Integrated Analysis of NMR and Electrical Resistivity Measurements for Enhanced Assessment of Throat-Size Distribution, Permeability, and Capillary Pressure in Carbonate Formations: Well-Log-Based Application
Authors:   The University of Texas at Austin, August Howard , Azizoglu Zulkuf ,   The University of Texas at Austin, Heidari Zoya ,   The University of Texas at Austin, Goncalves Leonardo ,   Petróbras, Blanes de Oliveira Lucas Abreu ,   Petróbras, do Nasicmento Neto Moacyr Silva ,   Petróbras, Victor Rodolfo Araujo ,   Petróbras
Published: October 2022

The genesis and diagenesis of carbonate rocks usually comprise changes in depositional sequences and multiple post-depositional processes. The interplay of these factors drives complexities in terms of texture, composition, and pore structure. Conventional petrophysical models do not reliably assess these complexities, requiring extensive calibration efforts or pore-scale image analysis. A recently developed method enabled 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. This method was tested earlier in the pore- and core-scale domains. However, the workflow for its application to the well-log-scale domain is yet to be developed and evaluated. This paper focuses on (a) depth-by-depth quantification of rock fabric features (i.e., constriction factor, effective pore-body-size, and pore-throat-size distribution), (b) depth-by-depth estimation of permeability and capillary pressure by integrating electrical resistivity and NMR well logs, and (c)validate the reliability of the new workflow against experimental measurements. We introduced a method that integrates electrical resistivity and NMR measurements for assessment of throat-size distribution, capillary pressure, and permeability. We first estimate constriction factor from electrical resistivity measurements. Then, we use NMR T2 distribution to estimate pore-body-size distribution. Next, we calculate depth-by-depth pore-throat-size distribution integrating the constriction factor and pore-body-size distribution. Subsequently, we estimate the capillary pressure from pore-throat-size distribution. Finally, we use effective pore-body-size, constriction factor, electrical tortuosity, and connected porosity to calculate permeability on a depth-by-depth basis. We successfully applied the new method to pre-salt carbonates of Barra Velha Formation, Santos’s basin, Brazil. We validated the reliability of the introduced workflow for assessment of permeability, capillary pressure, and throat-size distribution against core measurements obtained from different rock types. The applications of the new method for assessment of 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 22%, 110%, and 135% improvement when compared against those obtained from porosity-permeability correlations and NMR-based permeability assessment methods (e.g., Timur-Coates and Schlumberger-Doll-Research (SDR) methods), respectively. The introduced method provides the option of performing real-time and depth-by-depth assessment of pore-throat-size distribution and capillary pressure, which was not possible in the past. This leads to enhancement in in-situ permeability assessment and rock classification. Moreover, the proposed method minimizes the need for core-based calibration efforts and eliminates the need for detecting cutoff values in NMR-based permeability assessment.