Fracture treatment system for wells

A well fracturing system in which a fracturing fluid storage vessel, high pressure pump and high pressure conduit are connected in series to a well. A pressure vessel is connected to the high pressure conduit for injecting proppant carrying fracturing fluid into the well without the proppant carrying fracturing fluid passing through the high pressure pump. The pressure vessel may be formed of a pipe with a free floating piston dividing the pressure vessel into a drive side and driven side. Proppant carrying fracturing fluid is stored in the driven side, and may be emplaced within the pressure vessel at a remote site from the well. Drive fluid is injected into the drive side to force the proppant carrying fracturing fluid into the high pressure conduit and down the well. The drive fluid may be a diverted portion of fracturing fluid pumped by the high pressure pumper. Several pressure vessels may be used in parallel, and proppant may be stratified in the pressure vessel.

FIELD OF THE INVENTION 
The invention relates to a method and apparatus for conducting oil and gas 
well fracturing (frac) operations. 
BACKGROUND OF THE INVENTION 
In conventional fracturing of wells, proppant and fracturing fluid are 
mixed in a blender and then pumped into a well. As a consequence, the 
blender, pump and lines carrying the fracturing fluid downstream of the 
blender are subject to wear from the proppant, which is frequently highly 
abrasive material such as sand. In addition, any flow meters or other 
apparatus in the lines are subject to wear and possible wash out due to 
the proppant. Such wear may result in failure of the equipment, or leaks 
that may be hazardous. The potential for wear and the possibility of fire 
resulting from a leak makes it necessary to inspect equipment regularly. 
In addition, when proppant is mixed with fracturing fluid in a blender the 
proppant may clump together and it is difficult to ensure uniform mixing 
of the proppant in the fracturing fluid. Since fracturing operations are 
carried out very quickly, there is little time to rectify an error in 
proppant concentration, with the result that too little or too much 
proppant may be injected into the formation at a critical part of the 
fracturing operation. This may ruin the fracturing operation, or require 
it to be repeated, in the case that is feasible. 
SUMMARY OF THE INVENTION 
Objects of the invention therefore include providing a fracturing system 
that reduces wear on the equipment used, is safer, avoids use of blenders 
at the well site, with consequent reduction in expense and manpower 
required, and provides precise control of proppant injection into a well. 
There is therefore provided in accordance with an aspect of the invention, 
an apparatus for fracturing a formation penetrated by a well. The 
apparatus comprises in series a fracturing fluid storage tank, a high 
pressure pump and a high pressure conduit leading from the pump to the 
well. In parallel with the high pressure conduit and in series with the 
high pressure pump and the well there is provided a pressure vessel. The 
pressure vessel includes dispensing means for dispensing fluid from the 
pressure vessel into the high pressure conduit. Flow is regulated in the 
high pressure conduit by a control means. 
In a further aspect of the invention, the dispensing means comprises a 
movable piston disposed in the pressure vessel and separating the pressure 
vessel into a drive end and a driven end, the driven end including a 
vessel outlet and drive means for driving the movable piston towards the 
vessel outlet. 
In a further aspect of the invention, the drive means comprises supply 
means for supplying fluid from the high pressure pumper to the drive end 
of the pressure vessel. 
In a further aspect of the invention, the supply means comprises a diverter 
on the high pressure conduit for diverting a portion of flow in the high 
pressure conduit to the pressure vessel. 
In a further aspect of the invention, the diverter has first and second 
outlets and fluid flow in the first and second outlets is controlled by 
first and second valves respectively. 
In a further aspect of the invention, the supply means comprises control 
means for opening one of the first and second valves while closing the 
other of the first and second valves. 
In a further aspect of the invention, the drive means comprises a high 
pressure pump operably connected to the drive end of the pressure vessel. 
