Laboratory Research on Wet Combustion
Authors: Burger Jacques G. , Sahuquet Bernard C. 
Published: December 2007

In this study it was found that combustion-front velocity was higher in wet combustion than in dry, even at the lowest water-air ratio considered. This velocity increased as the ratio increased. It was also found that as long as a high-temperature zone existed, no coke deposit remained behind the combustion front.

Introduction

A marked interest in the use of thermal methods for recovering crude oils has been observed during recent year. A number of steam injection and forward in-situ combustion tests have been carried out on a pilot or industrial scale. However, some specific pilot or industrial scale. However, some specific technological problems as well as economic considerations have limited the application of these techniques. The use of steam injection is limited by the heat losses in the wellbore and in the formation, and the use of in-situ combustion is limited by the great amount of air required when the fuel content is high. In-situ combustion with the addition of water combines the advantages of dry combustion (i.e., in-situ heat generation) and those of steam injection (i.e., the high heat transport capacity of steam). Indeed, the addition of water during forward in-situ combustion significantly increases the amount of heat transported by the fluids from the burned-out zone to the region downward from the combustion front. Combustion tests with the addition of water have been classified according to the characteristics of the temperature and saturation profiles. Three main types of processes may be cited, corresponding to increasing values of the water/air ratio in the injected fluids:normal wet combustion,incomplete wet combustion, andsuperwet combustion.

In the case of wet combustion (normal or incomplete), the water that flows through the combustion zone is in the gas phase; that is, the peak temperature is higher than the vaporization temperature of water. The process is called normal wet combustion when the coke deposit is completely burned. But when the cooling effect of the injected water upward from the combustion front is great, the residence time of coke in the high-temperature zone may not be sufficient for its complete combustion to be achieved. This we shall refer to as incomplete wet combustion. Superwet combustion is obtained when the amount of heat available in the burned-out rock is too low to vaporize the water reaching the combustion front; the temperature peak disappears, and a vaporization-condensation plateau is likely to move through the porous medium. The use of the term "partially quenched combustion" will be avoided since it refers either to the disappearance of the temperature peak or to the incomplete combustion of the available fuel. The influence of the water/air ratio on the amount of air required for the combustion front to sweep a defined volume of the porous medium is of great importance for estimating the benefit obtained by water injection. In a simple scheme, the effect of the improved heat transport is to increase the length of the steam plateau, and the eventual decrease in the air requirement is a consequence of the incomplete combustion of the coke deposit. Under such conditions, the air requirement and the propagation velocity of the combustion front would remain unchanged in the normal wet-combustion range. However, this assumption seems questionable since the improved sweep in the steam plateau is likely to affect the oil saturation and fuel availability.

JPT

P. 1137