Biotreatment of Hydrate-Inhibitor-Containing Produced Waters at Low pH

Abstract

Summary With proper treatment to remove organics and inorganics, one can use the produced water (PW) generated during oil-and-gas extraction as process water. Biotreatment is generally regarded as the most cost-effective method for organics removal, and although widely used in industrial wastewater treatment, PW biotreatment installations are limited. This paper follows up to an earlier paper published in the SPE Journal (Janson et al. 2014). Although the earlier paper assessed the biotreatability of PW from a Qatari gas field from the summer season, this paper focuses on assessing the biotreatability of PW during the winter season [i.e., containing the thermodynamic hydrate inhibitor monoethylene glycol (MEG) and a kinetic hydrate inhibitor (KHI)]. Tests were conducted in batch and continuous reactors under aerobic mixed-culture conditions without pH control during 31 weeks. The results indicated that one could remove >80% of the chemical oxygen demand (COD) and total organic carbon (TOC) through biological treatment of PW with 1.5% MEG added. In contrast, biotreatment can remove only ≈43% of COD and TOC present in PW when 1.5% KHI was added as a hydrate inhibitor; 2-butoxyethanol, a solvent in KHI, is extremely biodegradable; it was reduced in concentration from >5000 to <10 mg/L by biotreatment; the KHI polymer though was only partially biodegradable. Cloudpoint tests conducted on PW with 1.5% KHI added showed only an 8°C increase in cloudpoint temperature (from 35 to 43°C). The target cloudpoint temperature of >60°C was not achieved. Although the feed to the reactors (PW with either KHI or MEG) was at pH 4.5, the reactors stabilized at a pH of 2.6, considered extremely acidic for aerobic bioactivity. The successful operation of an aerobic biological process for an extended period of time at a pH of 2.6 was unexpected, and published reports of bioactivity at that pH are limited. After extensive analytical tests, it was concluded that the pH decrease was caused by the production of an inorganic acid. A mechanism by which hydrochloric acid could be produced biologically was proposed; however, further research in this area by the academic community is recommended.