Reaction kinetics experiments are conducted to estimate important combustion parameters for crude oils. However, at elevated temperatures not only crude oil, but also reservoir rock is reactive, and the interaction of reservoir rocks with fluids may change the fate of the In-Situ Combustion (ISC) process. This study investigates the role of carbonates on the reaction kinetics of a bitumen sample from Canada.
To reach this goal, Thermogravimetric Analysis/Differential Scanning Calorimetry (TGA/DSC) experiments were conducted at a constant heating rate on a bitumen sample and the blends of bitumen with calcite (CaCO3) and dolomite (CaMg(CO3)2) minerals. The bitumen sample has been divided into its saturates, aromatics, resins, and asphaltenes (SARA) fractions. TGA/DSC experiments were conducted on the individual fractions and their pseudo blends in the presence and absence of carbonates to understand the contribution of each fraction in ISC success and their mutual interactions. Model fitting approach was used to analyze TGA/DSC graphs analytically to obtain activation energy and heat of reaction for each pseudo fraction, their blends, and initial bitumen samples at low (LTO) and high (HTO) temperature oxidation regions.
It has been observed that among all SARA fractions, the aromatics fraction alone generated the greatest amount of energy. Saturates are known as the ignitor for the combustion and its ignition characteristics are enhanced with the presence of carbonates. Similarly, the energy generation at low temperature oxidation (LTO) region for saturates becomes more significant for the saturates-aromatics pseudo blend. While the aromatics heat generation increased more for the pseudo blend with asphaltenes in the presence of carbonates, the energy generation of aromatics is negatively affected for the pseudo blend prepared with resins and carbonates. Thus, it was concluded that for the specific bitumen sample worked in this study, resins are the critical fraction determining the ISC fate in a carbonate reservoir. Moreover, we found that thermal decomposition of carbonate minerals negatively affects asphaltenes cracking and combustion reactions since both asphaltenes cracking and thermal decomposition of carbonate rock start at around the same temperature.
Our findings indicate that reaction kinetics studies should be conducted in the presence of all reservoir components (rock and fluids). However, because it is difficult to understand the contribution of each component to overall ISC performance, we recommend conducting reaction kinetics experiments on pseudo blends of reservoir fluid components. This procedure has been introduced for the first time with this study and enhanced our understanding towards ISC kinetics but should be extended to different crude oil and reservoir rock pairs.