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

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 Pas] 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. 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 importance of the specific surface area available to the crude oil for determining whether a self-sustained combustion could be achieved.