WO2015057664A1

WO2015057664A1 - Composition and method including mixed oxidants for treating liquids injected into or received from subterranean formations

Info

Publication number: WO2015057664A1
Application number: PCT/US2014/060423
Authority: WO
Inventors: Jason E. PETERS; Dane H. MADSEN
Original assignee: Blue Earth Labs Llc
Priority date: 2013-10-14
Filing date: 2014-10-14
Publication date: 2015-04-23

Abstract

A liquid biocide includes a blend of (i) a stabilized mixed oxidant solution component that may be produced by flowing a starting solution (e.g., salt brine, hypochlorous acid, and/or sodium hypochlorite) through a flow-through electrochemical module including first and second passages separated by an ion permeable membrane while electric power is applied between an anode and cathode in electrical communication with the first and second passages, respectively, followed by pH elevation, and (ii) a disinfectant solution portion including at least one source of chlorine ions (e.g., sodium hypochlorite solution), suitable for treating hydraulic fracturing fluid, flowback fluid, and produced water. Methods of using the biocide solution are further provided.

Description

COMPOSITION AND METHOD INCLUDING MIXED OXIDANTS FOR TREATING LIQUIDS INJECTED INTO OR RECEIVED FROM SUBTERRANEAN FORMATIONS STATEMENT OF RELATED APPLICATION(S)

[0001] This application claims benefit of U.S. Provisional Patent Application No. 61/890,865 filed on October 14, 2013, which is hereby incorporated by reference herein.

TECHNICAL FIELD

[0002] The present application relates generally to the field of chemicals including biocides for treating liquids injected into or received from subterranean formations such as oil fields, gas fields, and mining deposits, with certain embodiments having applicability to liquids used in or produced by hydraulic fracturing.

BACKGROUND

[0003] To enhance or increase the production of oil and gas hydrocarbons from wells bored into subterranean formations, significant amounts of water with viscous fluids are forced through pumps at high pressure into well bores to crack subterranean formations.

Fracturing fluid is used to carry sand, ceramics, or other particles, called "proppants," to hold the cracks open after the fluid pressure is reduced. The cracks held open by proppants (also known as propped fractures) provide additional paths for the hydrocarbons such as oil or natural gas (including shale gas, tight gas, and coal seam gas), to reach the wellbore, thereby increasing the production of oil and/or natural gas from the well.

[0004] Hydraulic fracturing (also known as "tracing") injection fluid contains water- soluble gelling agents (such as guar gum) that increase viscosity and efficiently deliver the proppant into the formation. Fracing fluid may vary in composition depending on the type of fracturing proppant used, the conditions of the specific well being fractured, and the fracturing water characteristics. In addition to gelling agents, commonly used chemical additives include: acids (such as hydrochloric acid or acetic acid - commonly used in the pre-fracturing stage for cleaning perforations and initiating fissures in near- wellbore rock); sodium chloride (which delays breakdown of gel polymer chains); polyacrylamide and other friction reducers (which minimize friction between fluid and pipes, thereby reducing load and power consumption of injection pumps, and also promote maintenance of proppant in suspension); ethylene glycol (which prevents formation of scale deposits in pipes); borate salts (which are used to maintain fluid viscosity at increased temperatures); sodium and potassium carbonates (used to maintain effectiveness of crosslinkers); citric acid (used for corrosion prevention); isopropanol (which increases viscosity of the tracing fluid); and biocides such as glutaraldehyde and quaternary ammonia (among others).

[0005] Fracing typically utilizes multiple (e.g., 10 or more) stages or "fracs", and each stage requires process water that may range up to 1 -3 million gallons for a typical natural gas well. After fracing, roughly 35% of the water returns to the surface as "flowback" in the first few weeks. Additional liquid known as "produced water" - which embodies a mix of fracing fluid and groundwater - comes up to the surface with the hydrocarbons (e.g., oil or gas) for most of the life of the well. Cleaning and recycling flowback and produced water can be challenging because types and levels of contamination may vary from well to well, and from basin to basin. In addition to presence of fracing fluid constituents, common pollutants in flowback and produced water include salts, bacteria, dissolved solids, and suspended solids. Contaminated water is commonly recovered, trucked off- site for processing, and subsequently pumped into injection wells (or evaporated in some instances).

[0006] A significant fraction (e.g., with estimates ranging from 30% to 70%) of spent fracing fluid is not recovered and stays in the ground. Although most fracing fluid is injected to a depth of at least one thousand to several thousand feet, and drinking water aquifers typically reside at a depth of less than 1000 feet, potential remains for fracing fluid to seep over time through natural fractures into drinking water aquifers.

[0007] Bacteria are a highly important issue in fracing because source water is typically obtained from surface sources such as ponds or rivers. Surface water routinely contains large populations of microorganisms, such as sulfate-reducing bacteria and acid-producing bacteria. Presence of bacteria in water used for fracing introduces risks into the fracing process. Fracing can open new parts of a reservoir for hydrocarbon production, but the process simultaneously injects intro fractured zones microbes that can become established and cause serious problems - including formation damage, generation of biogenic hydrogen sulfide, microbiologically influenced corrosion, and low- quality flowback water.

[0008] To inhibit growth of microbes, hydraulic fracturing fluids typically contain a biocide. Desirable biocides should control various organisms including sulfate reducing bacteria, slime producing bacteria, and acid producing bacteria. The growth of these bacteria can hinder or even block the flow of oil or natural gas through channels formed by hydraulic fracturing, and the presence of microbes with salts or other solids can potentially form a variable matrix of organic and inorganic deposits that may be difficult to remove. Presence of microbes can also cause the produced natural gas to become "sour" (i.e., more acidic), thereby requiring the resulting gas to undergo further treatment before being suitable for use in commerce.

[0009] Biocides used in hydraulic fracturing should not only inhibit the growth of microbes, but also have several other properties. Biocides selected for tracing treatments must be robust enough to eliminate a broad spectrum of bacteria that might be present in the source water. Biocides must also be compatible with high concentrations of salts (e.g., potassium, calcium, barium, magnesium, and sodium salts) and high temperatures (e.g., 85°C or more) that are experienced in subterranean environments. At the same time, a biocide must be compatible with stimulation fluids and must fit the economic constraints of the specific application.

[0010] Unfortunately, conventional biocides have various shortcomings as applied to tracing fluids. Some are toxic to the environment and/or to personnel handling them. Others do not work well in the presence of high salt concentrations or at higher temperatures. Still others are corrosive to equipment or interfere with other chemistries (e.g., crosslinkers or breakers) used in oil and gas production. Still others have very limited use life.

[0011] Glutaraldehyde, which is known for its fixative/crosslinking properties, is a known toxin that presents environmental concerns, and can have a deleterious effect on the fluid viscosity. Quaternary ammonium compounds are cationic surfactants with toxic effects by all routes of exposure, are corrosive in concentrated solutions, can be inactivated by anionic compounds, and have corrosive effects on metals (e.g., brass and copper) and vinyl. Sodium hypochlorite (also known as liquid bleach) is a strong oxidizing agent, is corrosive, and is reactive with other chemical agents (e.g., reactive with ammonia to produce chloramines, reactive with acids to produce chlorine gas, reactive with organic matter to produce various disinfectant by-products, etc.). Electrochemically activated solutions (e.g., produced by equipment of MIOX Corp., Albuquerque, NM) are generated on site (i.e., by electrolyzing a brine solution produced from water and salt), have a short shelf life, are strong oxidizing agents, and exhibit reduced efficacy with organic loads. Chlorine dioxide must be generated on site due to its limited shelf life and its explosiveness at concentrations of 15% or higher, is a strong oxidizing agent, and shows reduced efficacy with organic loads.

[0012] Production of biocides with limited shelf life (such as electrochemically activated solutions and chlorine dioxide) using portable machines at a hydraulic fracturing jobsite can present significant challenges, including the need to provide starting materials (e.g., acids, chlorides, and highly purified water), the need to maintain biocide production personnel on-site, and the need to dispose of waste (including unused biocide with short shelf life and/or intermediate products in situations when hydraulic fracturing is interrupted). Additionally, if biocide production machines should be rendered inoperable (e.g., due to clogging or equipment failure), then interruption in supply of biocide can result in costly interruptions in hydraulic fracturing.

[0013] It would be desirable to provide methods and additives for treating (e.g., disinfecting) liquids injected into or received from subterranean formations that overcome limitations associated with conventional additives. Various compositions and methods disclosed herein address limitations associated with conventional compositions and methods. SUMMARY

[0014] Various aspects of the disclosure relate to production and use of biocides that include, in combination, a disinfectant component and a mixed oxidant solution component, with the resulting biocides being useful for treating liquids injected into or received from subterranean formations such as oil fields, gas fields, and mining deposits.

[0015] In one aspect, the disclosure relates to a method for performing hydraulic fracturing, the method comprising: forming a hydraulic fracturing fluid comprising a biocide solution, and injecting the hydraulic fracturing fluid into a subterranean formation, wherein the biocide solution is produced by a process including the steps of: flowing at least one starting solution that comprises at least one of salt brine, hypochlorous acid, and sodium hypochlorite through at least one flow-through electrochemical module comprising a first passage and a second passage separated by an ion permeable membrane while electric power is applied between (i) an anode in electrical communication with the first passage and (ii) a cathode in electrical communication with the second passage, wherein a first solution or first portion of the at least one starting solution is flowed through the first passage to form an anolyte solution having an acidic pH, and a second solution or second portion of the at least one starting solution is simultaneously flowed through the second passage to form a catholyte solution having a basic pH; contacting the anolyte solution with a hydroxide solution to attain a pH value of at least about 9.0 to yield a mixed oxidant solution comprising a plurality of different oxidants; and combining the mixed oxidant solution with a liquid disinfectant solution to yield the biocide solution.