In a further aspect of the invention, the apparatus further comprises a 
pressure sensor sensitive to pressure in the high pressure conduit, a 
bypass circuit connected to the high pressure conduit and connected to 
return fluid from the high pressure conduit to the pump inlet, a bypass 
valve on the bypass circuit and a controller responsive to the pressure 
sensor for opening the bypass valve when the pressure in the high pressure 
conduit exceeds a pre-set amount. 
In a further aspect of the invention, there is provided a method of 
preparing a proppant carrying fracturing fluid for use in fracturing 
wells, the method comprising the steps of mixing proppant with a 
fracturing fluid to form a proppant carrying fracturing fluid such that 
the proppant is uniformly distributed within the fracturing fluid; and 
storing the proppant carrying fracturing fluid in a pressure vessel. 
In a further aspect of the invention, the method further comprises admixing 
gelation chemicals with the fracturing fluid prior to injecting the 
proppant carrying fracturing fluid into the pressure vessel, whereby the 
fracturing fluid forms a gel in the pressure vessel. 
In a further aspect of the invention, there is provided a method of 
fracturing a formation penetrated by a well, the well being located at a 
well site. The method comprises the steps of creating a proppant carrying 
fracturing fluid at a site remote from the well site and storing the 
proppant carrying fracturing fluid in a pressure vessel, transporting the 
pressure vessel to the well site and injecting the proppant carrying 
fracturing fluid into the well. 
In a further aspect of the invention, the pressure vessel has a drive end 
and a driven end, and the method further comprises the step of storing the 
proppant carrying fracturing fluid in the driven end of the pressure 
vessel. 
In a further aspect of the invention, the method further comprises forming 
a first stream of a proppant free fracturing fluid and pressurizing the 
drive end of the pressure vessel to drive the proppant carrying fracturing 
fluid from the driven end of the pressure vessel into the first stream to 
form a fluid for injection into the formation at a rate and pressure to 
cause fracturing of the formation. 
In a further aspect of the invention, the pressure vessel is pressurized 
with a drive fluid, such as a proppant free fracturing fluid, without 
mixing of the drive fluid and the second fracturing fluid. 
In a further aspect of the invention, the pressure vessel is pressurized by 
diverting a portion of the first stream. 
In a further aspect of the invention, the drive end of the pressure vessel 
is separated from the driven end by a movable piston, and the drive fluid 
acts upon the movable piston. 
In a further aspect of the invention, there is provided an apparatus for 
storage and injection of proppant containing fracturing fluids, in which 
the apparatus comprises a pressure vessel having an inlet end and an 
outlet end, a movable fluid dividing interface, such as a free floating 
piston, disposed within the pressure vessel, the movable fluid dividing 
interface being movable from the inlet end towards the outlet end and 
dividing the pressure vessel into a drive side and a driven side and a 
port for filling the driven side of the pressure vessel. In use, the 
driven side of the pressure vessel is initially filled with proppant 
carrying fracturing fluid. 
In a further aspect of the invention, there is provided a method of 
fracturing a formation penetrated by a well, the well being located at a 
wellsite, a pressure vessel being located at the wellsite, the pressure 
vessel having a drive end and a driven end, the method comprising the 
steps of forming a first stream of a first fracturing fluid; and 
pressurizing the drive end of the pressure vessel to drive a second 
fracturing fluid from the driven end of the pressure vessel into the first 
stream to form a fluid for injection into the formation at a rate and 
pressure to cause fracturing of the formation. 
In a further aspect of the invention, the pressure vessel is pressurized 
with a drive fluid, such as the first fracturing fluid, without mixing of 
the drive fluid and the second fracturing fluid. The pressure vessel may 
be pressurized by diverting a portion of the first stream. In a further 
aspect of the invention, the drive end of the pressure vessel is separated 
from the driven end by a movable piston, and the drive fluid acts upon the 
movable piston. 
One or more pressure vessels may be used in parallel. In addition, the 
invention permits use of a gas such as methane, ethane and nitrogen as a 
proppant free fracturing fluid used in combination with a proppant 
carrying fracturing fluid. 
