Temperature Dependence of the Electron Spin–Lattice Relaxation Time of Vanadyl Porphyrins in Asphaltenes from the Ashalcha Oilfield

Abstract

Oil asphaltenes are known as the cholesterol of petroleum because of their ability to precipitate, deposit, and interrupt the continuous production of oil from underground reservoirs. Studies of asphaltenes by various analytical techniques allow fundamental understanding of their structure for optimizing the processes of geologic exploration and enhanced oil recovery. The purpose of this work is to analyze the temperature dependences of the electron relaxation times of the intrinsic for oil asphaltenes paramagnetic vanadyl-porphyrin (VP) complexes. Asphaltenes from the viscous Ashalcha oil (2500 mPa·s) were investigated using pulsed electron paramagnetic resonance (EPR) techniques at X-band (9 GHz) in the temperature range T = 10–300 K. It is found that at T > 100 K electron spin–lattice (longitudinal) relaxation rate obeys the power law Tⁿ with n ≈ 3.0 while at T < 100 K direct relaxation process is more effective. An attempt to describe the relaxation data in the frameworks of the solid-state Debye model and fractal model are done. These models have little applicability to the type of asphaltenes studied.