, the desired temperature of the energized
methanol-
surfactant-proppant or foam 47 is within a range of about 0 F to about -100
F as it enters pipe
20c but somewhat warmer as it arrives at the fissures 19, due to heat gain
from the surrounding
geology between the surface equipment and the subterranean fissures to be
fractured. One criterion
among others for selecting the temperature of foam 47 concerns the
configuration and materials
used in the vertical piping 20c and/or the lining or tubing within 20c (not
shown) that delivers
foam 47 to the subterranean formation 18 by way of horizontal perforated pipe
20d. Another
exemplary embodiment could provide pipe 20c extended in a vertical direction
into the formation,
rather than a horizontal direction, with pipe 20d being "under" pipe 20c. A
second criterion could
be the water content of the methanol. The less water the colder high-pressure
methanol-surfactant-
proppant stream 46 can be without freeze up.
[0079] The total volume of foam 47 can be as little as 50,000 gallon-
equivalent per fracturing
section to many times more than that, and may depend on the local formation
characteristics and
the capacity of the surface equipment, including pump 35 (as a single unit or
grouped as several
pumps) and the meta-NO production equipment 12 to produce a steady, high-
volume stream of
foam 47. The volume can be anything that the equipment can handle and is
considered appropriate
for the frac job.
[0080] The methanol or other non-aqueous liquid 43 in the non-aqueous
liquid supply or buffer
tank 41 may receive recycled methanol 48, as discussed below and illustrated
in FIG. 4. That
recycled methanol stream 48 may also receive make-up methanol (not shown) to
offset any losses
in the methanol delivery and recycle loop.
[0081] FIG. 4 is a schematic illustration of an exemplary embodiment of a
method of
recovering proppant delivery liquid. Point A represents an exit opening from
the subterranean
formation from which liberated NC, the solution of proppant delivery liquid
dissolved in liberated
hydrocarbon stream 3 that is rising from the completed well, carrying with it
methanol vapor and
some amount of water vapor, at a temperature that is dependent on the down-
hole formation
temperatures, which can range from about 100 F to about 350 F. The liberated
proppant delivery
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solution 3 enters heat exchanger 33, which is designed to allow condensates to
form and to drip,
by gravity, to a collection point(s) at the bottom of the heat exchanger 33.
[0082] Such heat exchangers are sometimes called reflux condensers, where
the orientation,
density and other aspects of the heat exchanger's internal surface
arrangements are designed to
enhance condensation, including by adjusting the flow rate of the stream out
of which the
condensates will be derived. Those familiar with condensing heat exchangers
will be able to
optimize the design of heat exchanger 33 to achieve the optimal condensation
results. As in all
such heat exchangers, the two fluids that move through heat exchanger 33 never
mix, and are
always on separate paths, separated by thin, heat-conducting surfaces.
[0083] Here, the proppant delivery liquid-hydrocarbon solution 3 flows in
one direction
through heat exchanger 33 while the counter-flowing meta-NG stream 5 enters
from point B and
flows in the opposite direction carrying refrigeration through heat exchanger
33, thereby
condensing proppant delivery liquid-hydrocarbon stream 3. The purpose of meta-
NG stream 5 is
to deliver refrigeration to heat exchanger 33, condensing the methanol and
water carried by
proppant delivery liquid-hydrocarbon stream 3. Meta-NG stream 5 is pumped to a
high pressure
in a motor-driven cryogenic pump 31 (during which it warms one or two
degrees), but remains in
its metacritical state, above its critical pressure and colder than its
critical temperature, as defined
above. It should be noted that other methods for separating methanol from
methanol and NG can
also be applied. Such methods may include various types of phase separators.
However, to the
extent that such separation methods require refrigeration to condense the
methanol at any point in
the process, the available refrigeration content of meta-NG could still be
used. In other words, an
important aspect of VRGE, the on-site production of meta-NG, which is cold but
may be warmed
after pumping to pressure, is a widely advantageous step in the methanol
recovery system.
[0084] It should also be noted that any moisture that returns with the
methanol laden NG,
(including water known as "formation water") may travel and condense with the
methanol in the
reflux condenser (or similar arrangement) outlined above. In that event the
wet methanol liquid
could undergo a second separation step where the methanol would be driven off
from the water by
heat, much like a distiller, and where the refrigeration content of meta-NG
would condense the
vaporized methanol. The heat source for the vaporization of the methanol could
be the waste heat
from the prime mover (gas turbine or gas engine) that drives the on-site VX
Cycle (or equivalent)
meta-NG production equipment.
