Exploration of the difference in molecular structure of n-C₇ and CO₂ induced asphaltenes

Determination of the molecular structure of asphaltenes, especially in their native environment, is a formidable challenge in petroleum chemistry. Here we demonstrate that a combination of different spectroscopy and imaging based experimental techniques can be utilized to determine structures of asphaltenes, which have precipitated out of a crude oil, in an environment similar to real field conditions. A high pressure–high temperature quartz crystal microbalance (HPHT-QCM) setup can be used to detect asphaltene onset at oil production conditions. HPHT-QCM can also simulate CO2 injection conditions mimicking gas injection methods used to enhance oil recovery from depleted oil reservoirs. In this paper, we present the first compositional and structural research study on the QCM asphaltene deposits under gas injection conditions and compare it to n-C7 asphaltenes from the same crude oil precipitated in the laboratory. This study combines the use of Fourier transform infrared (FTIR) spectroscopy, nuclear magnetic resonance (NMR), environmental scanning electron microscopy (ESEM), and energy dispersive X-ray (EDX) analysis. Furthermore, deposits collected from chemically treated fluids were also studied. The HPHT-QCM asphaltene deposits from parent crude oil are richer in oxygen species, such as the Ox and R–OH polar groups, relative to the n-C7 asphaltenes. The results of this study provide high-pressure information that leads to better understanding of asphaltene precipitation and deposition phenomena and could be taken into account when designing prevention strategies to avoid asphaltene problems throughout the oil production process.