Nuclear Magnetic Resonance Relaxation Times Research Articles

Application of Solid-State NMR Relaxometry for Characterization and Formulation Optimization of Grinding-Induced Drug Nanoparticle.

The formation mechanism of drug nanoparticles was investigated using solid-state nuclear magnetic resonance (NMR) techniques for the efficient discovery of an optimized nanoparticle formulation. The cogrinding of nifedipine (NIF) with polymers, including hydroxypropyl methylcellulose (HPMC) and polyvinylpyrrolidone (PVP), and sodium dodecyl sulfate (SDS) was performed to prepare the NIF nanoparticle formulations. Then, solid-state NMR relaxometry was used for the nanometer-order characterization of NIF in the polymer matrix. Solid-state NMR measurements revealed that the crystal size of NIF was reduced to several tens of nanometers with amorphization of NIF by cogrinding with HPMC and SDS for 100 min. Similarly, the size of the NIF crystal was reduced to less than 90 nm in the 40 min ground mixture of NIF/PVP/SDS. Furthermore, 100 min grinding of NIF/PVP/SDS induced amorphization of almost all the NIF crystals followed by nanosizing. The hydrogen bond between NIF and PVP led to the efficient amorphization of NIF in the NIF/PVP/SDS system compared with NIF/HPMC/SDS system. The efficient nanosizing of the NIF crystal in the solid state, revealed by the solid-state NMR relaxation time measurements, enabled the formation of large amounts of NIF nanoparticles in water followed by the polymer dissolution. In contrast, excess amorphization of the NIF crystals failed to efficiently prepare the NIF nanoparticles. The solid-state characterization of the crystalline NIF revealed good correlation with the NIF nanoparticles formation during aqueous dispersion. Furthermore, the solid-state NMR measurements including relaxometry successfully elucidated the nanometer-order dispersion state of NIF in polymer matrix, leading to the discovery of optimized conditions for the preparation of suitable drug nanoparticles.

Use of multiparametric magnetic resonance microscopy for discrimination among different processing protocols and anatomical positions of Slovenian dry-cured hams

A novel multiparametric magnetic resonance microscopy (MRM) approach was applied to the Slovenian Kraški pršut dry-cured ham samples in order to evaluate its potential for discrimination among biceps femoris and semimembranosus muscle from two hams, differing in processing (salting duration) and thus in water and salt content. The approach is based on apparent diffusion coefficient (ADC) mapping as well as on longitudinal (T1) and transversal (T2) nuclear magnetic resonance relaxation time mapping. Three-dimensional maps were acquired and analyzed by one dimensional (1D) ADC, T1, and T2 distributions as well as by paired two-dimensional ADC-T1, ADC-T2 and T1–T2 distributions. The discriminating potential of the applied MRM approach was confirmed by differences among both 1D and 2D distributions of different ham samples. In addition, distribution peak positions highly correlated with the conventionally determined moisture content.

On the microscopic fluctuations driving the NMR relaxation of quadrupolar ions in water.

Nuclear Magnetic Resonance (NMR) relaxation is sensitive to the local structure and dynamics around the probed nuclei. The Electric Field Gradient (EFG) is the key microscopic quantity to understand the NMR relaxation of quadrupolar ions, such as (7)Li(+), (23)Na(+), (25)Mg(2+), (35)Cl(-), (39)K(+), or (133)Cs(+). Using molecular dynamics simulations, we investigate the statistical and dynamical properties of the EFG experienced by alkaline, alkaline Earth, and chloride ions at infinite dilution in water. Specifically, we analyze the effect of the ionic charge and size on the distribution of the EFG tensor and on the multi-step decay of its auto-correlation function. The main contribution to the NMR relaxation time arises from the slowest mode, with a characteristic time on the picosecond time scale. The first solvation shell of the ion plays a dominant role in the fluctuations of the EFG, all the more that the ion radius is small and its charge is large. We propose an analysis based on a simplified charge distribution around the ion, which demonstrates that the auto-correlation of the EFG, hence the NMR relaxation time, reflects primarily the collective translational motion of water molecules in the first solvation shell of the cations. Our findings provide a microscopic route to the quantitative interpretation of NMR relaxation measurements and open the way to the design of improved analytical theories for NMR relaxation for small ionic solutes, which should focus on water density fluctuations around the ion.

