Asphaltenes precipitation during carbon dioxide injection to enhance recovery has been considered as one of the major challenges in the tertiary production phase. How CO2 would change the asphaltenes structure is still unknown. The present study investigates the effects of CO2 on the isolated asphaltenes by means of various analytical techniques. Chemical structure of precipitated asphaltenes in the presence and absence of CO2 were characterized and compared. These results were coupled with the results of the stability assessment to determine the effects of structural alteration on asphaltenes stability in the oil matrix.
Four different crude oils were used to implement this experiment. In the first step, asphaltenes were precipitated by n-heptane. The asphaltenes were then dissolved in toluene and CO2 was injected (at 870 psi) to these solutions and they were mixed at 752°F. This process was repeated for three days, and one week to identify the effect of time on the possible reaction between CO2 and asphaltenes at elevated temperature and pressure. Next, CO2 was injected to the crude oils to determine whether it would react with other components of the oils other than asphaltenes. Same procedures were repeated with nitrogen as controlling experiments. For characterization, Fourier Transform Infrared Spectroscopy (FTIR spectroscopy) was conducted to specify the functional groups and their changes due to the addition of CO2. Finally, stability alteration of precipitated asphaltenes after reaction with CO2 was evaluated by UV-Vis spectroscopy.
FTIR results analyses demonstrated that in one tested sample the peak related to the amide functional group is created after injecting CO2. This peak was intensified by increasing the reaction time. To characterize the origin and mechanism of amide formation, 1,4-diazabicyclo[2.2.2]octane (DABCO) was added to this asphaltenes sample during reaction with CO2. Neither escalation of carbonyl group nor generation of aldehyde functional group was detected in the presence of DABCO. Such an observation proves that the amide group was formed by the reaction of amine in the asphaltenes and CO2. The stability of this sample in model oil was decreased after reaction with CO2. On the contrary, FTIR spectrums of the other three samples were not altered after reaction with CO2. Interestingly, one of these three asphaltenes samples became unstable in the model oil after reaction with CO2.
This study shows that the asphaltenes instability in the presence of CO2 could be a consequence of either chemical structural alteration of asphaltenes or change of the oil matrix solubility. Hence a comprehensive characterization of an oil sample is essential before designing any CO2 injection treatment. Accordingly, these results can be utilized to select more efficient inhibitors and stabilizers to prevent asphaltenes precipitation.