Abstract:
A high-pressure injection proppant system for stimulating coal bed methane production preloads proppant, such as sand, into one or more high-pressure vessels, for delivery into a fluid stream, such a N 2  gas stream. A screw auger arrangement meters the proppant volumes and rates into the fluid stream. Two such vessels operationally mounted in parallel can function separately or concurrently depending on the demand for proppant in a particular formation. The system provides for the injection of surfactants into the fluid stream to enhance the performance of the proppant, to aid in the placement of the proppant into a fracture network, and to demote proppant flowback during production and embedment. The system can be operated manually or by computer automation to aid in the accuracy of the mixing of the fluid stream components.

Description:
FIELD OF THE INVENTION  
       [0001]     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.  
       BACKGROUND OF THE INVENTION  
       [0002]     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 CO 2  or N 2  gas injection. High rate N 2  gas injection technique is a common practice in North American coal bed methane exploited plays, and CO 2  is used as a flood medium.  
         [0003]     Although using CO 2  gas to stimulate a formation works fine, it has certain drawbacks, including: 
        1. Costly treatments; and,     2. CO 2  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.        
 
         [0006]     Using N 2  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 N 2  is pumped as a gas and will not suspend or carry proppant as do conventional fracturing fluid systems.  
         [0007]     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 N 2  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.  
       SUMMARY OF THE PRESENT INVENTION  
       [0008]     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 N 2  gas stream, via an arrangement, such as a screw auger, which meters the proppant volumes and rates into the fluid stream.  
         [0009]     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.  
         [0010]     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.  
         [0011]     In another aspect the invention provides a high-pressure injection proppant apparatus comprising:  
         [0012]     at least one pressure vessel;  
         [0013]     means for filling the vessel with proppant;  
         [0014]     means for delivering a fluid containing nitrogen gas to the vessel and pressuzing the vessel therewith; and,  
         [0015]     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.  
         [0016]     In yet another aspect the invention provides a method of injecting proppant into a target formation comprising:  
         [0017]     providing at least one pressure vessel and a metering arrangement operatively coupled to the vessel and in fluid communication therewith;  
         [0018]     charging the vessel with proppant;  
         [0019]     pressurizing the vessel with a fluid containing nitrogen gas; and,  
         [0020]     operating the metering arrangement to meter the proppant from the pressurized vessel into a fluid stream containing nitrogen for delivery to the target formation.  
         [0021]     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. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING FIGURES  
       [0022]     Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, wherein:  
         [0023]      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;  
         [0024]      FIG. 2  is a view of the system of  FIG. 1  with the pressure vessels in an elevated operating mode;  
         [0025]      FIG. 3  is a plan view of the rig and system of  FIG. 2 ;  
         [0026]      FIG. 4  is an elevational end view of the rig and system of  FIG. 2 ;  
         [0027]      FIG. 5  shows the system of  FIG. 4  in isolation, with the rig omitted;  
         [0028]      FIG. 6  is a view similar to  FIG. 4 , but shows a second embodiment of the system of the present invention, in operating mode;  
         [0029]      FIG. 7  is an elevational side view of the system of  FIG. 6 ;  
         [0030]      FIG. 8  is a plan view of  FIG. 6  with the front portion of the rig omitted;  
         [0031]      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;  
         [0032]      FIG. 10  is an elevational side view of the system of  FIG. 9 ;  
         [0033]      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;  
         [0034]      FIG. 12  is an elevational side view showing the vessel and piping of  FIG. 11  in isolation;  
         [0035]      FIG. 13  is an elevational side view from the right side of  FIG. 12 ;  
         [0036]      FIG. 14  is an elevational side view from the opposed back side of  FIG. 12 ;  
         [0037]      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 . 
     
    
     LIST OF REFERENCE NUMBERS IN DRAWINGS  
       [0000]    
       
           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  
       
     
       DESCRIPTION OF EMBODIMENTS  
       [0145]     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.  
         [0146]     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.  
         [0147]     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.  
         [0148]     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&#39;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&#39;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.  
         [0149]     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.  
         [0150]     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.  
         [0151]     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&#39;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 .  
         [0152]     The proppant metering means is defined by a high pressure sand screw  54  disposed generally perpendicularly to each vessel&#39;s longitudinal centerline and it&#39;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 .  
         [0153]     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.  
         [0154]     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&#39;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 .  
         [0155]     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 .  
         [0156]     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.  
         [0157]     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&#39;s wheel configuration is also designed to make the rig “road legal”, despite the extremely heavy weight of the system  10 .  
         [0158]     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.  
         [0159]     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.  
         [0160]     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.  
         [0161]     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.  
         [0162]     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.  
         [0163]     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&#39;s ability to mix proppant in the fluid stream, and in particular to mix sand with a N 2  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.    
 
         [0170]     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:  
         [0171]     The sphere is a more efficient shape for confining contents under high-pressure;  
         [0172]     A greater volume of proppant may be held than in a given cylindrical configuration; and,  
         [0173]     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 .  
         [0174]     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.  
         [0175]     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&#39;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&#39;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.  
         [0176]     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.  
         [0177]     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:  
         [0178]     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.  
         [0179]     The top proppant supply and vent valves  346 ,  390  are opened and proppant is blown or pumped into the vessel until nearly full.  
         [0180]     The top supply and vent valves  346 ,  390  (capped at  391 ) are closed.  
         [0181]     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.  
         [0182]     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 .  
         [0183]     The auger is started to bring proppant from the pressure vessel to the outlet  357  of the auger.  
         [0184]     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.  
         [0185]     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.  
         [0186]     At this point the pressure vessel is vented down to atmospheric pressure via the vent valve  390  and/or purge valve  393  (&amp; associated choke  395 ) and then refilled with proppant, and the above sequence is repeated.  
         [0187]     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&#39;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.  
         [0188]     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.