Independent effects of microwave irradiation and acidic solutions on asphaltene content and upgrading of heavy crude oil

Abstract

This research explores the individual effects of microwave irradiation and acidic aqueous solutions on the key physicochemical properties of heavy crude oil, including asphaltene concentration, viscosity, density, and interfacial tension (IFT). Crude oil samples were treated using microwaves at 780 W (60 %) and 1300 W (100 %) for 1 to 3 min. At 1300 W, the oil temperature rose from 26 °C to 85 °C within 3 min, resulting in a 9 % reduction in asphaltene content after just 1 min of exposure. Viscosity initially increased from 377 cP to 410 cP due to light hydrocarbon evaporation but later decreased to 295 cP as heavier components such as asphaltenes and resins fragmented under microwave influence. Acidic solutions (15 % HCl) prepared with distilled water (ADW), seawater (ASW), and formation water (AFW) displayed different behaviors. ADW led to the most effective sludge reduction, while AFW, with a TDS of 215,502 ppm, exhibited the highest sludge resistance due to its ionic content (e.g., Ca²⁺ = 28,036 ppm, Mg²⁺ = 6,561 ppm). FTIR analysis revealed significant increases in S=O and C=O functional group indices, and NMR confirmed increased aromaticity and polarity after microwave and acid treatments. In terms of interfacial tension, initial IFT values dropped from 24.5 mN/m to 15.2 mN/m under acidic conditions and further to 12.1 mN/m after microwave exposure at 100 % power. However, IFT began increasing again after 3 min due to surfactant evaporation and structural reorganization. Density first increased from 0.928 g/cm³ to 0.946 g/cm³ due to light component loss, then decreased to 0.917 g/cm³ after continued breakdown of heavier molecules. Overall, microwave treatment at 1300 W for 2–3 min proved most effective for reducing viscosity and asphaltene concentration, while acidic solutions especially ADW, were more impactful in modifying IFT and reducing sludge stability.