Nanometer-sized magnetite and its impact on 1H NMR petrophysical characterization of synthetic carbonates

Abstract

Magnetic minerals, such as magnetite, can significantly influence ¹H Nuclear Magnetic Resonance (NMR) measurements, introducing biases that can affect petrophysical interpretations in reservoir rocks. Understanding these effects is crucial for improving the accuracy of fluid content estimations in subsurface evaluations. In this study, we investigate how nanometric-sized magnetite impacts T₂ relaxation times in synthesized carbonate samples with controlled porosity and magnetite concentrations. Twelve carbonate samples were synthesized with varying magnetite content (0.0 %–0.8 % wt.), ensuring a controlled environment for evaluating NMR responses. These samples underwent petrophysical (bulk volume, pore volume, grain density, and NMR), mineralogical (XRD and SEM-EDS), and magnetic (low-field magnetic susceptibility, hysteresis loop, FORC, and IRM measurements) characterization to ensure the integrity of both the synthesis and the magnetite contamination. Our findings indicate that (1) the synthesis successfully produced samples with consistent properties, showing a decrease in pore volume with increasing cementing fluid and a corresponding enhancement of magnetic properties with higher magnetite contamination; (2) ¹H NMR-based porosity estimates were significantly affected by magnetite contamination, displaying a noticeable flattening of T₂ relaxation curves and a reduction in relaxation times, likely due to enhanced diffusional effects; and (3) increasing magnetite concentrations induced nonlinear distortions in porosity ϕ_NMR, leading to systematic deviations from expected values and, consequently causing porosity underestimation. These results underscore the need to account for magnetic mineral contamination in NMR analyses of carbonate reservoirs and highlight the importance of controlled research into magnetite's impact on petrophysical assessments.