Note: Descriptions are shown in the official language in which they were submitted.
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PROPPANT ADDITION SYSTEM AND METHOD
TECHNICAL FIELD
[0001] This document relates to proppant addition systems and methods, and
particularly to low or atmospheric pressure proppant addition systems and
methods.
BACKGROUND
[0002] In the conventional fracturing of wells, producing formations, new
wells or
low producing wells that have been taken out of production, a formation can be
fractured to
attempt to achieve higher production rates. Proppant and frac fluid are mixed
in a blender
and then pumped into a well that penetrates an oil or gas bearing formation.
High pressure is
applied to the well, the formation fractures and proppant carried by the
fracturing fluid flows
into the fractures. The proppant in the fractures holds the fractures open
after pressure is
relaxed and production is resumed. Various fluids have been disclosed for use
as the
fracturing fluid, including various mixtures of hydrocarbons, liquefied
petroleum gas,
nitrogen, and carbon dioxide.
[0003] Proppant addition can be added into a pressurized stream of frac
fluid, for
example liquefied petroleum gas, directly by having the proppant addition tank
itself
contained under pressure. Proppant addition systems into LPG, such as those
disclosed in
WO/2007/098606, often use centrifugal pumps to dynamically seal the proppant
from the
volatile stream of frac fluid. However, a pressure vessel is still required,
as the dynamic seal
is only present whilst the centrifugal pump is in operation. Systems have been
proposed to
avoid the use of a pressure contained proppant tank, for example by sending a
stream of
proppant blended with frac oils, and a stream of liquefied petroleum gas as
(LPG) to separate
frac pressure pumps, after which the two streams are combined at pressure and
then used to
frac a well. This system requires the use and coordination of multiple sets of
frac pressure
pumps, which are expensive and costly to operate. The outlet pressures from
the two sets of
frac pressure pumps must be balanced correctly, which makes the pumping
difficult to
control. It also requires that the mixture of proppant be mixed with
substantial amounts of
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low vapor pressure frac oils, which may seriously reduce the positive effects
of the LPG frac
fluid, namely easy clean up and recovery from the well.
SUMMARY
[0004] An apparatus for fracturing a formation penetrated by a well is
disclosed
comprising a frac pressure pump, a frac fluid source, and a proppant supply
source. The frac
pressure pump is connected to the well. The frac fluid source is connected to
supply a stream
of frac fluid to the frac pressure pump. The proppant supply source has a
proppant receiver, a
positive displacement pump, and at least an inlet into the proppant supply
source. The at
least an inlet is connected to one or more liquid hydrocarbon sources to
supply liquid
hydrocarbons to proppant in the proppant supply source. The positive
displacement pump is
connected to pump proppant into the stream of frac fluid before the frac
pressure pump.
[0005] A method is also disclosed. Proppant and liquid hydrocarbons are
supplied
into a proppant supply source to create a mixture of proppant and liquid
hydrocarbons. The
mixture of proppant and liquid hydrocarbons is pumped from the proppant supply
source
into a stream of flu fluid using a positive displacement pump. The stream of
frac fluid
containing the mixture of proppant and liquid hydrocarbons is then pumped to a
frac
pressure pump connected to a well.
[0006] An apparatus for fracturing a formation penetrated by a well is also
disclosed,
the apparatus comprising a frac pressure pump, a frac fluid source, and fluid
lines. The frac
pressure pump is connected to the well. The frac fluid source is connected to
supply a stream
of frac fluid to the frac pressure pump, The fluid lines connect the frac
pressure pump, the
well, and the frac fluid source, the fluid lines having isolation valves
spaced so that the
volume of fluid containable between any set of neighboring isolation valves is
less than or
equal to 500 L.
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BRIEF DESCRIPTION OF THE FIGURES
[0008] Embodiments will now be described with reference to the figures, in
which
like reference characters denote like elements, by way of example, and in
which:
[0009] Fig. 1 is a schematic illustrating an apparatus for fracturing a
formation
penetrated by a well.
[0010] Fig. 2 is a side elevation view, in section, of an embodiment of a
proppant
supply source that may be used in the system of Fig. 1.
[0011] Fig. 3 is a schematic illustrating a further apparatus for
fracturing a formation
penetrated by a well.
[0012] Fig. 4 is a flow diagram illustrating a method of supplying frac
fluid to a well.
