Practical kinetic modeling of petroleum generation and expulsion

Models for petroleum generation used by the industry are often limited by (a) sub-optimal laboratory pyrolysis methods for studying hydrocarbon generation, (b) over-simple models of petroleum generation, (c) inappropriate mathematical methods to derive kinetic parameters by fitting laboratory data, (d) primitive models of primary migration/expulsion and its coupling with petroleum generation, and (e) insufficient use of subsurface data to constrain the models. Problems (a), (b) and (c) lead to forced compensation effects between the activation energies and frequency factors of reaction kinetics that are wholly artificial, and which yield poor extrapolations to geological conditions. Simple switch or adsorption models of expulsion are insufficient to describe the residence time of species in source rocks. Yet, the residence time controls the thermal stresses to which the species are subjected for cracking to lighter species. The ShellGenex model for petroleum generation and expulsion has been developed since 1988. In this multi-species model, the rate-limiting step of primary migration is treated as the slow diffusion of the petroleum through the kerogen itself. Other key aspects of the model are: the reaction and transport behaviour of the heavy compounds; the role of free radicals in the cracking kinetics of hydrocarbon species in the model; and the evolving free volume of the kerogen. The last of these has a particularly strong effect on the primary migration of the generated petroleum molecules. Some implications of this model are that (a) compositions of expelled petroleum vary more strongly than the standard parallel-reaction model suggests, and (b) expulsion is not immediately extinguished by waning temperatures and pressures, but continues with decreasing GOR of the expelled petroleum and increasing density of the expelled oil. (C) 2009 Elsevier Ltd. All rights reserved.