Feasibility Study of the In-Situ Combustion Process Using TGA/DSC Techniques

Abstract

Summary The influence of reservoir rock on the in-situ combustion process has been recognized by many investigators as an important part of the process. Several experimental studies have shown the effects of reservoir rock on the in-situ combustion process. However, all the available feasibility studies neglect this important and decisive criterion. This paper describes how the reservoir rock affects the minimum oil content necessary for the self-sustained combustion, which is introduced as a new criterion for the selection of suitable reservoirs for the process. process. Differential scanning calorimetry (DSC) was used to determine the heat value of the oil in the presence of the reservoir rock. The minimum temperature required for the total consumption of the fuel was obtained by the use of thermogravimetric analysis (TGA) and DSC. The minimum amount of oil necessary to sustain the combustion was calculated from these two parameters and compared with the oil content of the reservoir. Reservoirs with oil contents greater than or equal to this minimum value were considered feasible. In-situ combustion-tube experiments performed on actual reservoir rocks obtained from Kansas fields confirmed the validity of the prediction. prediction. Introduction In-situ combustion is a thermal method of EOR whereby oil is ignited underground, creating a combustion front that is propagated through the reservoir by continuous air injection. The fuel necessary to sustain the combustion front is supplied by the heavy residual material (coke) that deposits on the sand grains during distillation, thermal and catalytic cracking, pyrolysis, etc., of the crude oil ahead of the combustion front. The most important controlling factors of the fuel deposition are the nature of the crude oil and reservoir rock mineral. Several research projects have been conducted using clean, graded sand as the packing material rather than native core material. This type of packing always gives a much lower fuel deposition. In some cases, difficulty in sustaining the combustion was noticed when sandpacks were used. Correlations can be used to determine the fuel burned in a field. However, these correlations exhibit a large deviation from the experimental data because of the variations in crude and rock properties. Bardon and Gadelle investigated the effects of the matrix on the oxidation reactions of a French oil. They used different grain sizes of sand and compared them to the original matrix. The activation energy reported for the original core was always lower than that of the sandpacks because of the presence of 5% kaolinite in the matrix. Fassihi and Brigham conducted several experiments using oil mixed with sand and the original core from the Jobo field in Venezuela and from Lynch Canyon field in California. They found that the molar CO2 /CO ratio was higher for the original core than that of the runs using sand mixtures. They also reported a lower activation energy for the combustion reactions in the original core than in the sand. In 1969, Bousaid and Ramey pointed out that the catalytic effect of clays in reservoir rock was significant in crude-oil combustion. They observed that adding 20% clay to the sand mixture caused a 10% reduction in the activation energy of the combustion reactions. Vossoughi et al. also concluded that the addition of clay to porous media significantly affected the combustion of crude oil. The clay content of the sand mixture influenced the amount of fuel (coke) deposited in the sand. Drici and Vossoughi Studied the effect of sand-grain surface area on crude-oil combustion by performing a thermal analysis of crude-oil combustion in the presence of silica and alumina with a wide range of specific surface area. A reduction of the activation energy was reported for the mixtures of high surface-area/crude-oil ratio. Burger and Sahuquet, using thermal analysis methods, found that derivatives of heavy metals such as copper, nickel, and iron catalyzed the oxidation of crude oil. Additional coke was deposited when metallic derivatives were added either to oil or to the porous matrix. Similar observations were also reported by other investigators. to Clays and silt material present in the cores have large surface areas and include several metallic derivatives that control coke deposition during the burning process. The total mineralogy of the host reservoir rock can have a great influence on the process variables of an in-situ combustion process. Several authors, such as Geffen, Iyoho, Lewin and Assocs. Inc., and Chu, have conducted feasibility studies for the in-situ combustion process, but none of these studies reflect the importance of the composition of the reservoir rock. JPT P. 1441