[0016] In another aspect, the disclosure relates to a method for treating flowback fluid or produced water received from a subterranean formation, the method comprising contacting the flowback fluid or produced water with a biocide solution, wherein the biocide solution is produced by a process including the steps of: flowing at least one starting solution that comprises at least one of salt brine, hypochlorous acid, and sodium hypochlorite through at least one flow-through electrochemical module comprising a first passage and a second passage separated by an ion permeable membrane while electric power is applied between (i) an anode in electrical communication with the first passage and (ii) a cathode in electrical communication with the second passage, wherein a first solution or first portion of the at least one starting solution is flowed through the first passage to form an anolyte solution having an acidic pH, and a second solution or second portion of the at least one starting solution is simultaneously flowed through the second passage to form a catholyte solution having a basic pH; contacting the anolyte solution with a hydroxide solution to attain a pH value of at least about 9.0 to yield a mixed oxidant solution comprising a plurality of different oxidants; and combining the mixed oxidant solution with a liquid disinfectant solution to yield the biocide solution.

[0017] In another aspect, the disclosure relates to a method for producing hydraulic fracturing fluid, the method comprising adding a biocide solution to source water to yield treated source water, and adding at least one of the following components to the treated source water: a water soluble gelling agent; a proppant; a friction reducing component, and a viscosity enhancing component; wherein the biocide solution is produced by a process including the steps of: flowing at least one starting solution that comprises at least one of salt brine, hypochlorous acid, and sodium hypochlorite through at least one flow-through electrochemical module comprising a first passage and a second passage separated by an ion permeable membrane while electric power is applied between (i) an anode in electrical communication with the first passage and (ii) a cathode in electrical communication with the second passage, wherein a first solution or first portion of the at least one starting solution is flowed through the first passage to form an anolyte solution having an acidic pH, and a second solution or second portion of the at least one starting solution is simultaneously flowed through the second passage to form a catholyte solution having a basic pH; contacting the anolyte solution with a hydroxide solution to attain a pH value of at least about 9.0 to yield a mixed oxidant solution comprising a plurality of different oxidants; and combining the mixed oxidant solution with a liquid disinfectant solution to yield the biocide solution. In certain embodiments, one, two, three, or more of the following items may be added to the treated source water: a water soluble gelling agent; a proppant; a friction reducing component; and a viscosity enhancing component. In certain embodiments, one, two, three, or more of the following additional items may be added to the treated source water: an acid; a chloride salt; a carbonate salt; and a borate salt. Certain embodiments are directed to a hydraulic fracturing fluid produced according to a method disclosed herein.

[0018] In certain embodiments, a biocide solution as referenced herein may comprise a pH of at least about 13. In certain embodiments, liquid disinfectant solution as referenced herein may comprise a source of chlorine ions (e.g. , including but not limited to hypochlorous acid and sodium hypochlorite). In certain embodiments, liquid disinfectant solution as referenced herein may comprise a sodium hypochlorite solution; in certain embodiments, sodium hypochlorite may be present in the biocide solution in a range of from about 4 wt.% to about 7 wt.%. In certain embodiments, a biocide solution may comprise from 30 to 90 percent by volume mixed oxidant solution and comprise from 10 to 70 percent by volume sodium hypochlorite solution, or the biocide solution may comprise from 50 to 70 percent by volume mixed oxidant solution and comprise from 30 to 50 percent by volume sodium hypochlorite solution. In certain embodiments, at least one starting solution may comprise at least one of hypochlorous acid and sodium hypochlorite. In certain embodiments, contacting the anolyte solution with a hydroxide solution attains a pH value of at least about 10.0, at least about 10.5, at least about 1 1 .0, at least about 1 1 .5, at least about 12.0, or at least about 12.5.

[0019] In certain embodiments, characteristics of the at least one starting solution, flow rate of the at least one starting solution, materials of construction of the at least one flow-through electrochemical module, dimensions of the at least one flow-through electrochemical module, number of the at least one flow-through electrochemical module, conformation of the at least one flow-through electrochemical module, and field density of the applied electric power are selected to yield a mixed oxidant solution comprising a plurality of different oxidants that comprise at least one, at least two, or all of the following characteristics (i) to (iii): (i) a total chlorine value of at least about 3,000 ppm ; (ii) an oxidation-reduction potential (ORP) value in a range of from 600 mV to 800 mV; and (iii) a ratio of Na+ (in g/L according to Method EPA 300.0) to CI- (in g/L according to Method EPA 6010) of at least about 1 .5.

[0020] In another aspect, the disclosure relates to a hydraulic fracturing fluid comprising a biocide solution as disclosed herein.

[0021 ] In another aspect, any of the foregoing aspects, and/or various separate aspects and features as described herein, may be combined for additional advantage. Any of the various features and elements as disclosed herein may be combined with one or more other disclosed features and elements unless indicated to the contrary herein.

[0022] Other aspects, features, and embodiments of the disclosure will be more fully apparent from the ensuing description and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] FIG. 1 is a flow chart showing various stages involved in making a biocide including a mixed oxidant solution component and a disinfectant solution component, according an embodiment of the present disclosure. [0024] FIG. 2 is a simplified schematic cross-sectional view of a flow-through electrochemical module including flow chambers separated by an ion-permeable membrane and arranged to produce separate anolyte and catholyte streams by electrolysis of a salt brine solution, for producing a mixed oxidant solution of a biocide according to the present disclosure.

[0025] FIG. 3 is a schematic diagram showing arrangement of a production system for mixed oxidant solution (as a component of a biocide according to the present disclosure) including flow-through electrochemical modules and associated components.

[0026] FIG. 4 is a cross-sectional view of an exemplary flow-through electrochemical module such as may be used in the system of FIG. 3.

[0027] FIG. 5 is a line chart depicting total chlorine (ppm) versus time (days) for mixed oxidant solutions produced using the system of FIG. 3.

[0028] FIG. 6 is a table summarizing characteristics including total chlorine, pH, oxidation-reduction potential, conductivity, sodium ion concentration, chloride ion concentration, and sodium/chloride ion ratio for the following five products: (1 ) 12.5% hypochlorite bleach, (2) 6% hypochlorite bleach, (3) Clearitas® mixed oxidant solution, (4) Miox™ mixed oxidant solution, and (5) a stabilized mixed oxidant solution useful as a component of a biocide according to the present disclosure.

[0029] FIG. 7 is a schematic diagram showing components of a subsystem arranged to receive a stream of stabilized mixed oxidant solution component from the mixed oxidant solution production system of FIG. 3, and blend the mixed oxidant solution component with a disinfectant component (e.g., a chlorine ion source such as a sodium hypochlorite solution) to yield a biocide according to an embodiment of the present disclosure.

[0030] FIG. 8 is a schematic diagram showing components of at least a portion of a water treatment system arranged to receive a biocide solution according to the present disclosure.

[0031] FIG. 9 is a schematic diagram of various components of a hydraulic fracturing system, including a hydraulic fracturing fluid production/treatment subsystem, a flowback fluid treatment subsystem, a produced water treatment subsystem, and a hydrocarbon processing subsystem.

[0032] FIG. 10A is a line chart of luminometer relative light units (for detection of adenosine triphosphate (ATP)) versus concentration for five different doses of a first conventional disinfectant (including 25% gluteraldehyde and 12% quaternary ammonia) in 100 ml of dairy waste. [0033] FIG. 10B is a line chart of luminometer relative light units (for detection of ATP) versus concentration for five different doses of a second conventional disinfectant (including 12% gluteraldehyde and 3% quaternary ammonia) in 100 ml of dairy waste.

[0034] FIG. 10C is a line chart of luminometer relative light units (for detection of ATP) versus concentration for five different doses of a third conventional disinfectant (including 25% gluteraldehyde) in 100 ml of dairy waste.

[0035] FIG. 10D is a line chart of luminometer relative light units (for detection of ATP) versus concentration for five different doses of a fourth conventional disinfectant (5% chlorine dioxide produced from a mixture of Petrofid chlorite and hydrochloric acid in a 5:1 ratio) in 100 ml of dairy waste.

[0036] FIG. 10E is a line chart of luminometer relative light units (for detection of ATP) versus concentration for five different doses of a fifth conventional disinfectant (5.25% sodium hypochlorite (a/k/a liquid bleach)) in 100 ml of dairy waste.

[0037] FIG. 10F is a line chart of luminometer relative light units (for detection of ATP) versus concentration for five different doses of a biocide according to one embodiment of the present disclosure (including 60% mixed oxidant solution blended with 40% hypochlorite solution to yield 5.25% sodium hypochlorite) in 100 ml of dairy waste.

[0038] FIG. 10G is a table embodying results of Free ATP and Total ATP measurements on 1 gallon per thousand treatments of 100 ml dairy waste after 22 hours of exposure to the disinfectants described in FIGS. 10A-10F.