DETAILED DESCRIPTION
[0014] Proppant may be required to be supplied into a stream of fluid, for
example a
stream of frac fluid. In some cases it is desirable to supply the proppant as
a mixture of
proppant and liquid. This wets the proppant, allowing it to be more easily
transferred from
the proppant supply source and into the stream of frac fluid. In cases where
the proppant is
being supplied into a high pressure stream of fluid such as liquefied
petroleum gas, the
proppant supply source may need to be under positive pressure. The liquid in
the mixture of
proppant and liquid can then act as a liquid seal to prevent gas breakthrough
from the
proppant supply source into the frac fluid. In some cases the proppant supply
source must be
under positive pressure when the liquid itself in the proppant has a high
vapor pressure, such
as when liquefied petroleum gas is added to the proppant. LPG will vaporize at
atmospheric
pressure creating a hazardous situation.
[0015] Referring to Fig. 1, an apparatus 10 for fracturing a formation 12
penetrated
by a well 14 is illustrated. Apparatus 10 comprises a frac pressure pump 16, a
frac fluid
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source 18, and a proppant supply source 20. Frac pressure pump 16 is connected
to the well
14.
[00161 A frac fluid source 18 is connected to supply a stream of frac fluid
to the frac
pressure pump 16, through line 28 for example. In some embodiments the stream
of frac
fluid is volatile, for example if frac fluid source 18 comprises LPG. For cost
effectiveness,
the LPG may be predominantly propane or butane or a propane and butane mix.
The frac
fluid may also contain minor amounts of pentane and higher hydrocarbons. In
some
embodiments, the frac fluid comprises liquefied gas, such as LPG or CO2.
Referring to Fig.
1, liquefied CO2 may be supplied to the stream of frac fluid via source 30. In
some
embodiments, source 30 may supply other frac fluids, such as lower vapor
pressure
hydrocarbons. Gas, such as inert gas, may be supplied to each of tanks 18, 30,
via lines 32,
34 from gas source 36 as needed. Inert gas may be required to maintain
liquefying or drive
pressure on the LPG contained in tank 18. Various additives can be introduced
into the
stream of frac fluid, such as gelling agents, breakers, and activators for
example, via additive
sources 38A-38B. Additives may be added to the stream before or after the
introduction of
proppant. A pump 40 may be provided in order to provide the pumping pressure
required to
move the stream of frac fluid through line 28.
[0017] Proppant supply source 20 is illustrated as having a proppant
receiver 21, a
positive displacement pump 26, and at least an inlet into the proppant supply
source 20
(shown for example as inlet 48). The at least an inlet is connected to one or
more liquid
hydrocarbon sources, for example source 46, to supply liquid hydrocarbons to
proppant in
the proppant supply source 20. Proppant supply source 20 is illustrated as
containing a
mixture of proppant and liquid hydrocarbons (shown as mixture 22). The liquid
hydrocarbons may comprise hydrocarbons having six or more carbons. In some
embodiments, the proppant receiver 21 has an auger 24 for supplying at least
proppant, and
preferably a mixture of proppant and liquid hydrocarbons, to pump 26.
Referring to Fig. 1,
the proppant receiver 21 may comprise an outlet 42 for supplying the mixture
of proppant
and liquid hydrocarbons to the auger 24. Referring to Fig. 2, in other
embodiments the auger
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24 is located at least partially inside the proppant receiver 21, for example
along the base of
receiver 21. This way, proppant may be easily channeled from receiver 21 to
pump 26. The
proppant supply source 20 may be at or below atmospheric pressure, for example
if proppant
supply source 20 is open to the atmosphere. In Fig. 2, the proppant receiver
21 may be an
open topped 100 tonne hopper, which makes for easy addition of proppant into
proppant
receiver 21.
[0018] Referring to Fig. 1, the positive displacement pump 26 is connected
to pump
proppant, for example a mixture of proppant and liquid hydrocarbons, into the
stream of frac
fluid before the frac pressure pump(s) 16. hi some embodiments, pump 26 is
connected to
pump the mixture of proppant and liquid hydrocarbons from the auger 24 into
the stream of
frac fluid. Positive displacement pumps cause a fluid to move by trapping a
fixed amount of
it and then displacing the trapped volume into a discharge zone, for example
line 28. Positive
displacement pumps are advantageous because they provide a pressure seal
between the inlet
and the outlet Thus, a mixture of wetted proppant may be added at atmospheric
pressure to a
= pressurized stream of frac fluid. This is advantageous over the use of a
centrifugal pump in
that, should the pump fail, the pressure seal is maintained. Thus, there is no
requirement that
the proppant supply source 20 be contained under pressure. Further, positive
displacement
pumps are advantageous because they are capable of providing relatively stable
flow rates
regardless of varying pressures in the outlet stream. Thus, a positive
displacement pump
allows a user more control over the amount of proppant added to the frac
stream, and hence
more control over the frac itself.
