Abstract:
Methods and apparatus of embodiments of the invention relate to a system for treating a subterranean formation including mixing equipment to form a fluid comprising sodium hypochlorite and sodium diacetate; and pumps and a tubular to introduce the fluid into the subterranean formation, wherein a surface of the subterranean formation contains at least 15 percent less microorganisms than if no sodium hypochlorite were in the fluid. Methods and apparatus of embodiments of the invention relate to a method of producing a petroleum product from a wellbore including using a well treatment system comprising mixing equipment, pumps, and a tubular, forming a fluid comprising sodium hypochlorite and sodium diacetate; and introducing the fluid to the well treatment system to achieve a reduced population of microorganisms in the system. Methods and apparatus of embodiments of the invention relate to a system, comprising: a subterranean formation, a well treatment apparatus comprising mixing equipment, pumps, and a tubular, and a fluid comprising sodium hypochlorite and sodium diacetate to achieve a reduced population of microorganisms in the system. Methods and apparatus of embodiments of the invention relate to a method for treating a subterranean formation, comprising forming a fluid comprising sodium hypochlorite, a buffer, and a polymer; introducing the fluid to a surface of a subterranean formation; and decreasing a population of microorganisms, wherein the surface of the subterranean formation contains at least 15 percent less microorganisms than if no sodium hypochlorite were in the fluid, and wherein the fluid exhibits a pH of about 4.0 to about 7.5. Methods and apparatus of embodiments of the invention relate to a method for treating a subterranean formation, comprising forming a fluid comprising sodium hypochlorite and sodium diacetate; and introducing the fluid to a subterranean formation, wherein forming the fluid does not include introducing an acid, and wherein forming the fluid does not include forming a precipitate.

Description:
PRIORITY 
       [0001]    This application claims priority to U.S. Provisional Application No. 61/217,899, filed Jun. 5, 2009, which is incorporated by reference herein in its entirety. 
     
    
     BACKGROUND 
       [0002]    Hydraulic fracturing uses fluid additives such as slickwater additives. The demand for this type of well services has increased over the past decade, especially because of its successful application for shale gas. Horizontal wells are often standard, requiring as much as 4.2 million gallons of water per well in as many as 6 to 9 fracture stages. Because of environmental concerns and fresh water availability, the flowback and produced water are collected and used for subsequent fracture treatments. Produced water is a perfect environment for sulfate reducing bacteria (SRB) and acid forming bacteria (AFB) due to its anaerobic nature (&lt;2 ppm oxygen content) and high nutrient content (organics, free iron, etc.). Reuse of water introduces enough oxygen through regular pumping operations to allow aerobic bacteria to grow—mostly slime forming bacteria (SFB). The oxygen content is high enough for aerobic bacteria to grow but too low to kill anaerobic bacteria. The oxygen content will cause the anaerobic bacteria to stay in a biostatic state which does not kill them but prevents them from multiplying. 
         [0003]    As soon as the bacteria find an environment that is conducive to their growth, they will become active again and start multiplying. The anaerobic environment in the formation is ideal for growth of bacteria like SRBs and AFBs. The aerobic environment of the wellbore is conducive for SFBs. The growth of SRBs will not only lead to health and safety concerns due to increased sour gas or hydrogen sulfide (H 2 S) production but also to a slow souring of the reservoir. This also increases operation expenses because of corrosion (H 2 S pitting, stress cracking, etc.) in surface and subsurface tubulars. Other challenges in production can be related to AFBs (pitting) and SFBs (emulsion like materials may form). 
         [0004]    Various different methods can be applied to prevent bacteria growth and reduce operational expenses related to corrosion prevention, remediation of corrosion effects, and remediation of emulsion-like produced fluids. Common biocides are quaternary amines, glutaraldehyde, tetra-kis-hydroxylmethylphosphonium sulfate, and tetrahydro 3,5-dimethyl-1,3,5-thiadiazinane-2-thione. The issues with traditional non-oxidizing biocides like those described above are that they each have compatibility issues with common additives in stimulation fracturing treatments (e.g. quat amines are not compatible with quaternary and zircontate crosslinked fluids fluids or anionic friction reducing polymers) and that they are very toxic. Despite the treatment of water with these biocides, post-fracture treatment reservoir souring has been reported. The re-growth of SRB under reservoir conditions may lead to reservoir souring. An effective, low cost biocide that is compatible with other fluid additives and that is easily transportable is needed. 
     
    
     
       FIGURES 
         [0005]      FIG. 1  is a bar graph of free active chlorine as a function of hydrochlorous acid concentration for three time periods. 
           [0006]      FIG. 2  is a bar graph of bacterial population as a function of time for three types of bacteria when a fluid comprises a friction reducer. 
           [0007]      FIG. 3  is a bar graph of bacterial population as a function of time for three types of bacteria when a fluid comprises a biocide. 
           [0008]      FIG. 4  is a bar graph of bacterial population as a function of time for three types of bacteria when a fluid comprises a hypochlorous acid. 
           [0009]      FIG. 5  is a photograph comparing produced water before and after addition of hypochlorous acid. 
           [0010]      FIG. 6  is a chart illustrating the percent drag reduction as a function of rate that compares a fluid comprising a viscosity modifying agent with and without hypochlorous acid. 
           [0011]      FIG. 7  is a chart illustrating viscosity as a function of time for the fluid identified by Table 2 and varied concentrations of hypochlorous acid. 
           [0012]      FIG. 8  is a chart illustrating viscosity as a function of time for the fluid identified by Table 3 and varied concentrations of hypochlorous acid. 
           [0013]      FIG. 9  is a chart illustrating viscosity as a function of time for the fluid identified by Table 4 and varied concentrations of hypochlorous acid. 
           [0014]      FIG. 10  is a chart illustrating viscosity as a function of time for the fluid identified by Table 5 and varied concentrations of hypochlorous acid. 
           [0015]      FIG. 11  is a schematic view of mechanical equipment configured to perform an embodiment of the invention. 
           [0016]      FIG. 12  is a chart illustrating bacterial population as a function of types of bacteria in a field trial comparing the microbe content of fresh water, produced water, mix water, mix water and hypochlorous acid, and flowback water and acid after 21 days. 
           [0017]      FIG. 13  illustrates viscosity as a function of time for a guar fluid that contains no sodium hypochlorite and two different concentrations of sodium hypochlorite. 
           [0018]      FIG. 14  shows titration curves for addition of sodium diacetate buffer to various solutions of concentrated industrial sodium hypochlorite in tap water. 
           [0019]      FIG. 15  shows titrations of some produced water samples treated with sodium hypochlorite (0.21 gpt) and one sample of tap water that was pre-acidified using citric acid prior to treatment with concentrated industrial sodium hypochlorite. 
           [0020]      FIG. 16  shows drag reduction in a 0.5″ pipe using 0.25 gpt friction reducer, versus water. 
           [0021]      FIG. 17  provides friction reduction curves at 0, 15, and 30 minutes. 
       
