Advanced NMR methodologies in rock core analysis

<p>¹H NMR techniques have gained extensive acceptance in petrophysics for the evaluation of fluid-saturating reservoir rocks. This thesis presents the development of new NMR methods regarding the reserves (determination of pore length scales and surface relaxivities), productivity (estimates of permeability) and recovery of fluids (resolves of saturation evolution) in rocks. Traditionally, pore lengths are evaluated from the ground relaxation eigenmodes of spin-bearing molecules in pore space. This evaluation is not straightforward since it is affected by surface relaxivity. Here, we use an approach to determine pore length from detecting the high relaxation eigenmodes, in which way the eigenvalue spectrum directly scales to the pore size distribution. Based on this, we extend this approach for the use with low-field NMR spectrometers and 2D NMR eigenmode correlation methods. Surface relaxivity can be further extracted from these 2D correlation maps, which is in agreement with an independent NMR measurement. Permeability is generally estimated from surface relaxation via empirical pore-network models. However, for heterogeneous rocks a single (or averaged) permeability value may not be adequate. Therefore, we measure surface relaxation in conjunction with MRI techniques. Permeability profiles can then be obtained from spatially resolved relaxation maps yielding local connectedness between adjacent slices. The results are confirmed by the comparison of brine-permeability measurements. MRI experiments of fluids in rocks at reservoir-like conditions may yield optimized recovery strategies of reservoir fluids. In this context we combine MRI with diffusion-relaxation correlation measurements during flooding intervals. The results provide substantial information, such as flooding front and saturation profiles of immiscible fluids discriminated by fluid type.</p>

Advanced NMR methodologies in rock core analysis

<p>¹H NMR techniques have gained extensive acceptance in petrophysics for the evaluation of fluid-saturating reservoir rocks. This thesis presents the development of new NMR methods regarding the reserves (determination of pore length scales and surface relaxivities), productivity (estimates of permeability) and recovery of fluids (resolves of saturation evolution) in rocks. Traditionally, pore lengths are evaluated from the ground relaxation eigenmodes of spin-bearing molecules in pore space. This evaluation is not straightforward since it is affected by surface relaxivity. Here, we use an approach to determine pore length from detecting the high relaxation eigenmodes, in which way the eigenvalue spectrum directly scales to the pore size distribution. Based on this, we extend this approach for the use with low-field NMR spectrometers and 2D NMR eigenmode correlation methods. Surface relaxivity can be further extracted from these 2D correlation maps, which is in agreement with an independent NMR measurement. Permeability is generally estimated from surface relaxation via empirical pore-network models. However, for heterogeneous rocks a single (or averaged) permeability value may not be adequate. Therefore, we measure surface relaxation in conjunction with MRI techniques. Permeability profiles can then be obtained from spatially resolved relaxation maps yielding local connectedness between adjacent slices. The results are confirmed by the comparison of brine-permeability measurements. MRI experiments of fluids in rocks at reservoir-like conditions may yield optimized recovery strategies of reservoir fluids. In this context we combine MRI with diffusion-relaxation correlation measurements during flooding intervals. The results provide substantial information, such as flooding front and saturation profiles of immiscible fluids discriminated by fluid type.</p>

Carbon and Neon Ion Bombardment Induced Smoothing and Surface Relaxation of Titania Nanotubes

