Experimental Study of Compositional Factor on Asphaltene Deposition for Heavy Crude Oil Dilution in Offshore Production and Transportation

Heavy oil dilution has been widely used in the oil production and transportation due to its high efficiency in viscosity and drag reduction, and great convenience in operation. The composition of the heavy oil will change while being mixed with some diluents, thus the stability of the asphaltene in the heavy oil might be destroyed, which leads to a tremendous threat to the safe and economic operation of the production and transportation system. The asphaltene contents of eight onshore and offshore oil samples were measured using n-heptane and toluene, then the asphaltene deposition onset points of the oils diluted with n-alkanes (n-C7) were evaluated using viscosity methods. The reliability of the asphaltene deposition predicted by the refractive index of the diluted oils was verified, and meanwhile the impacts of n-C5, n-C7 and n-C8 on the asphaltene precipitation behavior were measured and analyzed. And then the status of the asphaltene deposition, suspending particle distribution and adhesion of the heavy oil diluted with diesel in the stainless wire mesh located in the visible loop pipe layout was investigated. The studied results demonstrate that the asphaltene deposition onset point has no direct relation to its content, and those of the eight diluted oils ranged from 15% to 30% at 70 °C. The onset point prediction method was verified to be reliable because it is based on that the critical solubility parameter and the square root of the diluent molar volume in the Asphaltene-Instability-Trend (ASIST) curve present a good linear relation. The relationship between the refractive index of the diluted oil and its asphaltene deposition onset point depends on the light oil type, and the smaller its carbon number, the more serious the asphaltene deposition in its diluted oil. The reasonable amount of a light oil blended with a heavy oil should well consider the light oil source, its economy and the asphaltene deposition risk at the same time.