FIELD OF THE INVENTION
- BACKGROUND OF THE INVENTION
The present invention relates generally to systems for injecting substances into underground formations, and in particular relates to novel systems and methods of combining fluids and proppant under high-pressure, and for injection of the resultant fluid stream into formations such as coal beds.
The Horseshoe canyon coal formations in Alberta have been difficult to stimulate for coal bed methane production. These formations have been through a plethora of conventional stimulation treatments, ranging from foams to crosslink polymers. Due to the nature of the low reservoir pressures of these coal formations, or seams, and their sensitivity to damage by conventional stimulation fluids (defined herein as a liquid and/or gas), stimulation fluid recovery becomes almost impossible. The only other economically viable choices appear to be straight CO2 or N2 gas injection. High rate N2 gas injection technique is a common practice in North American coal bed methane exploited plays, and CO2 is used as a flood medium.
Although using CO2 gas to stimulate a formation works fine, it has certain drawbacks, including:
- 1. Costly treatments; and,
- 2. CO2 does not clean up quickly, and since water is commonly produced during stimulation, it will turn into carbonic acid which is extremely hard on surface production manifolding.
Using N2 gas works the same way all fluids do to stimulate a formation, although extremely high rates are required to create enough stress to overcome the natural formation mechanics and actually fracture, or “frac”, the formation. Enhanced conductivity of a formation relies on the effect of hysteresis, namely when the frac faces come back together under stress, that these faces will not heal back to their original orientation. It would be desirable to use proppant (e.g. a sand or other suitable materials) to hold the fractured, or “fraced”, faces apart as used in conventional frac theory. However, the problem with this is that N2 is pumped as a gas and will not suspend or carry proppant as do conventional fracturing fluid systems.
- SUMMARY OF THE PRESENT INVENTION
What is desired therefore is a novel method of fracturing, or “fracing”, a target formation (such as a coal or shale formation) using gases and proppants, and a novel system for mixing such gasses and proppants in a manner that would result in an “impregnated” fluid stream suitable for such fracing. Preferably, the method and system should be capable of combining N2 gas and a proppant material, such as sand, to produce a suitable fluid stream for fracing a coal formation. The method and system should further provide for introduction of surfactants to the fluid stream to further enhance the performance of the proppant in the target formation.
According to the present invention, there is provided in one aspect a high-pressure injection proppant system (also referred to as “HIPS”) in which proppant, such as sand, is preloaded into one or more high-pressure cylindrical or spherical vessels, and such proppant is delivered into a fluid stream, such a N2 gas stream, via an arrangement, such as a screw auger, which meters the proppant volumes and rates into the fluid stream.
In another aspect the invention provides two vessels operationally mounted in parallel which can function separately or concurrently depending on the demand for proppant in a particular formation. When operated seperately, one vessel can be in use for fracing a formation while the other vessel is isolated, de-pressurized and reloaded with proppant via a pneumatic bulk proppant system. The other vessel is then ready for operation when the first vessel is depleted of proppant.
In yet another aspect the invention provides for the injection of surfactants (i.e. chemicals or like substances) into the fluid stream to enhance the performance of the proppant, to aid in the placement of the proppant into the fracture network, and to demote proppant flowback during production and embedment.
In another aspect the invention provides a high-pressure injection proppant apparatus comprising:
at least one pressure vessel;
means for filling the vessel with proppant;
means for delivering a fluid containing nitrogen gas to the vessel and pressuzing the vessel therewith; and,
a metering arrangement operatively coupled to the vessel and in fluid communication therewith for metering the proppant from the pressurized vessel into a fluid stream containing nitrogen gas for delivery to a target formation.
In yet another aspect the invention provides a method of injecting proppant into a target formation comprising:
providing at least one pressure vessel and a metering arrangement operatively coupled to the vessel and in fluid communication therewith;
charging the vessel with proppant;
pressurizing the vessel with a fluid containing nitrogen gas; and,
operating the metering arrangement to meter the proppant from the pressurized vessel into a fluid stream containing nitrogen for delivery to the target formation.
- BRIEF DESCRIPTION OF THE DRAWING FIGURES
Further, the system of the present invention can be operated manually or by computer automation to aid in the accuracy of mixing of the components of the fluid stream.
Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, wherein:
FIG. 1 is an elevational side view of a mobile carrier carrying a high-pressure injection proppant system (“HIPS”) according to a first embodiment of the present invention, showing the cylindrical pressure vessels of the system in a reclined transportation mode;
FIG. 2 is a view of the system of FIG. 1 with the pressure vessels in an elevated operating mode;
FIG. 3 is a plan view of the rig and system of FIG. 2;
FIG. 4 is an elevational end view of the rig and system of FIG. 2;
FIG. 5 shows the system of FIG. 4 in isolation, with the rig omitted;
FIG. 6 is a view similar to FIG. 4, but shows a second embodiment of the system of the present invention, in operating mode;
FIG. 7 is an elevational side view of the system of FIG. 6;
FIG. 8 is a plan view of FIG. 6 with the front portion of the rig omitted;
FIG. 9 is a perspective view, from the rear, of a third preferred embodiment of the system of the present invention showing a pair of spherical pressure vessels mounted on a mobile trailer;
FIG. 10 is an elevational side view of the system of FIG. 9;
FIG. 11 is a perspective view, from the front, of a fourth embodiment similar to the third embodiment, but having a single spherical pressure vessel;
FIG. 12 is an elevational side view showing the vessel and piping of FIG. 11 in isolation;
FIG. 13 is an elevational side view from the right side of FIG. 12;
FIG. 14 is an elevational side view from the opposed back side of FIG. 12;
- LIST OF REFERENCE NUMBERS IN DRAWINGS
FIG. 15 is an elevational side view from the left side of FIG. 12; and, FIG. 16 is a plan view from the top of FIG. 12.
DESCRIPTION OF EMBODIMENTS
- 10 high-pressure injection proppant system
- 12 trailer
- 14 truck
- 15 hydraulic wet kit
- 16 axles of 12
- 18 wheels on 12
- 20 proppant bulk storage tank
- 22 low-pressure blower pump
- 24 first low-pressure air line
- 26 second low-pressure bulk load line
- 28 surfactant storage and pumping assembly
- 30 delivery tubing for 28
- 32 hydraulic lift cylinders
- 34 pivots
- 36, 36 a, 36 b pressure gauges
- 38 densometer
- 40 pressure vessel(s)
- 42 outer wall of 40
- 43 reinforced portion of 42
- 44 inner chamber of 40
- 46 first vessel inlet for proppant
- 48 first/top end of 40
- 50 second vessel inlet/outlet
- 52 first vessel outlet
- 53 flange of 52
- 54 screw(s)
- 56 radial inlet of 54
- 57 radial outlet of 54
- 58 motor of 54
- 60 piping arrangement
- 61 high-pressure fluid stream
- 62 first inlet of 60
- 64 first (Y) diverter
- 66 first fluid stream
- 68 second fluid stream
- 70 venturi-type orifice
- 72 first outlet of 60
- 74 second (four way) diverter
- 76 first fluid sub-streams
- 78 second fluid sub-stream
- 80 piping
- 82 first valves of 60
- 84 third (T-shaped) diverter
- 86 third fluid sub-streams
- 87 fourth fluid sub-streams
- 88 second valves
- 90 third valves
- 92 piping
- 94 Y-joint
- 96 pressure vessel isolation valve
- 98 upstream injection port
- 99 downstream injection port
- 130 delivery line of second embodiment
- 140 pressure vessel(s) of second embodiment
- 142 outerwall of 140
- 144 a first inner chamber of 140
- 144 b second inner chamber of 140
- 144 c third inner chamber of 140
- 145 first bottomopening of 144 a
- 146 first vessel inlet
- 147 second top opening of 144 a
- 150 second vessel inlet
- 152 bottom vessel outlet of 144 c
- 154 screw(s) of second embodiment
- 158 motor of 154
- 160 piping arrangement of second embodiment
- 162 inlet
- 166 first fluid stream
- 167 Y-shaped diveter
- 168 second fluid stream
- 170 orifice
- 183 first valves
- 190 pressure relief valve
- 192 piping
- 196 isolation valve(s)
- 220 proppant storage tank
- 228 storage and pumping assembly
- 231 lower legs
- 240 spherical pressure vessel(s)
- 254 sand screw
- 280 valves
- 311 retractable arms
- 326 proppant supply line
- 327 proppant supply valve
- 340 pressure vessel
- 341 cap
- 346 proppant supply valve
- 350 top fluid inlet port
- 354 screw/auger
- 357 auger outlet
- 358 drive motor and seal assembly
- 360 piping arrangement
- 361 high pressure fluid stream/line
- 364 first fluid diverter
- 372 outlet
- 374 second fluid diverter
- 376 fluid auger by-pass line
- 380 piping for fluid by-pass
- 382 fluid by-pass line valve
- 388 top fluid supply valve
- 390 vent valve
- 391 cap for vent line
- 393 purge valve
- 394 y-joint (auger outlet by-pass)
- 395 choke
- 396 auger outlet valve
- 399 surfactant inlet