These and other aspects of the invention are described in the detailed 
description of the invention and claimed in the claims that follow.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
In this patent document, the term "proppant carrying fracturing fluid" 
means a fluid useful for the fracturing of wells in which proppant 
material is uniformly suspended. The fluid should have sufficiently high 
viscosity that the proppant does not settle during temporary storage such 
as may occur between filling of a pressure vessel with the fluid and use 
of the fluid at a wellsite, yet should still be pumpable. Typically, such 
a fluid will be gelled, and may be for example a gelled hydrocarbon. 
Techniques for gelation of fracturing fluids are well known and need not 
be further described here. Any conventional proppant may be used, 
depending on the well requirements. Also in this patent document, "high 
pressure" means pressures suitable for fracturing a formation penetrated 
by a well. 
Referring to FIG. 1, a pressure vessel 10 is used to store proppant 
carrying fracturing fluid and to inject the proppant carrying fracturing 
fluid into a well 12 (FIG. 2). The pressure vessel 10 is preferably made 
from a pipe or cylinder 11 and should have sufficient capacity to hold the 
fluid to be used in fracturing a well. For example, a pipe 30 inches in 
diameter and 45 feet long is believed suitable for many applications. 
Pressure tight caps 13, 16 are secured to the pipe 11 for example by 
flanges 14, 17 respectively to form a pressure vessel. An inlet 18 is 
provided in cap 16 and is controlled with a valve 20. An off axis outlet 
15 is provided at cap 13 and is controlled with a valve 22. The outlet 15 
may be located at a lower side of the pressure vessel 10 for ease of 
draining the vessel. A pig or movable plug 24 with seals 41 (FIG. 4) 
functioning as a free piston separates the interior of the cylinder into 
isolated drive and driven sides 26 and 27 respectively. A drain port 28 
and associated valve 29 are provided in a lower portion of the pressure 
vessel 10, and a filling port 30 with control valve 32 are also provided. 
The drain port 28 may be located so that the pig 24 may be placed on 
either side of the drain port 28, or a drive side drain 31 (FIG. 4) may be 
used. The filling port 30 is preferably placed close to the cap 13 so that 
no matter where the pig 24 is located, the driven side 27 of the pressure 
vessel 10 may be filled with fluid. The pressure vessel 10 is also 
provided with conventional skids or other suitable supporting structure 
(not shown). As shown in FIG. 4, the pressure vessel 10 also desirably 
includes a drive side drain 31 and drain valve 33 at a lower portion of 
the drive end of the pressure vessel 10, as well as a drive side filling 
port 35 and drive side filling valve 37 at an upper portion of the drive 
side. 
Referring now to FIG. 2, at the well site, a high pressure pumper 40 of 
conventional construction has a pump inlet 42 connected via lines 44 and 
46 to a fracturing fluid storage tank 39. Line 46 is a low pressure line 
and a conventional pressurizer 48 is located between lines 44 and 46 to 
supply pressurized fluid along line 45 to the high pressure pumper 40. The 
high pressure pumper 40 has a pump outlet 50 connected via line 52 to a 
proppant concentration control unit 54, having a first outlet 56 and a 
second outlet at line 64. Outlet 56 is connected via line 60 to well 12. 
Outlet line 64 is connected to the pressure vessel inlet 18. The pressure 
vessel outlet 15 is connected via line 66 to line 60. Together, lines 52 
and 60 and the unit 54 form a conduit leading from the high pressure 
pumper to the well 12. 
The proppant concentration control unit 54 comprises a flow diverter 70 
with an inlet port 72 controlled by valve 74, a first outlet port 76 
controlled by valve 78 and a second outlet port 80 controlled by valve 82. 
