New IFP Compositional Kinetics of Oil Generation and Degradation

Françoise Behar and François Lorant
Institut Français du Pétrole, 1-4 Avenue de Bois Préau, Rueil-Malmaison, France

The present work summarizes the state of the art for IFP compositional modeling of oil generation and degradation. It includes the methodologies for describing kerogen and oil cracking, the elaboration of complex kinetic schema and their reduction to 8 and 6 lumped schema.

The study of kerogen cracking in either open or closed system enables to understand (i) the main steps of kerogen thermal decomposition and (ii) the mass balances obtained in the two systems. In both systems, NSOs are major compounds generated from kerogen cracking. In closed system, these compounds are generated earlier than the main phase of hydrocarbon generation. Thus, the following primary kinetic schema was elaborated:

This new kinetic schema actually enables to account for kerogen cracking in both closed and open system pyrolysis. In open system (e.g., Rock Eval pyrolysis), due to the very high content of the NSOs produced at the onset of kerogen cracking, a major part of the generated NSOs are not swept away by the carrier. The NSOs which cannot be vaporized because of their high molecular weight remain in the pyrolysis chamber and undergo secondary cracking with increasing severity. Consequently, the kinetics obtained in open system is a combination between the kerogen cracking itself and the decomposition of NSOs generated during kerogen cracking. Thus, two successive reactions might be taken into account for the total conversion of both the kerogen and the total generated NSOs in open system:

For oil cracking, a new compositional kinetic schema was derived from a new set of pyrolysis conditions according to the relative thermal stability of the different chemical classes involved. Initial guesses in the optimization procedure were constrained by kinetic study on representative model compounds. Oil composition is described as a mixture of C₆-C₁₄ saturates, C₆-C₁₄ aromatics, C₁₄₊ saturates, C₁₄₊ unstable aromatics, and the NSOs (resins + asphaltenes). Among the generated products are included H₂S, C₁, C₂, C₃ and C₄ gases, 2 classes of poly aromatic compounds which are char precursors and solid residue. Results show that the optimized kinetic schema for oil cracking fits perfectly experimental data with a maximum deviation of 2 %. Extrapolated to geological conditions, the model predicts the observations done on natural fluids: aromaticity strongly decreases between laboratory and geological conditions whereas aliphaticity increases. Moreover, GOR values is at low temperatures whereas gas dryness is higher. Finally, the kinetic schema was lumped into 6 classes, making the use of the kinetic model more applicable to basin simulation studies, while preserving the same accuracy for predicting oil and gas properties in geological conditions than the complete kinetic schema.

AAPG Search and Discover Article #90066©2007 AAPG Hedberg Conference, The Hague, The Netherlands