Bound Water Content and Pore Size Distribution in Swollen Cell Walls Determined by NMR Technology

Nuclear magnetic resonance (NMR) relaxation time distributions can provide detailed information about the moisture in wood. In this paper, the bound water content and pore size distributions in swollen cell wall of two kinds of softwoods (Pinus sylvestris and Cunninghamia lanceolata) and three kinds of hardwoods (Populus sp., Fraxinus excelsior L., and Ochroma lagopus) were determined by NMR cryoporometry. The total bound water content of swollen cell wall almost exceeds 35%, based on dry mass, which is obviously higher than the fiber saturation point (FSP) (appr. 30%) measured by the extrapolation method. The bound water content of different species is consistent with the hypothesis that with the decrease of basic density, the more bound water could be contained in wood. The proportion of the pore diameter smaller than 1.59 nm is higher than 70%, and the proportion of the pore diameter larger than 4 nm is no more than 10%.

Evaluation of protein structural changes and water mobility in chicken liver paste batters prepared with plant oil substituting pork back-fat combined with pre-emulsification

Protein structural changes and water mobility properties in chicken liver paste batters prepared with plant oil (sunflower and canola oil combinations) substituting 0–40% pork back-fat combined with pre-emulsification were studied by Raman spectroscopy and low-field nuclear magnetic resonance (NMR). Results showed that pre-emulsifying back-fat and plant oil, including substituting higher than 20% back-fat with plant oil increased the water- and fat-binding (p<0.05) properties, formed more even and fine microstructures, and gradually decreased the NMR relaxation times (T21a, T21b and T22), which was related to the lower fluid losses in chicken liver paste batters. Raman spectroscopy revealed that compared with a control, there was a decrease (p<0.05) in α-helix content accompanied by an increase (p<0.05) in β-sheet structure when substituting 20–40% back-fat with plant oil combined with pre-emulsification. Pre-emulsification and plant oil substitution changed tryptophan and tyrosine doublet hydrophobic residues in chicken liver paste batters.

Understanding NMR relaxometry of partially water-saturated rocks

Abstract. Nuclear magnetic resonance (NMR) relaxometry measurements are commonly used to characterize the storage and transport properties of water-saturated rocks. Estimations of these properties are based on the direct link of the initial NMR signal amplitude to porosity (water content) and of the NMR relaxation time to pore size. Herein, pore shapes are usually assumed to be spherical or cylindrical. However, the NMR response at partial water saturation for natural sediments and rocks may differ strongly from the responses calculated for spherical or cylindrical pores, because these pore shapes do not account for water menisci remaining in the corners of desaturated angular pores. Therefore, we consider a bundle of pores with triangular cross sections. We introduce analytical solutions of the NMR equations at partial saturation of these pores, which account for water menisci of desaturated pores. After developing equations that describe the water distribution inside the pores, we calculate the NMR response at partial saturation for imbibition and drainage based on the deduced water distributions. For this pore model, the NMR amplitudes and NMR relaxation times at partial water saturation strongly depend on pore shape, i.e., arising from the capillary pressure and pore shape-dependent water distribution in desaturated pores with triangular cross sections. Even so, the NMR relaxation time at full saturation only depends on the surface-to-volume ratio of the pore. Moreover, we show the qualitative agreement of the saturation-dependent relaxation-time distributions of our model with those observed for rocks and soils.

Uncertainty estimates for surface nuclear magnetic resonance water content and relaxation time profiles from bootstrap statistics

A method for estimating uncertainty in surface nuclear magnetic resonance (NMR) water content and relaxation times utilizing bootstrapping statistics is presented. Bootstrapping is particularly well suited for assigning uncertainty to the surface NMR data set due to the primary factor that degrades surface NMR data quality: ambient electromagnetic noise. We use synthetic forward modeled data with various noise levels applied (the “known uncertainty”), and then demonstrate that a bootstrap resampling of the observed synthetic data can produce an uncertainty estimate that closely represents the “known uncertainty”. Finally, we present two field data sets collected under different magnitude ambient noise levels as examples illustrating the result of this approach under realistic noise conditions. This approach for estimating uncertainty is computationally intensive, but straightforward to implement and produces useful uncertainty estimates on both water content and relaxation time results for smooth surface NMR sounding models.