    
    
     SUMMARY 
       [0022]    Methods and apparatus of embodiments of the invention relate to a system for treating a subterranean formation including mixing equipment to form a fluid comprising sodium hypochlorite and sodium diacetate; and pumps and a tubular to introduce the fluid into the subterranean formation, wherein a surface of the subterranean formation contains at least 15 percent less microorganisms than if no sodium hypochlorite were in the fluid. Methods and apparatus of embodiments of the invention relate to a method of producing a petroleum product from a wellbore including using a well treatment system comprising mixing equipment, pumps, and a tubular, forming a fluid comprising sodium hypochlorite and sodium diacetate; and introducing the fluid to the well treatment system to achieve a reduced population of microorganisms in the system. Methods and apparatus of embodiments of the invention relate to a system, comprising: a subterranean formation, a well treatment apparatus comprising mixing equipment, pumps, and a tubular, and a fluid comprising sodium hypochlorite and sodium diacetate to achieve a reduced population of microorganisms in the system. Methods and apparatus of embodiments of the invention relate to a method for treating a subterranean formation, comprising forming a fluid comprising sodium hypochlorite, a buffer, and a polymer; introducing the fluid to a surface of a subterranean formation; and decreasing a population of microorganisms, wherein the surface of the subterranean formation contains at least 15 percent less microorganisms than if no sodium hypochlorite were in the fluid, and wherein the fluid exhibits a pH of about 4.0 to about 7.5. Methods and apparatus of embodiments of the invention relate to a method for treating a subterranean formation, comprising forming a fluid comprising sodium hypochlorite and sodium diacetate; and introducing the fluid to a subterranean formation, wherein forming the fluid does not include introducing an acid, and wherein forming the fluid does not include forming a precipitate. 
       DETAILED DESCRIPTION 
       [0023]    At the outset, it should be noted that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developer&#39;s specific goals, such as compliance with system related and business related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time consuming but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. The description and examples are presented solely for the purpose of illustrating the preferred embodiments of the invention and should not be construed as a limitation to the scope and applicability of the invention. While the compositions of the present invention are described herein as comprising certain materials, it should be understood that the composition could optionally comprise two or more chemically different materials. In addition, the composition can also comprise some components other than the ones already cited. 
         [0024]    In the summary of the invention and this description, each numerical value should be read once as modified by the term “about” (unless already expressly so modified), and then read again as not so modified unless otherwise indicated in context. Also, in the summary of the invention and this detailed description, it should be understood that a concentration range listed or described as being useful, suitable, or the like, is intended that any and every concentration within the range, including the end points, is to be considered as having been stated. For example, “a range of from 1 to 10” is to be read as indicating each and every possible number along the continuum between about 1 and about 10. Thus, even if specific data points within the range, or even no data points within the range, are explicitly identified or refer to only a few specific, it is to be understood that inventors appreciate and understand that any and all data points within the range are to be considered to have been specified, and that inventors have disclosed and enabled the entire range and all points within the range. 
         [0025]    Embodiments of the invention relate to the use of sodium hypochlorite as an effective biocide in combination with sodium diacetate for use in operations related to recovering hydrocarbons from subterranean formations, such as fracturing operations, especially those fracturing operations that use fluid additives for viscosity modification. That is, embodiments of this invention relate to the use of sodium hypochlorite and sodium diacetate for killing and managing microbes in water used for fracturing including slickwater fracturing. In some embodiments, hypochlorous acid can be delivered in a dilute and stable form, such as by using EXCELYTE™ composition, which is commercially available from Benchmark of Houston, Tex. Calcium hypochlorite may be selected for some embodiments. It also will form hypochlorous acid upon exposure to water. 
       Hypochlorous Acid 
       [0026]    Generally, when chlorine is added to water, hypochlorous acid is formed according to the equation: 
         [0000]      Cl 2 +H 2           HOCl+HCl 
         [0027]    Hypochlorous acid has outstanding bactericidal power. This is generally attributed to its ability to diffuse through cell walls and thereby reach the vital parts of the bacterial cell. A widely accepted theory credits the death of the cell to a reaction between hypochlorous acid and enzyme. The hypochlorite ion has little if any bactericidal effect since its negative charge impedes penetration of the cell wall. 
         [0028]    The bactericidal power of a solution of chlorine, a hypochlorite, or a chloramine is directly proportional to the hypochlorous acid concentration of the solution. The percent available chlorine as un-dissociated hypochlorous acid is therefore the true measure of the bactericidal effectiveness of a solution containing one of the chemicals of the available chlorine family. 
         [0029]    The available chlorine family is comprised of the group of chemicals which, when dissolved in water, yield solutions of hypochlorous acid. These compounds may be further subdivided into those which contain free available chlorine and those which contain combined available chlorine. 
         [0030]    Oxidizing power of a hypochlorite and/or hypochlorous acid solution is attributable to the amount of active oxidant, measured as Free Available Chlorine (FAC), irrespective of pH. Organic chloramines are also a source of FAC, where the low rate of hydrolysis of dissolved organic chloramines to give hypochlorite and/or hypochlorous acid contributes little to the rate of oxidation while maintaining the total oxidizing power, which relates to the amount of organic chloramines present. Thus, organic chloramines and other reagents that contribute to FAC supply more hypochlorite and/or hypochlorous acid as these oxidizers are depleted. 
         [0031]    Hypochlorous acid is 25 to 100 times more effective than bleach as a disinfectant without being corrosive. The key active ingredient, hypochlorous acid, is a naturally occurring molecule synthesized from an electrolyzed solution of salt and water. When exposed to atmospheric conditions, it quickly degrades into saltwater, therefore not leaving ecological damage at field locations. 
         [0032]    Hypochlorous acid does not fully dissociate and has a neutral pH. (around 7.5). In aqueous solutions, hypochlorous acid partially dissociates into a salt (the hypochlorite ion), therefore its use in oil field service application does not leave an undesirable ecological footprint. In contrast, the most commonly used oxidizers do not sterilize and completely kill bacteria. Hypochlorous acid, on the other hand, reacts quickly with any organic-based or readily oxidizable materials (Fe, H 2 S) present in the water. Further, hypochlorous acid is noncorrosive compared to other biocides. 
         [0033]    In some embodiments, hypochlorous acid will have a concentration of about 1 to 8,500 ppm in a fluid. The pH of hypochlorous acid influences the free available chlorine concentration. The relationship between pH and the degree of dissociation acid is illustrated by Table 1. Hydrolysis increases rapidly as the pH rises above neutrality. 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Dissociation of Hypochlorous Acid as a Function of pH at 25° C. 
               