Reference is first made to FIGS. 1 to 3 which show a high-pressure injection proppant system, or “HIPS”, (generally designated by reference numeral 10) according to a first embodiment of the present invention. The system is mounted on a carrier, which is preferably a wheeled trailer 12 adapted to be pulled by a motorized vehicle, or truck 14. It will be understood that the carrier may take various alternate forms, namely the trailer may itself be self-propelled, the truck and trailer may form one non-detachable unit, the system may be mounted on a skid for transport between sites, or the like. However, since the system is extremely heavy, not all carriers will be suitable for road transport as prescribed load limits for roads may be exceeded. Hence, in the present embodiment, the 24 wheeled trailer 12 is specifically designed to remain within such load limits (i.e. is “road legal”) by having three axles 16 with eight tires 18 per axle. Different axle and wheel combinations and quantities may be equally suitable, depending on the load to be transported. Likewise, the truck is suitably designed to haul the trailer 12, and should include a hydraulic “wet kit” 15 to power the system 10 on the trailer.
The preferred system 10 includes a proppant storage means in the form of a cone-shaped tank 20 located on the trailer 12. A relatively low-pressure blower pump 22, conveniently mounted on the truck 14 close to a power source (i.e. the hydraulic wet kit 15), communicates with the tank 20 via a first low-pressure line 24. The pump 22 permits the bulk transfer of proppant from the tank 20 at the front of the trailer to the two high-pressure vessels 40 at the back of the trailer via at least one second loading line 26 (FIG. 2). Although one line 26 may be configured for suitable delivery of proppant, each vessel has a designated line 26 in the present embodiment.
The system further includes a surfactant storage and high pressure pumping assembly 28 located on the trailer. This assembly stores one or more surfactants for injection or “misting” (via a delivery tubing generally indicated by 30) into the high-pressure fluid stream associated with the pressure vessels 40, as will be discussed later. The pumping assembly may employ as many high-pressure surfactant pumps as required. It is noted that in alternate embodiments, the assembly may be located elsewhere than on the trailer 12, such as on another trailer, but must be capable of communicating with the fluid stream during operation for the desired misting. Likewise, the proppant storage tank 20 may be remotely located, but in communication with the vessels 40 during operation.
The surfactant referred to herein should be a chemical or like substance for enhancing the performance of the fluid stream proppant, for aiding in the placement of the proppant into a formation's fracture network, and/or for reducing proppant flowback during production and embedment. The proppant should be any material suitable for achieving the desired fracturing, or “fracing” of a target formation. The preferred system of the present invention is specifically geared toward fracing a coal formation for enhancing gas production therefrom, and the desired proppant is a form of sand. The use of the terms “proppant”, “surfactant”, “front”, “back” and the like is not intended to limit the present system's use or operation, nor the scope of the invention. Further, when describing the invention, all terms not defined herein have their common art-recognized meaning.
Referring now as well to FIG. 4 (showing the trailer 12) and FIG. 5 (omitting the trailer), a particular aspect of the system is the arrangement at the back of the trailer which has a means for directing/diverting a high pressure fluid stream 61 into the pair of pressure vessels 40 operationally arranged in parallel, and a means for metering/feeding proppant into the fluid stream. Specifically, a piping arrangement 60 below the vessels 40 has a first inlet 62 for receiving a desired fluid. In a preferred embodiment that fluid is nitrogen gas pumped under high pressure from a nitrogen source, such as a pumper truck. A first Y-shaped diverter 64 downstream of the inlet splits the incoming nitrogen 61 into first and second fluid streams 66, 68 respectively. An adjustable venturi-type orifice 70 downstream of the diverter 64 is adapted to create a pressure drop, say in the range of 300 psi (or other desired amount), in the second fluid stream 68 passing therethrough. The orifice 70 should have the effect of diverting more volume of fluid into the first stream than the second stream, and for maintaining a positive fluid pressure in the screw(s) 58, as will become apparent later. The second fluid stream 68 then proceeds under relatively lower pressure toward a first outlet 72 for discharge to a coiled tubing rig or like apparatus in communication with the target formation.
