Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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CONTROL SYSTEM AND APPARATUS FOR DELIVERY
OF A NON-AQUEOUS FRACTURING FLUID
Field of Invention
[0001] A control system and associated methodology and apparatus for the
implementation of an eductor-mixer technique providing the capability for
injecting proppant material into a non-aqueous fracturing fluid stream
utilized in
hydraulic fracturing operations is described. The system and apparatus
includes
an eductor, an enclosed vessel that provides a proppant reservoir, valving
disposed between the eductor and the enclosed vessel, and a pressure control
system for modifying the pressure in the enclosed proppant vessel during the
fracturing operation. The control system employs a combination of control
valve
position and proppant reservoir pressure to adjust and set proppant feed rates
into
an eductor to be mixed with a non-aqueous fluid and to control proppant
concentrations into a fracturing fluid stream.
Background of the Invention
[0002] The use of carbon dioxide for enhanced production of oil and gas
from
reservoirs is well known. Liquefied gas based fracturing is unique as compared
to
conventional fluids such as water and have certain advantages in water
sensitive
and low pressure formations, including the promotion of fluid flowback (i.e.,
retrieval of water/fluid used in fracture treatment) which minimizes formation
damage caused by water. Michael J. Economides, T. M. (2007). Modern
Fracturing: Enhancing Natural Gas Production. (S. Weiss, Ed.) Houston, Texas,
USA: Energy Tribune Publishing Inc. LCO2 used in fracturing treatments is
typically added to a high pressure stream of water and proppant (typically
solids,
such as sand, polymer pellets, tracers, gravel, etc. of various sizes and
density) at
the well-head. Combining water with proppant and adding a separate pressurized
LCO2 stream is the most conventional method of forming a CO2-energized
fracture fluid. This is due, in large part, because it is simpler to mix
proppant with
water at atmospheric pressure then it is to add proppant to liquid carbon
dioxide at
a pressure above the triple point of carbon dioxide, (i.e., greater than 75.1
psia).
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[0003] Equipment is available and can be used for small fracture
treatments
(e.g. to place up to approximately 20 tons of proppant) to mix proppant
directly
with a liquid carbon dioxide-based fracturing fluid. This equipment includes a
pressurized vessel and manifold system that blends the proppant into a liquid
CO2
stream prior to the high-pressure pumps. Proppant is loaded into the CO2
blender.
The blender is sealed and then filled with CO2. During the fracturing process,
proppant is mixed into the fracturing fluid by either hydraulically driven
augers or
gravity fed through a control valve. Michael J. Economides, T. M. (2007).
Modern Fracturing: Enhancing Natural Gas Production. (S. Weiss, Ed.) Houston,
Texas, USA: Energy Tribune Publishing Inc. Once the batch of LCO2 and
proppant is exhausted, the fracture treatment must either be completed or
suspended to refill the blender with additional proppant.
[0004] Earlier efforts, as described in U.S. Pat. No. 4,374,545, provide
for a
batch process creating a proppant and LCO2 fracturing slurry. Each unit is
capable of metering up to 20 tons of a single type of proppant and addresses
the
control of proppant supply through the use of a metering auger. LCO2 additions
made to the bottom of the tank allow for a flowable and vapor-free proppant
slurry leaving the system as well as maintaining pressure in the vessel.
[0005] Another system is described in U.S. Patent Nos. 8,408,289 and
U.S.
8,689,876 which depict an upright standing vessel where proppant is metered
into
LPG (liquefied petroleum gas) as a base fracturing fluid. Proppant loadings
are
varied into the LPG fracturing fluid stream through the use of gravity
(through a
control valve) or via one or more augers disposed within and along the bottom
of
the proppant supply source or arranged outside of the proppant supply source.
Inert gas (in the form of nitrogen) is pumped into the vessel during operation
to
maintain vessel pressure to ensure the LPG mix remains in the liquid phase to
prevent back flow into the vessel.
[0006] A non-mechanical pump, such as an eductor, can be used to mix a
proppant into a fracturing fluid stream. Non-mechanical pumps have the benefit
of no moving parts, are generally low cost and simple pieces of equipment, and
are already commonly used in related material introduction. For instance,
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International Publication No. WO 2012087388 describes an eductor system for
introducing and blending polymer additives into a fracturing fluid stream.