             
          
           
               
                   
                 pH 
                 % HOCl Undissociated 
               
               
                   
                   
               
             
          
           
               
                   
                 5.0 
                 99.6 
               
               
                   
                 6.0 
                 96.5 
               
               
                   
                 7.0 
                 73.0 
               
               
                   
                 7.4 
                 50.0 
               
               
                   
                 8.0 
                 21.0 
               
               
                   
                 9.0 
                 2.7 
               
               
                   
                 10.0 
                 0.3 
               
               
                   
                   
               
             
          
         
       
     
         [0034]    Hypochlorous acid may be commercially manufactured using several methods. In some embodiments, hypochlorous acid may be made by exposing water containing sodium chloride to an electrolytic cell. It can also be made in a more concentrated form in the field by using a buffer, such as sodium diacetate, to lower the pH of a sodium hypochlorite solution in water. Finally, in some embodiments, hypochlorous acid may be generated by dissolving chlorine gas in water. 
         [0035]    Hypochlorous acid can also be formed by introducing sodium hypochlorite into a solution that has a pH that can be synthesized from an electrolyzed solution of salt and water, or generated by lowering the pH of a hypochlorite solution to a pH below 7.5, often tailored to have a pH of 4 to 7. For example, a continuous process that includes continuous addition of sodium hypochlorite and pH modifying agent such as a weak acid such as on the fly mixing in oil field service applications may be selected. PH modifying agents such as weak acid, a buffer and/or a strong acid may be used to tailor the pH. In some embodiments, the preferred pH modifying agent may comprise water-soluble organic acids with twelve or fewer carbon atoms. The weak acid is an acid that exhibits a pKa of less than 6. Weak acids include potassium dihydrogen phosphate, phthalic acid, phthalates such as potassium hydrogen phthalate and related acid salts, chelates, citric acid, sulfamic acid, ascorbic acid, octanoic acid, nonanoic acid, propionic acid, erythorbic acid, succinic acid, glutaric acid, adipic acid, polyacrylic acid, maleic acid, cyanuric acid, orthophosphoric acid, acetic acid, and sodium, potassium, and calcium salts of these acids. A weak acid, a buffer, or a combination thereof may be used to tailor the pH. In some embodiments, the preferred weak acid may comprise water-soluble organic acids with twelve or fewer carbon atoms. The preferred weak acid exhibits a pKa of less than 6. 
         [0036]    In some embodiments, a pH modifying agent may include a strong acid that does not contain a halogen, such as sulfuric, nitric, or phosphoric acid may be used in very dilute concentration, such as nanomolar concentration. Other buffers, buffer solutions, or buffer systems may be selected. 
         [0037]    The pH modifying agent may be selected to activate upon the passage of time or temperature, such that the hypochlorous acid is present in solution after the solution containing sodium hypochlorite and pH modifying agent is pumped into a wellbore. Generally, however the hypochlorous acid is manufactured, the pH modifying agent may be selected to modify the pH of the fluid over a tailored time or temperature. Agents most likely to be effective include polylactic acid, polyglycolic acid, or similar hydrolytic polyesters. Delay may be enhanced by isolating the agent in an oil phase and the sodium hypochlorite in the water phase in some embodiments, the acid may be encapsulated. Upon temperature and downhole mixing, delayed formation of hypochlorous acid may be achieved. Fumeric acid encapsulated in wax may also be selected. 
         [0038]    However the hypochlorous acid is formed, to maintain the hypochlorous acid concentration within a fluid, the fluid may be tailored to exhibit a pH of 4.0 to 7.5 using a buffer or weak acid. In some embodiments, the preferred weak acid may comprise water-soluble organic acids with twelve or fewer carbon atoms. A weak acid is an acid that exhibits a pKa of less than 6. Weak acids include potassium dihydrogen phosphate, thallic acid, phthalates, chelates, citric acid, sulfamic acid, ascorbic acid, octanoic acid, nonanoic acid, propionic acid, erythorbic acid, succinic acid, glutaric acid, adipic acid, polyacrylic acid, maleic acid, cyanuric acid, orthophosphoric acid, acetic acid, and sodium, potassium, and calcium salts of these acids. In some embodiments, a strong acid that does not contain a halogen, such as sulfuric, nitric, or phosphoric acid may be used in very dilute concentration, such as nanomolar concentration. Other buffers, buffer solutions, or buffer systems may be selected. 
         [0039]    Additional chemicals may be added to a hypochlorous acid composition to stabilize the hypochlorous acid concentration and/or to reduce the reactivity of the bacteria&#39;s residual enzymes. Dichloroisocyanuric acid, cyanuric acid, sulfamic acid, potassium iodate, ethylenediaminetetraacetic acid, or a combination thereof may be selected for some embodiments. 
         [0040]    The method can also include contacting the aqueous medium with an enzyme activity minimizer including a metal. In an embodiment, the metal can include a heavy metal compound in the aqueous medium including oilfield produced water. In an embodiment, the heavy metal can include zirconium compound. Zirconium containing chemicals may be used to reduce the reactivity of residual bacteria enzymes. Examples of zirconium containing chemicals that act as enzyme activity minimizer include zirconium nitrate, zirconyl chloride, zirconium phosphate, zirconium potassium chloride, zirconium potassium fluoride, zirconium potassium sulfate, zirconium pyrophosphate, zirconium sulfate, zirconium tetrachloride, zirconium tetrafluoride, zirconium tetrabromide, zirconium tetraiodide, zirconyl carbonate, zirconyl hydroxynitrate, zirconyl sulfate, zirconium complexed with amino acids, zirconium complexed with phosphonic acids, hydrates thereof and combinations thereof. Organo-zirconium compound examples include zirconium acetate, zirconyl acetate, zirconium acetylacetonate, zirconium glycolate, zirconium lactate, zirconium naphthenate, sodium zirconium lactate, triethanolamine zirconium, zirconium propionate, hydrates thereof and combinations thereof. Zirconium dichloride oxide may be selected for some embodiments. 
       Other Fluid Additives 
       [0041]    The carrier fluid, such as water, brines, or produced water, may contain other additives to tailor properties of the fluid. Rheological property modifiers such as friction reducers, viscosifiers, emulsions, stabilizers, solid particles such as proppant or fibers, or gases such as nitrogen may be included in the fluid. The fluid may include viscosity modifying agents such as guar gum, hydroxyproplyguar, hydroxyelthylcellulose, xanthan, or carboxymethylhydroxypropylguar, diutan, chitosan, or other polymers or additives used to modify viscosity for use in the oil field services industry. Water based fluids may include crosslinkers such as borate or organometallic crosslinkers. In some embodiments, the fluid may contain viscosity modifying agents that comprise viscoelastic surfactant. Viscoelastic surfactants include cationic, anionic, nonionic, mixed, zwitterionic and amphoteric surfactants, especially betaine zwitterionic viscoelastic surfactant fluid systems or amidoamine oxide viscoelastic surfactant fluid systems. 
       Applications 
