Study of the Surface Area Effect on Crude Oil Combustion by Thermal Analysis Techniques

Summary Prediction of underground fireflood (in-situ combustion) Prediction of underground fireflood (in-situ combustion) performance requires reliable kinetic models for the performance requires reliable kinetic models for the chemical reactions occurring during the process. The composition of the host rock, especially its clay content, seems to have a significant effect on the process. Clays possess a large surface area and also contain heavy metal possess a large surface area and also contain heavy metal derivatives. This study was conducted to investigate the effect of surface area on crude oil combustion. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were performed on crude oil alone and in the presence of clays, silica, and alumina with variable presence of clays, silica, and alumina with variable specific surface areas ranging from 30 cm2/g to 24.3 m2/g [4.6 sq in./g to 262 sq ft/g]. Decreasing the crude oil/surface area ratio enhanced the low-temperature oxidation (LTO) peak. It also was noticed that additives with large specific surface area shifted a large portion of the exothermic heat from a higher to a lower temperature range. The fractional shift correlates with the crude oil/surface area ratio of the mixture. Activation energies calculated for the crude oil combustion of the samples with a low crude oil/surface area ratio were significantly lower than those of samples with a high value of crude oil/surface area ratio. The surface area of the additives seemed to affect the crude oil combustion regardless of the composition of the additives. Introduction Crude oil combustion and the factors affecting it play an important role in the performance prediction of an in-situ combustion process. In-situ combustion, sometimes called "fireflood," is an EOR technique mostly suitable for medium to heavy crude oil reservoirs. In this process the residual oil undergoes significant physical and chemical changes, which lead to a solid or semiliquid type material called "coke." Coke eventually is burnt and acts as a fuel to sustain the firefront. The overall transition of crude oil to coke and the eventual combustion of the coke are of utmost importance. It is the same characteristic nature of the process that makes thermal analysis techniques such as TGA and DSC highly viable research tools in this area. We realize that TGA and DSC experiments lack the fluid flow characteristics of a fireflood. However, the physical and chemical changes that crude oil undergoes during an in-situ combustion process can be simulated easily in a TGA and/or DSC process can be simulated easily in a TGA and/or DSC experiment by controlling environment and heating rate. More importantly, the effect of different variables, such as crude oil composition and the physical and chemical nature of the host rock, can be depicted and studied easily. This paper is concerned with one aspect of the physical nature of the host rock-surface area. physical nature of the host rock-surface area. Effect of Solid Surface on Chemical Reaction Several laboratory investigations have shown that the presence of clay in the oil-bearing rock enhances the fuel presence of clay in the oil-bearing rock enhances the fuel deposition. In general, clays are very fine and, therefore, have high specific surface areas. In a recent paper, Vossoughi et al. studied the effect of clay on paper, Vossoughi et al. studied the effect of clay on crude oil combustion by using TGA and DSC techniques. They observed a significant reduction in activation energy of the crude oil combustion resulting from addition of clay to the crude oil/sand mixture. Reduction of activation energy was inferred to the catalytic properties of the clay. In Ref. 7, Vossoughi et al. demonstrated properties of the clay. In Ref. 7, Vossoughi et al. demonstrated the effect of the sand-grain specific surface area on the in-situ combustion process in the absence of clay. They performed a series of in-situ combustion tube runs with performed a series of in-situ combustion tube runs with sand grains of different specific surface areas. The runs with sand grains of low specific surface areas did not sustain, while those of high specific surface areas produced strong self-sustained combustion fronts. produced strong self-sustained combustion fronts. Therefore, it is evident that crude oil combustion is influenced not only by clays but by any granular material with high specific surface area. This is not surprising. The naturally occurring clay minerals mostly consist of silica and alumina. Clays, silica, alumina, and synthetic silica/alumina are classified as solid acid catalysts. Their catalytic activities are related to their acid site density and acid strength. Further, a relationship exists between their acidity and activation energy. Activation energy decreases with increasing acid amount. Literature on catalytic cracking process, reveals that increased catalyst site density and acid strength favor increased rates of coke formation. This, although detrimental to the catalytic cracking process, may be beneficial to the in-situ combustion process in view of greater fuel deposition, especially when light crude oil reservoirs are involved. JPT P. 731