Abstract:
A method and apparatus for making up fracturing fluid with coated proppant applies opposed electrical charges to the proppant and its coating to create a uniform coating without extensive mechanical mixing, and additionally, the method applies opposed electrical charges to the coated proppant and fracturing fluid to enhance mixing.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     None. 
     BACKGROUND 
     1. Technical Field 
     The present inventions relate to methods and apparatus for making fracturing fluid containing coated particulate materials, such as proppants. 
     2. Background Art 
     Hydraulic fracturing is a common stimulation treatment. The purpose of a fracturing treatment is to provide an improved flow path for oil or gas to flow from the hydrocarbon-bearing formation to the wellbore. A treatment fluid adapted for this purpose is sometimes referred to as a “fracturing fluid.” The fracturing fluid is pumped at a sufficiently high flow rate and pressure into the wellbore and into the subterranean formation to create or enhance one or more fractures in the subterranean formation. Creating a fracture means making a new fracture in the formation. Enhancing a fracture means enlarging a pre-existing fracture in the formation. 
     A newly-created or newly-extended fracture will tend to close together after the pumping of the fracturing fluid is stopped. To prevent the fracture from closing, a material is usually placed in the fracture to keep the fracture propped open and to provide higher fluid conductivity than the matrix of the formation. A material used for this purpose is referred to as a “proppant.” 
     A proppant is in the form of a solid particulate, which can be suspended in the fracturing fluid, carried downhole and deposited in the fracture to form a proppant pack. The proppant pack props the fracture in an open condition, while allowing fluid flow through the permeability of the pack. The proppant pack in the fracture provides a higher-permeability flow path for the oil or gas to reach the wellbore compared to the permeability of the matrix of the surrounding subterranean formation. This higher-permeability flow path increases oil and gas production from the subterranean formation. 
     A particulate for use as a proppant is usually selected based on the characteristics of size range, crush strength, and solid stability in the types of fluids that are encountered or used in wells. Preferably, a proppant should not melt, dissolve, or otherwise degrade from the solid state under the downhole conditions. 
     The proppant is selected to be an appropriate size to prop open the fracture and bridge the fracture width expected to be created by the fracturing conditions and the fracturing fluid. If the proppant is too large, it will not easily pass into a fracture and will screen out too early. If the proppant is too small, it will not provide the fluid conductivity to enhance production. See, for example, McGuire and Sikora, 1960. In the case of fracturing relatively permeable or even tight-gas reservoirs, a proppant pack should provide higher permeability than the matrix of the formation. In the case of fracturing, ultra-low permeable formations, such as shale formations, a proppant pack should provide for higher permeability than the naturally occurring fractures or other microfractures of the fracture complexity. 
     One common problem is that the proppant may not be sufficiently strong by itself to prop open a fracture. Crushing of proppant by fracture closure would generate fine particulates that migrate in the propped fracture, plug up pore spaces in the proppant pack, choking the flow path and decreasing well production. Proppant flowing back along with the production fluid often result in plugging or eroding of downhole equipment, such as electrical submersible pumps, or erosion of choke valves and other surface equipment. Another common problem is that the surface of the proppant may have an undesirable wettability characteristic for producing oil or gas from a particular subterranean formation. Another common problem is that, as the oil or gas moves through the subterranean formation, it can dislodge and carry particulate with the fluid into the wellbore. The migration of this particulate can plug pores in the formation or proppant pack, decreasing production, in addition to causing abrasive damage to wellbore pumps, tubing, and other equipment. 
     To help alleviate some of the common problems mentioned above, a resinous material can be coated on the proppant before the proppant is inserted in the formation. The term “coated” does not imply any particular degree of coverage on the proppant particulates, and the coverage can be partial or complete. 
     For example, some or all of the proppant can be coated with a curable resin. The curable resin can be allowed to cure on the proppant prior to the proppant being introduced into the well. The cured resin coating on the proppant provides a protective shell encapsulating the proppant and keeping the fine particulates in place, if the proppant were crushed or provides a different wettable surface than the proppant without the coating. 