       [0042]    The fluid may be used as a fracturing fluid, drilling fluid, completions fluid, coiled tubing fluid, sand control fluids, cementing operations fluid, fracturing pit fluid, or onshore or offshore water injector fluid, or any other fluid that is introduced into a subterranean formation primarily for the recovery of hydrocarbons. The fluid is introduced to the subterranean formation by drilling equipment, fracturing equipment, coiled tubing equipment, cementing equipment, or onshore or offshore water injectors. During, before, or after the fluid is added to a subterranean formation, the formation may benefit from fracturing, drilling, controlling sand, cementing, or injecting a well. 
         [0043]    An oil field services application of a hypochlorous acid fluid may include delivery of the fluid to the following mechanical equipment. Hypocholorous acid fluid may be delivered to the low pressure side of the operation, that is, into any low pressure hose, connection, manifold, or equipment; before or during treatment. Examples of the location for addition include into pond, pit, or other water containment source; into inlet hose/manifold of water tanks (upstream of water tanks); frac tanks—all together or separate; into water tanks (frac tanks) themselves; into hose/manifold of outlet side of water tanks; into batch mixing unit; into hose/manifold in between batch mixing unit and blender; into blender itself; into exit side of blender (upstream of fracturing pumps); hose/manifold; directly into low pressure side of pump manifold (missile). Hypochlorous acid fluid may be delivered to the high pressure side of an operation including into any high pressure iron, anywhere. Pumps that may be used, either solo or combined, include positive displacement pumps, centrifugal pumps, and additive pumps. The hypochlorous acid fluid may be added to the water stream in any way. (i.e. pour from a bucket, pump it into the water, etc.). 
         [0044]      FIG. 11  is a schematic view of mechanical equipment configured to perform an embodiment of the invention. Working tanks  1101  contain water or liquid that is introduced to water line  1102 . Water line  1102  may include an entry port  1103  for acetic acid or sodium diacetate or other pH controlling agent. The entry port  1103  includes a connection to the pH control agent line  1104  which is connected to additive skids  1105 , which may be any type of pump or other delivery device. The line  1104  may include acetic acid, sodium diacetate, or other pH controlling agent or any other chemical. The skids  1105  are controlled, in part, by feedback from a control device  1106  as illustrated by lines  1107 . The water line  1102  is also in communication with a pH meter or other online or offline sampling entity  1108  which may be used to determine the pH or other property of the water as it enters water line  1102 . The entity  1108  sends a signal via a line  1109  or using a transmitter that does not require lines to a pH meter  1110  or other property measurement device which sends a signal to the control device  1106  via lines  1111  or using a transmitter that does not require lines. The entities  1108  and  1112  may be any type of probe such as an electrode. The pH meter  1110  also collects information from pH meter or other online or offline sampling entity  1112  via line  1114  or using a transmitter that does not require lines which is connected to a blender  1113 . The pH meter  1110  sends a signal via line  1115  or using a transmitter that does not require lines to the control device  1106 . In any event, as the water or liquid flows through line  1102 , it continues on into the blender  1113  where additional chemicals are introduced via line  1116 , which delivers sodium hypochlorite and/or other biocide and/or any other relevant chemical. The delivery of sodium hypochlorite is, in part, controlled by the skids  1105 , which receive a signal from the controller  1106  via lines  1117  or using a transmitter that does not require lines. The fluid flows from the blender  1113  on to the manifold  1118  via lines  1119  or using a transmitter that does not require lines, then on to the wellbore through the pumps or other lines or other equipment of the manifold  1118  and on to tubulars or other wellbore equipment. 
         [0045]    In some embodiments, the pH control or component concentration control of the system may be performed using an electronic control system as described above. In some embodiments, manual control may be used, including measuring the pH and/or composition of the water in the tanks  1101  or the line  1102  or the line  1119 . In some embodiments, no pH metering may be performed at all and the concentration of components may be established based on volume of material. In some embodiments, a hybrid manual/electronic control system may be used with sampling and addition partially manually controlled, partially electronically controlled. In some embodiments, addition of one component may be using the skids  1105  described above or using equipment configured for addition at other points in the blender, line  1119 , or in the manifold. In some embodiments, the controller  1106  and/or pH meter  1110  and/or skids may be the same piece of equipment. In some embodiments, the controller  1106  and/or pH meter  1110  may be omitted altogether, especially if the volume of material is fixed. In some embodiments the blender  1113  may be a blender, a tubular, a line, a static mixer, or any other equipment that may provide static or agitated mixing or blending. In some embodiments, the order of mechanical equipment including mixing, blending, introducing components, measuring pH may be altered. Further, the control system may be configured in alternative ways to accommodate changes in the mechanical equipment. 
         [0046]    In some alternative embodiments, delivering the components to form the hypochlorous acid fluid to the mechanical equipment in the field must be selected based on the source of the acid. Commercially available hypochlorous acid, such as EXCELYTE™, is delivered premixed into any size storage containers. It may be added to the system with any way into any of the above points of addition. Sodium hypochlorite may be combined with a weak acid on the fly or by batch mixing. In on the fly applications, the material may be added by separate add lines—one for sodium hypochlorite, one for acid/buffer (any order); by a combined system—concentrated mixture of sodium hypochlorite and acid/buffer; or by a slurry system—combined mixture of water, sodium hypochlorite and acid/buffer. In batch mixing applications, the components may be mixed together before or during the fracturing job and stored in any type of container. It may be added to the system with any way into any of the above points of addition. In some embodiments, hypochlorous acid may kill or retard the reproduction of microorganisms. In some embodiments, hypochlorous acid in the fluid will result in a fluid with at least 25 percent less microorganisms or at least 25 percent less bacteria than if no hypochlorous acid were present. 
       EXAMPLES 
       [0047]    The following examples are presented to illustrate the preparation and properties of fluid systems, and should not be construed to limit the scope of the invention, unless otherwise expressly indicated in the appended claims. All percentages, concentrations, ratios, parts, etc. are by weight unless otherwise noted or apparent from the context of their use. 
         [0048]    Several analytical tools were selected to confirm the effectiveness of hypochlorous acid, its compatibility with other fluid additives, and its stability over time with optional stabilizing additives. 
       Water Quality—Water Analysis (Produced Water) 
       [0049]    The type of water used in these Examples, unless described otherwise, was produced water from the Piceance Basin, which is considered to be among the dirtiest, recycled, produced water with poor quality. The sample water was provided to us by a supplier and yielded a pH of 8.0 and a TDS of 142,000 ppm. Titrimetric methods were used to determine the anions present while Inductively Coupled Plasma spectrometry was used for the detection of cations in the sample water. 
       Free Available Chlorine (FAC) Demand Test 