[0007] General use of a liquid eductor for solids handling and blending
relies
heavily on the relationship of motive flow (i.e., the incoming flow of fluid
to the
eductor (without proppant addition)) to the rate of solids entrainment for the
control of solids concentration. As liquids pass through the converging nozzle
of
the eductor, potential energy is converted into kinetic energy resulting in a
high
velocity jet flow. This change in energy results in a localized decrease in
static
pressure that creates suction within the body of the eductor. This suction
allows
material to be drawn into the eductor and entrained by the fluid (LCO2, etc.).
The
eductor serves a dual purpose: mixing within the nozzle as well as drawing
material into the fluid to ensure intimate mixing. With more conventional
methods, such as using sand or similar material proppants to provide water-
based
slurries, the viscous properties of the water aids in drawing solid materials
into the
body of the eductor where suction occurs. Difficulty arises when it is
necessary to
establish a particulate suspension in a relatively low viscosity fluid (as
compared
to water), such as liquid carbon dioxide (LCO2). The present invention
addresses
the need to add proppant to such fluids on a more fully controlled basis by
delivering a homogeneous fracturing fluid to high pressure pumpers prior to
wellhead injection.
[0008] A system and method described in U.S. Pat. No. 7,735,551 is used
to
blend nitrogen gas with proppant to fracture an underground oil and gas
formation
or coal seam. The proppant and gas mixing occurs at a pressure sufficient to
fracture the formation. In one embodiment, an eductor is employed to introduce
proppant into the vapor stream and is in communication with the well bore.
Proppant material is either gravity fed from a proppant reservoir into the
eductor
with the use of a control valve or regulated in with the use of an auger. The
system described provides for the use of either valve position or auger speed
to
regulate proppant into the vapor stream to achieve specified proppant
loadings.
Pressure in the head space of the proppant reservoir is maintained at a
constant
value during the entirety of the stimulation.
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[0009] To overcome the disadvantages of the related art, it is an object
of the
present invention to provide a control mechanism for operating a system for
the
delivery of proppant into a liquefied gas, such as LCO2, for the purpose of
fracturing a subterranean formation. Although the liquefied gas discussed
herein
is in relation to LCO2, by way of example, it can be combination of immiscible
and non-immiscible fluids such a CO2 and methanol, CO2 and biodiesel, or CO2
and water. Specifically, the control mechanism developed utilizes an eductor
along with a proppant control valve and the pad pressure (as defined below) in
the
proppant reservoir to control proppant loading at specified concentrations in
a
substantially homogeneous fashion.
[00010] It is another object of the present invention to provide a system
designed to mix proppant and fracturing fluid at pressures significantly below
that
of the surface treatment pressure (e.g. at or below 400 PSI).
[00011] It is yet another object of the present invention to provide a system
where the eductor can be used with a liquid, and wherein said system does not
utilize an auger for purposes of metering proppant into fracturing fluid.
[00012] Other objects and aspects of the present invention will become
apparent to one skilled in the art upon review of the specification, drawings
and
claims appended hereto.
Summary of the Invention
[00013] The present invention describes a system and associated apparatus for
modifying entrainment rates of proppant with liquefied gas or a relatively low
viscosity (that is less than water at 1 centiPoise ¨ cP) liquid, (e.g. carbon
dioxide)
using an eductor. More specifically, this system employs the use of a proppant
reservoir, valving, an eductor, and a pressure source to provide the proper
concentration of proppant in a flowing stream of fracturing fluid for use in
stimulating subterranean formations such as new and existing oil and gas
wells.
An auger is not used to meter proppant flow in the present invention. The
vessel
is sealed from the atmosphere in order to achieve proper pressure
modification.
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Operating pressure of the equipment in the present invention, including the
proppant reservoir and the eductor, is in the range of about 100 to 400 PSI.