[0019] Pump 26 may be a progressive cavity pump. Progressive cavity pumps
are
used downhole as sand pumps, and are advantageous because they are capable of
moving
fluid containing large quantities of solids. A progressive cavity pump is also
known as a
progressing cavity pump, eccentric screw pump or even just a cavity pump.
Names can vary
from industry to industry and even regionally, including, MoynoTm pump, Mohno
pump,
Nemo pump, and Seepexim pump. This type of pump transfers fluid by means of
the
progress, through the pump, of a sequence of cavities as its rotor is turned
in relation to a
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stator. This leads to the volumetric flow rate being proportional to the
rotation rate and to
low levels of shearing being applied to the pumped fluid. Hence these pumps
have
application in fluid metering and pumping of viscous or shear sensitive
materials. In some
embodiments, positive displacement pump 26 may be another type of pump, for
example a
screw pump or lobe pump.
[0020] Referring to Fig. 2, apparatus 10 (shown in detail in Fig. 1) may
further
comprise a pressure seal between the proppant receiver 21 and the positive
displacement
pump 26. Referring to Fig. 2, in some embodiments the pressure seal
(illustrated as pressure
seal 57) may simply be created by the positive displacement pump. In other
embodiments, a
pressure seal 55 may be in place for example after auger 24, pressure seal 55
allowing fluids =
to pass into pump 26.
[0021] Referring to Fig. 1, a first inlet 48 of the at least an inlet may
be connected
into the proppant supply source 20 before the pressure seal. The first inlet
may be at least
one inlet. Referring to Figs. 1 and 2, liquid hydrocarbons can be supplied to
proppant in
proppant supply source 20 from a variety of locations. Referring to Fig. 1,
liquid
hydrocarbons are supplied into proppant receiver 21. Referring to Fig. 2,
liquid hydrocarbons
may be supplied through first inlets 48 and 49 into the proppant receiver 21
and auger 24,
respectively, The first inlet has its liquid hydrocarbons supplied by a liquid
hydrocarbon
source 46 of the one or more liquid hydrocarbon sources. In some embodiments,
each of first
inlets 48 and 49 may have different liquid sources, The liquid hydrocarbon
source 46
connected to supply the first inlet may comprise hydrocarbons having six or
more carbons.
Suitable liquid hydrocarbons added to the proppant supply source 20 from the
one or more
liquid hydrocarbon sources may include hydrocarbons having between eight and
ten carbons,
or for example eleven to fourteen carbons. It may be advantageous to use
hydrocarbons with
the least number of carbons possible that are non-volatile, for example when
the frac fluid
comprises LPG. Non-volatile hydrocarbons have at least one of a low vapor
pressure and a
high boiling point. The liquid hydrocarbons may have a vapor pressure of less
than 200 mm
Hg at room temperature, for example a vapor pressure of less than 15 mm Hg at
room
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temperature. Because higher weight hydrocarbons, for example C6-C20 are harder
to remove
from the formation in contrast to LPG, an amount of liquid hydrocarbons
sufficient to only
wet the proppant may be added to minimize the liquid hydrocarbons supplied
into the stream
of frac fluid. Wetted may refer to only enough liquid hydrocarbon to saturate
the pores of the
proppant contained within vessel 20. Because sand has around 30% porosity an
exemplary
load of 15 tonnes (15000 kg) of sand would contain 3 m3 of propane, or 200 L
per tonne of
sand. However, the low vapor pressure means that the liquid hydrocarbon and
proppant
mixture does not have to be contained within a pressure vessel, particularly
when the
hydrocarbons have seven or more carbons. For hydrocarbons having five or six
carbons,
addition of the hydrocarbons under sealed conditions is desirable.
[0022] Referring to Fig. 2, apparatus 10 may further comprise a second
inlet 54 of
the at least an inlet connected to supply liquid hydrocarbons into the
proppant supply source
20 after the pressure seal, for example seals after at least one of 55 and 57.