Nuclear magnetic resonance average pore-size estimations outside the fast-diffusion regime

The pore-size distribution (PSD) of geologic materials is an important rock parameter to understand the flow of water in the subsurface. PSDs can be obtained from sieving analyses, mercury porosimetry measurements, and imaging techniques, but none of these methods is available for in situ measurements. Nuclear magnetic resonance (NMR) measurements are controlled by rock parameters such as the surface-area to pore-volume ratio. NMR is available for in situ measurements. State-of-the-art NMR relaxation time measurements need a calibration of the surface relaxivity [Formula: see text] to extract pore-size information. State-of-the-art NMR diffusion measurements avoid the calibration of [Formula: see text] but are limited to small pores. We developed an approach that estimates the average pore size without calibrating [Formula: see text] by means of incorporating higher order modes into the signal interpretation of NMR relaxation times. We conducted forward-modeling studies using an analytic solution for cylindrical tubes, 2D finite-element simulations to incorporate fractal pore spaces, and laboratory experiments on synthetic and natural samples. Our experimental data indicated that relaxation can occur outside the fast-diffusion regime not only for coarse-grained materials, but also for fine- to medium-grained unconsolidated sandy materials due to high surface relaxivities. We found that the rock-fluid interface’s roughness had a significant impact on the diffusion regime and led to an apparent increase in [Formula: see text], which may cause intermediate or slow diffusion. The methodology was limited to materials with a narrow PSD and uniform distribution of [Formula: see text] because we assumed multiexponential decay due to diffusion in single isolated pores.

Quantitative evaluation of the restructuring effect of a warm mix additive on bitumen recycling production

Warm mix asphalt (WMA) is a promising technique that allows the production of asphalt at a noticeably lower temperature. One of the methods in use is to add small quantities of specific organic compounds to the asphalt conglomerate. Although it is well established that this procedure leads to an effective reduction of the temperature of asphalt conglomerate production, there is still a lack in the comprehension and description of the real mechanism of the additive action on the colloidal structure. In this work 3D micro-scale atomic force microscopy (AFM) images of fresh, aged and doped bitumen were taken to characterise and study the interaction of a commercial WMA additive with bitumen. We also combined the AFM results with the inverse Laplace transform of the nuclear magnetic resonance relaxation times (T2). Results revealed that the temperature lowering in the WMA recycling procedure is due to the restructuring action of the additive that leads to a re-organisation of the colloidal network of bitumen.

Adaptive truncation of matrix decompositions and efficient estimation of NMR relaxation distributions

The two most successful methods of estimating the distribution of nuclear magnetic resonance relaxation times from two dimensional data are data compression followed by application of the Butler–Reeds–Dawson algorithm, and a primal-dual interior point method using preconditioned conjugate gradient. Both of these methods have previously been presented using a truncated singular value decomposition of matrices representing the exponential kernel. In this paper it is shown that other matrix factorizations are applicable to each of these algorithms, and that these illustrate the different fundamental principles behind the operation of the algorithms. These are the rank-revealing QR (RRQR) factorization and the LDL factorization with diagonal pivoting, also known as the Bunch–Kaufman–Parlett factorization. It is shown that both algorithms can be improved by adaptation of the truncation as the optimization process progresses, improving the accuracy as the optimal value is approached. A variation on the interior method viz, the use of barrier function instead of the primal-dual approach, is found to offer considerable improvement in terms of speed and reliability. A third type of algorithm, related to the algorithm known as Fast iterative shrinkage-thresholding algorithm, is applied to the problem. This method can be efficiently formulated without the use of a matrix decomposition.

Model for the interpretation of nuclear magnetic resonance relaxometry of hydrated porous silicate materials.