A second four-way diverter 74 downstream of the diverter 64 allows the first fluid stream to split again into first and second fluid sub-streams 76 and 78 respectively. Elongate piping 80 carries the second sub-stream 78 toward the top of the vessels, while the first sub-streams 76 are directed to the bottom of the vessels through respective first valves 82. If only the left vessel is operating, then only the left valve 82 (as viewed in FIG. 5) is open for fluid entry, and the right valve 82 is closed, and visa versa. If both vessels are operating, then both valves 82 should be open. A third T-shaped diverter 84 further splits the second fluid sub-stream 78 into third fluid sub-streams 86 directed to the top of the vessels through respective second valves 88. The diverter 84 and valves 88 also act as a pressure equalization manifold between the vessels 40. Further, the piping 80 and associated valves 82, 88 and 90 (discussed below) are used to equalize the fluid pressures at the top and bottom of the vessels 40, and to de-pressurize the system to atmosphere when required.
Each pressure vessel 40 is formed by an elongate cylindrical tank having relatively thick outer walls 42 (e.g. 5 inches solid steel) to accommodate the high operating pressures (up to 9000 psi/63 MPa or more). The walls form an elongate interior cavity or chamber 44 for holding the desired proppant. The proppant is introduced into the chamber through a first vessel inlet 46 (shown in FIG. 2) at a first top end 48 of the vessel. A second vessel inlet 50 is provided at the top end of each tank for entry of the respective third fluid sub-streams 86, and to communicate with a respective third pressure relief valve 90 for bleeding pressure from the respective vessel to atmosphere prior to receiving proppant through the proppant inlet 46. A first vessel outlet 52 at the bottom of the vessel allows proppant and fluid to exit the vessel's chamber 44 and to encounter the first fluid sub-stream 76, and to then proceed to the proppant metering means. It is noted that the identifiers such a “top” and “bottom” as used herein refer to the vessel in its generally vertically oriented operating position, as shown in FIGS. 2-5, rather than when it is reclined about the pivot 34 by the hydraulic lift cylinders 32 into its generally horizontal transport position (as in FIG. 1). The vessels should be reinforced at 43 where they engage the hydraulic cylinders 32 and pivots 34.
The proppant metering means is defined by a high pressure sand screw 54 disposed generally perpendicularly to each vessel's longitudinal centerline and it's outlet 52. Other orientations of the screws should also be suitable. The screw has a flanged radial inlet 56 for attachment to a respective flange 53 of the vessel outlet 52, and for receiving the proppant and fluid therefrom. A variable rate electric or other suitable motor 58 operates the screw to discharge, or meter, a desired amount of proppant through a radial screw outlet 57 into piping 92. A Y-shaped joint 94 allows the proppant and fluids exiting the screw 54 to enter the second fluid stream 68 prior to exiting the first outlet 72. A pressure vessel isolation valve 96 on each piping 92 upstream of the Y joint 94 operates to isolate a respective vessel from the second fluid stream 68 as desired (e.g. when that vessel is inoperative and depressurized for proppant recharging), to prevent fluid backflow into the vessel through the screw. Each screw may be readily removed from the system for servicing, repair, or switching to a different screw size by uncoupling the flanges 53, 56 at one end, and at the other end by uncoupling from the isolation valve 96.
The piping arrangement 60 further incorporates an “upstream” surfactant injection port 98 at the first inlet 62 for introducing surfactants from the delivery tubing 30 into the fluid stream 61 prior to its split into the first and second fluid streams 66, 68. Such introduction may also be accomplished further downstream after the fluid and proppant have been mixed, such as at a “downstream” surfactant injection port 99 located immediately prior to the first outlet 72. Both ports 98, 99 may also be used concurrently, and other ports may be added in the system if required.