Liquid may be
supplied through inlet 54 from liquid hydrocarbon source 52 of the one or more
liquid
hydrocarbon sources. Suitable liquids include hydrocarbons having six or more
carbons, and
other frac oils. Other liquids may be present as desired, for example
alcohols. In some
embodiments, the liquid hydrocarbon source 52 connected to supply the second
inlet 54
comprises liquefied petroleum gas, including, for example, propane, butane or
pentane or
mixtures thereof. This way, the proppant may be wetted with liquefied
petroleum gas prior to
being supplied into the stream of frac fluid. In other embodiments, other high
vapor pressure
liquids may be added via second inlet 54. It should be understood that at
least one of inlets
48, 49, and 54 may be present. The inlet 54 is illustrated as being connected
directly into
pump 26, although this is not required.
[0023] Referring to Fig. 1, the pressure applied by the frac pressure pump
16 may be
a pressure suitable for fracturing the formation 12. An example frac pressure
pump is a
diesel QuintuplexTM pump with water cooled turbines, or an electrically
powered Triplex(tm)
piston pump, but any suitable pump may be used. As illustrated, more than one
pumping
device may be used as the pump 16.
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[0024] Referring to Fig. 1, the stream of frac fluid may have a boost pump
56 for
pumping the stream of frac fluid in high ambient temperatures, for example
those seen in
Texas in the daytime in summer. Boost pump 56 may be positioned at any point
along line
28 and provides extra pressure, for example 300 psi, in order to retain the
LPG or other
liquefied gas in the liquid state in the stream of frac fluid. The stream of
frac fluid may then
pass into a blender (not shown) where other chemicals may be added to the
stream of frac
fluid, and then on to the frac pressure pumps.
[0025] Referring to Fig. 3, an exemplary system is illustrated where
proppant supply
source 20 is provided. Liquid hydrocarbons are supplied to proppant receiver
21 from liquid
hydrocarbon source 46 and inlet 48. The proppant receiver 21 may be a rotary
tub, and
supplies a mixture of proppant and liquid hydrocarbons to positive
displacement pumps 26A,
26B through line 60. At least one pump 26, in this case two, is connected to
pump the
mixture of proppant and liquid hydrocarbons supplied from the proppant
receiver 21 into the
stream of frac fluid in line 28. Line 60 feeds lines 60A, 60B into pumps 26A,
26B,
respectively. A circulation pump 62 may be provided on inlet 48 to ensure that
the frac fluid,
for example heavy frac oils are pumped to proppant receiver 21.
[0026] Referring to Fig. 4, an exemplary method is illustrated. Referring
to Fig. 1, in
stage 100 (shown in Fig. 4), proppant and liquid hydrocarbons are supplied
into a proppant
supply source 20 to create a mixture of proppant and liquid hydrocarbons. The
liquid
hydrocarbons may comprise hydrocarbons having six or more carbons. Auger 24
may be
provided to allow a thick, highly solids laden mixture to be channeled from
receiver 21 to
pump 26 without requiring pressurization. In stage 102 (shown in Fig. 4) the
mixture of
proppant and liquid hydrocarbons is pumped from the proppant supply source 20
into the
stream of frac fluid in line 28 using positive displacement pump 26. In stage
104 (shown in
Fig. 4) the stream of frac fluid containing the mixture of proppant and liquid
hydrocarbons is
supplied to frac pressure pump(s) 16 connected to well 14.
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[0027] Table 1 below illustrates various slurry rates required to create a
stream of
frac fluid with specific a wellhead density. The exemplary data is constructed
using sand
(Regular density 2650 kg/m3) contained as a mixture of proppant and liquid
hydrocarbons
having 1325 kg of sand and 500 L of liquid hydrocarbons per m3 of mixture in
proppant
supply source 30. Wellhead flow rate indicates the flow rate of the frac fluid
slurry pumped
down the well. Wellhead density indicates the density in kg of sand per m3 of
frac fluid sent
down the well. The third column refers to the amount of sand required to be
added to the frac
fluid, and the fourth column indicates the amount of sand required to be added
to the frac
fluid each minute, both in order to achieve the desired wellhead density.