[0039] FIG. 1 1 is a line chart providing results of guar viscosity testing showing effects on viscosity (in centipoise) at elevated temperatures for two different concentrations of biocides according to embodiments of the present disclosure and one conventional disinfectant (including 25% gluteraldehyde and 12% quaternary ammonia) as combined with a 30 ppt guar gel crosslinked with a 1 .5 gpt delayed crosslinker, in comparison to a control solution including gel and crosslinker in the absence of biocide.

DETAILED DESCRIPTION [0040] Described herein are methods for making and using biocides including a mixed oxidant solution component and a disinfectant solution component, that are particularly useful for treating liquids injected into or received from subterranean formations such as oil fields, gas fields, and mining deposits.

[0041 ] Biocides as disclosed herein preferably include solutions in liquid form that are amenable to being produced at a centralized location, packaged in sealed containers (e.g., drums, tankers, totes, or the like) suitable for shipment to a point of use, and transported to a point of use for treating liquids, with sufficient stability and shelf life to permit the biocides to be stored (if necessary) at the point of use and to eliminate need for on-site generation of mixed oxidant chemistries.

[0042] In certain embodiments, the liquid disinfectant solution component (of a biocide solution) comprises a source of chlorine ions - such as, but not limited to, a sodium hypochlorite solution.

[0043] In certain embodiments, a biocide solution comprises from 30% to 90% by volume mixed oxidant solution and comprises from 10% to 70% by volume of a disinfectant solution. In certain embodiments, a biocide solution comprises from 50% to 70% by volume mixed oxidant solution and comprises from 30% to 50% by volume of a disinfectant solution. In certain embodiments, a biocide solution comprises from 55% to 65% by volume mixed oxidant solution and comprises from 35% to 45% by volume of a disinfectant solution.

[0044] In certain embodiments, a biocide solution comprises a pH of preferably at least about 12, more preferably at least about 12.5, still more preferably at least about 13, and still more preferably at least about 13.5.

[0045] A preferred mixed oxidant solution component exhibits enhanced effectiveness and enhanced stability compared to prior mixed oxidant solutions, including but not limited to RE-Ox® chemical solution described in U.S. Patent No. 8,366,939 and Clearitas® mixed oxidant solution commercialized by Blue Earth Labs, LLC (Las Vegas, Nevada, US) that are formed by electrolyzing a brine solution in a flow-through cathode chamber followed by electrolysis of the catholyte solution in a flow-through anode chamber. In contrast to prior mixed oxidant solutions, preferred mixed oxidant solution components described herein beneficially contain anolyte solution produced by flowing at least one starting solution (i.e., comprising at least one of salt brine, hypochlorous acid, and sodium hypochlorite) through an anode chamber without prior or subsequent transmission through a cathode chamber, wherein the resulting anolyte solution is immediately treated with a hydroxide solution to attain a mixed oxidant solution having a basic pH - preferably with a pH value of at least about 9.0, at least about 9.5, at least about 10.0, at least about 10.5, at least about 1 1 .0, at least about 1 1 .5, at least about 12.0, at least about 12.5, or at least about 13.0 - to yield the mixed oxidant solution. Elevated pH of the resulting mixed oxidant solution component has been found to significantly increase the effective shelf life of the component, even in the presence of high concentrations of mixed oxidants. The stabilized mixed oxidant solution component can be produced, blended with a disinfectant solution component to form a biocide, packaged in at least one container (e.g., suitable for shipment), and delivered to a customer without necessity for the biocide to be manufactured at the point of use. [0046] Biocides according to certain embodiments include as components thereof a mixed oxidant solution (comprising a plurality of different oxidants) that may be produced from a starting solution comprising at least one of salt brine, hypochlorous acid, and sodium hypochlorite, with the mixed oxidant solution production including: flowing at least one starting solution through at least one flow-through electrochemical module comprising a first passage and a second passage separated by an ion permeable membrane while electric power is applied between (i) an anode in electrical communication with the first passage and (ii) a cathode in electrical communication with the second passage, wherein a first solution or first portion of the at least one starting solution is flowed through the first passage to form an anolyte solution having an acidic pH, and a second solution or second portion of the at least one starting solution is simultaneously flowed through the second passage to form a catholyte solution having a basic pH; and contacting the anolyte solution with a hydroxide solution to attain a pH value of at least about 9.0 (or another desired pH value such as preferably at least about 10.0, preferably at least about 1 1 .0, preferably at least about 12.0, or preferably at least about 13.0) to yield said mixed oxidant solution. The mixed oxidant solution component may be combined with a disinfectant solution component to yield a biocide solution. In certain embodiments, the disinfectant solution component may comprise a source of chlorine ions. In certain embodiments, the disinfectant solution component may comprise a sodium hypochlorite solution (including, but not limited to, sodium hypochlorite in a range of from 6% to 12.5%). Other disinfectant solutions may be used, with preferred solutions comprising at least one source of chlorine ions. In certain embodiments, the biocide solution comprises sodium hypochlorite in a range of from 4 wt.% to 7 wt.%. In certain embodiments, the biocide solution comprises sodium hypochlorite in a range not exceeding 5.25%, thereby permitting the solution to be designated as non-hazardous and thereby eligible for shipping and containment methods suitable for non-hazardous agents.

[0047] Since the production and composition of the mixed oxidant solution component itself are novel (as disclosed in pending U.S. Patent Application No. 13/950,147, which is hereby incorporated by reference herein), details of the mixed oxidant solution component and its production are described herein.

[0048] In certain embodiments, at least one starting solution for producing a mixed oxidant solution comprises salt brine. In certain embodiments, at least one starting solution for producing a mixed oxidant solution comprises at least one of hypochlorous acid and sodium hypochlorite. [0049] In certain embodiments, catholyte solution produced by the at least one flow- through electrochemical module is discarded, preferably following partial or full neutralization by contacting the catholyte solution with an acid.

[0050] In certain embodiments, at least one flow-through electrochemical module useful for producing a mixed oxidant solution includes a centrally-arranged anode, a membrane surrounding the anode, a cathode surrounding the membrane, a first passage comprising an inner passage between arranged between the anode and the membrane, and a second passage comprising an outer passage arranged between the membrane and the cathode. Electrochemical modules having different geometries and conformations may be used.

[0051 ] In certain embodiments, characteristics of the at least one starting solution, flow rate of the at least one starting solution, materials of construction of the at least one flow-through electrochemical module, dimensions of the at least one flow-through electrochemical module, number of the at least one flow-through electrochemical module, conformation of the at least one flow-through electrochemical module, and field density of the applied electric power are selected to yield a mixed oxidant solution having desired properties. Such properties may include one or more of the following: an oxidation-reduction potential (ORP) value in a range of from 500 mV to 900 mV (or in a range of from 600 mV to 900 mV, or in a range of from 600 mV to 800 mV); a ratio of Na+ (in g/L according to Method EPA 300.0) to CI- (in g/L according to Method EPA 6010) of at least about 1 .5; and total chlorine value of at least about 1 ,000 ppm, at least about 3,000 ppm, at least about 5,000 ppm, in a range of from about 1 ,000 ppm to about 3,500 ppm, or in a range of from about 1 ,000 ppm to about 6,000 ppm. In certain embodiments, multiple values in the foregoing ranges for ORP, Na+:CI-, and total CI (e.g., one value for ORP, another value for Na+:CI-, and/or another value for total CI) may be present in the same mixed oxidant solution.

[0052] Mixed oxidant solutions described herein as components of biocides may be beneficially used to reduce formation of, and/or remove, scale and biofilm deposits from fluid conduits (e.g., pipes, wellbores, etc.) and other wetted surfaces (e.g., tanks, geologic formations, etc.) during hydraulic fracturing operations. The mixed oxidant solution component readily penetrates inorganic deposits as well as organic deposits/biofilms to break down and remove the organic "glue" that holds such deposits and films together.

[0053] As illustrated in FIG. 1 , a system 100 for producing, transporting, and/or using a biocide as disclosed herein may involve multiple stages, including mixed oxidant solution production 101 (encompassing starting solution creation 102, starting solution supply 104, electrochemical processing 106, waste processing 108 (e.g., as applied to a catholyte stream), and stabilization 1 10 (e.g., as applied to an anolyte stream)) to form a stabilized mixed oxidant solution) followed by blending 1 12 with a disinfectant component (provided by disinfectant supply 1 1 1 ) and output/blending 1 12 (preferably including packaging), transportation 1 14 of the blended biocide, and subsequent usage 1 16. In certain embodiments, one or more of the foregoing stages may be eliminated; two or more stages may be consolidated; and/or one or more additional stages may be added.

[0054] The starting solution creation stage 102 may include production of one or more starting solutions or precursors thereof. The starting solution supply stage 104 may include mixing and/or diluting starting solution precursors, and supplying the resulting one or more starting solutions to the electrochemical processing stage 106. The waste processing stage 108 may include neutralizing a basic catholyte stream produced by the electrochemical processing stage 106. The stabilization stage 1 10 may include elevating pH of an acidic anolyte stream produced by the electrochemical processing stage 106. The output/blending stage 1 12 may include venting, blending, and/or packaging steps. The transportation stage 1 14 may include transporting biocide solution to a point of use. The usage stage 1 16 may include applying the biocide oxidant solution to a fluid system (e.g., for treating liquids injected into or received from subterranean formations such as oil fields, gas fields, and mining deposits) at a point of use.

