Influence of Chemical Reactions on In Situ Combustion

Abstract

In-situ combustion (ISC) is an enhanced oil recovery process during which air or oxygen-enriched air is injected into a reservoir. The oil in the reservoir reacts with the oxygen and the so-called combustion front is formed and propagates through the reservoir, generating heat and flue gases. During the process, numerous chemical reactions take place in different zones and temperature ranges. For the description of the process the oil is represented by pseudo components. The definition of the pseudo component defines the reaction schemes implemented in the numerical simulator. The reaction kinetics are described by relative simple order reactions for which the reaction rates are calculated using the Arrhenius-type equations. Estimating the input parameters of the Arrhenius equation is a giant obstacle in ISC modelling. Combustion tube experiments are performed to acquire oil, water and gas production data, the effluent composition and temperature profiles which depend on the oil and reservoir rock properties. Estimating the Arrhenius parameters can be done by history matching these experiments. Due to the quite large amount of parameters non-unique solutions are found. Unfortunately, so far the resulting adjusted parameters are not tested if they describe a chemical-physical sound and realistic behavior. In this research an ISC tube experiment with an Athabasca bitumen was simulated using a commercial thermal simulator (CMG STARS). The cumulative oil and gas production and the temperature profiles of the experiment were used for verification of the simulations. The first simulation was done with the input parameters as stated by Yang and Gates (2009). In this simulation the reaction rate parameters were chosen such that coke formation from asphaltene by cracking already commences at temperatures of around 343 K and coke formation from asphaltenes by oxidation at temperatures of around 650 K. Further, in the applied reaction schemes methane combustion is assumed to be up to a factor 1030 slower than hydrocarbon gas combustion. In this study, the reaction kinetics were changed to see the influence of the reaction kinetics parameters of asphaltene cracking and asphaltene oxidation at lower temperatures. Further, the reaction rates describing methane combustion was set equal to the kinetic parameters of hydrocarbon gas combustion. From these simulations it was found that the hydrocarbon gas combustion reaction does not significantly influence the ISC process. Changing the reaction kinetics of asphaltene cracking and oxidation does influence the ISC process significantly; asphaltene cracking occurs fasters and starts at lower temperature, more coke is formed and combusted in the simulation but less oil is produced than in the base case. Furthermore, the injection rate of the air was varied to identify the impact of the fuel/oxygen ratio on the production data. A higher air injection rate shows that the combustion front moves through the reservoir in a shorter amount of time; which indicates that it is possibly economically favorable to inject air at a higher rate into an oil reservoir in which ISC is conducted.