As exploration and production (E&P) companies continue to focus on improving the efficiency of water usage and disposal operations, operators are looking for opportunities to inexpensively treat produced water, as well as improve the use of treated-produced water for drilling and well completion activities.
Treatment of produced water commonly has several objectives, including the removal of chemical contaminants such as hydrogen sulfide, iron sulfide, soluble iron species, manganese, ammonia, barium and suspended solids. Many of these contaminants can be removed using sodium hypochlorite as a chemical oxidant.
Hydrogen sulfide reacts rapidly with sodium hypochlorite to produce either sulfate ions or elemental sulfur. The exact nature of the reaction between hydrogen sulfide and sodium hypochlorite depends on the relative amount of the two species present, as well as the pH of the produced water.
Sulfate ions remain in the treated water while, in instances where elemental sulfur is formed, it can be removed through flocculation, settling, or filtration processes after sodium hypochlorite treatment.
Iron, Iron Sulfide and Manganese
Both Iron and Iron Sulfide are oxidized by sodium hypochlorite to produce a variety of iron oxide species. Iron oxide has low solubility in water and will eventually form particles that are large enough to precipitate or be removed through filtration processes. Similarly, manganese present in the produced water will reach with sodium hypochlorite to produce insoluble manganese oxide species that can be removed through filtration.
Removal of suspended solids can be facilitated through chemical oxidation treatment processes initiated by the addition of sodium hypochlorite. Iron oxide produced through the oxidation of iron can initiate a coagulation/flocculation process where the forming iron oxide particles sweep up other suspended particles in the water through electrostatic attraction. Once the coagulation/flocculation process is complete, the suspended material attached to the iron oxide particles is removed either through settling or filtration.
Ammonia engages in a series of chemical reactions with sodium hypochlorite, with the first reaction product being Monochloramine (NH2Cl). Monochloramine is a well-known disinfectant that is used to remove bacteria in drinking water. Once Monochloramine is formed, further treatment with additional sodium hypochlorite will result in the generation of other chloramine species and, finally, the removal of nitrogen through the production of nitrogen gas. Overall, this process is known as breakpoint chlorination and is a commonly used process to achieve the removal of ammonia from water.
While there will be variations on treatment outcomes based on raw produced water quality and chemical profile, the treatment of produced water with sodium hypochlorite combined with simple mechanical processes, such as aeration tanks and sand filters, is a highly cost-effective way to meet most common produced water treatment goals.
Beyond the removal of these chemical species, treatment with sodium hypochlorite will also eliminate oilfield bacteria. Clean brines resulting from this treatment process are readily used in additional hydraulic fracturing operations.
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