Novel Estimation of Pore-Throat Size and Permeability in Carbonates From Integration of NMR and Resistivity Data in the Santos Basin of Brazil

Due to the variability in depositional cycles and the active nature of the constituent minerals, carbonate rocks are commonly strongly modified by diagenetic processes which alter their original rock fabric and petrophysical properties. Conventional petrophysical models do not reliably assess these complexities, requiring extensive calibration efforts or pore-scale image analysis. We introduce a calibration-free method that enables the assessment of pore-network properties such as constriction factor, pore-body- and pore-throat-size distributions, as well as permeability and capillary pressure based on joint interpretation of Nuclear Magnetic Resonance (NMR) transverse relaxation time (T₂) distribution and electrical conductivity measurements. We successfully applied the introduced method to pre-salt carbonates of Barra Velha Formation, Santos's Basin, Brazil. The applications of the introduced method for assessing throat-size distribution in the core- and the well-log-scale domains have proven successful in 87% and 73% of the cases, respectively. The permeability estimates from the new method showed more than 42% improvement when compared against those obtained NMR-based permeability assessment methods. The new method provides a real-time and depth-by-depth assessments of pore-throat-size distribution and capillary pressure, minimizing the need for core-based calibration efforts and eliminating the need for detecting cutoff values in NMR-based permeability models.