Patent Publication Number: US-2015086307-A1

Title: Container system for hydraulic fracturing proppants

Description:
PRIORITY 
     The present non-provisional patent application claims benefit from U.S. Provisional patent application having Ser. No. 61/882,334, filed on Sep. 25, 2013, by Tim Stefan, and titled CONTAINER SYSTEM FOR HYDRAULIC FRACTURING PROPPANTS, wherein the entirety of said provisional patent application is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention is in the field of container systems that are used to store, ship, and dispense hydraulic fracturing proppants. More specifically, the present invention relates to such container systems fitted with a stretchable hopper that expands and contracts responsive to the amount of proppant material held by the container system. 
     BACKGROUND OF THE INVENTION 
     Hydraulic fracturing encompasses techniques for recovering oil from oilfields. Hydraulic fracturing also is referred to as fracking. In a typical process fluid is pumped at high pressure from the surface of an oil well down through a wellbore. The fluid is often an abrasive slurry comprising a fluid phase and one or more proppants dispersed in the fluid phase. The slurry is pumped to targeted regions to help create and maintain fractures within the underlying hydrocarbon formations. 
     The fracking fluid often is aqueous. A hydraulic fracturing proppant often is a solid material, typically sand, treated sand, man-made ceramic materials, or combinations of these, that are resistant to fracturing under high pressure and help to keep an induced hydraulic fracture open during or following a fracturing treatment. Proppants often are added to a fracking fluid which may vary in composition depending on the type of fracturing used. 
     Proppants desirably are permeable or permittive to gas under high pressures. Accordingly, the interstitial space between particles should be sufficiently large to allow such permeability. Yet, a proppant desirably has sufficient mechanical strength to withstand closure stresses to hold fractures open after the fracturing pressure is withdrawn. Large mesh proppants have greater permeability than small mesh proppants at low closure stresses, but could mechanically fail (e.g. get crushed) and produce very fine particulates (“fines”) at high closure stresses such that smaller-mesh proppants overtake large-mesh proppants in permeability after a certain threshold stress. Sand and treated sand are common proppant materials. Others include ceramic particles, glass, sintered bauxite, combinations of these, and the like. 
     In a typical hydraulic fracturing methodology, proppant materials are harvested and/or created at one location and then shipped to an oilfield to carry out fracking operations. This requires strategies to store, ship, and dispense the proppant material. Conventional strategies involve the use of large, rugged containers that hold substantial quantities of proppant materials. Because proppants such as sand are quite dense, the containers must be rugged and robust enough to support tons of material. Conventional containers suffer from significant disadvantages. 
       FIG. 1  shows a conventional container  10  that is used to store and dispense hydraulic fracturing proppants. Container  10  includes rigid body  12  having sides  14  and floor  16 . A lid  18  can be opened and closed to provide access to interior  20 . Floor  16  and lid  18  include ports  20  and  22 . Each of ports  20  and  22  has a door or other suitable closure (not shown) that can be opened and closed on demand. Proppant contents are dispensed through port  20  when the door of floor  16  is opened. Container  10  can be filled with proppant content by opening lid  18  (as shown) or through port  22  when lid  18  is closed. A problem with the design of container  10  is that residual proppant  24  remains in the lower corners when container  18  is emptied through port  20 . Either the residual proppant is unused, wasting the expense of storing and shipping the material, or extra labor involving more expense is needed to more completely empty container  10 . Given the weight of proppants, the volume used, the number of containers used in the course of a project, and the large size of the containers, the extra expense is significant. 
       FIG. 2  shows another conventional container  30  designed to avoid the problem of residual proppant remaining in the lower corners of the container. Container  30  includes rigid body  32  having sides  34  and floor  36 . A lid  38  can be opened and closed to provide access to interior  40 . Floor  36  and lid  38  include ports  40  and  42 . Each of ports  40  and  42  has a door or other suitable closure (not shown) that can be opened and closed on demand. Proppant contents are dispensed through floor  36  when the door of floor  36  is opened. Container  30  can be filled with proppant content by opening lid  38  (as shown) or through port  42  when lid  38  is closed. As an additional feature, container  30  includes rigid cone  44  that provides a hopper function to dispense proppant contents from container  30  without leaving residual proppant in lower corners. A problem with the design of container  30  is that substantial space  46  is wasted. To store and dispense the same volume of proppant as the design in  FIG. 1 , container  30  must be substantially larger in size adding significantly to the costs to manufacture, ship, store, and use the containers. 