[00014] A solids-conveying liquid eductor is used to mix and accelerate
proppant within the main liquid stream. The eductor can be varied in size
(with
different nozzle and tail) to accommodate the flow rates required for the
particular
well. Once the flow requirement for the motive stream has been determined, a
control system is implemented. The control system utilizes at least one valve
for
controlling the flow of proppant from one or more pressurized proppant
reservoir
into the eductor; thereby mixing the material with the motive stream. Gas
and/or
liquid is fed to the top of the proppant reservoir to control the static
pressure (as
defined below) inside the proppant reservoir. Modifying the static pressure
inside
the proppant reservoir extends the range of achievable proppant flow rates
from
the reservoir into the eductor.
[00015] In one aspect of the invention a method of controlling a proppant
concentration in a fracturing fluid that is utilized in stimulation of an
underground
formation is provided. The method includes:
[00016] supplying a motive fluid flow of liquefied gas at pressure between
about 150 to 400 psig to an eductor, wherein the liquefied gas is mixed with
the
proppant or proppant slurry in the eductor to form a fracturing fluid, wherein
the
pressurized proppant reservoir is disposed in a position to supply the
proppant
slurry to at least one eductor;
A. varying the pad pressure in the pressurized proppant reservoir from
about -30 to 40 psi; and
B. further varying a proppant control valve disposed between the eductor
and the pressurized proppant reservoir to control the proppant
concentration in a range from about 0.1 to 10 lbs/gal of proppant in the
fracturing fluid.
[00017] In another aspect of the invention, a system for controlling proppant
concentration in a fracturing fluid that is utilized in stimulation of an
underground
is provided. The system includes:
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A. providing a proppant reservoir having a proppant or proppant slurry
therein and disposed in a position to supply the proppant or proppant
slurry to an eductor;
B. providing an eductor to receive a motive fluid flow of liquefied gas at
a pressure between 150 and 450 psig, wherein the eductor is disposed
below the proppant reservoir and forms a fluid containing proppant at
the outlet of the eductor upon receiving the proppant or proppant slurry
from the proppant reservoir; and
C. providing a proppant control valve disposed between the proppant
reservoir and the eductor, wherein the pad pressure in the proppant
reservoir is varied from about -30 to 40 psi to attain a concentration
range from about 0.1 to 10 lbs/gal of proppant in the fracturing fluid.
Brief Description of the Drawings
[00018] The above and other aspects, features, and advantages of the present
invention will be more apparent from the following drawings, wherein:
[00019] Figure 1 is a plot that illustrates the differences between the motive
flow rate and the effect of sand/proppant mass flow comparing the use of water
and liquefied carbon dioxide.
[00020] Figure 2 is a plot showing the effect of pad pressure on the
concentration of proppant in the fracturing fluid stream through various
positions
of a computer controlled valve.
[00021] Figure 3 is a schematic depicting an embodiment of the
blender/reservoir system which provides controlled injection and mixing of
proppant with a liquefied gas fluid for fracturing a geological formation
utilizing
an eductor.
[00022] Figure 4 is a further illustration of another embodiment of the
overall
system indicating certain process control aspects.
[00023] Figure 5 is a graphical representation of various proppant control
valve
positioning at low pad pressures at a motive flow rate of 23 gal/min.
[00024] Figure 6 is a graphical representation of various proppant control
valve
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positioning at high pad pressures at a motive flow rate of 23 gal/min.
Detailed Description of the Invention
[00025] The present invention involves a system and apparatus for providing a
continuous or semi-continuous supply of fracturing liquid, where the flow rate
and
method of controlling the flow rate utilizes an eductor so that proppant can
be
thoroughly mixed with the fluid during creation of a fracturing fluid stream
and is
controlled through the use of control valves and proppant reservoir pressures.