The valves 74, 78 and 82 may each be conventional remotely controlled 
metering valves. A flow meter 73 is provided on line 52 for measuring flow 
in line 52. A flow meter 77 is provided on line 56 for measuring flow 
through the outlet 76 along line 60. A flow meter 81 is provided on line 
64 for measuring flow through the outlet 80 along line 64. The flow meters 
73, 77 and 81 are each conventional flow meters. Valves 20, 22, 74, 78 and 
82 are respectively connected via lines 90, 91, 92, 93 and 94 to a 
conventional controller 100 that is programmed to control the valves in 
accordance with the frac program described in this patent document. 
Signals representing flow rates in lines 52, 56 and 64 respectively are 
provided by flow meters 73, 77 and 81 respectively along lines 95, 96 and 
97 to the controller 100. Pumper 40 is connected to controller 100 via 
control line 98 and pressurizer 48 is connected to controller 100 via line 
99. A bypass line 102 is connected between line 52 and storage tank 39. 
Flow in line 102 is controlled by valve 104 which receives control signals 
from controller 100 along line 106. A pressure sensor 108 on line 52 is 
sensitive to pressure in the line and is connected via line 110 to supply 
signals representative of the line pressure to controller 100. In FIG. 4, 
the sand concentration control unit 54 comprises a conventional 
proportioning valve 71, with flow meters 77 and 81 on lines 60 and 64 
respectively. 
In a preferred manner of operation of the invention, the pressure vessel 10 
is first charged with proppant carrying fracturing fluid at a location 
remote from the well site. The proppant is mixed with a fracturing fluid 
in conventional manner to create the proppant carrying fracturing fluid 
either in the driven side 27 of the pressure vessel 10 or in another 
vessel and then transferred to the driven side 27 of the pressure vessel 
10. The fluid is injected into the pressure vessel 10 through port 30. 
Prior mixing may be done slowly to ensure uniform distribution of the 
proppant in the fracturing fluid. To ensure that the proppant does not 
settle, the fracturing fluid should have sufficient viscosity to prevent 
settling during transportation and temporary storage of the proppant 
carrying fracturing fluid. For this purpose, conventional gelation 
chemicals may be mixed with the fracturing fluid so that the fracturing 
fluid forms a gel in the pressure vessel. The proppant carrying fracturing 
fluid is then stored in the pressure vessel 10 while the pressure vessel 
10 is transported to a well site on a trailer. During transportation, the 
pressure vessel 10 may be rotated or its contents gently agitated with an 
internal stirring device to maintain the proppant uniformly distributed 
within the gelled fracturing fluid. 
At the well site, the pressure vessel 10 is used to inject or dispense 
proppant carrying fracturing fluid into the well 12 in a controllable 
fashion under pressure. The proppant carrying fracturing fluid is 
dispensed by driving the movable piston 24 with drive fluid 23 (FIG. 4 for 
example) injected into the drive side 26 of the pressure vessel 10. 
Pressure on drive side of the piston 24 urges the piston 24 along the pipe 
11 towards the driven side as shown in FIG. 4. The drive fluid may be 
supplied through lines 52 and 64 and driven by the pumper 40. The extent 
to which fracturing fluid is supplied along lines 64 and 60 is regulated 
by the sand concentration control unit 54. 
In the initial stages of a frac operation, a pad of liquid fracturing 
fluid, such as condensate, is injected into the well 12 by forming a 
stream of fracturing fluid in lines 52 and 60. Pressure on the fracturing 
fluid is gradually increased to initiate fracturing of the formation 
penetrated by the well in accordance with well established techniques. In 
this initial stage, the entire output of the pumper 40 is supplied along 
lines 52 and 60 into the well 12. Valve 82 is closed and valve 78 is open. 
The pressure vessel 10 is inoperative and no proppant is carried into the 
well 12. As the frac progresses, it becomes necessary to add proppant in 
order to keep open any cracks that have been opened in the formation. In 
the present invention, this is accomplished by opening valve 82 while 
closing valve 78. The rate at which valve 82 is opened, and valve 78 
correspondingly closed, depends on the proppant density distribution in 
the pressure vessel and the proppant density required by the frac program. 
