Estimation of combustion zone thickness during in situ combustion processes

Dry and wet combustion runs were conducted using three different crude oils having 12.4, 19.8 and 28.4 degrees API gravity. Combustion zone thickness measurements were made with four stationary in-situ sampling gas probes installed on the combustion tube at 36, 48, 60, and 72 cm from the top flange. Experiments were carried out in a thin-walled vertical tube having a 12.4 cm diameter and 124.5 cm length. Adiabatic control of the combustion tube was achieved during dry and wet combustion tests using external band heaters and an adiabatic Control system. The average air flux was 23.0 m(3) (St)/(m(2) h), and injected WAR changed from 0.209 to 0.607 m(3)/Mm(3)(St). Experimental results showed that the average zone thickness was 80 mm for a dry combustion process while it was 50 mm for a wet combustion process. The combustion zone thickness decreased with increasing degrees API gravity of crude oils. With water injection, the combustion zone thickness decreased, as expected in a wet combustion temperature profile. Temperature and oxygen concentration data were used to evaluate the commonly used combustion reaction rate expression. The experimentally determined relationship between oxygen, combustion temperature, and reaction rate could not be represented by the commonly used first-order, Arrhenius-type reaction rate expression. Relative reaction rate data showed that dry combustion was kinetically controlled, whereas the wet combustion was oxygen-diffusion limited.