Date Published: July 26, 2017
Publisher: Public Library of Science
Author(s): Mohita Sharma, Dongshan An, Tao Liu, Tijan Pinnock, Frank Cheng, Gerrit Voordouw, John M. Senko.
Coiled tubing corrosion was investigated for 16 field water samples (S5 to S20) from a Canadian shale gas field. Weight loss corrosion rates of carbon steel beads incubated with these field water samples averaged 0.2 mm/yr, but injection water sample S19 had 1.25±0.07 mm/yr. S19 had a most probable number of zero acid-producing bacteria and incubation of S19 with carbon steel beads or coupons did not lead to big changes in microbial community composition. In contrast other field water samples had most probable numbers of APB of 102/mL to 107/mL and incubation of these field water samples with carbon steel beads or coupons often gave large changes in microbial community composition. HPLC analysis indicated that all field water samples had elevated concentrations of bromide (average 1.6 mM), which may be derived from bronopol, which was used as a biocide. S19 had the highest bromide concentration (4.2 mM) and was the only water sample with a high concentration of active bronopol (13.8 mM, 2760 ppm). Corrosion rates increased linearly with bronopol concentration, as determined by weight loss of carbon steel beads, for experiments with S19, with filtered S19 and with bronopol dissolved in defined medium. This indicated that the high corrosion rate found for S19 was due to its high bronopol concentration. The corrosion rate of coiled tubing coupons also increased linearly with bronopol concentration as determined by electrochemical methods. Profilometry measurements also showed formation of multiple pits on the surface of coiled tubing coupon with an average pit depth of 60 μm after 1 week of incubation with 1 mM bronopol. At the recommended dosage of 100 ppm the corrosiveness of bronopol towards carbon steel beads was modest (0.011 mm/yr). Higher concentrations, resulting if biocide is added repeatedly as commonly done in shale gas operations, are more corrosive and should be avoided. Overdosing may be avoided by assaying the presence of residual biocide by HPLC, rather than by assaying the presence of residual surviving bacteria.
Flexible coiled tubing, which can be spooled onto a reel, is used in shale oil and shale gas operations for milling plugs from horizontal wells after hydraulic fracturing and other applications . Recirculation of fluids in such closed loop operations has been associated with increased coiled tubing failures . The presence of sand, fracturing chemicals, microbes, high rates of fluid injection and coiling-related material stresses may all contribute to high generalized and localized corrosion of coiled tubing [2–5], which is exposed to injection fluids internally and production fluids externally. When used in acidic and sour (H2S-containing) environments, coiled tubing is also susceptible to pitting and crevice corrosion, which may be a major cause of failure [3,6]. Microbially influenced corrosion (MIC), due to the action of sulfate reducing bacteria, has been observed in coiled tubing biased weld points , necessitating the use of biocides .
The high weight loss corrosion rate of carbon steel beads determined for S19 (1.25±0.07 mm/yr), an injection water sample from a coiled tubing operation in a shale gas field, was correlated to the high concentration of active bronopol in this sample (Fig 4). Corrosion rate measurements by weight loss and by electrochemical tests indicated that increasing concentrations of bronopol increased the corrosion rate. Increased pitting was also evident. Hence, overdosing of bronopol should be avoided.