Steam Assisted Gravity Drainage (SAGD) is a proven commercial thermal technology for oil sands recovery, but it has some limitations including low energy efficiency during generation and transport of steam to the producing formation and high capital and operating costs making SAGD projects vulnerable to low oil prices. In-situ combustion (ISC) provides an alternative to steam injection with the advantages of lower costs and higher energy efficiency. In recent years, ISC has been evaluated as a follow-up process to SAGD with the expectation to combine the advantages of steam injection and in-situ combustion. Before the design of such a hybrid process, it is important to understand the chemical reactions between air (or oxygen) and residual oil within a SAGD chamber in the presence of water and steam in order to simulate the process with a reasonable degree of confidence.

In this study, an improved reaction kinetics model, in terms of Saturates, Aromatics, Resins, and Asphaltenes (SARA) fractions, is proposed for modelling the ISC process in pre-steamed Athabasca oil sands. From the results of a set of laboratory Ramped Temperature Oxidation (RTO) tests, including temperature profiles along a reactor and produced gas composition, the oxidation behavior at different temperatures has been carefully analyzed. Based on the analysis, a reaction kinetics model consisting of Low Temperature Oxidation (LTO), thermal cracking, and High Temperature Oxidation (HTO), has been developed. This model was then incorporated into CMG STARS to simulate RTO experiments.

From the simulation study, it was found that the coke, which is formed through cracking reactions and traditionally considered to be the main source of fuel in ISC, reacts slowly at high temperatures in the RTO tests. The other source of fuel for combustion in the RTO tests is light hydrocarbons distilled or cracked from the original bitumen. These findings have been incorporated into the proposed reaction model. The experimental results of seven RTO tests, including temperature profiles, oxygen consumption, and carbon oxides production, were successfully matched by tuning kinetic parameters.

This work greatly increases understanding of fuel sources in an ISC process. The newly improved reaction kinetics model can predict oxidation and combustion behavior of ISC for pre-steamed Athabasca oil sands under a wide range of temperatures. It also provides a solid foundation for future simulation studies and technical guidance for the design of hybrid steam-combustion processes.

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