This paper introduces an improved method for permeability assessment using Nuclear Magnetic Resonance (NMR) measurements. Conventional techniques for permeability assessment from NMR measurements include empirical correlations such as SDR and Coates models. However, carbonate rocks are known for lack of good correlations between pore-body-size and pore-throat-size, which makes it challenging and often unreliable to estimate permeability from NMR T2 distribution in carbonate formations with complex pore structure. It has also been proposed that conventional permeability models can be improved by incorporating an estimated pore connectivity factor. However, the estimated permeability does not reflect the anisotropic characteristics of rock permeability.

This paper introduces a new NMR-based directional permeability model by incorporating a directional pore-connectivity factor into a conventional NMR permeability model. The objectives of this paper are (a) to introduce a quantitative directional pore-connectivity factor calculated from three-dimensional (3D) pore-scale images, and (b) to reliably estimate directional permeability in formations with challenging pore structures (e.g. complex carbonate formations) based on NMR measurements.

We successfully tested the introduced method on four carbonate rock samples with complex pore geometry, and six sandstone rock samples with anisotropic permeability. We compared the directional permeability estimated from the new NMR model against those calculated from Lattice Boltzmann Method (LBM) simulations, and showed good agreement between them. We also compared the estimates of permeability using the new NMR model and using conventional NMR models (e.g. SDR and Coates models), showing that the NMR directional permeability model significantly improves permeability estimation. The outcomes of this research can significantly improve permeability assessment in complex carbonate reservoirs and anisotropic sandstone reservoirs, and can be further extended to organic-rich source rocks.

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