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[0085] Some amount of moisture in the methanol is tolerable and such wet
methanol remains
suitable as the liquid used (along with a surfactant) in producing the foam
that delivers the
proppant. For example, foam produced with wet methanol (water mixed with
methanol), e.g., with
about 75% methanol and about 25% water, will, in some formations, behave much
like foam
produced by 100% methanol, and not require any mitigating chemicals. The exact
degree of
wetness of the NG-energized foam, or the ratio of methanol to water in the
liquid to be energized
by the high-pressure NG, can vary depending on the geological conditions at
the well. That ratio
of methanol to water may be 75/25, or 80/20 or 90/10 depending on local
formation conditions,
and laboratory tests of the formation samples. Those skilled in the art will
be able to set the wetness
limit of the liquid methanol, balancing the higher cost of methanol against
the lower cost of water
(especially formation water vs. imported water) against the ability of that
slightly wet methanol to
produce a foam to function as a proppant delivery vehicle that does no harm to
the formation and
avoids the need for chemicals to mitigate the effects of water.
[0086] Returning to FIG. 4, outflow NG stream 4 is the NG that has given up
its condensable
content, where most of the methanol and any water carried in proppant delivery
liquid-
hydrocarbon stream 3 has left heat exchanger 33 as outflow methanol stream 7,
and by way of a
valve 29 entered a liquid storage tank 2, from which it can be released by
another valve 1, and
used to produce more foam, or be transported to the next well.
[0087] In the event that the water content of proppant delivery liquid-
hydrocarbon stream 3
and outflow methanol stream 7 are higher than the amount of water that is
desired in the methanol
that is used to produce the foam, for example, because naturally occurring
water in the formation
("formation water") is returning in vapor or liquid form with the NG, the
liquids recovered in
storage tank 2 can be further separated. The separation of methanol from water
uses heat to boil-
off the methanol from the water, followed by condensation of the methanol by a
counter-flowing
source of refrigeration.
[0088] Returning to FIG. 4, stream 4 is the NG stream that has very little
methanol or water
content and which can travel to non-aqueous liquid supply or buffer tank 41.
Alternatively, it can
be directed to an NG pipeline that takes the recovered NG to market, or can be
directed to the VX
Cycle (or equivalent) equipment that produces the meta-NG used in VRGE or to
produce LNG
that can be shipped to market.
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[0089] Refrigerant outflow stream 6 is the somewhat warmed outflow
(formerly meta-NG
stream 5) from heat exchanger 33, leaving at point C. Depending on the flow
rates set for proppant
delivery liquid-hydrocarbon stream 3 and meta-NG stream 5, their respective
temperatures and
pressures, the outflow temperature for refrigerant outflow stream 6 will
remain cryogenic, e.g.,
about -100 F, but below the critical temperature of NG, thus no longer
metacritical. The cold,
high-pressure refrigerant outflow stream 6, exiting at point C, can now be
used to energize the
methanol + surfactant + proppant mixture, producing the "fracking foam"
described in connection
with FIG. 3 that would be used to complete the next fracking stage.
[0090] In the event that the proppant delivery liquid recovery systems and
methods are
operating when the well is completed, and no further stages of fracturing are
needed, refrigerant
outflow stream 6 (after point C) can become a product stream, such as by
further warming it (for
example from the waste heat produced by the prime mover that runs the VX
Cycle) and, after
reaching ambient temperatures, and adjusting the stream's pressure, depositing
that NG stream
into a nearby NG pipeline. Alternatively, the cold and high-pressure
refrigerant outflow stream 6
can exit the system at point C and be returned to the VX Cycle (or equivalent)
plant for liquefaction
into LNG, so that it can be transported to market outside of NG pipelines.
[0091] In other words, the outflow from point C is "pipeline quality"
because it was derived
from the VX Cycle (or equivalent) equipment, which removed any moisture and
CO2 content. The
outflow NG stream 4, with the moisture and methanol content of proppant
delivery liquid-
hydrocarbon stream 3 having been substantially removed, may be pipeline
quality, if the formation
from which proppant delivery liquid-hydrocarbon stream 3 is derived is
producing pipeline quality
gas. However, in some instances outflow NG stream 4, after leaving the system,
may require
further treatment, for example, to remove any heavier hydrocarbon liquids
carried by proppant
delivery liquid-hydrocarbon stream 3 and remaining in outflow NG stream 4. The
temperature and
pressure conditions at which the heat exchanger 33 will operate can be
adjusted to remove
methanol and water, leaving any heavier hydrocarbons in outflow NG stream 4,
where those
hydrocarbons can be removed by one of several well-understood methods that are
outside the
scope of this invention.
