Note: Descriptions are shown in the official language in which they were submitted.
CA 02649197 2008-12-24
PROPPANT CONTROL IN AN LPG FRAC SYSTEM
TECHNICAL FIELD
[0001] This document relates to methods and apparatus for controlling proppant
in a
well treatment system.
BACKGROUND
[0002] In hydraulic fracturing, frac fluids may be sent to a high pressure
pump to be
pumped down a well to fracture a formation. Typically, these frac fluids
contain proppant
supplied into the frac fluid for propping open fractures created in the
formation by the
pressure of the frac fluid. Proppant may be supplied into the frac fluid from
a proppant
supply source. Fluid may be added to the proppant supply source to assist in
controlling gas
break out. A method of monitoring fluid level in the proppant is required.
SUMMARY
[0003] An apparatus for supplying proppant is disclosed, comprising a vessel
and a
level monitor. The vessel has an interior containing a mixture of proppant and
liquid, an inlet
for supplying proppant to the interior, and an outlet for supplying the
mixture of proppant
and liquid from the interior of the vessel. The level monitor is associated
with the vessel for
monitoring at least a level of liquid and has a discriminator for
discriminating between the
level of liquid and a level of proppant in the mixture of proppant and liquid.
[0004] A method of supplying proppant is also disclosed. A vessel is provided
at
least partially filled with a mixture of proppant and liquid and having a
level of liquid and a
level of proppant. The level of liquid in the mixture of proppant and liquid
is monitored. The
mixture of proppant and liquid is supplied from the vessel through an outlet
in the vessel.
[0005] An apparatus for supplying proppant is also disclosed, comprising a
pressure
vessel and a level monitor. The pressure vessel has an interior containing a
mixture of
proppant and liquid, an inlet for supplying proppant to the interior, and an
outlet at or near a
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base of the interior for supplying the mixture of proppant and liquid from the
interior of the
pressure vessel. The level monitor is associated with the pressure vessel for
monitoring at
least a level of liquid of the mixture of proppant and liquid.
[0006] These and other aspects of the system and method are set out in the
claims,
which are incorporated here by reference.
BRIEF DESCRIPTION OF THE FIGURES
[0007] 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:
[0008] Fig. 1 is side elevation view, in section and not to scale, of a
proppant vessel
containing a level monitoring device.
[0009] Fig. 2 is a side elevation view, in section and not to scale, of a
portion of a
level discriminator.
[0010] Fig. 3 is a schematic of a proppant addition system.
[0011] Fig. 4 is a side elevation view, partially in section and not to scale,
of a
proppant supply source contained on a truck flatbed.
[0012] Fig. 5 is a side elevation view, in section and not to scale, of a
portion of a
further level discriminator.
[0013] Fig. 6 is a side elevation view, in section and not to scale, of a
pressure vessel
and a level monitoring device.
[0014] Fig. 7 is a perspective view, not to scale, of the lower end of a
discriminator.
[0015] Fig. 8 is a side elevation view, in section and not to scale, of a
proppant vessel
containing a level monitoring device having multiple systems.
[0016] Fig. 9 is a flow schematic of a method of supplying proppant.
DETAILED DESCRIPTION
[0017] Immaterial modifications may be made to the embodiments described here
without departing from what is covered by the claims.
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[0018] 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 may wet the proppant, allowing it to be more easily
transferred
from the proppant supply source. In cases where the proppant supply source is
under positive
pressure, the liquid in the proppant can act as a liquid seal to prevent gas
breakthrough into
the stream of frac fluid from the proppant vessel.
[0019] Referring to Fig. 1, an apparatus 10 for supplying proppant is
illustrated.
Apparatus 10 comprises a vessel 12 and a level monitor, for example a level
monitoring
device. Vessel 12 has an interior 16 containing a mixture of proppant and
liquid, an inlet 18
for supplying proppant to the interior 16, and an outlet 20 for supplying the
mixture of
proppant and liquid from the interior 16 of the vessel 12. Referring to Fig.