As
employed herein, "fracturing fluid" or "fracturing liquid" are used
interchangeably, and refers to the product routed downstream to the fracturing
pump. The eductor and associated valving must be properly sized in order to
provide efficient acceleration of the proppant and resulting combined fluid
proppant slurry at the desired concentration - depending on the required
fracturing
liquid flow rate. Eductors that may be employed includes for example, jet
pumps,
ejectors, venturi pumps, siphon pumps, aspirators, mixing tees, injector
pumps,
etc. The eductors can include a variable size nozzle or aperture, which may be
controlled through a programmable logic controller, or the like, to maintain
net
positive suction head (NPSH) pressure downstream of the proppant reservoir,
discussed below. This enables the use of a broad range of flow rates without
changing the nozzle or the eductor itself On the suction side of the eductor,
a
large reservoir (referred to as the proppant reservoir) is positioned for
holding
either dry proppant or proppant slurry (a mixture of proppant and liquefied
gas
potentially with other additives). The flow of proppant or slurry from the
reservoir to the fluid stream is controlled by a valve disposed between the
eductor
and the reservoir. For the purposes of the present disclosure, this valve will
be
referred to as the "proppant control valve". This proppant control valve can
be
one of many types including that of a sliding gate, knife valve, pinch valve,
and
choke valve. The proppant is loaded into the reservoir either via a hatchway
or
through pneumatic filling and then the vessel is sealed. Dry gas(es) or
liquefied
gases may then be added to the system. Dry gas is usually added to the top of
the
reservoir in order to prevent the aerosolization of proppant.
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[00026] Liquefied gas can be added through the bottom of the reservoir
through the separate liquid line (denoted as the liquid addition line)
attached to the
bottom of the vessel or alternatively into the suction side of the eductor.
Liquefied gas is added to the bottom of the reservoir initially to prevent the
formation of gas pockets. During the fracturing treatment, liquefied gas may
also
be added to the bottom of the reservoir in order to promote the formation of a
solid-liquefied gas suspension.
[00027] Preparation of the system and use of the apparatus to conduct the
process methodology is generally described as follows: proppant is loaded into
the
proppant reservoir and the reservoir is pressurized with gas to a pressure
above
the triple point pressure of the liquefied gas to ensure liquid remains in the
reservoir as liquefied gas is added.
[00028] Once the motive flow has been established, the proppant control valve
is opened to commence mixing proppant material with the fracturing fluid
stream.
The proppant loading in the fracturing fluid and/or the flow rate of the
combined
stream are normally measured by the use of a nuclear densitometer, a magnetic
flow meter, a Coriolis meter or other suitable measurement devices. In the
present invention, adjustments of the opening of the proppant control valve
position (i.e., between various size openings) is determined based on the
measured
concentration of the solids either via manual methods or through the use of an
automatic, computer controlled, control loop. The control of the opening and
closing of the valve allows for proper metering of the proppant to the
eductor.
The concentration of solids in the fracturing fluid is synonymous with
proppant
loading. Adjustment of the static pressure in the proppant reservoir is used
to
provide a greater range of operability of the valve (as described in detail,
below).
Metering of the proppant by adjusting the proppant control valve and static
pressure in the proppant reservoir allows for providing the desired loading of
the
proppant on a per gallon (or other unit of liquid measure) basis of fracturing
fluid.
This loading or concentration is normally in the range of at least 0.1 to 10
lbs per
gallon. An even more preferable range for certain fracturing operations is
between 0.1 and 4 lbs/gallon.
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[00029] The use of pad pressure (defined in this invention as the difference
in
pressure in the headspace of the proppant reservoir and the outlet of the
eductor)
provides the requisite static pressure that extends the overall capability to
attain
the desired proppant loadings. The static pressure in the reservoir, in the
present
invention, is measured as the difference in pressure at the bottom of the
reservoir
compared to the pressure measured at the discharge of the eductor pump.
[00030] Changes in static pressure are generally achieved by controlling the
flow of pressurized gas (such as gaseous carbon dioxide or nitrogen) or liquid
(such as liquefied carbon dioxide) fed to the top of the proppant reservoir.
In
addition, a pressure relief control valve may be used to release excess
pressure in
the reservoir's head space. Ideally, pad pressure is varied over the course of
the
fracturing operation and the range of operation is maintained between -20 and
30
psi. Excessive pad pressure can result in higher proppant loading than desired
in
the fracturing fluid stream. A pad pressure that is too high could result in
an
increased sensitivity of the proppant control valve and precise control of the
desired proppant concentrations could be more difficult to achieve. In this
case,
the pad pressure should be decreased. Alternatively, a pad pressure that is
too low
can result in limiting proppant flow from the proppant reservoir such that the
concentration of proppant in the fracturing fluid is lower than the set point.
