Industrial radioactivity in the UK's onshore oil and gas industry
2017-04-12T15:44:43Z (GMT) by
Naturally occurring radioactive material (NORM) is a common feature in North Sea oil and gas production offshore but, to date, has been reported from only one production site onshore in the United Kingdom. This site, Wytch Farm on the Dorset coast, revealed high activity concentrations of lead-210 in metallic form but little evidence of radium accumulation. NORM has now been discovered at two further onshore sites in the East Midlands region of the UK. The material has been characterised in terms of its mineralogy, bulk composition and disequilibrium in the natural uranium and thorium series decay chains. In contrast to Wytch Farm, scale and sludge samples from the East Midlands were found to contain elevated levels of radium and radioactive progeny associated with crystalline strontiobarite. The highest radium-226 and radium-228 activity concentrations found in scale samples were 132 and 60 Bq/g, with mean values of 86 and 40 Bq/g respectively, somewhat higher than the mean for the North Sea and well above national exemption levels for landfill disposal. The two East Midlands sites exhibited similar levels of radioactivity. Scanning electron microscope imaging shows the presence of tabular, idiomorphic and acicular strontiobarite crystals with elemental mapping confirming that barium and strontium are co-located throughout the scale. Bulk compositional data show a corresponding correlation between barium-strontium concentrations and radium activity. Scales and sludge were dated using the 226Ra/210Pb method giving mean ages of 2.2 and 3.7 years, respectively. The results demonstrate clearly that these NORM deposits, with significant radium activity, can form over a very short period of time. Although the production sites studied here are involved in conventional oil recovery, the findings have direct relevance should hydraulic fracturing or fracking for shale gas be pursued in the East Midlands oilfield. Highly saline flowback fluid containing above regulation levels of radium has already been observed as a result of exploratory fracking operations in the North West s Bowland Shale and now appears a likely outcome if similar operations are conducted in the East Midlands. Barite (barium sulphate) coprecipitation is a cost-effective method for removing radium from solution. For the first time, reagent quantities for radium recovery by barite coprecipitation have been optimised in saline solutions of ionic strength (IS) 0.3 and 3 M. The process is highly effective, removing a maximum of ~90% radium in a single optimised coprecipitation step, but requiring substantially more sulphate at 3 M than at 0.3 M ionic strength to reach this maximum. It has been shown that, if left, the radiobarite precipitate will continue to remove radium by post-precipitation (recrystallisation). A pattern of diminishing returns in radium recovery by post- precipitation at each IS was observed; the rate was faster in the first 24 hours then dropped significantly over 14 days. After 14 days >80% of the radium was recovered, slightly more radium being removed at IS = 0.3 M. Therefore, if >90% radium recovery is sought, further co- and post-precipitation steps could be undertaken to approach the UK s 0.01 Bq/dm3 radium aqueous waste discharge exemption limit. However, compliance with UK s aqueous waste regulations would require multiple coprecipitation steps to recover >99.9% radium, which may not be feasible in practice. The Bowland Shale has high shale gas prospectivity and may be subject to further fracking operations in the future. Should local groundwaters mix with flowback fluid, there is potential for drinking water supplies to become contaminated. Baseline monitoring of groundwater was conducted in the region, so that comparisons may be made following potential fracking operations. The determined mean baseline for radium-226 and uranium-238 were 15.5 and 11.1 mBq/dm3 respectively, both below their respective notifiable drinking water level in the UK. In the majority of the groundwaters, barite is supersaturated and geochemical modelling has shown that a considerable proportion of the aqueous radium could precipitate in the Ba1-xRaxSO4 solid solution. A further increase in dissolved radium, barium and sulphate as a result of fracking will greatly increase the likelihood of concentrating the radium in barite scales, commonly found in oil and gas production equipment. What this means in terms of the potential NORM inventory is that Ra-containing NORM could manifest itself as fluid or solid waste.