     The oilfield industry has a strong need for improved container systems for storing, shipping, and dispensing proppant materials used in hydraulic fracturing operations. 
     SUMMARY OF THE INVENTION 
     The present invention provides improved container systems for storing, shipping, and dispensing proppant materials used in hydraulic fracturing operations. Container systems of the present invention incorporate stretchable hopper structures into a container. The hopper expands and contracts responsive to the amount of proppant material held by the container. When filled with a sufficient amount of proppant, the hopper stretches to expand the storage volume for holding proppant material. When sufficient proppant material is dispensed from the container, the hopper contracts to lift and dispense container contents that otherwise might get trapped in container corners. 
     Thus, using a stretchable hopper rather than a rigid cone to provide a hopper function allows for greater storage capacity within the same overall volume. Using the stretchable hopper also makes it easier to fully dispense the full amount of proppant material in a storage volume compared to boxes with no cone. Container systems of the present invention provide the advantages of both boxes with rigid cones and boxes without cones but without their respective disadvantages. The container systems also are compatible with intermodal transport. The containers may be transported using rail cars, trucks, ships, container handling centers, etc. 
     In one aspect, the present invention relates to a container system for one or more hydraulic fracturing proppants, said container system comprising:
         (a) an expandable and contractable interior storage volume that holds one or more hydraulic fracturing proppants, said interior storage volume expanding and contracting responsive to an amount of the one or more proppants held in the storage volume; and   (b) a stretchable hopper defining at least a portion of the interior storage volume.       

     In another aspect, the present invention relates to a method of handling hydraulic fracturing proppants, comprising the steps of:
         (a) providing a container system according to claim  1 ; and   (b) at least partially filling the interior storage volume with one or more hydraulic fracturing proppants in a manner such that the stretchable hopper expands to increase the interior storage volume.       

     In another aspect, the present invention relates to a method of handling hydraulic fracturing proppants, comprising the steps of:
         (a) providing a container system according to claim  1 , wherein the interior storage volume holds a sufficient amount of one or more proppants such that the stretchable hopper is in an expanded state; and   (b) dispensing a sufficient amount of the one or more proppants such that the stretchable hopper contracts to form a cone shape that lifts and helps to dispense at least a portion of the one or more proppants from the interior storage volume.       

     In another aspect, the present invention relates to a container system for one or more hydraulic fracturing proppants, said container system comprising:
         (a) a support;   (b) a stretchable membrane coupled to the support in a manner effective to define at least a portion of a changeable storage volume, the size of the storage volume changing responsive to the amount of the one or more proppants held in the storage volume,   (c) an outlet fluidly coupled to the storage volume in a manner so that the one or more proppants can be dispensed from the storage volume through the outlet on demand; and wherein:
           i. the membrane expands to increase the storage volume as the storage volume is filled with more of the one or more proppants; and   ii. the membrane contracts to decrease the storage volume as the storage volume is emptied of the one or more proppants, said membrane contraction causing the storage volume to have an inverted, truncated cone-shape that converges towards the outlet to facilitate dispensing the one or more proppants from the storage volume through the outlet.   