In this
case, the pad pressure should be increased.
[00031] Operating static pressures and eductor discharge pressures must be
maintained in excess of the vapor pressure of the fracturing fluids at the
operating
temperature and/or exceed a required NPSH. For instance, maintaining the
proper
pressure to ensure liquid carbon dioxide (LCO2) remains a single phase fluid
(liquid) within the high pressure fracturing pumps requires approximately 50
psi
NPSH, or at least a pressure sufficiently above saturation conditions for
normal,
safe, and reliable operation of the high pressure pumps. Significant amounts
of
vapor or a provision of lower NPSH fluid risks vapor lock or cavitation. These
conditions will negatively affect performance and can damage the high pressure
pumps. Because of the risk of vapor lock and cavitation, operators must be
cognitive of pressure drops required to ensure proper eductor pump operation.
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[00032] Recommended operational pressure ranges for eductor pumps is
normally between 15 psi and 60 psi, depending on the available "disposable"
NPSH (or pressure available to the operator to ensure proper eductor
performance
while maintaining sufficient pressure above saturation as described above).
Motive flow rates failing to produce at least a 10 psi or greater pressure
drop in
the eductor will result in improperly cleared proppant or proppant flooding in
the
downstream piping.
[00033] A bypass line for the fracturing fluid is connected around the eductor
and may also be utilized to provide for increasing flow rate capabilities of
fracturing fluid without incurring higher pressure drops across the eductor
pump
or to further dilute the concentration of the proppant in the fracturing fluid
leaving
the eductor. This is especially beneficial when higher than expected flow
rates of
fracturing fluid are required so that an appropriate level of net positive
suction
head (NPSH) can be maintained. For instance, if a fracturing treatment
requires a
pumping rate of 40 BBLS/min and the installed eductor is only capable of
operating up to 30 BBLS/min before the discharging pressure is in danger of
maintaining the necessary NPSH, 10 BBLS/min flow can be bypassed around the
eductor, resulting in a total flow of 40 BBLS/min, at a cost of reducing the
maximum proppant concentration producible by the blending unit into the
fracturing fluid stream.
[00034] The actual operation of the system is described using two separate
stages;
A. The Pre-startup Stage:
During pre-start up, the following steps are followed:
(1) The proppant reservoir is isolated from the eductor and proppant/sand is
loaded into the proppant reservoir through either the port located on the
top of the reservoir or through pneumatic fill lines.
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(2) The proppant reservoir is then pressurized using the vapor addition line
at
the upper part of the reservoir.
(3) The proppant reservoir vessel is then filled with liquid through a liquid
line located at the lower part of the vessel.
a. Concurrently, liquid additions could be provided to the top of the
proppant reservoir either for filling or maintaining liquid levels in
the reservoir.
b. The pressure relief control valve is used to maintain a prescribed
pressure in the proppant reservoir during filling.
(4) Once filling is completed, the operational stage can begin.
B. Operational Stage:
(1) The fluid or motive is pumped down the main fluid line through the
eductor.
a. A bypass line which bypasses the eductor may be used to extend
fracturing fluid flows rates beyond the limitations caused by the
pressure drop through the eductor, and possibly prevent cavitation
of the downstream pumps.
(2) The proppant control valve is then opened and proppant is allowed to mix
into the main fluid line within the eductor.
a. An isolation valve could be located next to the proppant control
valve to act as a seal in the event that the proppant control valve
does not function as a leak tight valve.
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(3) Pad pressure is regulated to a set value. Pad pressure is increased by
flowing pressurized gas (or liquid) to the top of the proppant reservoir.
Pad pressure is decreased by opening the pressure relief control valve.
(4) The proppant control valve opening is adjusted to achieve the desired
proppant concentration in the fracturing fluid.
(5) The pad pressure can be adjusted to a new value to extend the range of
concentrations achievable.