6, outlet 20 may
be connected to supply proppant into a pressurized stream of fluid 24. The
vessel 12 may be
a positive-pressure vessel as illustrated in Fig. 6, for example if supplying
proppant into a
stream of pressurized fluid or if the liquid comprises liquefied petroleum
gas. Referring to
Fig. 1, vessel 12 may have a liquid inlet 22 for supplying liquid to the
interior 16 of the
vessel 12.
[0020] Referring to Fig. 1, level monitor 14 is associated with the vessel 12
for
monitoring at least a level of liquid 26. Level monitor 14 has a discriminator
28 as part of
level monitor 14 for discriminating between the level of liquid 26 and a level
of proppant 30
in the mixture of proppant and liquid. Referring to Figs. 4 and 1, level
monitor 14 may be
located at least partially outside the interior 16 of the vessel 12 (Fig. 4),
or within vessel 12
(Fig. 1) for example attached to a surface of the interior 16.
[0021] In some embodiments, the mixture of proppant and liquid is a mixture of
proppant wetted with liquid. In some embodiments, a suitable amount of liquid
is maintained
or supplied into interior 16 to provide at least one of a liquid seal and
saturation of the pores
of the proppant contained within. Because sand has around 30% porosity an
exemplary load
of 15 tonnes of sand would contain 3 m3 of propane.
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[0022] Referring to Fig. 1, in some embodiments the discriminator 28 comprises
a
sheath 34 having an upper end 36 and a lower end 38. Sheath 34 also has an
opening 40 at or
near the lower end 38 for allowing liquid in the interior 16 to flow into the
sheath 34.
Opening 40, which may be multiple openings, at least restricts the flow of
proppant into the
sheath 34, and preferably prevents proppant from entering sheath 34
altogether, thus at least
partially isolating the liquid from the proppant so the liquid level 26 can be
measured. Sheath
34 also has an equalization vent 42 for allowing the pressure inside the
interior 16 and inside
the sheath 34 to equalize. Vent 42 may be a hole or perforated surface for
example. In some
embodiments, discriminator 28 can discriminate between the level of liquid 26
and the level
of proppant 30 regardless of the relative level of either in the vessel 12.
[0023] Sheath 34 may be a radar rod, for example of the type sold by Endress
and
Hauser. Sheath 34 may also have a flange 35 for coupling to a head 13 of level
monitor 14
for example by bolting. Referring to Fig. 4, flange 35 may be adapted to
couple to at least
one of both inlet 18, for example the hatch of the vessel 12, and head 13.
This way, when
inlet 18 is opened, sheath 34 may be easily removed or inserted as desired.
Referring to Fig.
1, in some embodiments, sheath 34 is straight along a sheath length, which may
be from the
upper end 36 to the lower end 38. This may be necessary if the level monitor
14 operates on
a line-of-sight detection basis. Referring to Fig. 1, in some embodiments
sheath 34 spans
from an upper end 44 of the interior 16 to a lower end 46 of the interior 16.
This way,
discriminator 28 may discriminate between the proppant and the liquid over the
entire or
substantially the entire interior height of the inside of the tank. Referring
to Fig. 3, in some
embodiments sheath 34 only spans a portion of the interior height of the
vessel 12.
[0024] Referring to Fig. 1, opening 40 may comprise at least one slot as
shown. In
some embodiments, opening 40 may effectively consist of openings along a
portion or the
entire length of sheath 34, for maximum permeation of the liquid into the
sheath 34.
Referring to Fig. 7, opening 40 may comprise a proppant screen 48. Screen 48
is understood
to allow liquid to pass into sheath 34, while preventing proppant from
entering. In some
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embodiments, the entire length of sheath 34 may be a proppant screen, or a
perforated
surface for example.
[0025] Referring to Fig. 1, sheath 34 may be angled relative to vertical. This
may be
advantageous depending on the configuration of interior 16 of vessel 12. For
example, in the
embodiment illustrated, lower end 46 of vessel 12 feeds to an outlet 20, for
example an
auger. If outlet 20 is centrally located in the base of vessel 12, then there
may be no room for
lower end 38 of sheath 34 to extend into, and sheath 34 can be angled to the
nearby side of
outlet 20.
