Experimental design is a widely used statistical method for understanding the factors that impact an information gathering exercise such as mechanistic simulations. It is used here to gain insight into flow and operating conditions that affect in-situ combustion. Specifically, experimental design was applied to understand in-situ combustion of very heavy oil using a commercial thermal simulator. The design parameters investigated were selected based on combustion laboratory results and literature. These included activation energies of the reaction schemes, oil saturation, air injection rate and pressure control at the producer. A full factorial design was used to create the parametric space considering interactions between the parameters. The degree to which the combustion peak temperature, pressure, combustion front speed, recovery efficiency and coke deposited changed was used to determine the most critical parameters. Results showed the activation energy of the coke deposition reaction had the greatest influence on the combustion process. For the 3-reaction pathway assumed for crude-oil combustion, a slight increase in this activation energy inhibited the combustion reactions. The injection pressure of the system was impacted significantly by the initial oil saturation while the speed at which the front propagated was shown to be primarily a function of the air injection rate. Results also showed that for a given initial oil saturation, a limiting air flux existed below which oil plugging of the linear system occured. Given successful ignition and propagation of the front, optimal recovery was obtained between initial oil saturations of 45-70%. We also observed interaction between the parameters. The effect of the activation energy of the combustion reaction on the recovery was strongly dependent on the amount of oil initially in place. Recovery decreased with increasing activation energy at lower initial saturations, but was independent of activation energy at greater initial saturations. In addition, the velocity profile showed different incremental oil recovery rates with different initial oil saturation.
This paper presents a new workflow for the simulation of in-situ combustion (ISC) dynamics. In the proposed method, data from kinetic cell experiments, depicting the combustion chemistry, are tabulated and graphed based on the isoconversional principle. The tables hold the reaction rates used to predict the production and consumption of chemical species during in-situ combustion. This new method of representing kinetics without the Arrhenius method is applied on one synthetic and two real kinetic cell experiments. In each case, the new method reasonably captures the reaction pathways taken by the reacting species as the combustive process occurs. A data-density sensitivity study on the tabulated rates for the real case shows that only four experiments are required to capture adequately the kinetics of the combustion process. The results are, however, found to be sensitive to the size of the time step taken. The method predicts critical changes in the reaction rates as the experiment is exposed to different temperature conditions, thereby capturing the speed of the combustion front, temperature profile, and fluid compositions of a simulated combustion tube experiment. The direct use of the data ensures flexibility of the reaction rates with time and temperature. In addition, the non-Arrhenius kinetics technique eliminates the need for a descriptive reaction scheme that is typically computationally demanding, and instead focuses on the overall changes in the carbon oxides, oil, water and heat occurring at any time. Significantly, less tuning of parameters is required to match laboratory experiments because laboratory observations are easier to enforce.
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