[00035] Figure 1 shows the relationship of proppant entrainment rate versus
the
motive flow rate (i.e., the flow rate of the water or liquid CO2 flowing to
the
eductor) using a model 264 eductor manufactured by Schutte & Koerting. In
Figure 1, the line labeled "[1]" depicts the performance of the eductor
pumping a
proppant and water slurry using water as a motive fluid (as a "baseline" for
comparison). The area and points marked "[2]" indicate similar conditions but
instead LCO2 has replaced water as the motive and suspension fluid. The low
viscosity of liquid carbon dioxide (again as compared to that of water) is
believed
to account for the differences in trends between motive flow and entrainment
rates
and thereby requires a control strategy as provided in the present invention.
[00036] Figure 2 illustrates the proppant concentration as a function of pad
pressure and proppant control valve (for example, an equal-linear type valve)
position using liquid carbon dioxide as a fracturing fluid. Figure 2
illustrates
obtainable proppant concentration as a function of pad pressure and proppant
control valve openings. As shown herein, the control system functions over a
pad
pressure ranging from -25 to +30 psi, and may still function over a range of -
30 to
+40 psi. In the present invention pad pressure is used as a means of coarse
control
of proppant loading while proppant control valve opening is used as a means
for
fine tuning the proppant loading.
[00037] Figure 3 depicts an overview of the process using a flow diagram
showing the basic elements of the present invention. Liquid carbon dioxide
(LCO2) fluid is supplied as stream 101. Typically, stream 101 would be
supplied
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from a liquefied gas boost pump. The pressure of stream 101 is typically
between
200 and 400 psig. The LCO2 is routed through an eductor 104 and is mixed with
proppant from the proppant reservoir 102, which is oriented in a position
sufficient to provide proppant to the eductor, and preferably in a vertical or
near
vertical position. Moreover, the fluid in proppant reservoir 102, can be
subcooled
to provide the requisite NPSH downstream. For instance, decreasing the
pressure
in the reservoir and/or subcooling the liquid in the reservoir so the
requisite NPSH
is achieved. The eductor 104 serves the dual purpose of causing mixing within
process piping as well as providing suction for drawing the proppant from the
reservoir 102, thereby resulting in some degree of homogeneity in the product
stream 107. Typical LCO2 flow rates in a stream 101 for this system will be
between 10 and 80 BBLS/min. An appropriate converging nozzle size in the
eductor 104 is selected to produce a pressure drop of between 30 and 50 PSI
for a
selected liquid/motive flow 101. The recommended pressure drop in operation of
the eductor 104 is between 15 PSI and 60 PSI, depending on the available
"disposable" NPSH of the stream 107. During the fracturing operation, a
proppant control valve 105 regulates the flow of proppant or proppant slurry
from
the proppant reservoir 102 into the eductor 104. One or more of these eductors
can be placed and connected in parallel and perform as a single device. For
instance, the two seven inch eductors can be utilized in place of a single
nine inch
eductor, depending on the flow rate necessitated. The eductors and other
components of the system can be modularized, variable and switched out of the
system. The meter 106 could be any one of or a combination of a nuclear
densitometer, Coriolis meter, or other suitable measurement device that
provides
feedback on fracturing fluid loading concentration, density, or other
parameter
capable of determining proppant concentration prior to well head injection.
The
proppant control valve 105 opening can be adjusted based on the readings
provided by meter 106. The volume of pressurized liquid or gas 103 supplied to
the top of the proppant reservoir 102 allows for modification of the static
pressure
ranging from about 80-400 psi inside the proppant reservoir 102. An adjustment
in the system's static pressure changes the overall flow capacity of proppant
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control valve 105. The resulting LCO2 and proppant fracturing fluid is
supplied to
high pressure pumpers via stream 107. For a given or predetermined motive flow
rate, either the proppant control valve 105 or the pad pressure, or both, is
utilized
to achieve the desired concentration by metering the proppant solution into
the
motive flow. In an alternative embodiment, a phase separator (not shown) or
refrigeration system (not shown) can be utilized to remove vapor and provide
condensed fracturing fluid after the eductor to the high pressure pumpers.
