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Though petroleum has numerous benefits as a fuel, it has several adverse effects on the environment during production, distribution and consumption of petroleum. During the extraction of petroleum from reservoirs, a lot of water is also brought to the surface along with it. This water known as petroleum produced water (PPW) is the largest volume waste stream in the exploration and production processes. For crude oil wells, this can account for upto 98% of the extracted fluids during the later stages of production [1]. Worldwide, production of produced water associated with hydrocarbon recovery is more than 77 billion bbl (oil barrel) per annum [2]. PPW usually contains high concentrations of dissolved sodium chloride, dissolved hardness (calcium and magnesium carbonates), suspended solids, sulfate, and emulsified oils. With the increasing number of mature oil reservoirs, and the complex pollutants present in the PPW, its treatment and management of PPW requires urgent attention.
Apart from the huge volumes of PPW to be disposed, another issue that the oil industry has to deal with is that mature oilfields increasingly require water-based enhanced oil-recovery (EOR) methods. Thus, usage of PPW for water flooding leading to EOR was proposed [1]. However, due to the presence of sulfate in this water, there is proliferation of sulfate reducing bacteria (SRB), that not only cause reservoir souring and clogging, but also cause microbially influenced corrosion (MIC) in the pipelines or storage tanks leading to significant economic losses to the industry [3]. Apart from sulfate, the high total dissolved solids (TDS)/salinity of PPW is a concern for industries, especially when surface discharge is planned. Thus, a technology that removes sulfate and TDS is desirable if the PPW is to be used for reinjection to the reservoirs for EOR or other purpose by the energy companies, which was the motive behind the inception of this research work.
For the treatment of PPW a combinatorial approach was designed wherein anaerobic digestion, electro-oxidation and microbial fuel cell were sequentially used. Anaerobic digestion was used to convert sulfates to sulfides which were subsequently converted to sulfur during electro-oxidation. Sulfur can be easily removed from PPW due its low solubility in water. Through electro-oxidation, total dissolved solids (TDS) and conductivity were also reduced. Moreover, caustic soda was also generated during this stage. Lastly, microbial fuel cell was used to reduce the chemical oxygen demand (COD) of PPW. Thus, the novelty of the method used in this study lies in treating a complex waste water and simultaneously generating value added products from it. While treating real filed PPW, 78 % sulfates were removed and 64 % were converted to sulfur. Also, 95 % COD reduction took place post treatment. All the results obtained were further validated by electrochemical analyses. Therefore, PPW was successfully treated and the key goal of making this water usable for enhanced oil recovery from crude oil reservoirs was achieved.
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