[0055] In certain embodiments, production of stabilized mixed oxidant solution may be conducted in a minimally conditioned or unconditioned environment temperature (approximately 75 °F., +/- 25°F.). In other embodiments, one or more stages (e.g., electrochemical processing 106, stabilization 1 10, output/storage 1 12, transportation 1 14, and/or usage 1 16 may be performed in an air-conditioned or otherwise chilled environment.

[0056] Mixed oxidant solution components as described herein may be produced in one or more flow-through electrochemical modules. Within a flow-through electrochemical module, it is believed that a two-step oxidation process is performed. For example, if a NaCI (salt brine) solution is injected into a flow-through electrochemical module, the chloride ions are believed to undergo an initial oxidation step (e.g., to form hypochlorous acid and/or sodium hypochlorite), and the molecule(s) resulting from the initial oxidation step are believed to be further oxidized to generate the final molecule(s) of interest. Thus, if the starting solution includes hypochlorous acid and/or sodium hypochlorite in addition to or instead of salt brine, then the concentration of the final molecule(s) of interest may be enhanced. [0057] Traditional methods for identifying and/or quantifying the specific oxidants contained in the stabilized mixed oxidant solutions (useful as components of biocides) described herein have not been successful, due at least in part to the fact that chlorine is a strong oxidant and interferes with measurement. With respect to the two streams produced by flow-through electrochemical modules as disclosed herein, the anolyte stream is believed to include two or more of the following: HOCI, CI0₂, 0₃, Cl₂, 0₂, OH⁰, and/or OH^* (as may be supplemented with hydroxide (e.g., NaOH) upon execution of the stabilization step), and the catholyte stream is believed to include two or more of the following: NaOCI, NaOH, H₂, and H₂0₂.

[0058] The starting solution creation stage 102 (for forming mixed oxidant solution) involves the creation of a solution comprising at least one of salt brine, hypochlorous acid, and sodium hypochlorite. If the starting solution comprises salt brine, such brine may be created by mixing water and any suitable one or more type of salt, resulting in dissolution of salt in water. In one embodiment, such salt may consist of or include 99.9% pure food high grade Morton® brand sodium chloride (NaCI) (Morton Salt, Inc., Chicago, II). In other embodiments, various other types, brands, and grades of salt may be substitute. In certain embodiments, sodium chloride may be replaced or supplemented with one or more of sodium bromide, potassium chloride, potassium iodide, and calcium chloride. Substituting calcium chloride (CaCI₂) for some or all sodium chloride (NaCI) may be beneficial in certain embodiments, since the solubilized calcium ion is doubly charged in compared to a singly charged sodium ion.

[0059] Water used to make salt brine may include municipal tap water; alternatively, highly mineralized, low mineralized, chlorinated, and/or chloraminated water may be used. In certain instances, conductivity of a salt brine solution may be in a range of from 5-50 millisiemens as measured with a conductivity meter. Salt brine solution may be subject to one or more filtering steps after creation (e.g., by flowing brine through a screen, sand bed, a diffusion bed, and/or other filtration media). Further details regarding creation of salt brine solutions are provided in U.S. Patent No. 8,366,939, which is hereby incorporated by reference herein.

[0060] In certain embodiments, a starting solution may include at least one of hypochlorous acid and sodium hypochlorite, in combination with water and/or salt brine. Various methods for producing hypochlorous acid and sodium hypochlorite are known to those skilled in the art. In certain embodiments, hypochlorous acid and/or sodium hypochlorite may be manufactured at the same facility and/or in a substantially continuous process (i.e., without requiring intervening storage and/or transportation) for feeding such composition(s) to the electrochemical processing stage 106. In other embodiments, hypochlorous acid and/or sodium hypochlorite may be produced in a different facility and/or in a substantially discontinuous process relative to the electrochemical processing stage 106, whereby hypochlorous acid and/or sodium hypochlorite may be shipped to and/or stored in a facility prior to feeding of such composition(s) to the electrochemical processing stage 106.

[0061 ] The starting solution supply stage 104 may include blending and/or dilution of starting solution constituents. In certain embodiments, the starting solution creation stage 102 may include creation of a concentrated precursor solution that is subject to dilution with water and/or salt brine. In certain embodiments, hypochlorous acid and/or sodium hypochlorite may be blended with water and/or salt brine to form a starting solution. In certain embodiments, pH of a starting solution may be adjusted (e.g., raised or lowered) by addition of at least one acid or base. A suitable acid for addition to a starting solution may include HCI, and a suitable base for addition to a starting solution may include NaOH. Blending and/or dilution of constituents of a starting solution may be controlled responsive to one or more sensors, such as a pH sensor, a conductivity sensor, and/or one or more sensors arranged to sense chlorine content.

[0062] In certain embodiments, starting solution may be created and fed to the flow- through electrochemical processing stage 106 in a substantially continuous process (e.g., with minimal or no intervening storage). In other embodiments, one or more storage tanks may be arranged upstream of the electrochemical processing stage 106 in order to store starting solution.

[0063] The starting solution supply stage 104 preferably includes pressurization of starting solution, such as with at least one pump or other suitable apparatus. In the electrochemical processing stage 106, the oxidation and/or reduction reactions may include production of gaseous by-products (e.g., such as hydrogen gas, oxygen gas, chlorine gas, and/or by-products of other oxidized species). Under low pressure conditions, these gaseous molecules may appear as bubbles that might interfere with fluid flow through gas flow passages and/or contact one or more electrodes within a flow- through electrochemical processing apparatus and therefore interfere with electron flow and redox reactions. In certain embodiments, the starting solution is pressurized to a level exceeding the partial pressure of at least one gas (and more preferably exceeding partial pressure of all gases) subject to being created in a flow-through electrochemical processing apparatus and associated downstream components, thereby inhibiting formation of bubbles. Partial pressure preferably exceeds at least one of hydrogen gas, oxygen gas, and chlorine gas within a flow-through electrochemical processing apparatus as described herein. Pressure within a flow-through electrochemical processing apparatus may also be adjusted (e.g., using a pressure regulator or other pressure adjusting element(s)) to an appropriate level to adjust reaction kinetics within the apparatus. A bypass line may optionally be used to help adjust pressure before starting solution reaches a pressure regulator.

[0064] In certain embodiments, temperature of starting solution may be adjusted in the starting solution supply stage 104 and/or in the electrochemical processing stage 106 in order to enhance reaction kinetics. For example, temperature of starting solution and/or temperature within the a flow-through electrochemical processing apparatus may be adjusted (e.g., increased) in order to enhance the likelihood of a particular oxidation reaction, and increase the concentration of one or more desired molecules of interest.

[0065] In the electrochemical processing stage 106, at least one starting solution may be flowed through an electrochemical module including a first passage and a second passage separated by an ion permeable membrane while electric power is applied between (i) an anode in electrical communication with the first passage and (ii) a cathode in electrical communication with the second passage. In certain embodiments, composition and concentration of starting solution flowing through the first passage and the second passage may be substantially the same (e.g., with a first portion of a starting solution passing through the first passage, and a second portion of the starting solution passing through the second passage (wherein flow rate may be substantially the same or may be substantially different between the first passage and the second passage)). In other embodiments, at least one parameter of composition and concentration of starting solution may differ between the first passage and the second passage (e.g., with a first starting solution passing through the first passage, and with a second starting solution passing through the second passage), wherein flow rate may be substantially the same or may be substantially different between the first passage and the second passage. In certain embodiments, flow of starting solution through the anode chamber may be slower than flow rate through the cathode chamber, to permit longer residence time of starting solution (electrolyte) in the anode chamber and permit an increased number of oxidation reactions.

[0066] In certain embodiments, multiple flow-through electrochemical modules as described herein may be operated fluidically in parallel.

[0067] In certain embodiments, multiple flow-through electrochemical modules as described herein may be operated fluidically in series, with anolyte solution generated by a first module being used as a starting solution for at least the anode chamber of at least one downstream module, in order to promote an increased number of oxidation reactions.

[0068] In still further embodiments, multiple flow-through electrochemical modules as described herein may be operated fluidically in series-parallel. For example, one group of two or more modules may be arranged fluidically in series, and multiple series groups may further be arranged fluidically in parallel.

[0069] A simplified schematic cross-sectional view of a flow-through electrochemical module 225 suitable for producing a mixed oxidant solution (e.g., useful as a component of a biocide) is shown in FIG. 2. The module includes a first flow-through chamber 236 comprising an anode 230, a second flow-through chamber 238 comprising a cathode 234, and a membrane (e.g., an ion-permeable membrane) 232 arranged between the first chamber 236 and the second chamber 238. The anode 230 and the cathode 234 are in electrical communication with terminals 215A, 215B, respectively. In operation, a first starting solution or first starting solution portion is supplied to the first chamber 236 through a first chamber inlet port 221 A, and a second starting solution or second starting solution portion is supplied to the second chamber 238 through a second chamber inlet port 223A. Electric power is supplied across the anode 230 and cathode 234 to electrolyze the contents of the first chamber 236 and the second chamber 238 to yield an anolyte solution that exits the first chamber 236 through a first chamber outlet port 221 B, and to yield a catholyte solution that exits the second chamber 238 through a second chamber outlet port 223B.