[00038] Figure 4 is a schematic that illustrates another embodiment of the
present invention. In this embodiment, a parallel slipstream 302 of LCO2 can
be
provided that bypasses the eductor 305. This could be useful, for example,
during
the stages of the fracturing operation where no proppant is required (commonly
referred to as the pad or padding stage). This bypass stream 302 can also be
used
to assist in controlling the final proppant loading. The flow into stream 302
and
the motive stream 301 is controlled by flow control valves 304 and 303,
respectively. The flow of the motive stream 301 is routed into eductor 305
where
a proppant control valve 306 regulates the flow of proppant from the proppant
reservoir 315 into eductor 305. An isolation valve 307, located between the
control valve 306 and the proppant reservoir, is used to isolate the proppant
reservoir 315 from the eductor 305. LCO2 liquid is injected through line 308
to
the bottom of the reservoir 315 to promote a liquid-solid suspension. Flow in
line
308 is regulated by flow control valve 309 and actively provides for stirring
of the
proppant within the reservoir 315 during operation. This creates a dynamic
dispersion that aids removal of proppant from the reservoir 315 and promotes
uniformity and a degree of homogeneity of the slurry prior to entering eductor
305. A similar LCO2 line 310 regulated by another flow control valve 311
provides fluid to the top portion of reservoir 315. This fluid is used to
maintain a
liquid CO2 level above the proppant level in reservoir 315 to ensure that gas
from
the head space of the reservoir 315 does not enter eductor 305 and prevents
vapor
from passing through to the high pressure pumpers via line 317. Furthermore,
maintaining this liquid cap also facilitates the flow of proppant from the
reservoir
315 by reducing clumping and improving the flow behavior of the proppant. A
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pressurized gas line 312 can be utilized for injecting vapor to the top of the
reservoir 315 for modification and control of the static pressure of the
reservoir
315. Examples of gases that could be used to adjust the pressure include, but
are
not limited to carbon dioxide and nitrogen. The flow of pressurized gas into
the
proppant reservoir 315 is controlled through the use of a pressure control
valve
313. Working in conjunction with the pressure control valve 313 is a pressure
relief control valve 314. This valve works to relieve excess pressure stored
in the
head space of the proppant reservoir 315. The pressure in the head space of
the
proppant reservoir 315 can be both raised and lowered during operation via
control valves 313 and 314. Head space pressure changes in the reservoir 315
results in an alteration of the overall flow capacity of the proppant loading
control
valve 306. A density meter 316 is used to determine the proppant loading
during
operation. The density reading data is used to modify the proppant control
valve
306 opening in order to achieve a desired concentration. Fracturing fluid
stream
317 is then sent to the high pressure pumpers. The high pressure pumpers
further
increase the pressure of the proppant and liquefied gas stream to surface
treatment
pressure and are in communication with the well head.
[00039] The control system and methodology for arriving at the desired
proppant concentration is further explained in the Working Examples below.
These examples, however, should not be construed as limiting the present
invention.
Working Example 1: Motive Flow Rate of 20 BBLS/min
[00040] The data below in Table 1 provides a simulated example where the
reservoir pad pressure (PP) and percent valve opening (VP) requirements (for a
proppant control valve with a flow coefficient (CV) of 200) to obtain desired
proppant concentrations from 0.25 to 4 lbs of proppant per gallon of LCO2 in a
fracturing fluid slurry as prescribed by a fracturing treatment schedule. The
treatment schedule is utilized to provide a "pre-programmed" set of
instructions
(i.e., a PLC controller recipe is loaded into the system, and which
communicate
with the proppant control valve and adjust the pad pressure in the reservoir
via the
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control loops). Naturally, an operator may manually override the recipe if
necessary to modify the concentration of proppant in the slurry. Determining
the
control valve position and operating head pressure in the proppant reservoir
is first
determined through an iterative process carried out in the field. During
fracturing
operations, the pressure in the reservoir is adjusted to provide the
designated pad
pressure (PP) and valve position (VP) necessary in order to achieve the
desired
concentration based on a selected motive flow rate and the flow coefficient of
the
proppant control valve. The treatment schedule cannot be established without
the
proper determination of the pad pressure and proppant control valve position.