[0092] The choice of meta-NG vs. CNG for foaming will depend on the desire
to produce
foam that is more viscous with the colder meta-NG or less viscous with CNG.
Foam viscosity is
one of many factors that well-completion experts control in order to achieve
deep proppant
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delivery inside the fracture. The viscosity of foams, including the use of
methanol foam produced
with meta-NG as the gas source, is substantially higher than ambient
temperature foams. Higher
viscosity foams will perform better than lower viscosity foams. More
particularly, higher viscosity
helps create wider fractures and helps carry the proppant deeper inside the
fractures. Thus, one
embodiment is to calibrate the temperature of the foam to colder than ambient,
and preferably
colder than about 32 F, and most preferably colder than -20 F, achieving
viscosities that are not
possible with standard fluids at ambient temperature (except with special
additives), but where that
selected temperature range is within the tolerances of the piping (and casing)
that delivers the foam
to the formation to be fractured.
[0093] If the foam is sent down the well bore at, e.g., about -20 F, it
will arrive warmer at the
formation to be fractured, because the piping and the surrounding geology will
warm the foam, as
will the friction with the piping. Still, depending on the depth of the
hydrocarbon-bearing
formation, and the temperatures at those depths, the cold foam will arrive
significantly colder at
the formation than if it were sent down at a starting temperature that is at
ambient, arriving hotter
at the formation. In other words, the refrigerated foam will arrive colder
(and more viscous) in the
formation than fluids or foams produced at ambient temperatures.
[0094] In exemplary embodiments, the meta-NG-produced foam would be nearly
as cold as
the meta-NG (e.g., about -150 F), as long as the temperature of the foam does
not approach the
freezing temperature of the liquids used (about -140 F) and as long as the
foam is not excessively
viscous, and as long as the piping that carries the foam to the formation can
tolerate those
temperatures without cracking and without shrinking so much as to cause gaps
in the piping. Such
deeply chilled foam can have several other positive effects. First, it can
deliver thermal shock to
the formation, allowing for fracturing with less pressure. Secondly it may
cause some of the
formation water to freeze, expand and thus enhance the fracturing effort.
Third, to the extent that
the geology around the well bore freezes, it may add extra stability around
the casing.
[0095] For the above and other reasons, an exemplary embodiment is to send
down the coldest
possible foam that can be tolerated by the piping and casing system and is
still within the desired
viscosity limits. To that end, there are several approaches that can be
selected, including (but not
limited to) the following. Within a standard casing, deliver the cryogenic
foam within suspended
tubing, where spacers separate the cryogenic-tolerant tubing from the well
bore casing, allowing
the annular space between the tubing and the casing to form an insulation
barrier between the two.
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Such tubing may be of 9% nickel steel or other suitable material (including
high-density
polyethylene [HDPE] piping), with expandable (contracting) connections,
similar to "bellows,"
which would allow the tubing to contract and expand without leakage. '1'he
tubing would be
temporary, in place only during the multiple fracturing stages, and would be
removed after the
completion of the well and before the hydrocarbon stream is at production
levels. In that way, the
tubing could be reused at the next well.
[0096] A variation of the option above, but where the cryogenic-tolerant
liner is a wider
diameter than tubing, allowing the liner to be installed within the casing,
but with a rigid foam
liner between the two. As above, the liner would be removed after the
completion of the well and
reused at the next well. Another variation of the above is the use of
cryogenic "braided hose" that
is inserted in, e.g., 50' interconnected sections, from the surface to the
perforated piping through
which the cryogenic foam would be delivered. Such stainless steel braided
hoses are routinely used
to transfer cryogenic liquids such as LNG and liquid oxygen. The substantial
cost of this option
will likely be mitigated by the "quick connect" joints between sections of
hose, the ease of
installation and removal and the ability to reuse the hose sections many, many
times.
[0097] As mentioned above, disclosed non-hydraulic fracturing systems and
methods using
meta-NG to promulgate new fissures and expand existing ones will not result in
any methane
release to the atmosphere, because the fracturing process described here
requires tight seals
between the surface equipment and the underground formation in order to be
effective and because
the present invention does not use water for fracturing, thus eliminating the
return to the surface
of such water into which hydrocarbons have been dissolved. This is true even
in the context of
deep underground hydrocarbon formations containing large amounts of
hydrocarbons (including
methane), which have been contained for millennia by the overburden. Rather,
the cryogenic
methane that fractures the hydrocarbon-bearing formation 18 will allow the
trapped methane
(along with any methane used in the fracking) to rise to the surface-mounted
equipment through
the network of pipes 20, under controlled conditions, where it will be re-
refrigerated by the meta-
NG supply equipment 12 and/or inserted into an adjacent pipeline (or an LNG
tanker truck, ship,
or other LNG vessel) that will transport the methane to customers.