     A curable resin coating on the proppant can be allowed to cure after the proppant is placed in the subterranean formation for the purpose of consolidating the proppant of a proppant pack to form a “proppant matrix.” As used herein, “proppant matrix” means a closely associated group of proppant particles as a coherent mass of proppant. Typically, a cured resin consolidates the proppant pack into a hardened, permeable, coherent mass. After curing, the resin reinforces the strength of the proppant pack and reduces the flow back of proppant from the proppant pack relative to a similar proppant pack without such a cured resin coating. 
     A resin or curable resin can be selected from natural resins, synthetic resins, and any combination thereof in any proportion. Natural resins include, but are not limited to, shellac. Synthetic resins include, but are not limited to, epoxies, furans, phenolics, and furfuryl alcohols, and any combination thereof in any proportion. Examples of resins suitable for coating particulates are described in U.S. Pat. Nos. 6,668,926; 6,729,404; and 6,962,200. An example of a suitable commercially available resin is the EXPEDITE™ compound product sold by Halliburton Energy Services, Inc. of Duncan, Oklahoma. 
     By way of another example, some or all of the proppant can be coated with a tackifying agent, instead of, or in addition to, a curable resin. The tackifying agent acts to consolidate and helps hold together the proppant of a proppant pack to form a proppant matrix. Such a proppant matrix can be flexible rather than hard. The tackifying agent-coated proppant in the subterranean formation tends to cause small particulates, such as fines, to stick to the outside of the proppant. This helps prevent the fines from flowing with a fluid, which could potentially clog the openings to pores. 
     Tackifying agents include, but are not limited to: polyamides, polyesters, polyethers and polycarbamates, polycarbonates, and any combination thereof in any proportion. Examples of tackifying agents suitable for coating particulates are described in U.S. Pat. Nos. 5,853,048; 5,833,000; 5,582,249; 5,775,425; 5,787,986; and 7,131,491, the relevant disclosures of which are herein incorporated by reference. An example of a suitable commercially available tackifying agent is the SANDWEDGE™ product sold by Halliburton Energy Services, Inc. of Duncan, Oklahoma. 
     It is advantageous and common to coat proppant particles. The coated proppant particles are suspended in the fracturing fluid, preferably on the fly, and the resulting coated proppant particles are placed in one or more fractures formed in a subterranean zone. The term “on the fly” is used herein to mean that a flowing stream is continuously introduced into another flowing stream so that the streams are combined and mixed while continuing to flow as a single stream, as part of the on-going treatment. Coating the proppant particles with the hardenable resin or tackifying agents, and mixing the coated proppant particles with the fracturing fluid are preferably performed on the fly. Such mixing can also be described as “real-time mixing.” As is well understood by those skilled in the art, such mixing may also be accomplished by batch or partial batch mixing. 
     Coating compounds are typically mixed with proppant in the blender&#39;s sand screws; indeed, a common way to perform mechanical mixing is in a mill. An example of an on-the-fly mixing method is described in U.S. Pat. No. 6,962,200 wherein, a container holds a liquid hardenable resin component, while another container holds a liquid hardening agent component. The liquid materials in the containers are transported to a static mixer. Control of the total and relative amounts of resin component and hardening agent component is achieved through the use of liquid additive pumps and flow meters. In a preferred embodiment, the flow meters are computer-controlled to ensure accurate metering, corresponding to the concentration of proppant and pumping rate of proppant slurry and to allow for a rapid shutdown of on-the-fly mixing when necessary. 
     A static mixer mixes the resin component and hardening agent component into a single, hardenable resin composition that is then transported through a conveyance means which is partially located inside a proppant hopper. The mixer can be any means known in the art for mixing two liquid streams. The conveyance means can be any means known in the art for conveying particulate material, for example, a proppant screw conveyor. The proppant may be transported to the hopper by any suitable means known in the art. The proppant is removed from a bulk container via conveyor belt, after which it enters the hopper. Inside the conveyance means the coating is mixed with the proppant to form coated proppant particles. 
     Where conveyance means is a sand screw, the coating is coated onto the proppant by the auger action of the sand screw itself. The coated proppant particles are transported by the sand screw to a blender tub. The coated proppant particles from the hopper may be transported to a blender tub by any means known in the art. In a preferred embodiment, the transport rate of coated proppant particles from sand hopper to blender tub is computer-controlled to ensure accurate metering and to allow for a rapid shutdown of on-the-fly mixing when necessary. Also transported to the blender tub is a fracturing fluid. 