[0026] Level monitor 14 monitors the amount of liquid that is contained in the
proppant supply source 12. This may allow a user to estimate the effective
size of the liquid
seal provided by the liquid in the mixture at the outlet of the proppant
supply source. The
amount of liquid may be monitored by taking at least one of direct and
indirect
measurements of a level of liquid. As further described below, the level of
liquid may be the
actual height of the liquid in the proppant supply source, or it may be other
measurable levels
for example the level liquid rises to in sheath 34. In order to accurately
measure a level of
liquid and infer the amount of liquid present, calibration of the measuring
equipment may be
required, to take into differences in for example proppant, liquid, container
size, and outlet
location. The discriminator 28 is configured to distinguish between the level
of liquid and the
level of proppant in the mixture, so that at least the level of the liquid can
be measured.
[0027] This distinguishing may be accomplished at least in part by isolating
at least
one of the levels, for example by physically separating the liquid from the
proppant using
sheath 34. The distinguishing may also be done at least in part
electronically, for example by
analyzing measured data obtained from monitor 14 using a computing device. The
analyzation part may be provided for example in at least one of head 13 and
from a remote
location for example in a control console. In this way, monitor 14 obtains
measurements that
are then interpreted by another part of the level monitor 14. The final
information may be
then conveyed to at least one of a user and a control algorithm, and may be
used to make
CA 02649197 2008-12-24
further decisions. For example, if the level of liquid, and hence the
effectiveness of the liquid
seal is low, a decision may be made to increase the amount of liquid in the
mixture. Any
software used is contained on a computer readable medium.
[0028] Referring to Fig. 2, level monitor 14 (shown in Fig. 1) may be
configured to
use time domain reflectometry (TDR) to determine the level of liquid 26 within
the sheath
34. This type of measurement involves the use of a sonic device that emits
waves 52 from
for example head 13 which bounce off the level 26 and return as return waves
54, wherein
determining the height of liquid in the sheath is based on the time difference
between
sending waves 52 and detecting waves 54. The waves may be radio waves for
example. The
waves may be guided by the sheath 34. A suitable system that can accomplish
this is the
Levelflex M FMP41C from Endress and Hauser.
[0029] Referring to Fig. 5, other methods may be used such as a float 56 -
operated
system. Level monitor 14 may detect the position of float 56, and hence infer
level 26.
[0030] Referring to Fig. 6, another example of a method of detection is
illustrated
using seismic detection, where level monitor 14 sends out sound waves to
detect directly
levels 26 and 30. This is one example of an embodiment where the level monitor
is
configured to monitor both the level of liquid 26 and the level of proppant
30. In this case,
the discriminator comprises software in a computing device that distinguishes
between the
reflections from the liquid and from the proppant. This embodiment may also
work if level
26 is above level 30 as shown, and level monitor 14 is a TDR device that can
detect the
interface between liquid and proppant (ie level 30) as well as level 26
itself. Some TDR
devices can only detect the level of liquid in proppant if the level 26 of
liquid is above the
proppant level 30, and thus a physical discriminator 28 is required.
[00311 Referring to Fig. 8, another example of detecting both levels is
illustrated,
using a level monitor 14 having two systems 14A and 14B to detect levels 26
and 30,
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respectively. System 14A may be the same as level monitor 14 illustrated in
Fig. 1, while
system 14B may be a TDR device.
[0032] A further option is to use a camera such as a video camera and image
analysis
software. Image analysis software can detect edges in an image such as an edge
caused by
the levels 26 or 30. The camera can be inside the vessel 12 or directed
towards a sight tube
on the exterior of the vessel, though this complicates design of the vessel 12
and is not
preferred. Other embodiments may detect level 26 using refractive index
measurements for
example with fibre optic cables, or using infra-red detectors. In other
embodiments, a
density meter may be used.
