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Nuclear magnetic resonance (NMR) diffusion-relaxation correlation experiments (D-) are widely used for the petrophysical characterisation of rocks saturated with petroleum fluids both in situ and for laboratory analyses. The encoding for both diffusion and relaxation offers increased fluid typing contrast by discriminating fluids based on their self-diffusion coefficients, while relaxation times provide information about the interaction of solid and fluid phases and associated confinement geometry (if NMR responses of pure fluids at particular temperature and pressure are known). Petrophysical interpretation of D- correlation maps is typically assisted by the “standard alkane line”—a relaxation-diffusion correlation valid for pure normal alkanes and their mixtures in the absence of restrictions to diffusing molecules and effects of internal gradients. This correlation assumes fluids are free from paramagnetic impurities. In situations where fluid samples cannot be maintained at air-free state the diffusion-relaxation response of fluids shift towards shorter relaxation times due to oxygen paramagnetic relaxation enhancement. Interpretation of such a response using the “standard alkane line” would be erroneous and is further complicated by the temperature-dependence of oxygen solubility for each component of the alkane mixture. We propose a diffusion-relaxation correlation suitable for interpretation of low-field NMR D- responses of normal alkanes and their mixtures saturating rocks over a broad temperature range, in equilibrium with atmospheric air. We review and where necessary revise existing viscosity-relaxation correlations. Findings are applied to diffusion-relaxation dependencies taking into account the temperature dependence of oxygen solubility and solvent vapour pressure. The effect is demonstrated on a partially saturated carbonate rock.

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Capillary pressure measurements are an important part of the characterization of petroleum-bearing reservoirs. Three commonly used laboratory techniques, namely the porous plate (PP), centrifuge multi-speed experiment (CM), and mercury intrusion (MICP) methods, often provide nonidentical capillary pressure curves. We use high-resolution μ-CT images of Fontainebleau and Bentheimer sandstones to derive saturation profiles numerically in 3D at the pore scale through morphological distance transforms to simulate the above experiments. In the invasion simulation, the capillary pressure is realized by using as structural element a ball-whose diameter is a function of a local pressure potential, which in turn is a function of radial distance in centrifuge experiment and constant throughout the sample in MICP and porous plate measurements. To assess the effect on heterogeneous rock samples, we compare the computed saturation profiles of the relatively homogeneous sandstones to a highly heterogeneous numerical model of rock generated by a mixture of a Gaussian random field approach for the large-scale features and two Poisson particle processes at the small scale. The comparison of the image-based pore-scale numerical interpretation to capillary drainage experiments reveals their match and demonstrates the influence of boundary conditions and heterogeneity on the resulting saturation profiles and capillary pressure curves. Simulated centrifuge experiments may assist in estimation of experimental equilibrium times and provide a useful tool in speed schedule design.