       [0050]    The chlorine exists in the water as hypochlorous acid (free available chlorine, FAC). Chlorine is effective against all microorganisms and any readily oxidizable organic matter. If there is a lot of organic matter in the fracturing water, the chlorine will be consumed (or spent) and will be unavailable for killing the bacteria. Therefore, it is necessary to have a residual of FAC in the water to be effective as a biocide. The FAC demand test determines the dosage of hypochlorous acid necessary to treat the water and kill the bacteria in the frac water. The FAC demand test was used to determine the dosage of hypochlorous acid necessary to treat and kill the bacteria downhole. The FAC of the sample water was determined at several time points up to 45 minutes using various concentrations of hypochlorous acid. 5% (v/v) of hypochlorous acid solution containing 500 to 1000 ppm active ingredient was found to be the lowest effective concentration that showed a positive FAC residual necessary to sanitize and kill the microorganisms present in the produced water.  FIG. 1  shows data collected on a Piceance Basin water sample. As can be seen, 5% (v/v) or 50 gpt hypochlorous acid solution was the lowest effective concentration that showed a positive result of FAC residual which was necessary to sanitize and kill the bacteria. Consequently, 50 gpt hypochlorous acid solution was used as the concentration for all subsequent testing regarding hypochlorous acid. 
         [0051]    Bottle tests were used to evaluate the biocidal efficacy of hypochlorous acid against the three types of bacteria mentioned above as well as compare its performance with friction reducer and commonly used biocide, glutaraldehyde. The bacterial population was measured at time points up to seven days. 
       Effect of Friction Reducer on Bacterial Population 
       [0052]      FIG. 2  shows the effect of a viscosity modifying agent, that is, a friction reducer has on biological activity. As can be seen that friction reducer had little or no effect on the bacterial population. 0.25 gal/Kgal polyacrylamide emulsion was added to the Piceance Basin water sample for a period of seven days to see its effect on the biological activity. The above figure shows that the friction reducer had little or no effect on the bacterial population. 
       Effect of Gluteraldehyde on Bacterial Population 
       [0053]      FIG. 3  shows that glutaraldehyde is not very effective in killing bacteria in Piceance River water containing 0.25 gpt of friction reducer. Note a 2 log reduction in the population of SRB after 24 hours (a 3 log reduction is desirable). However, after 7 days there was re-growth of bacteria. 0.25 gal/Kgal glutaraldehyde was added to the produced water sample along with 0.25 gal/Kgal friction reducer to evaluate the effect of glutaraldehyde on bacterial activity for seven days. The above figure shows that glutaraldehyde in the presence of friction reducer was not effective in killing the bacteria in the water sample; however, there was a 2 log reduction in the SRB population after 24 hours. After seven days, regrowth of bacteria was apparent, suggesting the possibility of sour wells after fracturing treatment. 
       Effect of Hypochlorous Acid on Bacterial Population 
       [0054]      FIG. 4  shows the hypochlorous acid is very effective in killing all bacteria in the Piceance River water. After 7 days, the bacterial counts were blow detectable limits and no regrowth was apparent. 5 (v/v) hypochlorous acid solution (500-1000 ppm active) was added to the produced water sample containing 0.25 gal/Kgal friction reducer to evaluate the effect of hypochlorous acid activity on bacterial activity for seven days. Within five minutes, the bacterial population was significantly reduced from 10 6  cells/mL to 10 1  cells/mL. After 24 hours, the SRB population was not detectable and regrowth was not apparent after seven days. 
         [0000]    Compatibility with Slickwater Additives and Piceance River Water 
         [0055]    Visual tests were performed to illustrate that there were no incompatibilities between viscosity modifying additives and hypochlorous acid solution (500-1000 ppm active). Also, bottle tests were performed. Bottle tests (deionized water and produced water) were performed with deionized water and produced water, separately. 5% (v/v) hypochlorous acid solution was added to a series of individual bottles with slickwater additives, including clay stabilizer, scale inhibitor, friction reducer and a microemulsion. The compatibility of hypochlorous acid solution and the slickwater additives were observed at time 0 and 5 minutes. No incompatibilities were observed between the slickwater additives and hypochlorous acid solution in deionized water. Before adding hypochlorous acid solution to the produced water, there was a strong rotten egg odor in the water sample indicating the presence of SRB. After the five minute treatment of hypochlorous acid solution, a color change was observed and the rotten egg odor was eliminated. Additionally, the pH remained stable for all fluids tested.  FIG. 5  shows addition of hypochlorous acid to the produced water eliminated rotten odor and color changed to a lighter shade. The pH remained stable after the hypochlorous acid treatment. Apparently, the hypochlorous acid is very effective in improving the quality of produced water by oxidizing the contaminants. 
       Effects of Hypochlorous Acid on Friction Reducer 
       [0056]    A friction loop consisting of a ½″ and a ¾″ pipe was used for drag reduction measurements. Synthetic water was prepared based on the water analysis of the Piceance Basin produced water sample. Fifteen liters of the source water, along with the slickwater additives and hypochlorous acid solution, were stirred using an overhead stirrer at 1000 rpm for two minutes before being added to the friction loop for evaluation. Before analysis, the differential pressure gauges were purged and the pump was primed prior to recording the data for the test. The test fluid was then pumped for about 10 seconds at incremental intervals of about 6 Kg/min and the percent drag reduction was calculated. The figure shows the friction loop results of the slickwater additives and hypochlorous acid measuring the percent drag reduction as a function of flow rate (Kg/min). Varying the viscosity modifying additives with and without hypochlorous acid shows no incompatibilities as illustrated by  FIG. 6 .  FIG. 6  shows the friction loop results of slickwater additives and hypochlorous acid. Data are plotted as the percent drag reduction as a function of flow rate (Kg/min). Hypochlorous acid had no effect on the slickwater additives. This shows that the viscosity difference due to the presence of the hypochlorous acid in about 2 percent or less. 
         [0000]    Hypochlorous Acid in Combination with Common Fracturing Fluids 
         [0057]    The compatibility of hypochlorous acid was evaluated with common fracturing fluids currently used in field operations. The hypochlorous acid solution was used at concentrations of 0 gal/Kgal, 10 gal/Kgal, and 50 gal/Kgal. The fluid compositions are listed in Tables 2-5. Fluids were tested at 150 deg F. for a period of one hour. The mixing procedure for the fracturing fluids is as follows: 500 mL of deionized water was placed into a Waring blending cup; subsequently, the hypochlorous acid solution was added and allowed to mix for 20 seconds. The gelling agent was then added and allowed to mix for 10 minutes, after which the linear gel viscosity was checked and compared to the hydration chart (see below). The remaining additives were then added to the solution and the vortex was allowed to close (after the addition of the crosslinker). Rheology profiles of the four fluids may be found in  FIGS. 7 to 10  which illustrate the experimental results generated using the fluids of tables 2-5. The fluids did not result in a significant loss in viscosity when the hypochlorous acid solution concentration was increased from 0 gal/Kgal to 50 gal/Kgal. Additionally, the fluid is still viable and capable of transporting proppant. 
         [0058]    Common Fracturing fluids that may be utilized with hypochlorous acid are listed in the following tables. 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Fluid formulation 1 
               