[0033] Referring to Fig. 6, a further apparatus 10 is illustrated for
supplying proppant
comprising a pressure vessel 12 and a level monitor, for example level monitor
14. Pressure
vessel 12 has an interior 16 containing a mixture of proppant and liquid, an
inlet 18 for
supplying proppant to the interior 16, and an outlet 20 at or near a base of
the interior 16 for
supplying the mixture of proppant and liquid from the interior 16 of the
pressure vessel 12.
Level monitor 14 is associated with the pressure vessel 12 for monitoring at
least a level of
liquid 26 of the mixture of proppant and liquid. Referring to Fig. 1, this
type of apparatus 10
is also illustrated, with level monitor 14 having a discriminator 28. This
type of apparatus is
useful when the interior 16 is required to be under pressure, for example when
the liquid
comprises liquefied petroleum gas, or when the outlet 20 supplies the mixture
into a highly
pressurized stream of fluid 24. Because of the positive pressure of vessel 12,
it is important
to monitor the level of liquid 26 in order to ensure that enough liquid
remains at the base of
the interior 16 to adequately provide a liquid seal. Referring to Fig. 6, the
liquid seal acts
provides a barrier against which pressure from the interior 16, for example
provided through
a gas inlet 51, may press to supply the mixture from interior 16 while
preventing gas
breakthrough out of outlet 20.
[0034] Referring to Fig. 9, a method of supplying proppant is illustrated.
Referring to
Fig. 1, in stage 100 (shown in Fig. 9) a vessel, for example vessel 12 as
shown, is provided at
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least partially filled with a mixture of proppant and liquid and having a
level of liquid 26 and
a level of proppant 30.
[0035] In stage 102 (shown in Fig. 9) level of liquid 26 is monitored in the
mixture of
proppant and liquid. Vessel 12 may be pressurized, for example above
atmospheric pressure.
The liquid may comprise LPG. Monitoring may further comprise monitoring the
level of
liquid 26 within a sheath, for example sheath 34, in the vessel 12, the sheath
having at least
an opening 40 at or near a lower end 38 of the sheath 34 for allowing liquid
to flow into the
sheath 34 and at least restricting the flow of proppant into the sheath 34,
and an equalization
vent 42. Monitoring may further comprise monitoring using TDR as described
above for
example. The liquid may be supplied into the vessel 12 from an inlet 22 in the
vessel 12. The
method may further comprise controlling the supply of liquid into the vessel
12 based on the
monitored level of liquid 26 in the vessel 12.
[0036] In stage 104 (shown in Fig. 9), the mixture of proppant and liquid is
supplied
from the vessel 12 through an outlet 20 in the vessel 12. The mixture of
proppant and liquid
may be supplied from the vessel 12 to a pressurized stream of fluid, for
example stream of
fluid 24 illustrated in Fig. 6. The method may further comprise monitoring the
level of
proppant 30 in the vessel 12.
[0037] Referring to Fig. 1, vessel 12 may be mounted on a weight scale 58.
Apparatus 10 may be used in combination with weight scale 58 in order to
provide level of
liquid 26 readings and weight readings of the contents of vessel 12. This way,
signals may be
sent as output from each respective device to a controller (not shown) that
can then estimate
the amount of proppant and the amount of liquid contained within vessel 12.
This way, the
controller will know when it is time to refill vessel 12 with proppant or re-
supply liquid to
interior 16. In some embodiments, the mixture of proppant and liquid may be
supplied
through inlet 18, which may be a hatch as shown.
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[0038] Level of liquid 26 may not be the actual height of the liquid in the
vessel. For
example, when level 26 is below level 30, some liquid may be absorbed on to
the proppant
and may migrate to a higher level than in sheath 34. In this case, level 26
may be the level of
liquid as discriminated by the discriminator 28. Referring to Fig. 8, an
example of this is
illustrated as level of liquid 26 is below the actual height of the liquid 32.