[0070] In certain embodiments, an anode 230 may be formed of titanium coated with a material comprising iridium, rubidium, ruthenium, and tin. In one embodiment, the coating material includes iridium content of 48% - 24%, tin content of 40% - 54%, ruthenium content of 8% - 15%, and rubidium content of 4% - 7%. In other embodiments, the anode comprises a coating of platinum and iridium. The composition of the anode may be varied based on conductivity, durability, and cost considerations. In certain embodiments, coating materials provided by Siemens may be used.

[0071] In certain embodiments, a membrane 232 may comprise a ceramic material (e.g., including but not limited to glass bonded ceramic materials). In certain embodiments, the membrane 232 may comprise alumina. In other embodiments, the membrane may comprise a blend of alumina and zirconia materials. Various materials can also be used for the membrane 232 depending on considerations such as porosity, insulation characteristics, durability, and cost.

[0072] In certain embodiments, a cathode 234 may comprise titanium. In other embodiments, a cathode 234 may comprise different materials. The composition of the cathode may be varied based on conductivity, durability, and cost considerations.

[0073] Geometry and dimensions of the anode 230, cathode 234, membrane 232, and chambers 236, 238 may be varied in order to provide desired performance characteristics. In certain embodiments, anode, membrane, and cathode elements may be arranged as generally flat plates. In other embodiments, anode, membrane, and cathode elements may be arranged concentrically in a generally cylindrical apparatus (e.g., such as reactor cells made available by the VIIIMT Institute in Moscow, Russia). In certain embodiments, length of flow-through chambers may be adjusted (e.g., lengthened) and/or fluid flow rate may be adjusted (e.g., reduced) to increase residence time of starting solution in the chambers to increase the likelihood of contact of ions in solution with electrode (anode or cathode) surfaces for oxidation either once, twice, or three or more times. Anode and cathode surface areas may also be adjusted by altering geometry, size, and/or surface characteristics (e.g., texturing) in order to enhance likelihood of oxidation of ions either once, twice, or three or more times.

[0074] In certain embodiments, power supply components and/or electrode materials may be adjusted to allow increased power to be supplied to a flow-through electrochemical module. In an electrochemical cell, the number of oxidizing events will be related to the voltage applied (to overcome the electrochemical potential of a given molecule or atom) and the amperage through the cell (more electrons are able to flow through the cell and perform redox reactions). A given oxidation/reduction reaction will be based on both the number of interactions between solubilized molecules/atoms with a given electrode surface and the availability of electrons from the power supply (amperage).

[0075] An exemplary flow-through electrochemical module 325 is illustrated in FIG. 4. The module 325 includes a center anode 330. A membrane 332 (e.g., ceramic membrane) having an annular shape surrounds the anode 330. Beyond the membrane 332, and forming an exterior portion of the electrochemical module 325, is the exterior cathode 334. The length of the center anode 330 may be greater than the exterior cathode 334, and the membrane 332 may also be longer than the exterior cathode 334. A first (inside) passage 336 is arranged between the center anode 330 and the membrane 332. A second (outside) passage 338 is arranged between the membrane 332 and the exterior cathode 334.

[0076] At the ends of the module 325 are inside collectors 322A, 322B and outside collectors 324A, 324B, such as may be formed of polytetrafluoroethylene material or another fluropolymer material, or may be formed of polyethylene with addition of antioxidant materials. The upstream inside collector 322A receives starting solution from an inlet port 321 A and leads into the first (inside) passage 336 that supplies anolyte solution to the downstream inside collector 322B and outlet port 321 B. In a corresponding manner, the upstream outside collector 324A receives starting solution from an inlet port 323A and leads into the second (outside) passage 338 that supplies catholyte solution to the downstream outside collector 324B and outlet port 323B. In one embodiment, each port 321 A, 321 B, 323A, 323B may have female 1/8 inch national pipe taper fittings; in other embodiments, other sizes and/or types of fitting may be used - including, but not limited to, hose barb fittings.

[0077] FIG. 3 is a schematic diagram showing arrangement of a mixed oxidant solution production system 300 including multiple flow-through electrochemical modules 325A-325B (each according to the module 325 illustrated in FIG. 4) and associated components. (The system 300 may be operated to perform the stages of electrochemical processing 106, waste processing 108, and stabilization 1 10 as depicted in FIG. 1 ). At least one starting solution source 301 (which may include a pressure regulator (not shown)) supplies starting solution through at least one feed valve 302 arranged to supply one or more starting solutions to starting solution supply headers 307, 309 and inlet pipes 31 1 A, 313A. A first inlet pipe 31 1 A is arranged to supply starting solution to a flow-through anode chamber 336, and a second inlet pipe 313A is arranged to supply starting solution to a flow-through cathode chamber 338, wherein the anode chamber 336 and cathode chamber 338 are separated by a membrane 332. A power supply 308 is arranged to supply electrical direct current (DC) via terminals 315, 316 arranged to apply voltage between an anode in electrical communication with the anode chamber 336 and a cathode in electrical communication with the cathode chamber 338, to electrolyze starting solution present in the flow-through electrochemical module 325. Catholyte solution generated by cathode chamber 338 flows to an outlet pipe 313B, catholyte header 319, and needle valve 360 for subsequent neutralization (i.e., by reducing pH). Anolyte solution generated by the anode chamber 336 flows to an outlet pipe 31 1 B, anolyte header 317, and three-way valve 340 for subsequent stabilization (i.e., by increasing pH). Catholyte solution generated by each module 325 has a basic pH (e.g., typically a pH value in a range of from 9 to 12), and anolyte solution generated by each module has an acidic pH (e.g., typically a pH value in a range of from 1 to 4).

[0078] In one embodiment, ten groups of four flow-through electrochemical modules 325 (such the module 325 as illustrated in FIG. 4) may be employed, for a total of forty flow-through electrochemical modules. Each reactor cell or module 325 may receive 12 volts and 10 amps. Within each group, two of the four modules 325 may be wired electrically in parallel, with the two modules of each group being wired in series with another two modules in the group of four. FIG. 3 illustrates only two modules 325A- 325B. In other embodiments different wiring configurations are employed, including all reactor modules 325 being operated electrically in series or in parallel.

[0079] A large number of modules 325 form a module bank that allows for the production of large quantities of mixed oxidant solution. With this number of modules 325, in one embodiment the pressure and aggregate flow rate of starting solution entering the modules may be adjusted to 5-10 psi and 1 -2 gal/minute. The number of modules used can be increased or decreased to meet production needs, and the pressure and/or flow rate or starting solution supplied to the module bank may be varied depending on factors including the number, size, and configuration of modules 325, the characteristics of the at least one starting solution, and the desired characteristics of the resulting anolyte solution.

[0080] The power supply 308 may comprise a linear unregulated unit (e.g., produced by Allen-Bradley), a linear regulated power supply, or an AC/DC/AC/DC switching power supply. Multiple power supplies 308 can also be employed. The electric power to each module 325 from the power supply 308 can also be varied as needed.

[0081 ] Continuing to refer to FIG. 3, the catholyte stream received from the outlet pipe 313B, catholyte header 319, and needle valve 360 flows past a pH meter 361 , a three-way valve 362, and a flow sensor 364 to reach a waste neutralization element 365 arranged to receive a flow of acid from an acid source 366 and an acid flow control valve 368. Various types of acid may be used, including but not limited to hydrochloric acid. Acid may be supplied to the waste neutralization element 365 (which may include a mixer, such as a flow-through mixer) responsive to signals from the pH meter 361 and flow sensor 364 to neutralize or at least partially neutralize the catholyte (e.g., preferably to a pH value in a range of from 7 to 9, or more preferably in a range of from 7 to 8) to permit disposal of the neutralized catholyte product (e.g., by directing such product to a sewer).

[0082] The anolyte stream received from the 31 1 B, anolyte header 317, and three- way valve 340 flows past a pH meter 341 then through a needle valve 342, a stabilization (e.g., base addition) element 344, a mixer 349, another pH meter 351 , and a flow meter 354 before flowing to an output stage 500. The stabilization element 344 is arranged to receive a flow of base (preferably one or more hydroxides, such as but not limited to sodium hydroxide, potassium hydroxide, and the like) from a base (e.g., hydroxide) source 346 and a base flow control valve 348. Base (e.g., hydroxide) may be supplied to the stabilization element 344 responsive to signals from one or both pH meters 341 , 351 and a flow sensor (not shown) to elevate pH of the anolyte from a starting acidic value (e.g., in a pH range of from 2 to about 4) to an elevated pH value in the basic range, (preferably a pH value of at least about 9.0, at least about 9.5, at least about 10.0, at least about 10.5, at least about 1 1 .0, at least about 1 1 .5, at least about 12.0, at least about 12.5, or at least about 13.0) - to yield the mixed oxidant solution.

[0083] The pH stabilization step is preferably performed a very short distance downstream of the flow-through electrochemical modules 325 to permit such stabilization to be performed immediately after anolyte production - thereby suppressing chlorine gas and minimizing degradation of mixed oxidants the anolyte solution. Preferably, pH stabilization is performed on anolyte solution within less than about 5 seconds (more preferably within less than about 3 seconds) after anolyte exits the flow-through electrochemical modules 325.

