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Characterization of Crude Oils That Naturally Resist Hydrate Plug Formation

Aman, Zachary M., Syddall, William G. T., Haber, Agnes, Qin, Yahua, Graham, Brendan, May, Eric F., Johns, Michael L., Pickering, Paul F.
Energy & Fuels 2017 v.31 no.6 pp. 5806-5816
autoclaves, dispersions, emulsions, mechanics, oils, petroleum, rheometers
The high operating pressures and distances of deep water tiebacks increase the likelihood of hydrate blockage during transient operations such as shut-in and restart. In many cases, complete hydrate avoidance through chemical management may become cost prohibitive, particularly later in the field’s life. However, a subclass of crude oils has been observed in which hydrate blockages do not form during restart, rendering active hydrate prevention unnecessary. Over the past 20 years, limited information has been reported about the chemical or physical mechanisms that enable this plug-resistive behavior. This study presents an extensive and systematic method of characterizing whether an oil may naturally resist hydrate plug formation, including (i) chemical and physical property analysis, (ii) water-in-oil emulsion behavior, and (iii) the effect of the oil on hydrate blockage formation mechanics. This last set of experiments utilizes both a high-pressure rheometer and a sapphire autoclave to allow direct observation of hydrate aggregation and deposition, combined with resistance-to-flow measurements. The effect of shut-in and restart on the oil’s plugging tendency is also studied in these experiments. The method was tested with seven petroleum fluids, some of which naturally resisted plugging-type behavior even at hydrate volume fractions up to 50%. Most of the fluids presented in this study did not form stable water-in-oil emulsions but did form stable, nonagglomerating hydrate-in-oil dispersions. The oils suppress hydrate formation rates, and their resistance-to-flow does not increase significantly even when the amount of hydrate present would normally result in a plug.