An alternate second embodiment of the present invention is shown in FIGS. 6 to 8 where the screws 154 are located longitudinally within the pressure vessels 140. The reference numerals used in these figures are similar to those used to describe the components of the system 10, with the addition of a prefix “1”. Each vessel has in essence three longitudinally aligned chambers. A first elongate chamber 144 a is defined by the vessel's outer wall 142 for holding the proppent received through the first vessel inlet 146 via the delivery line 130. A pressure relief valve 190 bleeds excess pressure before filling the chamber 144 a. A second elongate chamber 144 b is longitudinally disposed within the first chamber 144 a in a parallel relationship, and houses the screw 154 operated by the motor 158. The bottom end of the second chamber 144 b has a first bottom opening 145 into the first chamber 144 a to allow entry of the proppant. The screw raises the proppant to the opposed top end where it is discharges out of a second top opening 147 into the open end of a hollow third chamber 144 c. The third chamber 144 c is also located within the first chamber 144 a and extends downwardly alongside the second chamber 144 b and opens at a bottom vessel outlet 152 where the proppant and high-pressure fluid exit the vessel into the piping arrangement 160.
The piping arrangement 160 is similar to the piping arrangement 60 in that high pressure fluid, such as nitrogen gas, enters at the inlet 162 and is divided into first and second fluid streams 166 and 168 with the aid of orifice 170. The first fluid stream is then directed to one or both vessels at the Y-shaped diverter 167 by controlling the first valves 183. The first fluid stream enters the bottom of the first chamber 144 a via the second vessel inlet 150. The pressurized fluid is urged through the proppant and up the screw where it proceeds through the top opening 147 and then down the third chamber 144 c to exit the bottom outlet 152. When the screw is activated to discharge proppent through the top opening 147, the proppant is entrained in the high-pressure fluid flow and is carried down the third chamber 144 c to the outlet 152. The fluid and proppent exiting the outlet 152 proceed through piping 192 and the respective pressure vessel isolation valve 196 to rejoin the second fluid stream 168 moving to the first piping outlet 172.
This system is not preferred over the first embodiment for several reasons. First, for a given size of pressure vessel, the vessel 140 holds less proppent than the vessel 40 since internal volume is lost to the second and third chambers 144 b, 144 c. Second, a longer and more costly screw must be employed in the vessel 140, and such screw is more difficult to access or remove than in the first embodiment. The screw 154 must lift proppent against gravity, whereas the negative effects of gravity are reduced in the arrangement of the first embodiment.
The operation and advantages of the present invention may now be better understood, with reference to the first embodiment. For illustrative purposes it will be assumed that nitrogen and a form of sand are to be pumped into a coal formation. In the first embodiment, the rig is brought to the work site in an advantageous reclined transportation mode (as in FIG. 1) to avoid road clearance limitations. The trailer's wheel configuration is also designed to make the rig “road legal”, despite the extremely heavy weight of the system 10.
The vessels 40 and associated components are then elevated into the operating mode (FIG. 2) for use. If the vessel chambers 44 require charging with sand, then it is pumped from the tank 20 into at least one of the chambers via the line 26 and through respective first vessel inlet 46. An advantage of this two vessel arrangement is that fracing may commence once one vessel is charged with sand. There is no need to wait for the second vessel to be filled to begin operations. Likewise, there is no need to disrupt ongoing operations once the first vessel is emptied of sand since pumping may readily switch to the second filled vessel. In the meantime, the first vessel can be refilled with sand and be ready for when the second vessel is emptied. In unusual circumstances where the rate and volume of sand injection requires both vessels to operate simultaneously, then operations may be disrupted periodically while the vessels are refilled.
