Integrated Workflow Using NMR and Electrical Resistivity Measurements for Reliable Permeability Assessment in Vuggy Carbonates
Authors: Arrieta D. C. , Heidari Z. , Azizoglu Z. , Sahu P. , Nassau G. , Câncio P. , Trevizan W. 
Published: September 2024

Efficient exploitation of spatially heterogeneous reservoirs (e.g., vuggy carbonates) relies on accurate characterization of fluid flow patterns and reservoir behavior. The integration of electrical resistivity and nuclear magnetic resonance (NMR) measurements was recently introduced to reliably estimate permeability in carbonate formations. However, the accuracy of this method is affected by the formation fluids and the solid particles in the mud filling large pores. To resolve these challenges, it is necessary to correct the NMR response for the impact of these fluids prior to permeability assessment in vuggy zones. The objective of this paper is to develop an advanced workflow that integrates NMR and electrical resistivity measurements for depth-by-depth calibration-free assessment of permeability in carbonate rocks with complex pore structure.

To enable calibration-free NMR-based assessment of permeability, we integrate NMR and electrical resistivity measurements through a new rock physics model. To account for the presence of hydrocarbons and mud particles within a partially-water-saturated zone, we perform fluid substitution to correct the initial NMR transversal relaxation times (T2) data into a corresponding fully water-saturated response. Next, we use the corrected NMR data for depth-by-depth assessment of the pore-body-size distribution. Then, we use electrical resistivity measurements to assess the global conductance-reduction factor (i.e., combined effect of constriction factor and electrical tortuosity). Finally, we use the aforementioned estimated properties for depth-by-depth assessment of permeability. Additionally, we create an optional procedure to enhance permeability estimates in the presence of mercury injection capillary pressure (MICP) and micro-computed tomography scan images (micro-CT). This optional procedure, also can be applied if the pore structure of the rock presents low connectivity impacting the values of electrical resistivity measurements.

We successfully verified the accuracy of the introduced workflow in a vuggy carbonate formation in the Santos Basin, Brazil. The permeability estimates were in agreement with the experimental measurements from core samples on each identified rock type. Permeability estimates from the new method indicated 51.2% improvement when compared to those obtained conventional NMR-based permeability models (e.g., Timur-Coates and Schlumberger-Doll Research (SDR)) in spatially heterogeneous carbonates. Results also confirmed that the integration of variable rock types obtained from acoustic image logs with the permeability assessment method, improved permeability estimates by indirectly capturing flow units though rock textural features. The novelty of this workflow is integration of multi-scale and multi-physics rock measurements (i.e., resistivity measurements and pore-/log-scale image data) for enhanced interpretation NMR T2 well-log measurements (after imposing corrections for the impacts of formation and mud fluids invaded in the vuggy portion of the pore structure) for the purpose of permeability assessment. This new method also provides a reliable quantitative description of pore structure, which is required for reliable formation evaluation of complex carbonate formations. Additionally, the introduced method enabled minimal core-based calibration efforts for cut-off values required for interoperation of NMR measurements compared to conventional methods.