Effects of Pressure on Oil Thermal Cracking: Insights on the Data from High-Pressure Optical Cell (HPOC) Experiments

Abstract

Abstract The role of pressure in the thermal cracking of crude oil are still under debate. In this study, a novel method employing high-pressure optical cell (HPOC) and fluorescence spectrometry was used to in situ monitor the thermal cracking of oil under elevated temperatures and pressures, and investigate the effects of pressure. Specifically, two HPOC cells loaded with crude oil samples were simultaneously heated on the same heating-cooling stage but at different pressures (i.e. 30 MPa and 100 MPa). Changes in the attributes of pyrolytic products (i.e. physical appearance, fluorescent colors and intensity, etc.) were real-time recorded. The fluorescence spectra were measured every half hour to semi-quantify the chemical evolution of pyrolytic products. The results showed that the fluorescent color of pyrolytic products generated at 100 MPa evolved from blue to greenish yellow as heating, and finally turned to yellow at the end of pyrolysis experiments. In contrast, the fluorescent color in the 30 MPa cell finally turned to deep blue, instead of yellow, after 12 hours’ heating. A light blue bubble of primarily light, gaseous cracking products was observed after 5 hours’ heating at 30 MPa, whose volume expanded and finally filled up the whole cell. In contrast, no such bubbles were observed in the 100 MPa cell. The λmax values derived from fluorescence spectra shifted initially toward longer but later toward shorter wavelengths in the 30 MPa cell (i.e. an initial redshift followed by a later blueshift), distinctly contrasting the evolutional pattern of monotonically shifting of λmax toward longer wavelengths as observed at 100 MPa (i.e. a constant redshift). The red-green quotient (Q =I650/I550) based on the λmax values can be used to semi-quantify the thermal maturity and stability of chemical substitute. The smaller Q values in the 30 MPa cell clearly suggested that the chemical compounds in the 30 MPa held higher thermal maturity and probably subject to a more intense thermal cracking. The distinction between optical, physical, and chemical properties of pyrolytic products at different pressures are direct evidence proving that high pressure can hinder the thermal cracking of oil to gases. Therefore, the burial depth range at which petroleum can still remain as liquid phase might be extended at high pressure, which is vital for the establishment of exploitation strategies of deep reservoirs a high pressure.