[0084] FIG. 5 is a line chart depicting total chlorine (ppm) versus time (days) for mixed oxidant solutions produced using the system of FIG. 3. As shown in FIG. 5, chlorine content of an initially acidic anolyte solution having pH adjusted (e.g., with addition of sodium hydroxide) to a value of 7.73 degraded rapidly, from an initial chlorine value exceeding 4500 ppm to a value of approximately 1300 within 8 days. Increasing the pH of anolyte solutions resulted in enhanced stability, as shown in the data generated for pH-modified anolyte solutions having pH values of 9.1 , 10.04, 1 1 .05, and 12.09, respectively. At a pH value of 10.04, total chlorine content of a pH-modified anolyte solution diminished by less than about 10% (from a starting value of approximately 4700 ppm) after 28 days. At a pH value of 1 1 .05, total chlorine content of a pH-modified anolyte solution diminished by less than about 5% or less (from a starting value of approximately 4900 ppm) after 28 days At a pH value of 12.09, total chlorine content of a pH-modified anolyte solution was substantially unchanged after 28 days at a value of approximately 5000 ppm. FIG. 5 therefore shows that modifying pH of initially acidic anolyte to elevated pH (e.g., at least about 9.0, at least about 10.0, at least about 1 1 .0, at least about 12.0, or another intermediate value or value exceeding 12.0) beneficially improves stability of chlorine species in mixed oxidant solutions.

[0085] Referring back to FIG. 3, various elements of the system 300 may be automated and controlled via a controller 390. The flow-through electrochemical modules 325 may be periodically cleaned by suspending production of mixed oxidant solution, and circulating one or more solutions through the modules via recirculation lines 368, 369 and recirculation element 370. In certain embodiments, cleaning may involve three cycles: (a) an initial rinse cycle, (b) an acid rinse cycle, and (c) a final rinse cycle. Cleaning may be performed according to any suitable schedule, such as hourly, once every few hours, once per day, or any other suitable interval. Increased frequency of cleaning cycles are expected to enhance quality of the resulting mixed oxidant solution. An initial rinse cycle may last approximately 80 seconds, followed by an acid rinse cycle (e.g., using 0.1 to 5% hydrochloric acid (HCI)) that may last for approximately five minutes, followed by circulation of starting solution for approximately 160 seconds before the power supply 308 is reactivated for continued production of stabilized mixed oxidant solution. Timing and duration of cleaning cycles may depend on factors such as module size, flow rates, cleaning frequency, cleaning solution concentration, and desired results. [0086] FIG. 6 is a table summarizing characteristics including total chlorine, pH, oxidation-reduction potential (ORP), conductivity, sodium ion concentration, chlorine ion concentration, and sodium/chloride ion ratio for the following five products: (1 ) 12.5% hypochlorite bleach, (2) 6% hypochlorite bleach, (3) Clearitas® mixed oxidant solution, (4) Miox™ mixed oxidant solution, and (5) a stabilized mixed oxidant solution (i.e., prior to combination with a disinfectant component to form a biocide) produced by a method as disclosed hereinabove. Various differences between the five compositions are apparent.

[0087] Relative the stabilized mixed oxidant solution, both hypochlorite (liquid bleach) compositions have extremely high total chlorine (e.g., 37 to 90 times higher than the stabilized mixed oxidant solution), high pH, high conductivity (e.g., 7.5 times higher than the stabilized mixed oxidant solution), but lower ORP and lower ratio of sodium/chloride ion ratio. It is understood that hypochlorite (bleach) does not contain a significant number of mixed oxidants. Applicants have observed that hypochlorite (liquid bleach) has very limited ability to control deposits composed of both organic and inorganic constituents in water systems, in comparison to the high efficacy in controlling deposits characteristic of the stabilized mixed oxidant solution.

[0088] Clearitas® mixed oxidant solution (previously sold as RE-Ox^® scale control additive) has been commercialized by the assignee of the present disclosure for a period of multiple years. Such solution may be produced substantially in accordance with the method described in U.S. Patent No. 8,366,939. Relative to the stabilized mixed oxidant solution, Clearitas® solution has significantly lower total chlorine (about 600 ppm versus 1550 ppm for the stabilized mixed oxidant solution), substantially lower conductivity, and substantially lower pH (i.e., 6.69 versus 9.12), but increased ORP and increased ratio of sodium/chloride ion ratio. Tests performed by the assignee of the present application confirm that a lower concentration of the stabilized mixed oxidant solution provides comparable scale control benefits to the use of Clearitas® solution at higher concentration, with the stabilized mixed oxidant solution further exhibiting significantly increased effective shelf life (e.g., on the order of at least 2-5 times greater than Clearitas® solution).

[0089] Miox™ mixed oxidant solution is typically generated at a point of use through operation of an on-site mixed oxidant production apparatus commercially available from Miox Corporation (Albuquerque, New Mexico, USA). A two-month old refrigerated sample of a mixed oxidant solution produced by a Miox mixed oxidant production apparatus (believed to have utilized a production method according to at least one of U.S. Patent Nos. 5,316,740 and U.S. 7,922,890) was analyzed as the basis for comparison. Relative to the stabilized mixed oxidant solution, the Miox™ mixed oxidant solution has higher total chlorine (about 3780 versus 1550 ppm for Applicants' stabilized mixed oxidant solution), lower pH (about 9.12 versus about 10.46), higher ORP, higher conductivity, and similar sodium/chloride ion ratio. Effectiveness of the Miox™ mixed oxidant solution in performing scale control was not evaluated.

[0090] FIG. 7 is a schematic diagram showing components of a subsystem 500 arranged to receive a stabilized mixed oxidant solution from the production system 300 of FIG. 3, and blend the mixed oxidant solution component with a disinfectant component (e.g., a chlorine ion source such as a sodium hypochlorite solution) to yield a biocide according to an embodiment of the present disclosure. After passage through the pH sensor 351 and flow sensor 354, a stabilized mixed oxidant solution may flow past a vent line 502 to vent any gas produced during the process. The stabilized mixed oxidant solution 350 enters a holding tank 510, where it may be monitored for quality (e.g. , to confirm that the pH value desirable is at least about 9.0, at least about 9.5, at least about 10.0, at least about 10.5, at least about 1 1 .0, at least about 1 1 .5, at least about 12.0, at least about 12.5, or at least about 13.0). Titration may also be conducted (e.g., using a Hach digital titrator Method 8209 (Hach Co., Loveland, CO)) to measure the total chlorine content, to preferably yield a total chlorine value of preferably at least about 1 ,000 ppm, or preferably at least about 2,000 ppm, or preferably at least about 3,000 ppm, or preferably at least about 4,000 ppm, or preferably at least about 5,000 ppm. In certain embodiments, the total chlorine value of the stabilized mixed oxidant solution may desirably be in a range of from about 1 ,000 ppm to about 3,500 ppm.

[0091 ] In certain embodiments, the stabilized mixed oxidant solution 350 may be pumped (using pump 515) to an insulated storage tank 520, wherein insulation 522 helps keep the temperature of the solution 350 consistent. A desired temperature for the solution is in a range of from 50 °F-80 °F. Degradation of the mixed oxidant solution 350 depends on temperature and time, with degradation being more rapid at high temperatures (and particularly in direct sunlight). Reducing solution temperature may enhance shelf life. The anolyte solution exiting the flow-through electrochemical modules may have a temperature of approximately 100^eF. Chilling the mixed oxidant solution immediately after stabilization is believed to permit further enhanced shelf life. The insulated storage tank 520 may optionally be refrigerated, such as by using a fluoroplastic heat exchanger constructed utilizing polyvinylidene fluoride and/or polytetrafluoroethylene materials.

[0092] From the insulated storage tank 520, the stabilized mixed oxidant solution may be pumped (using pump 525) through a first flow regulating device (e.g., mass flow controller) 527 to a mixer 540, which is further arranged to receive a stream of disinfectant solution 531 supplied from a disinfectant solution tank 530, a pump 535, and a second flow regulating device (e.g., mass flow controller) 537. Any suitable type of mixer 540 may be used, with certain embodiments preferably embodying one or more static mixer or other flow-through mixing elements to blend the mixed oxidant solution component 350 and the disinfectant solution component 531 to yield a biocide solution. At least one sensor 542 may be arranged at or downstream of the mixer(s) 540 to sense one or more properties of the biocide solution (e.g., flow rate, pH, conductivity, chlorine content, temperature, etc.). In certain embodiments, flow of the mixed oxidant solution component 350 and/or the disinfectant solution component 531 through the flow regulating devices 527, 537 may be controlled responsive to output signals of the at least one sensor 542. Downstream of the at least one sensor 542, the biocide flows into at least one container 545 (such as totes, barrels, tanks, or the like) that may be subsequently sealed and suitable for transport to a point of use. The biocide is a somewhat dilute oxidizer and can be corrosive over time. Suitable materials for packaging and handling biocide solutions as disclosed herein include fluoroplastics, PVC, and polyethylene.