     While these conventional methods of producing fracturing mixtures containing coated proppant have been successful, vigorous agitation and mixing must be performed in the blender and sand screw to ensure uniform coating of the abrasive proppant with viscous coatings. This results in wear and tear on the mixing equipment. 
     Problems associated with using sand screws for coating chemicals on proppant: 1) tackiness of coating chemicals tends to cause the sand screw to lock up during fracturing treatment with slow turning of the screw at low proppant concentrations, and 2) tackiness or curing of coating chemicals on sand screw requires its thorough cleaning to prevent it from locking up after each usage. The constant maintenance of sand screw requires time and resources. 
     Accordingly, there is a need for improved methods and apparatus for making mixtures of fracturing fluid with coated proppant therein. 
     SUMMARY OF THE INVENTIONS 
     The present inventions will allow formation of fracturing fluids containing proppants coated with materials, such as SANDWEDGE™ and EXPEDITE™ compounds, without the need to mix these compounds with the sand in a blender&#39;s sand screws. Not using the sand screws to mix EXPEDITE™ compound with sand will save on sand screw wear and clean up. Proppant delivery systems which do not use sand screws can be used to coat proppant with EXPEDITE™ compound and tackifying agents. 
     Dry proppant (sand) in motion can pick up a static electric charge, and this static electricity can have enough voltage to cause a spark which can ignite flammable compounds. Sand equipment and blenders are typically grounded together to prevent a large build up of static electricity. This invention would apply opposite electrical charges to the moving proppant and coating compound. For example, SANDWEDGE™ compound would be given a charge of opposite voltage to the sand voltage before it was mixed with the proppant falling vertically from a sand bin or from the sand screw on a blender to the mixing tub. The opposite charges of the proppant and SANDWEDGE™ compound would cause them to accelerate towards each other and bind together. The opposite charges would also force the SANDWEDGE™ compound to more evenly coat the proppant particle, rather than beading up on one side. 
     Where necessary, a power source could be used to increase the voltage differential between the proppant and coating compound. 
     The coating compound could be applied to the proppant after the proppant passed through the sand screws eliminating the need to clean up the sand screws to remove the coating compound at the end of the job. 
     Future blending systems which do not use sand screws will be able to use Sand Wedge® and Expedite® coated proppant. 
     The method and apparatus of the present invention could also be used to better combine sand and fracturing fluid in a blender by applying opposite electric charges to the sand and fracturing fluid. Less mechanical mixing would be necessary and air entrainment reduced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawing is incorporated into and forms a part of the specification to illustrate at least one embodiment and example of the present invention. Together with the written description, the drawing serves to explain the principles of the invention. The drawing is only for the purpose of illustrating at least one preferred example of at least one embodiment of the invention and is not to be construed as limiting the invention to only the illustrated and described example or examples. The various advantages and features of the various embodiments of the present invention will be apparent from a consideration of the drawing in which: 
         FIG. 1  is a schematic view of the fracturing fluid mixing system, including the electrostatic coating apparatus of the present invention; 
         FIG. 2  is a schematic view of one embodiment of the electrostatic coating apparatus of the present invention; and 
         FIG. 3  is a schematic view, similar to  FIG. 1 , of an alternative embodiment of the fracturing fluid mixing system, including the electrostatic coating apparatus of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention provides an improved apparatus and method for preparing proppant containing fracturing fluids. The present invention&#39;s particular applicability is to the coating of proppants, mixing proppant with fracturing fluids. 
     Referring more particularly to the drawings, wherein like reference characters are used throughout the various figures to refer to like or corresponding parts, there is shown in  FIG. 1  a diagram of one embodiment of the apparatus for preparing fracturing fluids containing coated proppant. 
     In accordance with one improved method and apparatus of this invention, proppant particles are coated with the hardenable resin composition, preferably on the fly, the coated proppant particles are suspended in the fracturing fluid, preferably on the fly, and the resulting hardenable resin composition-coated proppant particles are pumped into a well and placed in one or more fractures formed in a subterranean zone and then allowed to harden and consolidate into one or more high-strength permeable packs. Forming the hardenable resin composition, coating the proppant particles with the hardenable resin composition, and mixing the hardenable resin-coated proppant particles with the fracturing fluid are all preferably performed on the fly on an as-needed basis. However, the present inventions could be used to prepare batches of coated proppant and fracturing fluids. 