In
order to create the ability to provide a range of low end proppant loading to
high
end proppant loading, it is necessary to vary pad pressure to achieve proppant
loading within predetermined ranges. The motive flow rate is set by
determining
the specific pumping rate required for the fracture treatment.
[00041] The system (such as described in any one of the exemplary
embodiments above) is initially set at a low pad pressure, in this given
example, a
low pad pressure of -15 PSI is used. Setting the system at this low pressure
allows for achieving better control of low proppant loadings (e.g. 0.25, 0.50
lbs/gal) using the proppant control valve. The proppant control valve is
initially
adjusted to increase proppant concentration in the fracturing fluid stream as
prescribed by the treatment schedule, which is loaded in the PLC controller.
In
the example given the valve is adjusted from 10% to 40% open to achieve
proppant loadings from 0.25 to 1.5 lbs/gal. After 1.5 lbs/gal is reached, the
pad
pressure is increased in order to better achieve higher proppant loadings
(e.g. 3.5,
4.0, 4+, lbs/gal). In the example the pad pressure is adjusted from -15 PSI to
15
PSI. The pressure increase is done in a fashion were it has minimum impact on
the proppant control valve position (in the example given this is done at 1.5
to 2.0
lbs/gal) and therefore is done at a specified loading. Once the new pad
pressure
has been established the process is completed through adjustments with the
proppant control valve.
[00042] The following is done to minimize operational complexity: the head
pressure is changed only once through the process; the system is adjusted
using
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only one parameter at a time (either head pressure or proppant control valve
position is changed, not both) or if two parameters are adjusted, one is
changed
minimally; the proppant control valve and pad pressure is adjusted in one
direction (the head pressure is always increased and the proppant control
valve
opened only).
Table 1: Operating Conditions for Various Proppant Concentrations at a
motive flow of 20 BBLS/Min Delivery to the Well Head
1111111111111111110114i11111111111111111111111111111111111111110111111111111111
111111111111:111111111:6111
1mincgottvgmai
(LN/0401ENAIMMEAOliiog/POOROMM
0.25 -15 10%
0.5 -15 19%
........
0.75 -15 25%
........
:.:.:.:.
1 -15 30%
-15 40%
============== ============
2 15 40%
:.:.:.:.:.:. :.:.:.:.:.
2.5 15 48%
.:.:.:.:.:
............
15 56%
35 15 67%
15 82%
0.5 15 14%
0 15 0%
Working Example 2: Pilot Tests Conducted at 23 GPM of Motive Flow.
[00043] Results from the operation of a pilot plant system similar to the one
described above and shown in Figure 3 are given in this Working Example 2. In
this system, the concentration of the fracturing fluid was controlled by
varying the
proppant control valve opening while operating the proppant reservoir at "low"
(i.e. between -5 and -27 psi) and "high" (i.e. between 11 and 27 psi) pad
pressure
conditions. The motive flow was 23 gallons per minute for both pad pressure
conditions.
[00044] Figure 5 illustrates the resulting concentration from pilot plant
operations for the "low" pad pressure range. The proppant control valve varied
from 8% to 70% open position and proppant concentrations from 0.25 to 3.27
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lbs/gal were observed. The proppant concentration did not increase above 3.27
lbs/gal when the proppant control valve open position was increased above 70%.
Figure 6 illustrates the results of varying the proppant control valve
position for
the "high" pad pressure range. The control valve position varied from 10% to
23% open and concentrations from 0.75 to 4.04 lbs/gal were observed. The
minimum achievable concentration for the "high" pad pressure condition was
0.75
lbs/gal.
[00045] The outcome of the "low" and "high" pad pressure pilot tests described
in this example illustrates that it is necessary to change both the pad
pressure and
the proppant control valve position to achieve the full range of proppant
loadings
required for a fracturing treatment (e.g. 0.25 to 4.0+ lbs/gal).
[00046] While the invention has been describe in detail with reference to
exemplary embodiments thereof, it will become apparent to one skilled in the
art
that various changes and modifications can be made, and equivalents employed,
without departing from the scope of the appended claims.
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