TOWARDS A NEW METHOD OF GEOCHEMICAL KINETIC MODELING - IMPLICATIONS FOR THE STABILITY OF CRUDE OILS

Usual geochemical models that describe the thermal decomposition of oi ls in reservoirs use first order rate laws with activation energies th at are constant over the range 100-450 degrees C. Because these empiri cal models cannot account for numerous observations such as hydrocarbo n stability in high temperature reservoirs, we seek to develop a novel non-empirical method. This method describes cracking and alkylation s toichiometric reactions that account for the free radical nature of th e reactions taking place in oils, and is thus not empirical. It is ill ustrated by a simple case that uses a simplified mechanism of hexane p yrolysis. It is shown that below 200 degrees C; rate laws are of order 0.5 in reactant, contrary to what geochemical models imply. It is als o shown that the activation energies of the stoichiometric reactions i ncrease as temperature decreases, which is another reason why geochemi cal models are incorrect. Below about 250 degrees C, the activation en ergies of cradling stoichiometries are about 70 kcal/mol, and those of alkylation stoichiometries are about 48.5 kcal/mol. The overall pyrol ysis rate will also have an apparent activation energy of about 70 kca l/mol. It is shown that according to this model, hydrocarbons in reser voirs should be much more stable than previously thought: the half-lif e of pure hexane at 180 degrees C is several hundred million years, an d it is argued that, contrary to the predictions of usual oil decompos ition models, hydrocarbons in crude oils should be stable to at least 180 degrees C, and that the complete cracking of oil to dry gas at 200 degrees C is not possible. A strategy is proposed to extend the prese nt approach and to build a universal model of the secondary cracking o f oils that will include about 10000 reactions. (C) 1998 Elsevier Scie nce Ltd. All rights reserved.