               

     In another aspect, the present invention relates to a container system for one or more hydraulic fracturing proppants, said container system comprising:
         a) a housing having an interior volume;   b) a stretchable membrane coupled to the housing in a manner effective to define at least a portion of a changeable storage volume, wherein the membrane stretches and contracts to change the size of the storage volume responsive to the amount of the one or more proppants held in the storage volume;   c) an outlet fluidly coupled to the storage volume in a manner so that the one or more proppants can be dispensed from the storage volume through the outlet on demand; and   d) wherein, when the amount of the one or more proppants held in the storage volume is sufficiently low, the membrane has a contracted state in which the storage volume has an inverted, truncated cone shape that converges towards the outlet to form a hopper that facilitates dispensing the one or more proppants from the storage volume through the outlet; and   wherein, when the amount of the one or more proppants held in the storage volume is sufficiently high, the membrane has a stretched state in which the membrane stretches sufficiently so that the one or more proppants held in the storage volume defined by the stretched membrane substantially fill the interior volume of the housing.       

     In another aspect, the present invention relates to a method of handling hydraulic fracturing proppants, comprising the steps of:
         a) providing a container system according to any preceding claim, wherein the container system is substantially empty, the outlet is closed, and the stretchable membrane is in a contracted state in which the storage volume has an inverted, truncated cone shape that converges towards the outlet to form a hopper that facilitates dispensing the one or more proppants from the storage volume through the outlet when the outlet is opened; and   b) filling the container with at least one proppant, wherein the membrane stretches to increase the size of the storage volume as the storage volume is filled with the at least one proppant.       

     In another aspect, the present invention relates to a method of handling hydraulic fracturing proppants, comprising the steps of:
         a) providing a container system according to any preceding claim, wherein the container system is substantially filled with at least one proppant, the outlet is closed, and the stretchable membrane is in a stretched state to hold the at least one proppant and wherein a housing supports at least a portion of the stretched membrane;   b) opening the outlet to allow the at least one proppant to be dispensed from the container system; and   c) dispensing the at least one proppant such that, when a sufficient amount of the proppant has been dispensed, the membrane contracts to cause the storage volume to have an inverted, truncated cone shape that converges towards the outlet to form a hopper that facilitates further dispensing the one or more proppants from the storage volume through the outlet.       

     In another aspect, the present invention relates to a method of handling hydraulic fracturing proppants, comprising the steps of:
         a) providing a first container system according to any preceding claim, wherein the container system is substantially filled with at least one proppant, the outlet is closed, and the stretchable membrane is in a stretched state to hold the at least one proppant and wherein a housing supports at least a portion of the stretched membrane;   b) stacking the first container system on a second container system according to any preceding claim, wherein the outlet of the first container is coupled to an inlet of the second container system;   c) dispensing the at least one proppant from the first container system into the second container system; and   d) further dispensing the at least one proppant from the second container system.       

     In another aspect, the present invention relates to a method of handling hydraulic fracturing proppants, comprising the steps of:
         a) providing a first container system according to any preceding claim, wherein the first container systems is substantially filled with a first proppant content, wherein the stretchable membrane in the first container system is in a stretched state and wherein a housing supports at least a portion of the stretched membrane;   b) providing a second container system according to any preceding claim, wherein the second container systems is substantially filled with a second proppant content, wherein the stretchable membrane in the second container system is in a stretched state and wherein a housing supports at least a portion of the stretched membrane;   c) stacking the first container system on the second container system, wherein the outlet of the first container is coupled to an inlet of the second container system;   d) dispensing the second proppant content from the second container system;   e) dispensing the first proppant content from the first container system into the second container system; and   f) dispensing the first proppant content from the second container system.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically shows a prior art container used to store hydraulic fracturing proppants. 
         FIG. 2  schematically shows an alternative prior art container used to store hydraulic fracturing components. 
         FIG. 3  shows a perspective view of a container system of the present invention. 
         FIG. 4  shows an exploded perspective view of the container system of  FIG. 3 . 
         FIG. 5  shows a perspective side view of a structural frame used in the container system of  FIG. 3 . 
         FIG. 6  shows a top view looking down into a box used in the container system of  FIG. 3 . 
         FIG. 7  schematically shows a perspective wireframe view of a stretchable hopper used in the container system of  FIG. 3 . 
         FIG. 8  shows a top view of an assembly in which the box of  FIG. 6  is installed in the frame of  FIG. 5 , with the walls of the box schematically shown as being partially transparent to allow the frame to be seen through the box. 
