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Abstract
This paper increases understanding of in-situ combustion (ISC) mechanisms based on experimental results for a Central European crude oil for which ISC has proven to be economically successful. Ramped temperature oxidation (RTO), or so- called kinetics, studies measure the rate of crude-oil oxidation. Similarly, combustion tubes packed with mixtures of sand, clay, water, and hydrocarbon measure our ability to propagate a combustion front. Through the combination of the isoconversional approach for estimation of reaction kinetics (apparent activation energy Ea, Arrhenius constant or pre-exponential factor A) and implementation of combustion tube runs under different conditions, the mechanisms behind the combustion process are elucidated. The results of seven combustion tube runs are presented and discussed in terms of repeatability, effect of grain surface area, effluent gas concentration oscillations, stoichiometry, minimum air flow rate and recovery efficiency. Based on experimental results, critical parameters for field application as well as for simulation are derived (hydrogen/carbon-ratio, air requirements). Opposed to previous publications, the ISC process is described in terms of stoichiometry for the entire tube run, giving insight into development of hydrogen/carbon-ratio and other important parameters over time. This helps to compare, verify, and tune simulation results obtained from commercial simulators. Results obtained point out the exceptional efficiency of ISC in terms of recovery and fuel consumed. Monitoring combustion stoichiometry over time gives an increased insight into flue-gas composition oscillations.

Introduction
Though in-situ combustion (ISC) projects have proven to be economically successful (e.g. Carcoana (1990)), industry has always been reluctant to apply ISC on a broader scale (Sarathi (1999)). Designing a field project is rather involved because of the complex nature of the process. Lab experiments, a prerequisite for screening, estimation of compressor capacity requirements, and simulations, are very challenging to be performed in a controlled environment. Ideally, kinetic cell and combustion tube experiments are carried out to determine whether or not an oil is a proper candidate for an ISC project (Cinar et al. (2009)). Kinetic cell experiments, usually carried out as RTO experiments combined with effluent gas analysis, give insight into the mechanism behind the kinetics of combustion (Cinar et al. (2009)). Combustion tube experiments are designed to study the combustion process in one dimension (Moore et al. (1997)).