Variations in the
height of the liquid 32 and the level 26 may be the result of varying
affinities between
proppant and liquid, due to the types of proppant and liquid used. Thus, level
monitor 14
may be used to directly measure the height of the liquid in the vessel 12, or
it may be used to
indirectly extrapolate the height of the liquid. This may be used to determine
the amount of
liquid in the vessel itself. In some embodiments, level monitor 14 may send as
output signals
indicative of either level 26 or the height of the liquid to, for example, a
controller (not
shown) that is part of the level monitor 14. The controller may control
operation of the entire
frac system (not shown), and may adjust the parameters of vessel 12 as needed.
These
methods give a good idea of the size of the liquid seal, and may be used to
determine when
and how much additional liquid to add to the proppant.
[0039] Liquefied petroleum gas (LPG) includes 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, hexane, and various mixes thereof. Further examples include i-butane,
i-pentane, n-
pentane, and n-butane. The LPG mixture may be controlled to gain the desired
hydraulic
fracturing and clean-up performance.
[0040] 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
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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.
[0041] Referring to Fig. 3 an exemplary proppant addition system is
illustrated. For
exemplary purposes, the frac fluid source comprises LPG. LPG is supplied into
suction
manifold 60 through connections S1-S4 to create the stream of frac fluid.
Additives, for
example gelling agents, can be introduced into the stream of frac fluid via
connections LA1-
LA3. The stream passes through a flow conditioner 62 and a flowmeter 64 before
passing a
proppant introduction region 66, at which point proppant from vessels 12A and
12B may be
introduced into the stream of frac fluid via augers 68A, 68B, respectively.
The stream of frac
fluid then continues along line 70 to discharge manifold 71, passing flowmeter
72 and
nuclear densitometer 74. Densitometer 74 measures the wellhead density of the
stream of
frac fluid and proppant. Further additives may be added at this point via
connections LA4-
LA6. The stream of frac fluid can then be sent to a frac pumping system via
connections D1-
D4, where it will be pumped into a well to fracture a formation.
[0042] Nitrogen may be introduced into the system via lines 74, 76 from at
least one
N2 pumper (not shown). Nitrogen pass through a surge tank 78, and can be
supplied to at
least one of the proppant vessels 12A, 12B, the LPG tanks, and all the lines
that carry LPG at
any point in the process. Nitrogen is supplied through a pressure regulator R1
to proppant
vessels 12A and 12B via lines 80, 82, respectively. Nitrogen can then be
supplied, via a
series of valves to vessels 12A, 12B via connections at the top (84A, 84B) or
bottom (86A,
86B) of the vessels. Nitrogen pressure may be controlled to add a suitable
amount of
pressure on the proppant and liquid in the vessels 12A, 12B. Nitrogen may also
be sent to the
LPG tanks via pressure regulator R2 to control the balance of pressure in the
LPG tanks.
[0043] Proppant, for example sand, may be supplied to vessels 12A, 12B from a
sand
loader through lines 88A, 88B through sand inlets SI1-S12. LPG can be
controllably supplied
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to vessels 12A, 12B through connections S 1-S4 of the suction manifold 60, and
connection
90, which feeds to lines 90A, 90B. LPG can be added to the vessels 12A, 12B
through a
series of valves and via connections at the top (84A, 84B) or bottom (86A,
86B) of the
vessels, similar to nitrogen addition. The pressure may be balanced and
equalized between
the LPG tanks and vessels 12A, 12B, in order to prevent over or under
pressuring vessels
12A, 12B. Once the system is ready to add proppant to the stream of frac
fluid, valves HV 1
and HV2, which may be electro-hydraulic valves, are opened, and augers 68A and
68B
supply the mixture of proppant and liquid into the stream of frac fluid at
region 66. During
the process, level monitoring devices 14C and 14D monitor the levels of liquid
in vessels
12A, 12B, sending feedback to a controller in charge of the entire operation.
Based on the
levels of liquid in each respective vessel, the controller may decide to add
more liquid to
each respective tank as required.
[0044] The system may use relief valves and mechanisms at any point in the
system
as mandated for safety by law, for example relief valves RV1-RV5. Upon
completion of the
frac, connections D1-D4 may be closed, and all lines bled, for example via
bleed valves
BV1-BV3, through line 92, hose reel 94, and to the flare stack.
[0045] 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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