         FIG. 9  schematically shows a lower gate assembly that can be used as a closure for the box of  FIG. 6 . 
         FIG. 10  schematically shows a side cross section view of the container system of 
         FIG. 3  in which the container system is empty and the stretchable hopper is in a contracted state in which the hopper has a truncated cone shape. 
         FIG. 11  schematically shows a side cross section view of the container system of  FIG. 3  in which the container system is full of proppant material and the hopper has expanded to allow the proppant material to fill the container system. 
         FIG. 12  schematically shows a side cross section view of the container system of  FIG. 3  in which the container system has been partially emptied but still includes a sufficient amount of proppant material so that the hopper is in a fully expanded state. 
         FIG. 13  schematically shows a side cross section view of the container system of  FIG. 3  in which the container system has been emptied sufficiently so that the hopper is contracted to form a cone shape to lift and dispense remaining proppant material. 
         FIG. 14  is a perspective view of a structural frame, box, stretchable hopper, and lid assembly used in the container system of  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION OF PRESENTLY PREFERRED EMBODIMENTS 
     The embodiments of the present invention described below are not intended to be exhaustive or to limit the invention to the precise forms disclosed in the following detailed description. Rather a purpose of the embodiments chosen and described is so that the appreciation and understanding by others skilled in the art of the principles and practices of the present invention can be facilitated. 
     An illustrative embodiment of a container system  100  of the present invention is shown in  FIGS. 3 to 14 . Container system  100  is useful for storing, shipping, and dispensing one or more hydraulic fracturing proppants. As main components, system  100  includes a housing  102  formed by structural frame  104  and box  106 . Container system  100  further includes; stretchable hopper  108 ; lid assembly  110  incorporating a first gate assembly  112 ; and a lower gate assembly  114 . Structural frame  104  holds box  106  and optionally can serve as a support for mounting lower gate assembly  114 . In some embodiments, lower gate assembly also could be mounted to box  106  or to both frame  104  and box  106 . Structural frame  104  can have a variety of shapes to correspond to the shape of box  106 . Exemplary shapes for frame  104  and box  106  include substantially cube-shaped or another rectilinear shape, cylindrical, cone and truncated cone shapes (including pyramids), combinations of these, or the like. As shown, structural frame  104  is substantially cube-shaped to match the box  106 , except that the bottoms of frame  104  and box  106  are slightly coned. The shallow cone shape is too shallow to function optimally on its own as a hopper to easily help empty the contents (See  FIGS. 10-13 ) of container system  100 . However, the cone shaped bottoms nonetheless are beneficial to add substantial strength and rigidity. In the practice of the present invention, stretchable hopper  108  more effectively provides a hopper function as described below. 
     The sides of structural frame  104  are formed from vertical stiles  116  and horizontal rails  118 . The bottom of frame  104  is formed by cross members  119 , truss members  120 , and gate frame  117  configured so that the bottom has a shallow cone shape that corresponds to a similar shallow cone shape on box  106 . The various stiles  116 , rails  118 , cross members  119 , truss members  120 , and gate frame  117  may be integrally formed as single components or may be individual components that are coupled together using any suitable coupling techniques such as welding, bolting, lashing, screwing, gluing, snap fit engagement, combinations of these, and the like. The components of frame  104  may be made from a wide variety of materials including steel or other metallic compositions, polymer(s), polymer composites (such as fiberglass composites, pultruded composites, long fiber reinforced extruded composites, wood, and man-made cellulosic products), combinations of these, and the like. In some modes of practice, industry standards (e.g., ISO standards or the like) may be applicable, and structural frame  104  desirably would be configured to meet such standards. 
     Gate frame  117  helps to support lower gate assembly  114 , wherein gate assembly  114  is coupled both to box  106  and gate frame  117  in this embodiment. In such modes of practice, components such as truss members  120  and/or frame  117  may include features to help hold, secure, and/or support the gate assembly  114 . 