Typically, in a frac program, the proppant density is low at the start of 
introducing proppant into the well, and then increases. Thus, the valve 82 
will typically be opened gradually, according to the proppant density 
required by the frac program. Use of the pressure vessel 10 for injection 
of proppant carrying frac fluid into the well allows for precise control 
of proppant density. The valve 78 acts as a variable choke on the line 60, 
forcing fluid flow into the line 64. It may be desirable in some 
circumstances to use a separate variable choke on line 60 to establish a 
pressure differential in line 60 between the control unit 54 and junction 
with the line 66. 
The opening of valve 82 and closing of valve 78 diverts some of the fluid 
from line 52 into line 64 and into the pressure vessel 10, causing an 
increased pressure on the drive side 26 of the pressure vessel 10. 
Pressure differential across the pig 24 causes the pig to move towards the 
driven side 27 of the pressure vessel and, without mixing of the drive 
fluid and driven fluid, force proppant carrying fracturing fluid out of 
the driven side into line 66 to mix with the proppant free stream of 
liquid in line 60 and thence be injected into the well 12. Mixing may take 
place in a mixing chamber 61 shown in FIG. 4. Chemicals may also be 
injected into line 60 through a line 63 by pump 65 and chemical storage 
unit 67 shown in FIG. 4. By control of the pressure in line 52 using 
pumper 40, the fluid may be injected into the formation at a rate and 
pressure sufficient to cause fracturing of the formation penetrated by the 
well. While control of the valves 78 and 82 may be accomplished manually, 
it is preferred to control them with controller 100. Pressure on the 
proppant carrying fracturing fluid may be increased during injection of 
the proppant carrying fracturing fluid into the well 12 as required by the 
frac program. To prevent fluid from the well entering the pressure vessel 
10, a one-way check valve (not shown) may be installed on the line 66. 
As the frac progresses, it becomes desirable to stop injecting proppant 
carrying fracturing fluid and flush what is in the well into the 
formation. Valve 82 is therefore closed while valve 78 is correspondingly 
opened until all flow in line 64 is shut off. The operator will know from 
the volume pumped when the proppant has been flushed into the formation. 
While use of the pumper 40, control unit 54 and their associated valves and 
lines comprises means for dispensing fluid from the pressure vessel, the 
pressure vessel 10 may also be driven by other means, such as shown in 
FIG. 3. In FIG. 3, a first high pressure pumper 40 is connected via line 
112, flow meter 114 and valve 116 to the well 12. In addition, a drive 
means for a pressure vessel 10, constructed according to the pressure 
vessel shown in FIG. 1, includes a second high pressure pumper 120 
connected via line 122 through flow meter 124 and valve 126 to pressure 
vessel 10. Valve 126 may be the same as valve 20. Both pumpers 40 and 120 
and the associate lines 112, 122 and valves 116, 126 are controlled by 
controller 100. As in the embodiment shown in FIG. 2, during initial and 
late stages of a frac, valve 126 is closed and valve 116 is open. When a 
controlled amount of proppant carrying fracturing fluid is to be mixed 
with the stream of fluid in line 112, valve 126 is opened while valve 116 
is closed. At the end of the frac, valve 126 is closed, while valve 116 is 
opened more. The drive fluid used by pumper 120 may be the same fracturing 
fluid used by pumper 40, or may be any other fluid capable of driving the 
pig 24 in pressure vessel 10. 
In a further embodiment, particularly if the pressure vessel 10 has a small 
diameter compared with its length, the pig 24 may be omitted and the 
proppant carrying fracturing fluid driven solely by fluid entering the 
drive end 26 of the pressure vessel as shown in FIG. 7. However, some 
mixing of the drive fluid 23 and the proppant carrying fracturing fluid 
136 is likely to occur at the interface 25 between drive fluid 23 and 
driven fluid 136, meaning that some of the fluid will not be usable, or 
only usable with care. If the mixing interface however is sufficiently 
small in relation to the volume of the pressure vessel, this may not be a 
problem in some applications where some fluid wastage is acceptable. 
