Pyrolysis and Oxidation of Asphaltene-Born Coke-like Residue Formed onto in Situ Prepared NiO Nanoparticles toward Advanced in Situ Combustion Enhanced Oil Recovery Processes

Pyrolysis and oxidation of asphaltene-born coke-like residue formed onto ex situ and in situ prepared NiO nanoparticles as initial steps toward developing advanced in situ combustion enhanced oil recovery (EOR) processes were studied. The in situ synthesized NiO nanoparticles in heavy oil matrix, containing coke-like residue, were characterized by X-ray diffraction, Brunauer-Emmett-Teller, field-emission scanning electron microscopy, and energy-dispersive X-ray mapping techniques. The pyrolysis and postpyrolysis oxidation of the coke residue were investigated by temperature-programmed pyrolysis (TPP) and temperature-programmed oxidation (TPO) methods, respectively. Oxidation kinetics of the coke residue was described by the Kissinger-Akahira-Sunose isoconversional method. The results showed a higher percentage of coke residue on the in situ prepared nanoparticles than the ex situ employed ones. Eventually, during the TPP of the coke residue, the amount of carbon oxides released per total amount of the coke is 18.6% higher for the in situ NiO as compared to the ex situ NiO nanoparticles. This may be attributed to the uniform dispersion of the in situ NiO in the coke residue. Furthermore, compared to the ex situ NiO, the in situ NiO nanoparticles shift the oxidation temperature of the coke residue by about 100 degrees C to lower temperature. Multistep kinetics was predicted with a significant drop of the activation energy of the oxidation of the coke residue in the presence of in situ and ex situ NiO nanoparticles, confirming their catalytic effect. However, the pre-exponential factor, as a representation of the collision efficiency, is significantly higher over the in situ NiO compared to the ex situ NiO, leading to the enhanced oxidation of the coke residue. This may be attributed to the loss of surface area due to particle aggregation for the case of ex situ preparation, as well as the orientation of asphaltene molecules during the adsorption onto the surface. The asphaltenes could be aligned mostly vertically over the in situ NiO surface; thus the vertical alignment provides good channels for diffusion of gas-phase oxygen onto the surface leading to high collision efficiency and catalytic activity.