Titania nanotube arrays with their enormous surface area are the subject of much attention in diverse fields of research. In the present work, we show that not only 60 keV and 150 keV ion bombardment of amorphous titania nanotube arrays yields defect creation within the tube walls, but it also changes the surface morphology: the surface relaxes and smoothens in accordance with a curvature-driven surface material’s transport mechanism, which is mediated by radiation-induced viscous flow or radiation-enhanced surface diffusion, while the nanotubes act as additional sinks for the particle surface currents. These effects occur independently of the ion species: both carbon and neon ion bombardments result in comparable surface relaxation responses initiated by an ion energy of 60 keV at a fluence of 1 × 1016 ions/cm2. Using atomic force microscopy and contact angle measurements, we thoroughly study the relaxation effects on the surface topography and surface free energy, respectively. Moreover, surface relaxation is accompanied by further amorphization in surface-near regions and a reduction in the mass density, as demonstrated by Raman spectroscopy and X-ray reflectivity. Since ion bombardment can be performed on global and local scales, it constitutes a versatile tool to achieve well-defined and tunable topographies and distinct surface characteristics. Hence, different types of nanotube arrays can be modified for various applications.

On the Validity of the Relationship 1/T = 1/TB+1/TS+1/TD in NMR Techniques With Regards to Permeability Estimation of Natural Porous Media

One of the most relevant feature of geophysical techniques based on nuclear magnetic resonance is their ability to estimate the permeability of natural porous media, since other geophysical techniques, as the use of the formation factor and neutron well-logs, allow to quantify the volume of water in the media. Permeability is conventionally obtained from decay time of the total resonance signal. However, the fluid in the pores of a medium normally has different mobility degree that can be differentiated by the NMR results. Therefore, a detailed estimation of permeability requires decomposing the total resonance signal as a function of the decay times corresponding to the three mechanisms that contribute to the signal: the intergranular free fluid, the surface layer, and the diffusion relaxation mechanism. The relationship currently used to make this decomposition states that the exponential decay rate attributed to the total resonance signal is the sum of the three existing decay rates. We demonstrate that this relationship is not generally applicable in porous media, showing the contradiction with the much more widely accepted relationships as well as computation examples from three typical decay rates in a single pore and from sandstone with bulk and surface relaxation mechanisms. Consequently, we conclude that the assertion whereby the permeability of any porous medium does not depend on the decay time of the free fluid is an overstatement, since it only applies to very small pore sizes.

Foam Flow in Different Pore Systems—Part 1: The Roles of Pore Attributes and Their Variation on Trapping and Apparent Viscosity of Foam

Summary Lateral propagation of foam in heterogeneous reservoirs, where pore geometries vary laterally, depends on the roles of pore geometries on the foam properties. In this paper, the pore attributes of 12 different rock samples were characterized in terms of porosity, permeability, pore shape, pore size, throat size, aspect ratio, coordination number, and log mean of surface relaxation times (T2LM). These were measured from gas porosimeter and permeameter, X-ray microcomputed tomography (CT)-basedpore-network models, thin-section photomicrographs, and nuclear magnetic resonance (NMR) surface relaxometry. The samples have a wide range of porosity: 12 to 29%; permeability: 1 to 5,000 md; average pore size: 3.7 to 9 µm; average throat size: 2.4 to 8 µm; average aspect ratio: 1 to 1.7; average coordination number: 2.6 to 5.2; and T2LM: 9.4 to 740 ms. Nitrogen foam flow experiments (without oil) were then conducted on each rock sample using a specialized coreflood apparatus. A graphical analysis of the coreflood data was used to estimate the total saturation of trapped foam (10 to 66%), flowing foam (3 to 14%), and apparent viscosity of foam (3.2 to 73 cp). Trapped foam saturation and apparent viscosity values were then correlated with each of the measured pore attributes. The results revealed that all pore attributes, except aspect ratio, have positive correlations with foam trapping and apparent viscosity. The best correlation with trapped foam saturation was obtained when the most influential pore attributes (pore size, throat size, aspect ratio, and coordination number) were combined into a single mathematical function. Foam apparent viscosity also appears to be mostly influenced by trapped foam saturation, permeability, and coordination number of pore systems. Trapping is also likely enhanced by the presence of fenestral or channel pores. Furthermore, the shape and angularity of pores seem to facilitate snap-off and trapping of foam, because rock samples with angular pores trapped the highest foam saturation compared with other samples with rounded and subrounded pores. It was also shown that the correlation between trapped foam saturation (and foam apparent viscosity) and the absolute permeability of porous media may reverse at some high-permeability values (greater than several darcies), when one or both of the following conditions exist: (1) The aspect ratio of a lower-permeability porous medium is lower than that of a higher-permeability porous medium, and (2) the coordination number of a lower-permeability porous medium is higher than that of a higher-permeability porous medium. Finally, T2LM showed a good correlation with foam trapping, making NMR logging a prospective tool for pre-evaluating foam performance in targeted reservoir sections.

