Prediction of In-Situ Combustion Process Variables By Use of TGA/DSC Techniques and the Effect of Sand-Grain Specific Surface Area on the Process

Abstract

Prediction of In-Situ Combustion Prediction of In-Situ Combustion Process Variables By Use of Process Variables By Use of TGA/DSC Techniques and the Effect of Sand-Grain Specific Surface Area on the Process Abstract This paper describes a new technique to predict the parameters that govern the performance of the in-situ combustion process. This prediction is accomplished by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) of the crude-oil combustion. The effect of surface area on the in-situ combustion-tube runs was also investigated. The crude oil studied was from Iola field, Allen County, KS. This oil has a gravity of 19.8 API [0.94 g/cm3] and a viscosity of 222 cp [0.222 Pa-s] at 100.4 F [38 C] and 98 cp [0.098 Pa-s] at 129.2 F [54 C]. Pa-s] at 100.4 F [38 C] and 98 cp [0.098 Pa-s] at 129.2 F [54 C]. At a bulk density of the combustion-tube pack of 104.26 lbm/cu ft [1.67 g/cm3], the minimum crude-oil content to support an adiabatic combustion process was estimated to be 7.1 wt%. This translates into 34.4% oil saturation for the sandpack of 37% porosity and the crude-oil gravity of 19.8 API [0.94 g/cm3]. However, the combustion front, in a sandpack of 70 mesh (specific surface area of 76 cm2/g [7.6 m2/kg]) with an oil content even greater than the required minimum oil content predicted by the present approach, did not sustain itself. Additional tube runs were performed with finer sand grains having specific surface areas of 317, 1,120 and 3,332 cm2/g [31.7, 112, and 333.2 m2/kg]. A strong, sustained combustion front was observed only in the last run-i.e., the greatest specific surface area. TGA was applied to the samples taken at 1- to 2-in. [2.54-to 5.08-cm] intervals ahead of the front to study crude-oil distribution. In the case of unsuccessful runs, the amount of the crude oil ahead of the front decreased to a level that sufficient fuel could not be laid down to sustain the front. In the self-sustained run with the greatest surface area, crude-oil content immediately ahead of the front was even higher than the original sand/oil mixture. Therefore, a minimum surface area is required to provide conditions for sufficient fuel to be laid down by the coking process. process. This finding is believed to be important in revealing the mechanism responsible for the lack of self-sustained combustion in sandpacks or porous rocks with low specific surface area. It also reveals the porous rocks with low specific surface area. It also reveals the importance of the specific surface area available to the crude oil for determining whether a self-sustained combustion could be achieved. Introduction In-situ combustion is a complex process that involves simultaneous heat and mass transfer in a multiphase environment coupled with chemical reactions of crude-oil combustion. Many studies on the thermal and fluid dynamics of the in-situ combustion process have been conducted, but little has been done to study chemical reaction kinetics and mechanisms involved in underground combustion. In a recent study, Fassihi et al. showed that the combustion of crude oil in porous media follows several consecutive reactions. They identified three groups of reactions (low-, middle-, and high-temperature reactions), and argued that the first was heterogeneous (gas/liquid), the second, homogeneous (gas phase), and the third, heterogeneous (gas/solid phase). They produced a model based on Weijdema's kinetic equation in which a simple reaction is assumed for each group of crude-oil reactions and Arrhenius-type dependency of the rate constant on temperature. This model, however, allowed only the prediction of crude-oil combustion parameters under very stringent and controlled conditions. The oxidative behavior of crude oils under varying conditions of temperature, pressure, and atmosphere may also be studied by thermal analysis. Most researchers took a qualitative approach and used thermal analysis techniques to study the thermo-oxidative behavior of crudes with specific reference to the temperatures at which each oxidation reaction occurs. Weckowska and Bogdanow, however, took a different approach to thermal analysis by investigating the thermal decomposition kinetics of the vacuum-distillation residue of crude oil. They used the kinetic model of Zsako to describe mathematically the kinetics of thermal decomposition of a Romashkino crude-oil residue. SPEJ p. 656