Improved approach for modelling reaction kinetics for organic-rich shales, particularly oil shales

Abstract

Two methods are compared to calculate credible reaction kinetic distributions that fit S2 pyrogram curves generated by multi-heating rate pyrolysis analysis of oil shales. The fixed-A method constrains all reactions in the distribution to conform to a single frequency factor. The variable E-A (Var E-A) method allows the activation energy (E) and A to vary independently for each reaction. The reaction kinetics are important as they determine the temperature ranges at specific heating rates at which oil and gas products are generated. This study evaluates the two kinetic fitting methods using published pyrogram details from three samples of kerogen extracts from oil shale (China, Jordan, U.S.A) and one whole rock oil shale sample (China). The component reactions of the Var E-A solutions conform to the lower end of an empirically observed E-lnA trend defined by a large number of organic-rich shale formations; the fixed-A solutions do not. The temperature ranges of some Var E-A solution reactions are broader than those of the fixed-A solution, meaning that fewer reactions are required to provide effective pyrogram fits. The Var E-A solutions provide more credible explanations of the aromatization processes governed by first- and second-order reactions. involved in forming bitumen, oil/gas products and carbon residues during the thermal maturation of kerogen. A conceptual model is proposed to explain this. The differences between the fixed-A and Var E-A kinetic solutions have important implications for the design of in-situ conversion processes (ICP) for generating oil and gas products from oil shales.