Complex pore structure in carbonate rocks causes the petrophysical characterization and formation evaluation a challenge. Permeability is one of the main evaluation parameters for determining the potential production of a reservoir because it defines the ability of fluids to flow through rocks. It is not possible to directly measure permeability using a wireline tool; one method to obtain it, however, is from Nuclear Magnetic Resonance (NMR) logs using various models such as Timur-Coates, T2LM (Kenyon) and P-connectivity which are adjusted using laboratory core analysis. Interpretation methods were developed to obtain permeability of the formation from NMR readings. These models relate permeability with other petrophysical properties such as porosity that can be directly estimated using well log information. The NMR models include parameters such as c, m, n and T2,cutoff, that must be calibrated with NMR laboratory analyses because they vary for each formation. In some cases, NMR models must be modified to estimate rock permeability more accurately. This paper presents a workflow to modify the NMR models to determine permeability in carbonate rocks. The correlation between conventional laboratory core analysis and NMR permeability from Timur-Coates and Kenyon models is improved using an adjustment factor applicable for each equation. The adjustment factor can be correlated with petrophysical properties of the rocks such as NMR porosity, the irreducible fluid volume (BVI) and the moveable fluid volume (BVM). The results of calculating NMR permeability in carbonate rocks applying the modified equations to laboratory data and NMR log is also presented. Permeability from formation tests is used to correlate with NMR permeability using log data.

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In this work, we present a characterization of phase configuration in water-saturated sintered glass bead samples after oil injection, through the analysis of time-dependent diffusion coefficients obtained from sets of one-dimensional pulsed field gradient nuclear magnetic resonance (PFG NMR) measurements, pre and post drainage. Estimates of samples surface-to-volume ratio and permeability from pre drainage PFG measurements in a water-saturated sample were compared with analytical and reported values, respectively, and a fair agreement was found in both cases. Short-time analysis of diffusion coefficients extracted from PFG measurements was used to quantify the increase in surface-to-volume ratio probed by the wetting phase after drainage. Analysis of water and oil diffusion coefficients from post drainage PFG experiments were carried out using a bi-Gaussian model, and two distinct scenarios were considered to describe fluids conformation within pores. For the case where non-wetting phase was considered to exhibit a poorly connected geometry, an analysis assuming the formation of oi-in-water droplets within pores was performed, and a Gaussian distribution of droplets radii was determined.