Nuclear magnetic resonance (NMR) relaxation experimentation is an effective technique for probing the dynamics of proton spins in porous media, but interpretation requires the application of appropriate spin-diffusion models. Molecular dynamics (MD) simulations of porous silicate-based systems containing a quasi-two-dimensional water-filled pore are presented. The MD simulations suggest that the residency time of the water on the pore surface is in the range 0.03-12 ns, typically 2-5 orders of magnitude less than values determined from fits to experimental NMR measurements using the established surface-layer (SL) diffusion models of Korb and co-workers [Phys. Rev. E 56, 1934 (1997)]. Instead, MD identifies four distinct water layers in a tobermorite-based pore containing surface Ca2+ ions. Three highly structured water layers exist within 1 nm of the surface and the central region of the pore contains a homogeneous region of bulklike water. These regions are referred to as layer 1 and 2 (L1, L2), transition layer (TL), and bulk (B), respectively. Guided by the MD simulations, a two-layer (2L) spin-diffusion NMR relaxation model is proposed comprising two two-dimensional layers of slow- and fast-moving water associated with L2 and layers TL+B, respectively. The 2L model provides an improved fit to NMR relaxation times obtained from cementitious material compared to the SL model, yields diffusion correlation times in the range 18-75 ns and 28-40 ps in good agreement with MD, and resolves the surface residency time discrepancy. The 2L model, coupled with NMR relaxation experimentation, provides a simple yet powerful method of characterizing the dynamical properties of proton-bearing porous silicate-based systems such as porous glasses, cementitious materials, and oil-bearing rocks.

Determining adsorbate configuration on alumina surfaces with (13)C nuclear magnetic resonance relaxation time analysis.

Relative strengths of surface interaction for individual carbon atoms in acyclic and cyclic hydrocarbons adsorbed on alumina surfaces are determined using chemically resolved (13)C nuclear magnetic resonance (NMR) T1 relaxation times. The ratio of relaxation times for the adsorbed atoms T1,ads to the bulk liquid relaxation time T1,bulk provides an indication of the mobility of the atom. Hence a low T1,ads/T1,bulk ratio indicates a stronger surface interaction. The carbon atoms associated with unsaturated bonds in the molecules are seen to exhibit a larger reduction in T1 on adsorption relative to the aliphatic carbons, consistent with adsorption occurring through the carbon-carbon multiple bonds. The relaxation data are interpreted in terms of proximity of individual carbon atoms to the alumina surface and adsorption conformations are inferred. Furthermore, variations of interaction strength and molecular configuration have been explored as a function of adsorbate coverage, temperature, surface pre-treatment, and in the presence of co-adsorbates. This relaxation time analysis is appropriate for studying the behaviour of hydrocarbons adsorbed on a wide range of catalyst support and supported-metal catalyst surfaces, and offers the potential to explore such systems under realistic operating conditions when multiple chemical components are present at the surface.

Combining Neutron and Magnetic Resonance Imaging to Study the Interaction of Plant Roots and Soil

The soil in direct vicinity of the roots, the root-soil interface or so called rhizosphere, is heavily modified by the activity of roots, compared to bulk soil, e.g. in respect to microbiology and soil chemistry. It has turned out that the root-soil interface, though small in size, also plays a decisive role in the hydraulics controlling the water flow from bulk soil into the roots. A promising approach for the non-invasive investigation of water dynamics, water flow and solute transport is the combination of the two imaging techniques magnetic resonance imaging (MRI) and neutron imaging (NI). Both methods are complementary, because NI maps the total proton density, possibly amplified by NI tracers, which usually corresponds to total water content, and is able to detect changes and spatial patterns with high resolution. On the other side, nuclear magnetic resonance relaxation times reflect the interaction between fluid and matrix, while also a mapping of proton spin density and thus water content is possible. Therefore MRI is able to classify different water pools via their relaxation times additionally to the water distribution inside soil as a porous medium. We have started such combined measurements with the approach to use the same samples and perform tomography with each imaging method at different location and short-term sample transfer.

Diffusional coupling between microfractures and pore structure and its impact on nuclear magnetic resonance measurements in multiple-porosity systems

Nuclear magnetic resonance (NMR) relaxation time measurements, although among the most accurate methods to estimate formation porosity, have been considered conventionally as insensitive to the presence of microfractures. Hence, the NMR responses in multiple-porosity systems, which may contain intergranular pores, microfractures, or channel-like inclusions, have not yet been thoroughly investigated. NMR pore-scale simulations using a random-walk algorithm enabled us to quantify the impact of microfractures/channels on NMR measurements and to propose a new concept of fracture-pore diffusional coupling in such heterogeneous systems. We randomly distributed and oriented microfractures (or channels) in 3D pore-scale images of different rock matrices. We then quantified the sensitivity of NMR [Formula: see text] (spin-spin relaxation time) distribution to the presence of microfractures (or channels) and compared the pore-scale simulation results against a previously published experimental study. The pore-scale simulation results from synthetic rock samples revealed that NMR [Formula: see text] distribution can be influenced not only by the pore-size distribution but also significantly by fracture-pore diffusional coupling. The intergranular pore size can be underestimated by up to 29%, and the volume fraction of intergranular pores can be underestimated by more than 10%, if the impact of diffusional coupling was not taken into account in interpretation of NMR measurements. Furthermore, we developed a simplified 1D analytical model for fracture-pore diffusional coupling. The analytical solutions of the 1D model were in agreement with the simulation results in the synthetic rock samples, which further demonstrated the existence of fracture-pore coupling in multiple-porosity systems. The developed 1D model enabled real-time evaluation of diffusional coupling effect in the presence of microfractures and complex pore-size distribution. The results were promising for future applications of NMR relaxometry for the assessment of microfracture content, when combined with other conventional well logs.