             
          
           
               
                   
                 Additive 
                 Concentration 
               
               
                   
                   
               
             
          
           
               
                   
                 Tetramethyl 
                 2 
                 gpt 
               
               
                   
                 ammonium chloride 
               
               
                   
                 (TMAC) 
               
               
                   
                 Slurried guar 
                 6.25 
                 gpt 
               
               
                   
                 Borate crosslinker 
                 3 
                 gpt 
               
               
                   
                 Hypochlorous acid 
                 0, 10, 50 
                 gal/Kgal 
               
               
                   
                 solution (500-1000 ppm 
               
               
                   
                 active) 
               
               
                   
                   
               
             
          
         
       
     
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Fluid Formulation 2 
               
             
          
           
               
                   
                 Additive 
                 Concentration 
               
               
                   
                   
               
             
          
           
               
                   
                 Tetramethyl 
                 2 
                 gpt 
               
               
                   
                 ammonium chloride 
               
               
                   
                 (TMAC) 
               
               
                   
                 Slurried guar 
                 6.25 
                 gpt 
               
               
                   
                 Boric acid 
                 5.5 
                 ppt 
               
               
                   
                 Sodium Hydroxide 
                 10 
                 ppt 
               
               
                   
                 d-Sorbitol 
                 2 
                 gpt 
               
               
                   
                 Hypochlorous acid 
                 0, 10, 50 
                 gal/Kgal 
               
               
                   
                 solution (500-1000 ppm 
               
               
                   
                 active) 
               
               
                   
                   
               
             
          
         
       
     
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 Fluid Formulation 3 
               
             
          
           
               
                   
                 Additive 
                 Concentration 
               
               
                   
                   
               
             
          
           
               
                   
                 Tetramethyl 
                 2 
                 gpt 
               
               
                   
                 ammonium 
               
               
                   
                 chloride 
               
               
                   
                 (TMAC) 
               
               
                   
                 Slurried guar 
                 6.25 
                 gpt 
               
               
                   
                 Sodium Borate 
                 1.3 
                 gpt 
               
               
                   
                 30% Sodium 
                 0.5 
                 gpt 
               
               
                   
                 Hydroxide 
               
               
                   
                 Hypochlorous 
                 0, 10, 50 
                 gal/Kgal 
               
               
                   
                 acid solution 
               
               
                   
                 (500-1000 ppm 
               
               
                   
                 active) 
               
               
                   
                   
               
             
          
         
       
     
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 5 
               
             
             
               
                   
               
               
                 Fluid Formulation 4 
               
             
          