Assuming that the left vessel 40 in FIG. 5 is charged and ready for operation, and the right vessel is not, then the operator should isolate the right vessel by closing the first and second valves 82, 88 leading to the right vessel, as well as the respective (right side) isolation valve 96. Conversely, the first and second valves 82, 88 and the isolation valve 96 for the left vessel should be opened or activated. Once a high-pressure nitrogen stream 61 is established from a nearby nitrogen truck into the first inlet 62, the orifice 70 should provide the necessary pressure drop and split into first and second nitrogen streams 66, 68. The first stream is then further split into the first nitrogen sub-stream 76 at the lower end of the vessel and into the third nitrogen sub-stream 86 which enters the vessel at the top. The first and second valves 82, 88 control the relative pressures of the nitrogen gas to ensure that the nitrogen moves downwardly through the sand in the chamber 44 and does not reverse to force the sand upwardly, particularly as the sand is being depleted in the vessel. Both gravity and the nitrogen flowing out of the vessel should urge the sand from the chamber 44 toward the screw 54. If the screw is not activated, the nitrogen should seep through the porous sand and around the stationary screw blades to escape out of the screw outlet 57. However, once the screw is activated to carry sand to the screw outlet 57, the sand should be carried in the fourth nitrogen sub-stream 87 to the (unsanded) second nitrogen stream at the Y-joint 94, where both streams commingle and exit the first outlet 72 to a coiled tubing rig and ultimately to the coal formation.
If desired or required, surfactants may be introduced at either one or both of the upstream and downstream injection ports 98, 99. Injection at the downstream port 99 avoids circulation of the surfactant through the vessels and most of the system 10. In contrast, injection into the relatively “dry” nitrogen stream at the upstream port 98 will “wet” the sand in the vessels.
This nitrogen and sand combination, mixed potentially with one or more surfactants, should enhance the stimulation of coal deposits for improved gas production over prior art methods, as discussed earlier.
It is noted that pressure gauges 36 and one or more densometers 38 are installed at selected locations in the system to monitor pressures and proppant concentrations in the fluid stream exiting the system, to ensure that the desired volume and rate of proppant is being delivered to a particular formation. In particular, the gauge 36 a measures the manifold inlet pressure to the screw 58, and the gauge 36 b measures the manifold outlet pressure near the outlet 72. If the exiting fluid stream is not satisfactory, then the orifice 70 and/or the various described valves and/or the speed of the screw(s) 58 for proppant delivery may be adjusted, either manually or preferably remotely by PLC (programmable logic controller) systems, to obtain the desired mix/values.
Further advantages of the present invention include:
- the system provides great flexibility for various pumping operations;
- the system allows for a wide range of proppant density in the fluid stream;
- the system can use various types of proppant;
- the system's ability to mix proppant in the fluid stream, and in particular to mix sand with a N2 gas stream, provides an important means of enhancing production of coal bed methane sales gas;
- the system is cost effective to build and operate; and,
- the trailer 12 carrying the system 10 is “street” (i.e. weight) legal.
An even more advantageous third preferred embodiment of the present system is shown in FIGS. 9 and 10. In general, the system of this embodiment in essence functions the same way as the first embodiment, except that the vessels 240 have a spherical configuration rather cylindrical. The reference numerals used for this embodiment are similar to those used to describe the components of the system 10, with the addition of a prefix “2”. There are several advantages to employing such spheres, including:
The sphere is a more efficient shape for confining contents under high-pressure;
A greater volume of proppant may be held than in a given cylindrical configuration; and,
The spherical configuration omits the need for separate operating and transporation modes. For holding a given volume of proppant, the sphere 240 need not be as tall as the cylinder 40 (when elevated in an operating position), and so the sphere provides a more advantageous road height clearance when mounted on the trailer. Hence, the spheres 240 are mounted in a single orientation on the trailer for both transport and operation, and need not be reclined for transportation nor inclined for operation as the cylindrical vessels 40.
Each spherical pressure vessel 240 has a sand screw 254 located therebeneath in a manner similar to the first embodiment, and the piping system for proppant and nitrogen gas delivery is also similar. However, the location of certain features on the trailer 212 have changed, such as placement the proppant storage tank 220 and the surfactant storage and high pressure pumping assembly 228 at the rear of the trailer. Each sphere 240 also has a plurality of legs 231 spaced about a bottom portion thereof for supporting the sphere on the trailer, and three valves 280 at a top portion thereof for connection to respective piping for delivery of proppant, for delivery of nitrogen gas, and for venting.
A fourth embodiment of the invention in FIG. 11 shows a trailer carrying a single spherical pressure vessel 340 which is of a similar design to the third embodiment. Some of the reference numerals used for this embodiment are those used to describe like components of the system 10, with the addition of a prefix “3”. The vessel's mounting assembly differs from the previous lower legs 231 in that retractable arms 311 are employed to engage a top portion of the sphere to hold it on the trailer. Also, the vessel has a single cap 341 which accesses the sphere's interior and operatively connects to the proppant and nitrogen gas supplies, and has a vent. Valves in either the cap, or in piping leading to the cap, control the flow of products into the sphere, and for venting of the vessel. Further, the auger 354 in this embodiment is inclined for better ground clearance. A drive motor and seal assembly 354 (shown in outline) is coupled to the upwardly inclined end of the auger to operate the auger.