Detection of Iron Nanoparticles using Nuclear Magnetic Resonance Relaxometry and Inverse Laplace Transform

Department of Bioindustrial Machinery Engineering College of Agriculture and Life Sciences, Chonbuk National University, 567 Baekje-daero, Deokjin-gu, Jeonju-si 561-756, Republic of Korea Received: November 14th, 2014; Revised: November 24th, 2014; Accepted: November 26th, 2014 Purpose: Rapid detection of bacteria is very important in agricultural and food industries to prevent many foodborne illnesses. The objective of this study was to develop a portable nuclear magnetic resonance (NMR)-based system to detect foodborne pathogens (E. coli). This study was focused on developing a method to detect low concentrations of magnetic nanoparticles using NMR techniques. Methods: NMR relaxometry was performed to examine the NMR properties of iron nanoparticle mixtures with different concentrations by using a 1 T permanent magnet magnetic resonance imaging system. Exponential curve fitting (ECF) and inverse Laplace transform (ILT) methods were used to estimate the NMR relaxation time constants, T1 and T2, of guar gum solutions with different iron nanoparticle concentrations (0, 10-3, 10-4, 10-5, 10-6, and 10-7 M). Results: The ECF and ILT methods did not show much difference in these values. Analysis of the NMR relaxation data showed that the ILT method is comparable to the classical ECF method and is more sensitive to the presence of iron nanoparticles. This study also showed that the spin-spin relaxation time constants acquired by a Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence are more useful for determining the concentration of iron nanoparticle solutions comparwith the spin-lattice relaxation time constants acquired by an inversion recovery pulse sequence. Conclusions: We conclude that NMR relaxometry that utilizes CPMG pulse sequence and ILT analysis is more suitable for detecting foodborne pathogens bound to magnetic nanoparticles in agricultural and food products than using inversion recovery pulse sequence and ECF analysis.

Pore size analysis from low field NMR spin–spin relaxation measurements of porous microspheres

The porous structure characteristics of twenty-one porous microspheres with agarose framework were investigated based on the low-field nuclear magnetic resonance (LF NMR) relaxation time (T 2) distribution measurements. The feasibility of the technique was confirmed by direct relationship between T 2 and the mean pore size of the polymer networks (Rh) of agarose hydrogel which was established using the “Fiber-Cell” model. The expected pore radius distribution curve was obtained and the reliability was validated by comparing them with the results obtained by inverse size-exclusion chromatography. Thereafter, the technique was applied to characterize the pore size distribution (PSD) of soft microspheres and the pore size transformation of microspheres with or without grafted polymers in the process of protein adsorption. All of these results strongly support LF NMR as a promising, rapid and nondestructive technique for the determination of PSDs in many kinds of soft porous microspheres.

Fractal dimension of pore-space geometry of an Eocene sandstone formation

The geometry of pore structure of reservoir rock is described by the shape, size, distribution and connection of pores, and pore throats of rocks. Pore-space properties are important for the description and characterization of fluid storage and transport in reservoir rocks. We used the fractal concept to describe the geometric structure of the pores of 24 samples from an Eocene sandstone formation in China. The fractal behavior of pore volume distribution was investigated by capillary pressure curves and nuclear magnetic resonance (NMR). Additionally, the fractal dimension of the pore surface was determined based on data of the specific surface area per unit pore volume ([Formula: see text]). The comparison of fractal dimensions determined by three different methods indicates a clear differentiation into the “surface dimension” and “volume dimension.” The fractal dimension resulting from longer transverse NMR relaxation times and lower capillary pressure reflects the volume dimension of larger pores. The fractal dimension derived from the short NMR relaxation times is similar to the fractal dimension of the internal surface. The surface dimension increases with rising [Formula: see text]. The average value of surface dimension was determined to be 2.31 for the set of Eocene sandstones. This value of fractal dimension was successfully applied in a model of permeability prediction that is based on formation factor and specific surface area ([Formula: see text]).

