Abstract: Petroleum acids are strong polar components in the heavy oil system. Taking the ultra-heavy oil samples from SAGD well group of Fengcheng oilfield in Xinjiang as the research object, the molecular structures of petroleum acids were analyzed by high-energy mass spectrometry to study their influence on types, viscosity, and emulsification characteristics of heavy oil fluids. The influence of petroleum acid content on viscosity-temperature characteristics and shear rheology of heavy oil was investigated, and the influence of water cut on the apparent viscosity of heavy oil and its corresponding oil-water emulsification characteristics were analyzed under the conditions of different petroleum acid contents through experiments. The study shows that the acid value (in terms of KOH) of the ultra-heavy oil sample is 5.46 mg/g, the petroleum acid content of the sample is about 1.71%, which are dominated by 2-3 cyclic monoacids, and the carbon number is mainly distributed in the range of 15-35; the viscosity of the heavy oil decreases with the increase of the petroleum acid content; and the inflection point temperature of the sample from the Newtonian to the non-Newtonian fluid decreases from 120 ℃ to 100 ℃ when the content of the petroleum acid increases from 0 to 50%. The apparent viscosity first increases and then decreases when the water cut increases. The water cut at the antiphase point decreases from 30%-40% to 20%-30%, and the viscosity decreases dramatically at different water contents and when the petroleum acid content increases from 0 to 50%. This study reveals the influence of molecular structure characteristics and petroleum acid contents of petroleum acids in ultra-heavy oil on the fluid properties of heavy oil in western China, which provides a reference for reducing emulsification and corrosion of heavy oil and improving the efficient development of heavy oil resources.

Key words: ultra-heavy oil, petroleum acid, viscosity-temperature relationship, rheology, emulsification, Fengcheng Oilfield