Ultrasonic studies of relaxation phenomena in crude oils and their solutions

Asphaltenes are operationally defined as non-volatile and polar components of crude oils soluble in toluene and insoluble in $n$-heptane. They gradually plug production wells and pipes because of flocculation and precipitation causing technical problems and financial losses. The flocculation is reversible. Absorption of ultrasound have been measured in three crude oils and their mixtures with toluene at frequencies from 2.4 to 80 MHz at $T\,\hbox{=}\,298.15$ K. Processes with relaxation times in the interval from 10 to 100 ns were found in the crude oils. The relaxation may be due to aggregation of asphaltenes, that in favourable conditions may lead to flocculation. Relaxation processes in the solutions are faster, with relaxation times partly outside the measurement range. Most probably, the molecules of asphaltenes easier translate and rotate in the solutions than in viscous crude oils. If the proposed model is correct, one may expect relaxation times shorter in deasphaltenated oils. On the other hand, the relaxation may be ascribed to bending long hydrocarbon chains. That process is also rather slow