           
               
                   
                 Additive 
                 Concentration 
               
               
                   
                   
               
             
          
           
               
                   
                 Polyvinyl acetate/ 
                 6.7 
                 gpt 
               
               
                   
                 polyvinyl alcohol 
               
               
                   
                 copolymer 
               
               
                   
                 Erucic amidopropyl 
                 40 
                 gpt 
               
               
                   
                 dimethyl betaine 
               
               
                   
                 Hypochlorous acid 
                 0, 10, 50 
                 gal/Kgal 
               
               
                   
                 solution (500-1000 ppm 
               
               
                   
                 active) 
               
               
                   
                   
               
             
          
         
       
     
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 6 
               
             
             
               
                   
               
               
                 Linear Gel Viscosities at Increased Biocide Concentrations 
               
               
                 Biocide 
               
             
          
           
               
                 Concentration 
                 Temperature 
                   
               
               
                 (gpt) 
                 (F.) 
                 Viscosity (511, sec −1 ) 
               
               
                   
               
             
          
           
               
                 0 
                 71.2 
                 19 
               
               
                 10 
                 74.1 
                 18.5 
               
               
                 50 
                 68.7 
                 18 
               
               
                   
               
             
          
         
       
     
       Stabilization of Hypochlorous Acid 
       [0059]    Using 100 mL of 3% (v/v) bleach, 29 mL of 5% (v/v) acetic acid was added to obtain a pH of 6.5 from an initial pH value of 8.48. The FAC residual was greater than 1000 ppm. Additionally, in a separate experiment, 22 mL of 1M sodium citrate was added to the bleach solution to obtain a pH of 6.5. The FAC value was then found to be 24 ppm. More details of this portion of the experimental data are presented below in paragraph 0061. 
         [0060]    Bottle tests were used to evaluate the stabilization of hypochlorous acid with the following chemicals: dichloroisocyanuric acid (DCCA) and cyanuric acid (CA). Cyanuric acid is known to stabilize the rate of decomposition of hypochlorous acid in ultraviolet conditions. Over a period of four days, a set of bottles with the following components were left open: 1) hypochlorous acid solution (500-1000 ppm active), 2) hypochlorous acid solution +30 ppm CA, 3) hypochlorous acid solution +50 ppm CA, 4) hypochlorous acid solution +30 ppm DCCA, and 5) hypochlorous acid solution +50 ppm DCCA. At the time of preparation, the initial pH and FAC were taken and recorded (see table below). The test points were then taken again after 1 day and four days. For all solutions prepared, the pH was stable (within 5% of the starting hypochlorous acid solution) after the addition of DCCA and CA. Additionally, the FAC residual value for all solutions decreased by 5%, with the DCCA-containing solutions obtaining a consistently higher FAC residual than hypochlorous acid solution alone. 
         [0000]                                                                                                            Time = 0   Time = 1 day   Time = 4 days                    FAC       FAC       FAC           pH   (ppm)   pH   (ppm)   pH   (ppm)                        Hypochlorous acid   6.80   726   6.86   692   7.18   628       solution       Hypochlorous acid   6.51   660   6.76   646   7.13   587       solution + 30 ppm       CA       Hypochlorous acid   6.47   670   6.77   637   7.16   580       solution + 50 ppm       CA       Hypochlorous acid   6.62   739   6.75   705   7.13   639       solution + 30 ppm       DCCA       Hypochlorous acid   6.79   739   6.99   705   7.20   639       solution + 50 ppm       DCCA                    
Hypochlorous Acid Solution Made from Sodium Hypochorite.
 
         [0061]    A tank is filled with 400 gallons of city water. To this is added 20 gallons of 12% sodium hypochlorite solution. This result in a 0.6% solution of sodium hypochorite. To this is added an excess of citric acid until the pH of the resulting solution reaches pH equal to 6.5. This stock solution is then added on the fly to the fracturing treatment. The concentration of the stock solution added to the fracturing fluid was 0.2 to 0.6 gallons per thousand gallons. Using 100 mL of 1% (v/v) sodium hypochlorite (10000 ppm), 12.8 mL of 5% (v/v) acetic acid was added to obtain a pH value of 7.0 from an initial pH of 9.7. The active concentration (FAC residual) of the resultant solution was then found to be 8500 ppm. After one hour, the active concentration remained the same. In 24 hours, the active concentration decreased by 3.5% to 8210 ppm. Similarly, 55.2 mL of 0.1M succinic acid solution was added to 100 mL 1% (v/v) sodium hypochlorite to obtain a pH value of 7.0. The active concentration was found to be 6040 ppm after titration. 
         [0062]    A fracturing treatment using hypochlorous acid lasted two days. Four stages, at 2 hours per stage, were pumped using a total of 1.86 million gallons of water. 1.6M pounds of proppant were used. In total, 19 k gallons of hypochlorous acid solution (500-1000 ppm active) was pumped. The concentration of hypochlorous acid solution that was required (10 gpt) also required bulk storage and high rate additive pumps. A 12,000 gallon fluid module (modified frac tank) was placed next to the water frac tanks. An additive skid with 2 large Waukesha pumps, capable of 45 gpm, added hypochlorous acid at a rate of 42 gpm. Hypochlorous acid solution was pumped from the bulk module tank and into the 250 bbl batch mixing tank. 
         [0063]    In another field test, 26 gpt hypochlorous acid solution was added with 1 gpt slickwater fluid and mixed for less 1 min at 80 bbls/min to a form a fluid. To be precise, the pH of the fluid was 6. Thus, the 26 gpt hypochlorous acid was 2.5 percent active hypochlorous acid and 0.075 percent hypochlorite ion.  FIG. 12  is a chart illustrating bacterial population as a function of types of bacteria in a field trial comparing the microbe content of fresh water, produced water, mix water, mix water and hypochlorous acid, and flowback water and acid after 21 days. That is,  FIG. 12  illustrates the microbe population demise over time. All data points were taken on location. Initial flowback data was taken 21 days after job completion. The final flowback data point was taken 51 days after job completion. 
         [0000]    Sodium Diacetate in Combination with Sodium Hypochlorite 
         [0064]    Several tests were performed to illustrate how sodium diacetate works as a buffer for maintaining the pH and thus the integrity of the sodium hypochlorite. Titrations were performed using Eppendorf-style micropipettors to dispense sodium diacetate into 100 ml fluid samples contained in glass jars. The mixture was continuously stirred at medium shear by a 15 mm Teflon stir bar actuated by an Ika stir plate. The pH was measured using a Fisher Scientific XL-15 pH meter that was calibrated freshly prior to the beginning of each titration. FAC was measured using the Hach spectrophotometer, which colorimetrically evaluates [OCl]. A friction loop consisting of a ½″ and a ¾″ pipe was used for drag reduction (DR) measurements. The pressure difference (denoted as ΔP) across the pipes, as well as the mass flow and the temperature were recorded or each fluid analyzed. Initially, the friction loop was calibrated with local tap water prior to the fluid testing and all tests were run at room temperature. Fluid was prepared by adding 0.25 gpt friction reducer to treated water and stirring for 2 minutes at 100 rpm using an overhead mixer. After the prepared fluid was added to the friction loop hopper, the differential pressure gauges were purged and the pump was primed prior to recording the data for the test. The test fluid was pumped for about 10 seconds at incremental intervals of about 6 Kg/min and the percent drag reduction (% DR) is calculated using the following equation (Eq 3): 
         [0000]    
       