NMR Fluid Substitution–A New Method of Reconstructing T2 Distributions Under Primary Drainage and Imbibition Conditions

Most nuclear magnetic resonance (NMR)-based petrophysics models, such as pore structure characterization and permeability prediction, were developed using T2 distributions measured at fully water-saturated conditions (i.e., Sw = 1). The downhole implementation of those models across the hydrocarbon zones is disputable due to partial saturation (Sw < 1) conditions; hence, a correction to such effects on T2 distributions is required. This paper provides a critical review of the fluid substitution methods currently available in the industry and presents an improved method for enhanced formation evaluation. In the new method presented, an effective irreducible water saturation model is used to account for the pore structure and capillary pressure effects, which were barely considered by the currently available NMR fluid substitution methods. For water-wet reservoir rocks, the typical NMR T2 distribution at the partial saturation condition displays a clear separation between the wetting and nonwetting phases. The water phase can be classified as irreducible and movable fluid volumes. Then, using a T2 mapping relationship and a total porosity constraint, the T2 distribution of movable water at Sw < 1 is shifted and amplified to determine the T2 distribution of movable water at Sw = 1. To validate the new method, NMR measurements were conducted on sandstone samples at Sw = 1 as well as Sw < 1. The reconstructed T2 distribution at Sw = 1 was compared with the measured T2 distribution at Sw = 1. Results showed that the reconstructed T2 distribution matched very well with the T2 distribution measured at Sw = 1, confirming the robustness of the new technique. Parameters used in the reconstruction methodology are observed to be a good indicator of pore connectivity. During desaturation, the water T2 in large pores shifts to a shorter T2 because of the enhanced surface relaxation as the water volume decreases while the surface area remains constant. Therefore, the amplitude at the short T2 increases. The increased amplitude was remapped to large pores in reconstructing T2 spectra of full saturation.

Structural Details of BaTiO3 Nano-Powders Deduced from the Anisotropic XRD Peak Broadening

In this study, nano-BaTiO3 (BTO) powders were obtained via the solvothermal method at different reaction times and were investigated using transmission electron microscopy (TEM), X-ray diffraction (XRD) and Raman spectroscopy. The results were compared with those obtained for a larger crystallite size BTO powder (BTO-m). The sizes of the cuboid crystallites (as determined by XRD and TEM) ranged from about 18 to 24 nm, depending on the reaction time. The evolution with temperature of the structure parameters of nano-BTO was monitored by means of X-ray diffraction and Raman spectroscopy and no signs of phase transition were found up to 170 °C. Careful monitoring of the dependence of the XRD peak widths on the hkl indices showed that the effect of the cubic crystallite shape upon the XRD peak widths was buried by the effect of hidden tetragonal line splits and by anisotropic microstrain. The good correlation of the line widths with the tetragonal split amplitudes, observed especially for BTO-m above the transition temperature, indicates tetragonal deformations, as also revealed by Raman spectroscopy. The large anisotropic microstrain shown by the nano-powders, which had a maximum value in the <100> directions, was considered evidence of the phenomenon of surface relaxation of cubic crystallites edged by {100} faces. The observed behavior of the nano-BTO structures with increasing temperature may suggest a correlation between the surface relaxation and tetragonal deformation in the nano-cubes. The experimental results for both nano-BTO and mezoscale-BTO are in agreement with the core-shell model.