It is noted that a configuration of a single vessel per trailer is not preferred as it will present certain disadvantages. If the capacity of the one vessel is insufficient to treat a particular formation, then fracing operations will have to be disrupted as the vessel is refilled with proppant.
A sample operating sequence of the fourth embodiment will now be set out, with reference to FIGS. 12-16 which show the vessel 340 and associated piping 360 in isolation from the trailer. The sequence is described for one pressure vessel, but is equally applicable to each vessel of a multi-vessel configuration:
Lower valves (such as the auger outlet valve 396) under the spherical pressure vessel are closed. The sand screw, or auger 354, is off (inoperative). The pressure vessel 340 is empty and unpressurized.
The top proppant supply and vent valves 346, 390 are opened and proppant is blown or pumped into the vessel until nearly full.
The top supply and vent valves 346, 390 (capped at 391) are closed.
The top fluid (nitrogen) valve 388 is opened and the pressure vessel is pressurized up to the line pressure of the main horizontal fluid line 361 running along the bottom of the trailer. In this embodiment the vessel has a pressure rating up to about 9000 psi, and a proppant capacity of about 5 tonnes.
The outlet valve 396 at the end of the auger 354 is opened and the fluid (nitrogen) bypass line valve 382 at the auger outlet is opened. This flow of fluid (nitrogen) clears the auger outlet 357.
The auger is started to bring proppant from the pressure vessel to the outlet 357 of the auger.
Since the top and bypass fluid (nitrogen) valves 388, 382 are open, the high-pressure flow of fluid (nitrogen) assists the flow of, namely helps push, the proppant through the auger.
Once the pressure vessel is empty, the top fluid (nitrogen) valve 388 is closed, then the auger 354 is stopped, then the bypass fluid (nitrogen) line 382 is closed and then the auger outlet valve 396 at the discharge 357 of the auger is closed.
At this point the pressure vessel is vented down to atmospheric pressure via the vent valve 390 and/or purge valve 393 (& associated choke 395) and then refilled with proppant, and the above sequence is repeated.
The fluid stream, namely all or mostly nitrogen, in the main fluid line 361 across the bottom of the trailer is pumped at very high pressure. With the use of in-line restrictors, a portion of the fluid stream is diverted (via the first diverter 364) to the pressure vessel's top fluid inlet port 350 and to the auger fluid by-pass line 376 (via the second diverter 374), and another portion to the auger outlet bypass 394, in a like manner to that shown in FIG. 5 for the first embodiment. After the first diverter 364 there is an inlet 399 for the surfactant where it is injected at high pressure into the fluid (nitrogen) stream in the main line 361. After this injection point there is an auger outlet by-pass 394 for discharging the proppant and combining it with the fluid stream in line 361. The resulting fluid stream at the outlet 372 of this line (analogous to the the first outlet 72 in FIG. 5) contains a mixture of nitrogen, suspended surfactant and proppant for use in a target formation.
The above description is intended in an illustrative rather than a restrictive sense, and variations to the specific configurations described may be apparent to skilled persons in adapting the present invention to other specific applications. Such variations are intended to form part of the present invention insofar as they are within the spirit and scope of the claims below. For instance, it may be possible to employ only one cylindrical vessel 42 per trailer, as in the FIG. 11 embodiment, but the single vessel configurations present certain disadvantages. If the capacity of the one vessel is insufficient to treat a particular formation, then fracing operations will have to be disrupted as the vessel is refilled with proppant. Likewise, three or more pressure vessels might be employed per trailer, but it is believed that the third vessel would be redundant, be cost inefficient, and would lead to weight restriction issues for the trailer. Any number of trailers with pressure vessels mounted thereon may be employed in series or parallel at a given site, but capacity and cost efficiency are among the factors that will dictate the optimal configuration. It should also be appreciated by those skilled in the art that, based on the above information, other vessel shapes may also provide suitable proppant storage and pressure capacities.