     Container system  100  is stackable for storage and shipping. Features of container system  100  also allow stacked containers to be filled and emptied on demand while stacked. For example, with gates appropriately opened, stacked containers can be filled with, e.g., sand and/or other proppant material. The sand can be poured or otherwise introduced into a top container of the stack, and the sand will fill all containers in the stack. Gates can be closed to seal the containers after the desired filling is completed. At a point of use, the gates can be opened so that the sand and/or other proppant material can be dispensed from all or some containers in a stack. Container systems of the present invention thus can be stacked like a silo, with proppant material flowing downward through the stack from one container to another either for filling the stack with proppant material or dispensing proppant material from the stack. 
     As illustrated, box  106  is schematically shown as being partially transparent so other components of system  100  can be viewed through box  106 . In practice, box  106  may be opaque, transparent and/or partially transparent depending on material(s) used to form box  106 . Box  106  helps to define a storage volume  121  to hold, ship, process, treat, dispense or otherwise handle or use one or more hydraulic fracturing proppant materials (see  FIGS. 10 to 13 ). Box  106  also helps to support stretchable hopper  108  when box  106  is filled with proppant material. In many embodiments, stretchable hopper  108  is mounted to box  106  by suitable mounting features (not shown) such as one or more clamp, snap fit, lashing, bolts, screws, cord, welds, adhesive, combinations of these, and the like. In some other embodiments, stretchable hopper  108  is attached to frame  104 . In other embodiments, stretchable hopper  108  may be secured to more than one other component such as being secured to both frame  104  and box  106 . 
     Box  106  may have any suitable shape. Exemplary shapes are cylindrical, conical (including pyramids), cubic or other rectilinear shape. Box  106  as shown is substantially cubic in shape with a bottom  122  having a shallow cone shape for strength and rigidity. In addition to bottom  122 , box  106  includes sides  124  and top rim  126 . Top rim  126  defines aperture  130  through which proppant material can be loaded into storage volume  121  directly with lid assembly  110  raised, through opened gate assembly  112 , or the like. Bottom  122  includes facets  128  to form the shallow cone shape for rigidity and strength. The shallow cone shape also makes cleaning easier as cleaning and rinsing liquids more easily drain from the sloped facets  128 . Bottom  122  has aperture  129  through which box contents can be dispensed. 
     The components of box  106  may be provided in several ways as desired. In some instances, box  106  is integrally formed as a single item via a suitable molding or other fabrication process. Alternatively, box  106  components can be fabricated as separate parts that are then assembled via welds, glue, bolts, lashing, screws, nails, pins, rivets, snap fit, combinations of these, or the like. 
     Box  106  may be formed from a wide range of materials. Exemplary materials include steel or other metal composition(s), one or more polymers (e.g., high density polyethylene), fiber reinforced polymer materials, wood, synthetic cellulosic material (e.g., plywood or other composite cellulosic sheet goods), synthetic lumber, combinations of these and the like. One or more components of box  106  optionally may be reinforced with fiberglass, carbon fiber, polyaramid fabric, reinforcing fibers, meshes, organic and/or inorganic particles, and the like. One or more components of box  106  also may include one or more additives to help facilitate manufacture and/or enhance performance and service life. Exemplary additives include antistatic agents, biocides, fungicides, coloring agents, UV protecting agents, antioxidants, fillers, and the like. 
     Box  106  may have a wide range of sizes. Desirably, box  106  has a size so that container system  10  can be transported via truck transport, shipping, rail, and or combinations of these. In exemplary embodiments, each of the height, depth, and width of box  106  independently is in the range from 1 foot to 40 feet, preferably 5 to 15 feet, more preferably 5 to 10 feet. 