[0098] Disclosed non-hydraulic fracturing systems and processes can be
deployed at wells
with nearby pipeline access, allowing the flowing NG to be delivered to market
in the standard
way. Alternatively, at locations too far from pipelines, VRGE allows for the
on-site liquefaction
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(or meta-NG production) of the liberated methane, thus allowing wells at such
locations to get the
recovered product to market in tanker trucks/trailers or ships, even in the
absence of a pipeline.
Where the NG is delivered by pipeline, VRGE will allow higher quantities of
methane to be
delivered, because meta-NG (and even moderately cold NG) has far greater
density than standard
NG, thus increasing the capacity of such pipelines. A given diameter pipeline
will carry more
product (in lbs and BTUs) if that product is denser.
[0099] Disclosed embodiments of VRGE are also suitable for "pipeline
quality" gas fields and
for gas that has higher concentrations of CO,, water, N2 or heavy
hydrocarbons, because VRGE
can include any degree of clean-up required to remove the non-methane
components from the
recovered NO. (Hydraulic fracturing by water or LN2 does not provide for that
option.) Many of
the techniques used to "knock out" CO2, water, N2 and heavy hydrocarbons
involve the use of
refrigeration. VRGE, using the meta-NG production system at the surface, can
allocate a portion
of its refrigeration capacity (and low-grade "waste" refrigeration output) to
those knock-out
processes. Indeed, the heavy hydrocarbons (often referred to as natural gas
liquids ("NGLs"), and
which include propane, butane, isobutane, pentane and ethane) often found in
NG streams have
substantial market value (sometimes greater than the value of the methane
obtained from the same
NG stream), and the use of the refrigeration inherent in VRGE to separate
these heavy
hydrocarbons for sale to the market would be part of exemplary embodiments of
VRGE.
[00100] For hydrocarbon-bearing formations located beyond the reach of cost-
effective
connections to existing NG pipelines, located where nearby pipelines have
limited capacity, or
located where the price of NG is relatively low compared to the price of the
product at the end-
user, the optimal solution for recovered methane is to liquefy it and to send
it to market in LNG
tanker trucks (or ship), outside of the pipeline system, as a "value added"
product. Disclosed
embodiments allow the same meta-NG plant that produces the fracking fluid to
also be the
LNG/meta-NG production facility that allows for the recovered methane to be
converted to LNG
or CCNG and brought to market outside of the natural gas pipeline network (via
tanker truck or
ship).
[00101] It also should be understood that disclosed non-hydraulic fracturing
systems and
methods can be adapted for use in tight geological formations that contain oil
and/or condensates.
Such deployments would have different sets of protocols as to when to use meta-
NG (for fracking),
when to send down the proppant and at what temperature, and when to use warm
CNG to induce
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the flow of oil and/or condensates. The liquid hydrocarbons that would arrive
at the surface would
include "associated" NG suspended in the liquid, which would be allowed to
"boil oft" the liquid
and thus be separated from it. The output from such a deployment would include
crude oil and/or
condensates in liquid form and NG that can be chilled to meta-NG or LNG.
[00102] When applied to formations that contain more oil than natural gas,
embodiments of
VRGE would be used with alternating downward meta-NG flow and warm CNG flow,
thermally
shocking the formation, and delivering pressure, but with the final step being
a warm CNG flow
to induce the flow of the liquid hydrocarbons (e.g., oil) formerly trapped in
the formation. The
rising oil would contain some amount of CNG, which would be separated by well-
known means
(such as well site heater treaters), with the recovered methane and other
gaseous hydrocarbons
used to fuel the equipment, or sent off-site as NG/LNG/meta-NG/LPG/NGLs to
markets seeking
those products, including gas processing plants.
[00103] Thus, it is seen that non-hydraulic fracturing systems, methods and
processes are
provided. It should be understood that any of the foregoing configurations and
specialized
components may be interchangeably used with any of the apparatus or systems of
the preceding
embodiments. Although illustrative embodiments are described hereinabove, it
will be evident to
one skilled in the art that various changes and modifications may be made
therein without
departing from the scope of the disclosure. It is intended in the appended
claims to cover all such
changes and modifications that fall within the true spirit and scope of the
disclosure.
31
