Modeling Molecular Interactions with Wetting and Non-Wetting Rock Surfaces by Combining Electron Paramagnetic Resonance and NMR Relaxometry.

Three types of natural rocks─Bentheimer and Berea sandstones, as well as Liège Chalk─have been aged by immersion in a bitumen solution for extended periods of time in two steps, changing the surface conditions from water-wet to oil-wet. NMR relaxation dispersion measurements were carried out on water and oil constituents, with saturated and aromatic molecules considered individually. In order to separate the different relaxation mechanisms discussed in the literature, 1H and 19F relaxation times were compared to 2H for fully deuterated liquids: while 2H relaxes predominantly by quadrupolar coupling, which is an intramolecular process, the remaining nuclei relax by dipolar coupling, which potentially consists of intra- and intermolecular contributions. The wettability change becomes evident in an increase of relaxation rates for oil and a corresponding decrease for water. However, this expected behavior dominates only for the spin-lattice relaxation rate R1 at very low field strengths and for the spin-spin relaxation rate R2, while high-field longitudinal relaxation shows a much weaker or even reverse trend. This is attributed in part to a change of radical concentration on the pore surface upon coverage of the native rock surface by bitumen as well as by the change of surface chemistry and roughness. EPR and DNP measurements quantify the change of volume vs surface radical concentration in the rocks, and an improved understanding of the role of relaxation via paramagnetic centers is obtained. By means of comparing different fluids and nuclei in combination with a defined wettability change of natural rocks, a refined model for molecular dynamics in conjunction with NMR relaxation dispersion is proposed.

Lattice Boltzmann framework for accurate NMR simulation in porous media.

Nuclear magnetic resonance (NMR) responses of fluids saturating porous media arise from complex relaxation-diffusion dynamics of polarized spins. These constitute a sensitive probe of the microstructure and are described by the Bloch-Torrey equations. An NMR simulation framework based on an augmented lattice Boltzmann method aimed at the fine-scale resolution of nuclear polarization density is presented. The approach encapsulates the time evolution of the full magnetization vector and naturally incorporates the mechanisms of diffusional transport. Spin dephasing mechanisms are fully resolved at tomogram voxel scale to account for magnetic field inhomogeneity. The approach is validated against analytical solutions of spin-echo decays for simple pore geometries. An application to a nano-computed-tomography image of chalk with inhomogeneous internal fields yields T_{2} spectral measures in good agreement with experiment and illustrates the spatial pore-scale dynamics of net magnetization. Findings establish the feasibility of the framework for pure diffusion and present an approach vector to modeling the evolution of magnetization under flow conditions.

Application of low-field, 1H/13C high-field solution and solid state NMR for characterisation of oil fractions responsible for wettability change in sandstones.

Asphaltene adsorption on solid surfaces is a standing problem in petroleum industry. It has an adverse effect on reservoir production and development by changing rock wettability, plugging pore throats, and affects oil transport through pipelines. Asphaltene chemistry constitutes important part of the ageing process as part of petrophysical studies and core analysis. The mechanisms and contribution of various oil components to adsorption processes is not fully understood. To investigate the kinetics of the ageing process and address the relative contribution of different oil components, we prepared three sets of sandstone core plugs aged in different oil mixtures over various time intervals. Cores were then re-saturated with decane to evaluate their wetting state using low-field NMR relaxometry by monitoring a change of surface relaxivity. Adsorbed deposits were then extracted from cores for solution-state NMR analysis. Their 1H and 1H-13C correlation spectra obtained using heteronuclear single quantum coherence (HSQC) technique were matched to spectra of four SARA (saturates, aromatics, resins and asphaltenes) components of oil mixtures to deduce components of deposits and inter-component interactions. We notice that wettability reversal of rock is inversely proportional to initial asphaltene concentration. Analysis of deposits reveals an increase in their aliphatic content over ageing time, which is accompanied by a change of the morphology of the pore space due to cluster aggregates forming a network. Results suggest that the ageing process in respect to the wetting state of rock samples consists of three distinctive stages: (i) an early-time period, when the fraction of most polar asphaltenes creates a discontinuous layer corresponding to mixed-wet state; (ii) an intermediate-time interval, at which the full grain coverage may be achieved (at favourable chemical environment) corresponding to strong oil-wetting; (iii) a late-time stage, where intense macro-aggregates accumulation occurs, changing the pore space integrity. It is likely asphaltene-aliphatic interactions leading to growth of sub-micron size macro-aggregates.

On the influence of wetting behaviour on relaxation of adsorbed liquids - A combined NMR, EPR and DNP study of aged rocks.

The influence of wettability modification in natural rocks has been studied by means of NMR relaxation dispersion of water and several organic liquids, employing 1H, 19F and 2H relaxometry. Berea, Bentheimer and chalk were aged with a bitumen solution, altering the samples from water-wet to mixed-wet. Relaxation measurements were supported by EPR and DNP experiments which are sensitive to bulk radicals and interfacial radicals, respectively. The effect of ageing on relaxation was found to be twofold: first, the change from water-wet to oil-wet affects the dynamics of molecules differently, in particular their immediate interaction with the surface, which is reflected in their relaxation times; second, the bitumen cover includes paramagnetic impurities which act as additional relaxation sinks to all molecules. EPR was used to confirm the amount of deposited material and the total radical content of the rock samples, whereas DNP revealed a small but significant signal enhancement due to the surface-bound bitumen containing stable radicals. The DNP enhancement is dominated by the Solid Effect despite the low viscosity of the interacting fluids.

About the connectivity of dual-scale media based on micro-structure based regional analysis of NMR flow propagators.

The characterisation of heterogeneous porous media at multiple length scales typically requires the classification of structure at some scale to allow the calculation of effective transport properties at a scale relevant for macroscopic description. While such a classification may be derived from various imaging methods, a shortcoming is often the simultaneous characterisation of the connectivity between regions representing different micro-structure. In this work we combine NMR based flow propagators with the simulations performed on corresponding reconstructed structure, and relate the NMR measurements to their simulated global and local representations to study fluid transport locally and the exchange between micro- and macro-porous regions. This is achieved by carrying out detailed lattice Boltzmann simulations and random walk method to track the displacements of tracers in each kind of region. Using Euclidean distance maps (EDT) we analyse the fluid invasion to regions of different scale and relate it to the connectivity of the system. We demonstrate that numerical simulation has great flexibility in providing additional sensitivity to the inference of region-region connectivity.

Temperature-Dependent Oxygen Effect on NMR D-[Formula: see text] Relaxation-Diffusion Correlation of n-Alkanes.

Nuclear magnetic resonance (NMR) diffusion-relaxation correlation experiments (D-[Formula: see text]) 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-[Formula: see text] 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-[Formula: see text] 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.