       FIGS. 3 ,  4 , and  7  schematically show stretchable hopper  108  in wireframe, but in actual practice stretchable hopper  108  is sufficiently non-permeable so as to help hold and dispense proppants held within hopper  108 . Stretchable hopper  108  has a first state in which hopper  108  is in the form of a truncated cone have one or more sides  134 , top rim  136 , and bottom rim  138 . Preferably hopper  108  has a frustrum shape so that top rim  136  and bottom rim  138  are parallel, although this is not required in all embodiments. As used herein, a cone shape generally refers to a shape having a first end and a second end, wherein the cross-sectional area of the shape gradually tapers from the first end towards the second end. The taper can be linear, convex, concave, undulating, combinations of these, or the like. The cross sections at the first and second ends and intermediate between these ends independently may be circular, oval, triangular, square or other polygonal shape, or any other suitable shape. 
     As shown, each of top rim  136  and bottom rim  138  defines a generally square cross section. Top rim  136  defines a relatively large first end, while bottom rim  138  defines a relatively smaller second end. The sides  134  of hopper  108  gradually taper from top rim  136  to bottom rim  138 . The taper is shown in  FIGS. 4 and 7  as being slightly convex when viewed from the exterior of hopper  108 . However, when installed in container system  100 , hopper  108  may be in tension so that the taper is linear. Such tension can help hopper  108  better support last portions of proppant being dispensed than if hopper  108  were slack at such time. 
     Top rim  136  defines a top opening  140  at the top of hopper  108 , while bottom rim  138  defines a bottom opening  142  at the bottom of hopper  108 . Bottom rim  138  is coupled to lower gate assembly  114  to facilitate dispensing proppant from storage volume  121  when lower gate assembly  114  is opened. Top rim  136  is in fluid communication with top gate assembly  112  and aperture  130  to allow storage volume  141  inside hopper  108  to be filled with proppant through top gate assembly  112  and/or aperture  130 . 
     Stretchable hopper  108  is in the first, relatively contracted state when container system  10  is empty or when sufficient proppant contents have been dispensed from container system  10 . In this state, hopper  108  has a truncated cone shape. Hopper  108  increasingly stretches as hopper  108  is filled with proppant, allowing substantially the entire volume of box  106  to be used to hold proppant. In this stretched shape, hopper  108  has a shape that more closely matches the contours of box  108 . As hopper  108  is sufficiently emptied, hopper  108  returns to the truncated cone shape to provide a hopper function to facilitate emptying substantially all proppant contents from hopper  108 . In other words, the stretchable hopper  108  stretches to occupy a greater volume of box  106  to increase storage volume when hopper  108  is filled with one or more proppants. Hopper  108  contracts to return to the hopper state when the amount of one or more proppants held in hopper  108  is sufficiently low. As the hopper  108  contracts as hopper  108  is emptied, the contraction causes hopper  108  to return to its inverted, truncated cone shape that converges from top rim  136  to bottom rim  138 . The cone helps to empty substantially all of the proppant contents, even the material that had been stored in the corners of the stretched hopper  108 . 
     Using a stretchable hopper  108  rather than a rigid cone allows for greater storage capacity within the same overall volume. Using the stretchable hopper  108  also makes it easier to fully dispense greater proportions of proppant material from container system  100  compared to boxes with no cone. Container system  100  of the present invention thus provides the advantages of boxes with rigid cones and boxes without cones but without their respective disadvantages. The function of container system  100  is described in more detail below with respect to  FIGS. 10 through 13 . 
     Hopper  108  incorporates a stretchable membrane material to help provide the ability of hopper  108  to repeatedly expand from and return to its first state in which hopper  108  has a truncated cone shape. Examples of such materials include thermoplastic and/or thermo set neoprene, natural and/or vulcanized rubber (e.g., including polyisoprene), polyurethane-polyurea copolymers, polybutadiene, polyisobutylene, polyurethane, polyester, combinations of these, and the like. 
     Neoprene elastomers are preferred. Membranes formed from materials including at least neoprene tend to be rugged and easy to clean. Neoprene as used herein refers to polychloreprene polymers and/or copolymers that incorporate 2-chlorobutadiene and optionally one or more other co-polymerizable constituents. Neoprene membranes can be selectively vulcanized to toughen up selected portions of the membrane such as at the bottom proximal to bottom rim  138  and/or at other stress points such as where the membrane is secured to the frame  104 , box  106 , and/or another portion of container system  100 . 