[0093] Following packaging of biocide in one or more containers 545, the biocide is ready for the transportation 548 to a customer / point of use 550. A customer may supply (e.g. , inject) the stabilized mixed oxidant solution into suitable conduits or containers at a point of use, such as (but not limited to) a facility or installation for performing hydraulic fracturing (e.g., for treating source water for tracing fluid, treating flowback fluid, and/or treating produced water) or other liquid processing of materials in subterranean formations. The customer 550 is able to utilize the biocide solution without requiring on-site generation thereof (with attendant difficulties of personnel care, maintenance of production equipment, and quality control). Moreover, due to the extended shelf life of the biocide, the customer has increased flexibility to store biocide at the customer site with reduced concern regarding waste or disposal of unused "expired" product.

[0094] The biocide can beneficially disinfect liquids (e.g. , source water for tracing fluid, flowback fluid, and/or produced water) as well as beneficially reduce, remove, or prevent formation of deposits in conduits, apparatuses, and/or formations containing such liquids. The mixed oxidant solution component of the biocide solution prevents nucleation, which is a key requirement for the crystallization of minerals from solution directly on surfaces. Nucleation is the beginning of scales, films and other deposits. Existing films and mineral scales cannot be sustained and new films or scales cannot form without continuous nucleation. The biocide solution disrupts the attachment mechanisms of biofilms, mineral scales and other deposit constituents in systems contacting water-containing liquids, thereby elevating liquid quality. [0095] Biocide may be added to liquid to be treated in a static or flowing environment. In certain embodiments, biocide and a liquid to be treated may both be flowed into or through (e.g., a pipe and/or flow-through mixer) to effectuate mixing. In other embodiments, biocide may be supplied to a tank housing liquid to be treated in a static state; such liquid may optionally be agitated following addition of biocide.

[0096] FIG. 8 illustrates components of an exemplary water treatment system 600 arranged to contact liquid with biocide as disclosed herein. A liquid (e.g., water) source 620 may be arranged to supply liquid through a flow sensor 631 to a (primary) mixing or treatment unit 640 arranged to receive biocide. The mixing or treatment unit 640 (which may optionally include static mixing, active mixing, and/or agitating elements) is arranged to receive biocide from a reservoir or container 642 and a pump 644 with passage of the biocide through a flow sensor 646. An optional secondary mixing or treatment unit 641 (which may embody a tank and/or long conduits, optionally supplemented with one or more mixing or agitating elements) may be arranged downstream of the primary mixing or treatment unit 640 to promote prolonged contact of biocide and liquid prior to subsequent use. In certain embodiments, the prolonged contact spans at least about one minute, more preferably least about two minutes, more preferably least about three minutes, more preferably least about four minutes, more preferably least about five minutes, or more preferably at least about ten minutes in order to provide sufficient time for the biocide to disrupt and/or eliminate pathogens in the source liquid. In certain embodiments, additional biocide may be added to the secondary mixing or treatment unit 641 . At least one water property sensor 633 (e.g., pH sensor, total chlorine sensor, and/or other type of sensor(s)) may be arranged downstream of the mixing or treatment unit(s) 640, 641 between the water source 620 and an output element 650. A controller 690 may be arranged to control the supply of stabilized mixed oxidant solution to the mixing or treatment unit(s) 640, 641 responsive to the flow sensor(s) 631 , 646 and the at least one water property sensor 633. In certain embodiments, additional chemistries (e.g., suitable for forming hydraulic fracturing fluid) may be added at the output element 650. In certain embodiments, the treatment unit(s) 640, 641 , controller 690, reservoir 642, pump 644, and sensors 646, 631 , 633 may be arranged in a single unit or module 681 (optionally mounted on a skid, trailer, or other mounting element) at a desired point of use.

[0097] FIG. 9 is a schematic diagram of various components of a hydraulic fracturing system 700, including a hydraulic fracturing fluid production/treatment subsystem 720, a flowback fluid treatment subsystem 740, a produced water treatment subsystem 760, and a hydrocarbon processing subsystem 780, with each subsystem including at least one connection to a production well arranged to stimulate production of hydrocarbons from a subterranean formation. One or more of the hydraulic fracturing fluid production/treatment subsystem 720, the flowback fluid treatment subsystem 740, or the produced water treatment subsystem 760 may include or embody a water treatment system 600 such as illustrated and described in connection with FIG. 8.

[0098] The hydraulic fracturing fluid production/treatment subsystem 720 includes a primary water storage element 722 (as may be embodied in one or more tanks or reservoirs), and a holding tank 724 arranged to receive water from the primary water storage element 722, with the holding tank 724 (or a conduit between the water storage element 722 and holding tank 724) arranged to receive biocide from a biocide source 725. Contents of the holding tank 724 may be optionally agitated. In certain embodiments, the holding tank 724 may be replaced with an elongated conduit to promote contact between biocide and source water to be treated for a desired time period to provide a desired biocidal effect. Downstream of the holding tank 724, one or more mixing elements 726 (which may be embodied in one or more elongated conduits and/or flow through mixers that may be static or active in character) may be arranged to receive chemical components from a chemical source 728 for producing fracturing fluid. Desirable chemical components include at least one (and preferably multiple) of the following: a water soluble gelling agent; a proppant; a friction reducing component; a breaker (e.g., for breaking crosslinked chains at a desired time); a viscosity enhancing component; an acid; a chloride salt; a carbonate salt; and a borate salt. The at least one mixing element may also be arranged to receive treated flowback fluid from flowback treatment output line 748 and/or treated produced water from produced water treatment output line 768, such as may be beneficial to reduce consumption of source water. Downstream of the mixing elements 726, one or more (preferably multiple) pumps 715 are arranged to inject hydraulic fracturing fluid into the production well 710 to stimulate production of hydrocarbons.

[0099] The flowback fluid treatment subsystem 740 is arranged to receive flowback fluid from the production well 710, and includes a flowback fluid collection element 742, at least one separation element 743 (e.g., arranged for gravitational, mechanical, and/or chemical separation of constituents of flowback fluid), and at least one treatment element 744 arranged to receive biocide from a biocide source 745 to promote disinfection of the processed flowback fluid. The at least one treatment element 744 may optionally include one or more elongated conduits, tanks, and/or mixing elements (not shown) to promote contact between the processed flowback fluid and the biocide for a desired time period, prior to circulation of the treated fluid via the flowback treatment output line 748 to the hydraulic fracturing fluid production/treatment subsystem 720. [00100] The produced water treatment subsystem 760 is arranged to receive produced water from the production well 710, and includes a produced water collection element 762, at least one separation element 763 (e.g., arranged for gravitational, mechanical, and/or chemical separation of constituents of produced water), and at least one treatment element 764 arranged to receive biocide from a biocide source 765 to promote disinfection of the processed produced water. The at least one treatment element 764 may optionally include one or more elongated conduits, tanks, and/or mixing elements (not shown) to promote contact between the processed produced water and the biocide for a desired time period, prior to circulation of the treated fluid via a produced water treatment output line 768 to the hydraulic fracturing fluid production/treatment subsystem 720.

[00101 ] The hydrocarbon processing subsystem 780 includes one or more hydrocarbon processing elements 782 (e.g., for thermal, mechanical, and/or chemical processing as may be necessary to separate hydrocarbons from liquids and other constituents) and one or more storage and/or transport elements 784 that may be embodied in vessels, pipelines, tankers, etc.

[00102] The biocide sources 725, 745, 765 associated with the hydraulic fracturing fluid production/treatment subsystem 720, flowback fluid treatment subsystem 740, and produced water treatment subsystem 760, respectively, are preferably arranged to supply a liquid biocide solution as disclosed herein to a respective liquid stream (e.g. , source water, flowback fluid, or produced water) for disinfection thereof. In certain embodiments, a single centralized source of biocide (not shown) may be arranged in fluid supplying relationship with the biocide sources 725, 745, 765 to simply storage, transfer, and logistics associated with biocide material management for the hydraulic fracturing system 700.

[00103] FIGS. 10A-10E embody line charts of luminometer relative light units (RLU) (for detection of adenosine triphosphate (ATP)) versus concentration for five different conventional disinfectants as applied to dairy waste, and FIG. 10F provides a similar line chart for a biocide as disclosed herein (including 60% mixed oxidant solution blended with 40% hypochlorite solution to yield 5.25% sodium hypochlorite content in the biocide). Each disinfectant was dosed into 100 ml of dairy waste at different dosages and allowed to sit for three minutes prior to testing. The testing sequence in each instance was: Total ATP, Free ATP, Duplicate Total ATP, and Duplicate Free ATP. All four tests were performed within three minutes after the above-mentioned three minute holding time. The averages were then plotted against control values (of 9420 Total ATP, and 6741 Free ATP). [00104] FIG. 10A is a line chart of RLU versus concentration for five different doses of a disinfectant including 25% gluteraldehyde and 12% quaternary ammonia. FIG. 10B is a line chart of RLU versus concentration for five different doses of a disinfectant including 12% gluteraldehyde and 3% quaternary ammonia). FIG. 10C is a line chart of RLU versus concentration for five different doses of a disinfectant including 25% gluteraldehyde). FIG. 1 0D is a line chart of RLU versus concentration for five different doses of a disinfectant including 5% chlorine dioxide produced from a mixture of Petrofid chlorite and hydrochloric acid in a 5:1 ratio). FIG. 10E is a line chart of RLU versus concentration for five different doses of a disinfectant including 5.25% sodium hypochlorite (a/k/a liquid bleach)). FIG. 10F is a line chart of RLU versus concentration for five different doses of a biocide according to an embodiment of the present disclosure (including 60% mixed oxidant solution blended with 40% hypochlorite solution to yield 5.25% sodium hypochlorite).

