Pressure Retardation of Oil Cracking from High Conversion Levels in Hydrocarbons

Haitham Al-Masroori¹, Colin Snape^1*, Will Meredith¹, Clement Uguna¹, Andrew Carr², Iain Scotchman³, and Robert Davis^4
1Nottingham Fuel & Energy Centre, School of Chemical and Environmental Engineering, University of Nottingham, University Park, Nottingham, NG7 2RD, UK
²Advanced Geochemical Systems, Burton-on-the-Wolds, Leicestershire, LE12 5TD, UK and British Geological Survey, Keyworth, Nottingham, NG12 5GG, UK
³Statoil (UK) Ltd, Statoil House, 11A Regent Street, London, SW1 4ST, UK
⁴Woodside Energy (USA) Inc, Sage Plaza, 5151 San Felipe, Houston, Texas 77056, USA
*Corresponding author: [email protected]

To explain the preservation of oil in high temperature high pressure basins, we recently conducted experiments on oil cracking in pressurised liquid water which better simulates actual sub-surface conditions than regimes where vapour is present. It was found that cracking of oil and n-hexadecane to hydrocarbon gases at 350^oC and moderate water pressures up to 500 bar is retarded significantly compared to experiments under normal hydrous conditions (both liquid and vapour present). Combination reactions were observed for the first time in pressurised water; involving the disappearance of lighter hydrocarbons (<C₁₂)and increases in the amount of unresolved complex material. These reactions favoured by increasing pressure provide a new mechanism for rationalising the thermal stability of oils and for producing heavy oils at temperatures above which biodegradation can occur. The extent of cracking is very small at 350^oC and to investigate the effects of water pressure at high conversions, we have now carried out experiments oil and n-hexadecane using anhydrous conditions, low pressure superheated steam and high pressure supercritical water (390 bar) at 420^oC. Under anhydrous conditions, coke and hydrocarbon gas yields of ca. 10 and 25 % are obtained with the remaining oil being very light. Despite the change in water phase, the coke yield was suppressed five fold and hydrocarbon gas yield two fold in supercritical water at 420^oC compared to low pressure steam and anhydrous conditions. Moreover, the oil remaining is considerably heavier in supercritical water, providing evidence that oils can survive considerably longer when pressurized in high temperature geological basins.

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