     In addition to or as an alternative to vulcanization, stretchable hopper  108  optionally may incorporate reinforcing components. Examples include a stretchable mesh integrated on and/or in the membrane, reinforcing fibers, organic or inorganic fibers, combinations of these, or the like. 
     Lid assembly  110  includes plate  152  with reinforcing frame  154  around the perimeter. Lid assembly  110  desirably is mounted to structural frame  104  or box  106  on hinges (not shown) or the like so that lid assembly  110  can be raised or lowered. Lid assembly  110  may be opened to service or maintain system  100  and/or to fill hopper  108  with one or more proppants. One or more latches (not shown) or other securement components can be used to secure lid assembly  110  in a closed position. Gate assembly  112  fits and is mounted to plate  152  around central opening  156 . 
     Gate assembly  112  includes large sliding door  160  that slides within frame  158 . Door  160  can be opened to provide one aperture  162  through which storage hopper  108  can be filled with one or more proppants. Door  160  can be closed to seal the contents. Aperture  162  is a large, elongate opening. Container system  100  can be placed on a moving conveyor while being filled with proppant material. The long axis of aperture  162  can be aligned with direction of movement as container moves on the conveyor to provide a suitable window of time during which filling can occur. In other modes of practice, container system  100  can be stationary while being filled. 
     Door  160  includes a frame  164  on which smaller door  166  slides open to provide another, smaller aperture  168  through which hopper  108  can be filled with one or more proppants. Door  166  can be closed to seal the contents. The small door  166  provides an opportunity to attach equipment to fill hopper  108  via one or more nozzles or the like. The small door  166  also facilitates silo functions when multiple container systems  100  are stacked. When containers are stacked, small door  166  may be opened and then fluidly coupled to a lower gate assembly on the container above. This allows proppant material to drain from one stacked container to the one(s) below. 
     Lower gate assembly  114  includes frame  170 , cross member  172 , sliding door  174  that slides back and forth on frame  170 , aperture  176  that is created when door  174  is opened, and actuating device  178 . 
     Doors  160 ,  166 , and  174  independently can be actuated manually or by automation, e.g., by hydraulic action. Gate assemblies  112  and  114  desirably includes features so that doors  160 ,  166 , and  174  can be sealed tight to help contain liquids (if any) included with the proppant material. The seals desirably also are weather resistant to protect the proppant contents from the environment. In some modes of practice, ceramic seals are used as these seal tightly to provide liquid tight closures and can tolerate the abrasive character of proppant materials such as sand. 
       FIGS. 10 through 13  schematically show one way in which container system  100  can be used to handle proppant material.  FIG. 10  schematically shows a cross section of container system  100  in which hopper  108  is empty and lid assembly  110  is closed. Without the weight of proppant material or other force, stretchable hopper  108  is in a first state in which hopper  108  has a truncated cone shape with the widest part of the cone at the top of box  106  and the narrowest part of the cone at the bottom of box  106 . For schematic purposes, box  106  is shown without floor  122  having a moderate cone shape. The hopper  108  converges towards gate assembly  114 . Zone  182  is between hopper  108  and box  106 . If hopper  108  were rigid, the volume associated with zone  182  could not be used to store proppant material. If no hopper  108  were present, the zone  182  could be filled with proppant material, but zone  182  would be difficult to empty through lower gate assembly  114 . 
     Hopper  108  has a cone angle  190 . Hopper  108  may have a wide range of cone angles  190 . If cone angle  190  is too shallow, however, the hopper  108  may be less effective at helping to dispense proppant material as described in  FIGS. 11-13 . If cone angle  190  is too steep, hopper  108  may not stretch as effectively as shown in  FIG. 11 . Accordingly, certain ranges of cone angles  190  may be more preferred to enhance performance. In some modes of practice, therefore, cone angle  190  is in a range from 20 degrees to 70 degrees, preferably 30 degrees to 50 degrees, more preferably 35 degrees to 45 degrees. By way of example, cone angles of 35 degrees and 41 degrees would be suitable. 