Alternatively, the interface formed between drive fluid 23 and driven 
fluid 136 may be formed with a bladder 140 as shown in FIGS. 6A-6C. In 
FIG. 6A, the bladder 140 is in position for commencing driving of proppant 
carrying fracturing fluid 136 into the well 12. The bladder 140 closely 
follows the interior surface 142 of the drive end 26 of the pressure 
vessel 10. The pressure vessel 10 is completely filled with proppant 
carrying fracturing fluid 136. In FIG. 6B, drive fluid 23 has been forced 
into the drive side 26 of the pressure vessel 10, forcing the bladder 140 
towards the driven side 27 of the pressure vessel 10 and forcing proppant 
carrying fracturing fluid 136 out of the pressure vessel 10 through outlet 
15. In this intermediate stage of injection, the bladder 140 is partly 
folded as shown at 144. In FIG. 6C, the bladder 140 is shown completely 
extended. The bladder 140 is not preferred since it is prone to 
overlapping and folding. 
The pressure vessel 10 is charged at the remote well site with a known 
density of proppant, for example 20 pounds per gallon. The precise rate of 
pumping of proppant may then be determined from the flow rates indicated 
by the flow meter 124. The proppant to fluid ratio in the fracturing fluid 
entering the well 12 may be readily controlled by varying the amount of 
flow permitted in lines 112 and 122. 
The fracturing fluid that forms a stream in lines 52 or 112 may be any 
conventional fluid used for fracturing wells such as diesel, condensate, 
oil of various kinds, liquid hydrocarbon, and alcohols. 
It is believed that the fracturing fluid may also be a gas, such as 
methane, ethane and nitrogen. In this case, the pumper 40 will be replaced 
by a conventional compressor. When the proppant free fracturing fluid is a 
gas, it is believed that gelled alcohol or gelled liquid hydrocarbon may 
be used for the proppant carrying fracturing fluid that is mixed with the 
stream of gas. The pressure vessel 10 may be driven by diverting gas from 
the compressor in this instance. The fracturing fluid may also be a 
liquified gas such as butane, propane and carbon dioxide, which is pumped 
into the well in a liquified state. 
The pumps may for example be Gardner Denver.TM. piston pumps or Reda.TM. 
centrifugal pumps or equivalents, which are commonly commercially 
available. The pressure vessel 10 may be made from aluminum, steel, 
titanium alloys, or other suitable material, depending on the application. 
Two or more pressure vessels 10, 10A may be used in parallel as shown in 
FIG. 5, for example to meet volume requirements. Features of the second 
pressure vessel 10A are the same as those of pressure vessel 10 but the 
reference numerals are supplied with the suffix A to allow the two 
pressure vessels to be distinguished in this patent document. The lines 
52, 60 and 112 may use chiksan swivel joints and do not carry abrasive 
material. In some embodiments of the method of the invention, mixing of 
proppant carrying fracturing fluid may be done on site as illustrated in 
FIG. 5, with the pressure vessel 10 separately trucked to the site. In 
FIG. 5, pressure vessel 10A has its outlet 15A connected via line 66A to 
line 66. Driven side drain 28A is connected via line 130 to pump 132, 
which in turn is connected to storage unit 134. Storage unit 134 contains 
a proppant carrying fracturing fluid 136 which may be pumped by pump 132 
into the pressure vessel 10A. Pump 132 may be a low pressure pump of 
conventional construction since it is not required to pressurized fluids 
in the well 12. The drive side drain 31A of pressure vessel 10A is 
connected via line 138 to fracturing fluid storage vessel 39. As the 
pressure vessel 10A is recharged, fracturing fluid returns to the storage 
vessel 39 through line 138. Upon recharging of the pressure vessel 10A, 
and any other pressure vessel 10 on site, a fracturing process may be 
continued. A large frac, requiring for example in the order of 60 m.sup.3 
of proppant carrying fracturing fluid, may then be accomplished with, say, 
five pressure vessels, each being recharged once in sequence, with 
recharging of one or more continuing while another is being discharged. In 
addition, the pressure vessel 10 or 10a may be charged on site with the 
system shown in FIG. 5. 