         
           
             
               
                 
                   
                     % 
                      
                     
                         
                     
                      
                     DR 
                   
                   = 
                   
                     
                       
                         
                           Δ 
                            
                           
                               
                           
                            
                           Pwater 
                         
                         - 
                         
                           Δ 
                            
                           
                               
                           
                            
                           Pfluid 
                         
                       
                       
                         Δ 
                          
                         
                             
                         
                          
                         Pwater 
                       
                     
                     · 
                     100. 
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
         [0065]    Each fluid had its friction pressure measured at time 0, at time=15 minutes, and at time=30 minutes to gauge the effect of the formation cleaning fluid and buffer on the acrylamide friction reducer. A control experiment without buffer and concentrated industrial sodium hypochlorite was run in a similar manner to evaluate the decrease in friction pressure inherent in running the fluid through the loop repeatedly. 
         [0066]    The first round of titrations were performed using various concentrations of concentrated industrial sodium hypochlorite in Sugar Land tap water to ensure the performance of sodium diacetate buffer was as expected in the presence of sodium hypochlorite. Several concentrations of sodium hypochlorite and other readily available reagents were tested. The results are summarized in  FIG. 14 .  FIG. 14  shows titration curves for addition of sodium diacetate buffer to various solutions of concentrated industrial sodium hypochlorite in tap water. 
         [0067]    In the simplest of these experiments, tap water with an initial pH of 7.82 has its pH changed to 5.43 by the addition of 0.5 gpt buffer. Addition of a further 0.5 gpt or even another 10 gpt preserve apH.of just under 5.4. There is a stir-rate dependence in the experiment—at low shear, the pH reported by the probe is not quickly representative of the bulk solution because the probe is immersed in ˜2.5″ of a 100 ml solution. With increased stirring, this feature went away. The same “high shear” stir rate was used in all the other experiments. Several water samples containing a level of concentrated industrial sodium hypochlorite appropriate to water cleaning were tested in the same manner, and all converge on pH of between 5.4 and 7 with minimal addition of buffer. Note that all the titrations trend out to final pH values between 5 and 7, even when as much as 12.5 gpt buffer is added. 
         [0068]      FIG. 15  shows titrations of some produced water samples treated with concentrated industrial sodium hypochlorite (0.21 gpt) and one sample of tap water that was pre-acidified using citric acid prior to treatment with concentrated industrial sodium hypochlorite.  FIG. 15  illustrates titration of various acidic and produced waters with sodium diacetate buffer. Even acidic produced waters can have pH correction up into a more benign range using sodium diacetate buffer. 
         [0069]    The objective of friction pressure measurements was to verify which combinations of friction reducer, formation cleaning agent (concentrated industrial sodium hypochlorite), and buffer have little or no effect on the friction reducing capacity. In order to establish this, a control experiment was performed to quantify the effect of recirculation in the loop on the acrylamide polymer in the friction reducer.  FIG. 16  shows drag reduction in a 0.5″ pipe using 0.25 gpt friction reducer, versus water. 
         [0070]    At high rate, the friction reducer reduces friction by about 65%. Test duration is about 3 minutes. The test was repeated after the fluid was simply left to sit in the loop under static conditions, giving the lower (30 min) trace, which shows ˜61% friction reduction. 
         [0071]    This experiment was then performed using a fresh sample with 0.25 gpt friction reducer, 0.21 gpt concentrated industrial sodium hypochlorite, and 0.5 gpt sodium diacetate. The friction reduction curves at 0, 15, and 30 minutes are shown in  FIG. 17 . It is notable that 30 minutes and two cycles of testing cause roughly the same depletion of friction reducing power when sodium diacetate and sodium hypochlorite are present as was observed for the friction reducer itself. One interpretation is that chemical activity of the sodium diacetate and sodium hypochlorite on the friction reducer is negligible as compared to the shear imposed by the test. When extrapolating from a lab scale to a field situation, the shear rates may be considerably higher but the chemistry should be the same. 
         [0072]    From this data, it may be concluded that sodium diacetate buffer can correct the pH of a hypochlorite solution in produced water from a high pH to below 5.5. Sodium diacetate buffer can correct the pH of a more strongly acidic hypochlorous acid solution in produced water from a pH near 3.0 to a pH of almost 5.0. Sodium diacetate buffer does not have an adverse effect on the stability of concentrated sodium diacetate solutions at concentrations relevant to slickwater fracturing. In fact, sodium diacetate buffer adjusts pH of alkaline fluids into a range where the active water cleaning chemical in concentrated industrial sodium hypochlorite is more stable than it would be if the fluid were nearer to neutral pH. Sodium diacetate buffer and concentrated sodium hypochlorite together do not have a measurable effect on the friction reducing ability of friction reducer as measured in a friction loop test. 
         [0073]    The particular embodiments disclosed above are illustrative only, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the invention. Accordingly, the protection sought herein is as set forth in the claims below.