“Magnetita em rochas carbonáticas: implicações para a recuperação de hidrocarbonetos e caracterização petrofísica por RMN”

Abstract

The presence of magnetite (Fe3O4) in petroleum systems represents an important factor in understanding inaccuracies related to carbonate formations, which influence both exploration and the petrophysical analysis of these reservoirs. In this doctoral thesis, we investigate the effects of synthesized magnetite, using the co-precipitation method, in various grain sizes and concentrations, exploring its relationship with the pore space in different porosities and the fluid present in the reservoir. The results consist of two main articles. The first article investigates how pH variations induced by acid affect the surface properties and catalytic and leaching activities of nanometric and micrometric magnetites, particularly in the presence of naphthalene. These changes are crucial for chemical decomposition processes in carbonate reservoirs. By analyzing catalytic performance under different pH conditions, the project provides valuable insights into the potential environmental impacts and practical applications of acid fracturing, particularly relevant to Enhanced Oil Recovery (EOR). It was noted that under acidic conditions, the Fe2+ ions leached from magnetite oxidize naphthalene, revealing their crucial role in the chemical decomposition of organic substrates in oil extracted from rocks containing magnetite. It was discovered that the chemical and crystallographic structure of magnetite significantly influences the decomposition of naphthalene, used as a model of Polycyclic Aromatic Hydrocarbon (PAH). Additionally, the morphology of magnetite grains, whether micro or nanometric, plays an important role as a heterogeneous catalyst, along with the acidity of the aqueous medium. Homogeneous catalysts demonstrated greater efficiency in decomposing naphthalene, followed by samples of nanometric and micrometric magnetite, which act as heterogeneous catalysts. The second article employs laboratory methods to investigate the interferences that magnetites may cause in Nuclear Magnetic Resonance (NMR) spectra and consequently in petrophysical estimates based on NMR. Twelve synthetic carbonates with different concentrations of magnetites and varying porosities were prepared, and mineralogical and magnetic characterizations were performed. Furthermore, the study included gas porosimetry analyses and permeability estimation using a permeameter. It was observed that shifts in the T2 spectrum are strongly correlated with increased magnetic susceptibility. The gradient of diffusive relaxation contributed to the overlap of the magnetic susceptibility of carbonates concerning the amount of porosity in samples with higher magnetite concentrations, while in carbonates with lower amounts of magnetite, the surface relaxivity modulated the T2 curve. These results show a strong correlation between the rate of diffusive relaxation and the concentration of magnetite. It is emphasized that to utilize relaxation rates from NMR in estimating porosity and permeability in carbonate rocks, one must consider the effects caused by magnetite. In summary, these studies represent a significant contribution to advancing knowledge in the characterization of magnetic minerals in carbonate formations of petroleum systems, exploring their interaction with pore space, their filling, and enhancing the understanding of pore potential. This provides insights for future research on chemical reactions in the magnetic characterization of carbonates, enhanced oil recovery, and the estimation of porosity and permeability based on T2 spectra.