     In  FIG. 1 , container  10  holds a liquid hardenable resin component while container  20  holds a liquid hardening agent component. The liquid materials in containers  10  and  20  are transported to a static mixer  30  through lines  11  and  21 , respectively. Control of the total and relative amounts of resin component and hardening agent component is achieved through the use of flow meter  12  on resin component line  11 , and flow meter  22  on hardening agent component line  21 . In a preferred embodiment, flow meters  12  and  22  are computer-monitored to provide precise control for the flow. Static mixer  30  mixes the resin and hardening agent into a single hardenable resin mixture that is to be used to coat proppant. Static mixer  30  can be any means known in the art for mixing two liquid streams, in one embodiment; mixer  30  may be a static mixer. The resin mixture is transported by line  31  to a sand hopper  50 . 
     Proppant is stored in container  40  which is transported to a container known as a sand hopper  50  as needed by conveyor  41 . Proppant from container  40  may be transported to sand hopper  50  by any suitable means known in the art. In one embodiment, the proppant is removed from container  40  via conveyor belt  41 , after which it enters sand hopper  50  from the top. 
     Proppant entering the hopper is coated with the hardenable resin mixture, using the electrostatic mixer  200  located in the sand hopper. As will be described in detail in regard to  FIG. 2 , the mixer  200  is connected to an electrical power source  100  and applies opposing charges to the proppant and the hardenable resin mixture to enhance coating. In principal, as the proppant enters the hopper, it moves (drops or is propelled) past or into contact with a set of charged plates while the liquid hardenable resin mixture is sprayed onto the proppant from a nozzle with an applied voltage opposite to the plates. The opposed charges cause the coating to be attracted to the proppant. 
     The coated proppant is transported by conveyor  54  to a container called a “blender tub.” The hardenable resin-coated proppant particles may be transported to blender tub  70  by any means known in the art. In a preferred embodiment, the transport of hardenable resin-coated proppant particles from sand hopper  50  to blender tub  70  is computer-controlled to ensure accurate metering and to allow for a rapid shutdown of on-the-fly mixing when necessary. 
     Also transported to blender tub  70  is a fracturing fluid from container  60 . The fracturing fluid from container  60  may be transported to blender tub  70  by any means  61  known in the art. In a preferred embodiment, the transport of fracturing fluid from container  60  to blender tub  70  is computer-controlled to ensure accurate metering and to allow for a rapid shutdown of on-the-fly mixing when necessary. 
     Inside blender tub  70 , the fracturing fluid is substantially mixed with hardenable resin-coated proppant particles to form a blended composition suitable for use in subterranean fractures in the present invention. In this embodiment, a second electrostatic mixer  300  is used to blend the fracturing fluid with the coated proppant. Mixer  300  is connected to the electrical power source  100 . Mixer  300  applies opposite electrical charges to the coated proppant and fracturing fluid to assist in blending. The blended coated proppant containing fracturing fluid is pumped by pump  80  into a well to form fractures. 
     In the  FIG. 1  embodiment, a curable resin coating can be used to form a protective shell to encapsulate the proppant. This provides a different wettable surface than the proppant without the coating. EXPEDITE™ compound can be used to coat the proppant. In another embodiment, a tackifying coating that acts to aggregate and helps hold together the proppant of a proppant pack to form a proppant matrix, such as SANDWEDGE™ compound can be used. 
     Referring to  FIG. 2 , electrostatic mixer  200  of the present invention will be described in detail. The mixer  200  is located in a housing  210  and functions by applying an electrical charge to the proppant flowing into the hopper  50  and by applying an opposite charge to the coating material. Plates  214  apply an electrical charge to the proppant and a spray bar  218  applies an opposite charge to the coating material. 
     The housing or mixing tube  210  is located above or in the upper portion of the sand hopper  50 . The cross section of the throat  212  of the tube  210  can be of any shape with the interior of the throat  212 . Tube  210  can be constructed from non conductive materials or from materials with the interior surface (or throat)  212  coated with insulating (non-conductive) material. Alternatively, tube  210  can be mounted such that it is electrically insulated from the spray bar  218 . 