A Review of the Principles and Applications of the NMR Technique for Near-Surface Characterization

This paper presents a comprehensive review of the recent advances in nuclear magnetic resonance (NMR) measurements for near-surface characterization using laboratory, borehole, and field technologies. During the last decade, NMR has become increasingly popular in near-surface geophysics due to substantial improvements in instrumentation, data processing, forward modeling, inversion, and measurement techniques. This paper starts with a description of the principal theory and applications of NMR. It presents a basic overview of near-surface NMR theory in terms of its physical background and discusses how NMR relaxation times are related to different relaxation processes occurring in porous media. As a next step, the recent and seminal near-surface NMR developments at each scale are discussed, and the limitations and challenges of the measurement are examined. To represent the growth of applications of near-surface NMR, case studies in a variety of different near-surface environments are reviewed and, as examples, two recent case studies are discussed in detail. Finally, this review demonstrates that there is a need for continued research in near-surface NMR and highlights necessary directions for future research. These recommendations include improving the signal-to-noise ratio, reducing the effective measurement dead time, and improving production rate of surface NMR (SNMR), reducing the minimum echo time of borehole NMR (BNMR) measurements, improving petrophysical NMR models of hydraulic conductivity and vadose zone parameters, and understanding the scale dependency of NMR properties.

Interpretation of NMR relaxation as a tool for characterising the adsorption strength of liquids inside porous materials.

Nuclear magnetic resonance (NMR) relaxation times are shown to provide a unique probe of adsorbate–adsorbent interactions in liquid-saturated porous materials. A short theoretical analysis is presented, which shows that the ratio of the longitudinal to transverse relaxation times (T1/T2) is related to an adsorbate–adsorbent interaction energy, and we introduce a quantitative metric esurf (based on the relaxation time ratio) characterising the strength of this surface interaction. We then consider the interaction of water with a range of oxide surfaces (TiO2 anatase, TiO2 rutile, γ-Al2O3, SiO2, θ-Al2O3 and ZrO2) and show that esurf correlates with the strongest adsorption sites present, as determined by temperature programmed desorption (TPD). Thus we demonstrate that NMR relaxation measurements have a direct physical interpretation in terms of the characterisation of activation energy of desorption from the surface. Further, for a series of chemically similar solid materials, in this case a range of oxide materials, for which at least two calibration values are obtainable by TPD, the esurf parameter yields a direct estimate of the maximum activation energy of desorption from the surface. The results suggest that T1/T2 measurements may become a useful addition to the methods available to characterise liquid-phase adsorption in porous materials. The particular motivation for this work is to characterise adsorbate–surface interactions in liquid-phase catalysis.

Research on the Remaining Oil Starting Mechanism in Different Pores of Polymer Surfactants Using NMR Analysis

With its unique structure and properties, polymer surfactants have been used in chemical flooding. Compared with ordinary polymer, polymer surfactants have a higher recovery degree. However, remaining oil starting mechanism in different pores using different polymer surfactants after water flooding is still unclear. NMR (Nuclear Magnetic Resonance) has a good effect on determination of rock oil saturation and analysis of pore structure. In this paper, oil displacement experiment using kerosene which contains no hydrogen was conducted and the problem caused by the similarity between oil phase relaxation time and water relaxation time in large pores was overcome. Through the change of NMR relaxation time, oil distribution situation in different pores of ordinary polymer, refining III polymer surfactant, and Haibo III polymer surfactant after polymer flooding was measured accurately. This paper also quantitatively analyzed the contribution degree to oil recovery in pores of different sizes, and evaluated oil displacement effect of the above three kinds of oil displacement systems. The results show that when only the swept volume is considered, recovery degree of Haibo III polymer surfactant is higher, reaching 53.66%. In different systems, middle pores contribute most to recovery degree. At the same time, the remaining oil in middle pores and large pores accounts more, which is the main attack direction towards tapping the potential of remaining oil in the oilfield.