In some well fracturing programs, it is desirable to have different sizes 
of proppant in the proppant carrying fracturing fluid during different 
stages of the fracturing treatment. For this purpose, the proppant 
carrying fracturing fluid 136 may be stratified as shown in FIG. 8, with 
different sizes 136A (largest), 136B, 136C (smallest), or density of 
proppant in different longitudinal zones 146, 147, 148 of the pressure 
vessel 10. For filling the pressure vessel 10 with proppant carrying 
fracturing fluid with different sizes of proppant, the pressure vessel 10 
is preferably charged through outlet 15 with the piston 24 initially as 
close to the outlet 15 as possible. The pressure vessel 10 is then charged 
with the reverse sequence of proppant sizes and density as to be 
discharged according to the frac program. Thus, a proppant carrying frac 
fluid with a first proppant size and density is injected into the pressure 
vessel 10, and the piston 24 is pushed back to accommodate this frac 
fluid. Then a proppant carrying fracturing fluid with a second proppant 
size and density is injected into the pressure vessel 10. A pressure 
vessel 10 to be charged in this manner should have a centrally located 
outlet 15A as illustrated in FIG. 4. 
The pressure vessel 10 preferably has sufficient volume to complete a 
fracturing treatment of a well, preferably more than about 5 m.sup.3, 
without being too large for transportation. In this way, fracturing of a 
formation may be completed with only a single cycle of driving proppant 
carrying fracturing fluid from the pressure vessel. The vessel does not 
need to be recharged at the well site. However, where larger fluid volumes 
are required, one or more additional pressure vessels 10 may be used, 
again without requiring recharging of any of the pressure vessels 10. On 
the other hand, recharging of the pressure vessel 10 at the well site may 
be required in some circumstances. 
In cases where settling of the proppant during transportation in the 
pressure vessel is a concern, the pressure vessel 10 may be provided with 
a system for continuous slow mixing of the proppant carrying fracturing 
fluid during transportation. 
In this way, blenders for high rate blending of sand and frac fluid are not 
required. Fewer personnel are required at the well site, less expensive 
equipment may be used, cheaper fluids may be used such as propane as the 
frac fluid and the frac may be carried out more safely with less wear on 
the equipment. 
Cycle time, being defined as the period during which proppant carrying 
fracturing fluid is discharged from the pressure vessel 10 without 
recharging, may be as low as 31/2 to 4 minutes but will typically be 
longer. At this slow cycle rate, very little wear is experienced by the 
pressure vessel in discharging fluid into the well, and no wear is 
experienced by the pump 40, and lines 52 and 60, upstream of the 
conjunction of lines 60 and 66, or any of the components on the lines 52 
and 60. Those items that are subject to wear, such as the tubing between 
the pressure vessel 10 and the well, may be made with wear resistant 
material such as tungsten carbide. 
Very low pressure differentials, for example in the order of 150-200 psi, 
across the piston 24 are required to cause the piston 24 to move, although 
the pressure vessel itself will be pressured at the high frac pressure. 
An alternative to using the pumper 40 or pump 120 as part of a means to 
drive fluid from the pressure vessel 10 is to use a mechanically driven 
piston. 
A person skilled in the art could make immaterial modifications to the 
invention described in this patent document without departing from the 
essence of the invention that is intended to be covered by the scope of 
the claims that follow.