     Preferably the proppant is delivered by the conveyor  41  to the housing  210  and moves or falls by the force of gravity past charged plates  214 , located in the throat  212 . In this figure, the top of the figure represents the upward direction. It is envisioned that other means of moving the proppant past the plates could be used, such as the proppant being propelled using a flowing gas, much like in sand blasting. In the present embodiment, the throat  212  has a cylindrical cross-section shape and plates  214  comprise a plurality of radially spaced concentric rings, however other plate shapes could be used. The plates are connected to and charged by an electrical power source  100 . As the proppant particles move past or contact the plates a charge is applied to the proppant particles. 
     As illustrated, a spray bar  218  is located below and in the path of the proppant. The spray bar  218  has a plurality of nozzles for spraying coating material into the path of the proppant. The spray bar  218  is also connected to the electrical power source  100  and has an electric charge applied to it that is opposite from the charge applied to the plates  214 . The opposed charges on the proppant and coating material passing the spray bar creates an electrical attraction between the proppant and coating which improves the coating process. 
     In a similar manner, the coated proppant leaving hopper  50  and entering blender tub  70 , is charged in electrostatic mixer  300  while the fracturing fluids are sprayed through a spray bar with the opposite charge. The charged fracturing fluids will be attracted to the coated proppant and will tend to displace gases from the surface of the particles. 
     As used herein “spraying” includes atomizing the treatment fluid into fine droplets. The liquid is charged in the nozzles  220 , such that, the liquid droplets exiting the nozzle are charged prior to contacting or “coating” the oppositely charged falling proppant. This coating process is performed without requiring physical mixing, augering or agitating the proppant and liquid mixture. 
     In  FIG. 1 , the coating process and the mixing proppant with fracturing fluid are accomplished in two separate steps at two different locations. In an alternative embodiment, illustrated in  FIG. 3 , the coating-proppant electrostatic mixer  200  could be located in the blended tub  170  positioned above the proppant-fracturing fluid electrostatic mixer  300 . Proppant would be fed from container  140  into the housing by conveyor  141  where it would fall through electrostatic mixer  200  and be coated with a tackifying fluid pumped from container  110  along lines  111  and  131  as controlled by flow meter  112 . The opposed charges on the tackifying fluid and the proppant will cause the tackifying fluid and proppant to be attracted and bind together. The opposed charges will cause the tackifying fluid to more evenly coat the particles of the proppant rather than bead up on one or more sides. The coated proppant would be recharged as it enters (falls through) electrostatic mixer  300  and be mixed with the oppositely charged fracturing fluid entering the tub  170  via conduit  161  from container  160 . Pump  180  pumps the blended fluid to a hydrocarbon well. In this alternative system, the process is simplified and unnecessary equipment is eliminated. 
     It is also envisioned that either of the electrostatic mixers could be eliminated, as required by the materials being used and the ease of coating and mixing. It is also envisioned that the embodiments of  FIGS. 1 and 3  could alternatively be used to coat proppant with any materials, including tackifyers, resins and the like. 
     While compositions and methods are described in terms of “comprising,” “containing,” or “including” various components or steps, the compositions and methods also can “consist essentially of” or “consist of” the various components and steps. As used herein, the words “comprise,” “have,” “include,” and all grammatical variations thereof are each intended to have an open, non-limiting meaning that does not exclude additional elements or steps. 
     Therefore, the present inventions are well adapted to carry out the objects and attain the ends and advantages mentioned as well as those which are inherent therein. While the invention has been depicted, described, and is defined by reference to exemplary embodiments of the inventions, such a reference does not imply a limitation on the inventions, and no such limitation is to be inferred. The inventions are capable of considerable modification, alteration, and equivalents in form and function, as will occur to those ordinarily skilled in the pertinent arts and having the benefit of this disclosure. The depicted and described embodiments of the inventions are exemplary only, and are not exhaustive of the scope of the inventions. Consequently, the inventions are intended to be limited only by the spirit and scope of the appended claims, giving full cognizance to equivalents in all respects. 
     Also, the terms in the claims have their plain, ordinary meaning unless otherwise explicitly and clearly defined by the patentee. Moreover, the indefinite articles “a” or “an”, as used in the claims, are defined herein to mean one or more than one of the element that it introduces. If there is any conflict in the usages of a word or term in this specification and one or more patent(s) or other documents that may be incorporated herein by reference, the definitions that are consistent with this specification should be adopted.