[0028] Table 1: Exemplary Slurry Rates
Wellhead Wellhead Kg of kg of sand/ Slurry rate
leaving
flow rate Density Sand minute Proppant Tank
(m3/min) (kg added needed (m3/min)
sand/m3 per m3
of slurry) of frac
fluid
0 0 0 0 0
3 100 96.4 289.2 0.218264151
3 200 186 558 0.421132075
3 300 269.6 808.5 0.610188679
3 400 347.5 1042.5 0.786792453
3 600 420.6 1261.8 0.952301887
3 800 614.5 1843.5 1.391320755
3 1000 726 2178 1.643773585
[0029] LPG may include a variety of petroleum and natural gases
existing in a liquid
state at ambient temperatures and moderate pressures. In some cases, LPG
refers to a
mixture of such fluids. These mixes are generally more affordable and easier
to obtain than
any one individual LPG, since they are hard to separate and purify
individually. Unlike
conventional hydrocarbon based fracturing fluids, common LPGs are tightly
fractionated
products resulting in a high degree of purity and very predictable
performance. Exemplary
LPGs used in this document include ethane, propane, butane, pentane, and
various mixes
thereof. Further examples include HD-5 propane, commercial butane, i-butane, i-
pentane, n-
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pentane, and n-butane. The LPG mixture may be controlled to gain the desired
hydraulic
fracturing and clean-up performance.
[0030] LPGs tend to produce excellent fracturing fluids. LPG is readily
available,
cost effective and is easily and safely handled on surface as a liquid under
moderate pressure.
LPG is completely compatible with formations and formation fluids, is highly
soluble in
formation hydrocarbons and eliminates phase trapping - resulting in increased
well
production. LPG may be readily and predictably viscosified to generate a fluid
capable of
efficient fracture creation and excellent proppant transport. After
fracturing, LPG may be
recovered very rapidly, allowing savings on clean up costs.
[0031] Referring to Fig. 1, an apparatus 10 is illustrated for fracturing a
formation 12
penetrated by a well 14, the apparatus 10 comprising a frac pressure pump 16,
and a frac
fluid source 18. Frac pressure pump 16 is connected to the well 14, and frac
fluid source 18
is connected to supply a stream of frac fluid to the frac pressure pump 16.
Fluid lines, for
example lines 28 and 29 connecting the frac pressure pump 16, the well 14, and
the frac fluid
source 18 are present. The fluid lines have isolation valves, for example
isolation valves
70C, 70E, 70G, 70H, and 701 spaced so that the volume of fluid containable
between any set
of neighboring isolation valves is less than or equal to 500 L, for example
less than or equal
to 400 L, 200 L, 100 L, 70 L, or 50 L. The isolation valves may be remotely
controlled by a
controller, and activated in the event of for example a leak, an emergency,
after pressure
testing, or at any suitable stage during the frac procedure. Referring to Fig.
1, apparatus 10
may include other components, such as proppant supply source 20, additive
source 38A-13,
gas source 36, source 30, pump 40, pump 56, and any other component required.
The
isolation valves 70A-K compartmentalize the apparatus 10 to define segments of
the system
that can contain the maximum amount of fluid. For example, valves 70C, 70D,
70J, 70K, and
70E define a segment 71 of line 28 that can be isolated. The isolation valves
may be spaced
so that each of the frac pressure pump 16, the well 12, the frac fluid source
18, and any other
component of the system if desired, may be independently isolated from one
another. Vents
72A-K may be spaced in between each set of neighboring isolation valves, in
order to
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provide an outlet for venting the fluid contained in between the isolation
valves should a
particular segment of the system be isolated. As illustrated, the vents may be
connected to
vent any fluid present, for example to a flare 74, isolation vessel (not
shown), or sales line
(not shown). At least some of the vents may be connected to a manifold (not
shown) prior to
flaring. In some embodiments, a pop tank (not shown) is provided prior to the
flare stack 74.
[0032] This system provides added safety to frac apparatus 10, especially
when the
frac fluid source comprises liquefied petroleum gas, since the entire system
can be isolated
into small segments should one or more components in the system fail. Thus, if
for example
a leak is detected, the isolation valves may be activated in order to reduce
the total amount of
frac fluid leaked to the environment to the volume contained in the segment
where the leak
occured. Also, should a leak occur in one or more segment and catch fire, the
amount of frac
fluid available as fuel to the fire can also be reduced by isolating the one
or more segments.
After a segment is isolated it may be safely vented, in order to clear away
any hazardous
fluid contained within the fluid lines.
[0033] It should be understood that the figures illustrated exemplary
systems, and
various valving, tubing, connections, and other devices may be necessary in
order to
properly operate the system.
[0034] In the claims, the word "comprising" is used in its inclusive sense
and does
not exclude other elements being present. The indefinite article "a" before a
claim feature
does not exclude more than one of the feature being present. Each one of the
individual
features described here may be used in one or more embodiments and is not, by
virtue only
of being described here, to be construed as essential to all embodiments as
defined by the
claims.
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