[00105] After 22 hours, the Free and Total ATP was again measured on the 1 gallon per thousand treatments to illustrate the effect of disinfectant on ATP over time. Results of such measurements (for the disinfectants and biocide described above in connection with FIGS. 10A-10F) are contained in FIG. 10G. FIGS. 10A-10G demonstrate the relative and absolute efficacy of Applicant's novel biocide in controlling a broad spectral of bacteria. Notably, Applicant's novel biocide (described in connection with FIG. 10F) performed markedly better than each of 5% chlorine dioxide (of FIG. 10D) and 5.25% sodium hypochlorite (of FIG. 10F) with respect to both average total ATP and free total ATP, but without the toxicity associated with gluteraldehyde and/or quaternary ammonia (contained in the disinfectants described in connection with FIGS. 10A-10C).

[00106] FIG. 1 1 is a line chart providing results of guar viscosity testing showing effects on viscosity (in centipoise) at elevated temperatures for two different concentrations of biocides according to embodiments of the present disclosure and one conventional disinfectant (Line D, including 25% gluteraldehyde and 12% quaternary ammonia) as combined with a 30 ppt guar gel crosslinked with a 1 .5 gpt delayed crosslinker, in comparison to a control solution (Line B) including gel and crosslinker in the absence of biocide. Line A represents 1 gallon per thousand and Line C represents 0.4 gallon per thousand of a biocide according to the present disclosure (including 60% mixed oxidant solution blended with 40% hypochlorite solution to yield 5.25% sodium hypochlorite). Line E represents temperature (in degrees Fahrenheit, represented on the right axis). FIG. 1 1 demonstrates that Applicant's biocide corresponding to Line C was comparable to the conventional disinfectant corresponding to Line D, by similar reduction of viscosity over a range of temperatures, whereas a slightly greater (but still acceptable level of) interference with viscosity was encountered with the higher concentration of Applicant's biocide corresponding to Line A. It has therefore been demonstrated that at appropriate concentration levels, biocides according to the present disclosure are suitably compatible with conventional crosslinkers used in hydraulic fracturing fluid.

[00107] Novel compositions and methods including mixed oxidants for treating liquids injected into or received from subterranean formations have been disclosed herein, with particular utility for hydraulic fracturing systems to promote water treatment (e.g., disinfection), with beneficial reduction or inhibition of deposits and biofilms.

[00108] Embodiments as disclosed herein may provide beneficial technical effects including provision of disinfection treatment and scale/biofilm control compositions suitable for liquids injected into or received from subterranean formations with non- toxicity, enhanced effectiveness, extended effective shelf life, and elimination of need for on-site generation of mixed oxidant solutions, while avoiding use of hazardous chemicals.

[00109] While specific aspects, features, and embodiments of the disclosure have been described herein, it will be appreciated that the utility of the disclosure is not thus limited, but rather extends to and encompasses numerous other variations, modifications and alternative embodiments, as will suggest themselves to those of ordinary skill in the field of the present disclosure. Various combinations and sub-combinations of the structures described herein are contemplated and will be apparent to a skilled person having knowledge of this disclosure. Any of the various features and elements as disclosed herein may be combined with one or more other disclosed features and elements unless indicated to the contrary herein. Correspondingly, the disclosure as hereinafter claimed is intended to be broadly construed and interpreted, as including all such variations, modifications and alternative embodiments, within its scope and including equivalents of the claims.

Claims

CLAIMS What is claimed is:

1 . A method for performing hydraulic fracturing, the method comprising:

forming a hydraulic fracturing fluid comprising a biocide solution; and

injecting the hydraulic fracturing fluid into a subterranean formation;

wherein the biocide solution is produced by a process including the steps of: flowing at least one starting solution that comprises at least one of salt brine, hypochlorous acid, and sodium hypochlorite through at least one flow-through electrochemical module comprising a first passage and a second passage separated by an ion permeable membrane while electric power is applied between (i) an anode in electrical communication with the first passage and (ii) a cathode in electrical communication with the second passage, wherein a first solution or first portion of the at least one starting solution is flowed through the first passage to form an anolyte solution having an acidic pH, and a second solution or second portion of the at least one starting solution is simultaneously flowed through the second passage to form a catholyte solution having a basic pH; contacting the anolyte solution with a hydroxide solution to attain a pH value of at least about 9.0 to yield a mixed oxidant solution comprising a plurality of different oxidants; and combining the mixed oxidant solution with a liquid disinfectant solution to yield the biocide solution.

2. A method for treating flowback fluid or produced water received from a subterranean formation, the method comprising contacting the flowback fluid or the produced water with a biocide solution, wherein the biocide solution is produced by a process including the steps of:

flowing at least one starting solution that comprises at least one of salt brine, hypochlorous acid, and sodium hypochlorite through at least one flow-through electrochemical module comprising a first passage and a second passage separated by an ion permeable membrane while electric power is applied between (i) an anode in electrical communication with the first passage and (ii) a cathode in electrical communication with the second passage, wherein a first solution or first portion of the at least one starting solution is flowed through the first passage to form an anolyte solution having an acidic pH, and a second solution or second portion of the at least one starting solution is simultaneously flowed through the second passage to form a catholyte solution having a basic pH; contacting the anolyte solution with a hydroxide solution to attain a pH value of at least about 9.0 to yield a mixed oxidant solution comprising a plurality of different oxidants; and combining the mixed oxidant solution with a liquid disinfectant solution to yield the biocide solution.

3. A method for producing hydraulic fracturing fluid, the method comprising:

adding a biocide solution to source water to yield treated source water; and adding at least one of the following components to the treated source water: a water soluble gelling agent; a proppant; a friction reducing component, and a viscosity enhancing component;

4. A method according to claim 3, comprising adding at least two of the following components to the treated source water: a water soluble gelling agent; a proppant; a friction reducing component, and a viscosity enhancing component.

5. A method according to claim 3, further comprising adding at least one of the following additional components to the treated source water: an acid; a chloride salt; a carbonate salt; and a borate salt.

6. A method according to claim 3, comprising adding at least two of the following additional components to the treated source water: an acid; a chloride salt; a carbonate salt; and a borate salt.

7. A method according to any one of claims 1 to 6, wherein the biocide solution comprises a pH of at least about 13.

8. A method according to any one of claims 1 to 6, wherein the liquid disinfectant solution comprises a source of chlorine ions.

9. A method according to any one of claims 1 to 6, wherein the liquid disinfectant solution comprises a sodium hypochlorite solution.

10. A method according to claim 9, wherein sodium hypochlorite is present in the biocide solution in a range of from about 4 wt.% to about 7 wt.%.

1 1 . A method according to claim 9, wherein the biocide solution comprises from 30 to 90 percent by volume mixed oxidant solution and comprises from 10 to 70 percent by volume sodium hypochlorite solution.

12. A method according to claim 9, wherein the biocide solution comprises from 50 to 70 percent by volume mixed oxidant solution and comprises from 30 to 50 percent by volume sodium hypochlorite solution.

13. A method according to any one of claims 1 to 6, wherein the at least one starting solution comprises at least one of hypochlorous acid and sodium hypochlorite.

14. A method according to any one of claims 1 to 6, wherein said contacting the anolyte solution with a hydroxide solution attains a pH value of at least about 10.5.

15. A method according to any one of claims 1 to 6, wherein said contacting the anolyte solution with a hydroxide solution attains a pH value of at least about 12.0.

16. A method according to any one of claims 1 to 6, wherein characteristics of the at least one starting solution, flow rate of the at least one starting solution, materials of construction of the at least one flow-through electrochemical module, dimensions of the at least one flow-through electrochemical module, number of the at least one flow- through electrochemical module, conformation of the at least one flow-through electrochemical module, and field density of the applied electric power are selected to yield a mixed oxidant solution comprising a plurality of different oxidants that comprises at least one of the following characteristics (i) to (iii): (i) a total chlorine value of at least about 3,000 ppm ; (ii) an oxidation-reduction potential (ORP) value in a range of from 600 mV to 800 mV; and (iii) a ratio of Na+ (in g/L according to Method EPA 300.0) to CI- (in g/L according to Method EPA 6010) of at least about 1 .5.

17. A method according to any one of claims 1 to 6, wherein characteristics of the at least one starting solution, flow rate of the at least one starting solution, materials of construction of the at least one flow-through electrochemical module, dimensions of the at least one flow-through electrochemical module, number of the at least one flow- through electrochemical module, conformation of the at least one flow-through electrochemical module, and field density of the applied electric power are selected to yield a mixed oxidant solution comprising a plurality of different oxidants that comprises each of the following characteristics (i) to (iii): (i) a total chlorine value of at least about 3,000 ppm; (ii) an oxidation-reduction potential (ORP) value in a range of from 600 mV to 800 mV; and (iii) a ratio of Na+ (in g/L according to Method EPA 300.0) to CI- (in g/L according to Method EPA 6010) of at least about 1 .5.

18. A hydraulic fracturing fluid produced by the method of claim 3.

19. A hydraulic fracturing fluid produced by the method of claim 4.

20. A hydraulic fracturing fluid produced by the method of claim 5.

21 . A hydraulic fracturing fluid produced by the method of claim 6.