Abstract
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.
Highlights
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 [1]
We have described an integrated approach of field water sample analyses, which included determination of water chemistry, most probable numbers of microorganisms, microbial community composition and general corrosion rates to determine possible causes for high corrosion rates in coiled tubing [8]
Sixteen field water samples (S5 to S20) from a coiled tubing milling operation in a shale gas field near Grande Prairie, Alberta, Canada were collected in autoclaved sterile 1 L plastic bottles. These were either SP1, SP2, SP3 or SP4 type, as explained in Fig 1, and were collected over 3 days of operation (Table 1), in which coiled tubing was used to mill out the plugs present in a horizontal well successively to clear the well for gas production after fracturing
Summary
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 [1]. Recirculation of fluids in such closed loop operations has been associated with increased coiled tubing failures [2]. Shell Global Solutions International, Suncor Energy Inc., Yara Norge AS and Alberta Innovates. STEP Energy Services, National Oilwell Varco and Fusion Technologies Inc supported this work and provided the field samples. Field operators from Fusion technologies Inc. collected the samples from the field and delivered them to our lab. The funders had no role in study design, data collection and analysis, decision to publish or preparation of the manuscript
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