       FIG. 11  schematically shows a cross section of container system  100  in which hopper  108  is filled with proppant material  184 . The weight of the proppant material stretches hopper  108  substantially to the full extent allowed by the walls of box  106 . Hopper  108  has expanded to increase its storage volume as the hopper  108  is filled with proppant material  184 . Even zones  182  (see  FIG. 10 ) are filled with proppant material  184 . A rigid cone could not allow the storage volume inside hopper  108  to be expanded in this manner. Hopper  108  is in a stretched state so that the proppant material  184  substantially fills the entire interior volume of box  106 . Box  106  and structural frame  104  (not shown in  FIGS. 10-13 ) support stretched hopper  108 . Lid assembly  110  and gate assemblies  112  and  114  are sealed to protect the contents of the filled box  106  from the environment. In this state, container system  100  can be stored, stacked, transported, or otherwise handled. 
       FIG. 12  schematically shows a cross section of container system  100  in which a portion  186  of proppant material  184  is being dispensed through opened lower gate assembly  114 . The proppant can be used in a variety of ways. In some illustrative modes of practice, proppant material  184  is dispensed directly at a point of use. In other modes of practice, proppant material can be dispensed onto a conveyor (not shown) and then conveyed to another location for further handling. In other modes of practice, container system  100  can be stacked on top of one or more other containers, so that the proppant material  184  is dispensed into one or more other containers. A substantial amount of proppant material  184  remains in hopper  108  so that hopper  108  is still substantially in the same fully stretched state as in  FIG. 11 . 
       FIG. 13  schematically shows a cross section of container system  100  in which additional portions  188  of proppant material  184  have been dispensed through lower gate assembly  114 . More proppant material has been dispensed. The amount of proppant material  184  remaining in hopper  108  is sufficiently low so that hopper  108  has contracted from the fully stretched state in  FIGS. 11 and 12 . In the contracted state, hopper  108  returns to having a truncated cone geometry that converges toward gate assembly  114  to facilitate dispensing proppant material  184 . The contraction of hopper  108  lifts proppant material out of zone  182 , to allow material from those zones to more easily dispense than if no cone were to be present. After proppant material  184  is full y  dispensed from storage hopper  108 , hopper  108  is sufficiently empty so that the emptied container system  100  is again in the state shown in  FIG. 10 . It can be seen therefore that the hopper  108  stretches to expand its storage volume and contracts to form a hopper responsive to the amount of proppant material in hopper  108 . The expansion and contraction of hopper  108  exploits the potential energy in the proppant material and in the stretched hopper  108  to help control the geometry of hopper  108 . 
     In an illustrative experiment, a wood box was made that was about 3 feet wide by about 3 feet deep by about 3 feet tall. A gate was coupled to the bottom of the box. The gate could be opened and closed. A stretchable membrane in the shape of a cone and made from neoprene sheeting was installed in the box. The top, larger end of the membrane was attached to the top rim of the box. The bottom, smaller end of the membrane was attached to the bottom gate. The membrane tapered from the top toward the gate at a cone angle of about 35 to 41 degrees. The box was filled with sand. As the sand filled the box, the membrane expanded until substantially the entire interior of the box was filled with sand. The box supported the expanded membrane. The gate at the bottom was opened to drain the sand from the box. When the amount of sand was sufficiently low, the membrane contracted and returned to having a cone shape. This helped to lift sound out of the bottom corners of the box and drain the sand through the open gate. Substantially all of the sand was drained from the box. 
     All patents, patent applications, and publications cited herein are incorporated by reference as if individually incorporated. Unless otherwise indicated, all parts and percentages are by weight and all molecular weights are number average molecular weights. The foregoing detailed description has been given for clarity of understanding only. No unnecessary limitations are to be understood therefrom. The invention is not limited to the exact details shown and described, for variations obvious to one skilled in the art will be included within the invention defined by the claims.