2D NMR技术在石油测井中的应用

近几年,2D NMR技术得到迅速发展,特别是在核磁共振测井领域. 该文将主要介绍2D NMR技术的脉冲序列、弛豫原理以及2D NMR技术在石油测井中应用. 2D NMR技术是在梯度场的作用下,利用一系列回波时间间隔不同的CPMG脉冲进行测量,利用二维的数学反演得到2D NMR. 2D NMR技术可以直接测量自扩散系数、弛豫时间、原油粘度、含油饱和度、可动水饱和度、孔隙度、渗透率等地层流体性质和岩石物性参数. 从2D NMR谱上,可以直观的区分油、气、水,判断储层润湿性,确定内部磁场梯度等. 2D NMR技术为识别流体类型提供了新方法.     

Quantitative evaluation of porous media wettability using NMR relaxometry

We propose a new method to determine wettability indices from NMR relaxometry. The new method uses the sensitivity of low field NMR relaxometry to the fluid distribution in oil-water saturated porous media. The model is based on the existence of a surface relaxivity for both oil and water, allowing the determination of the amount of surface wetted either by oil or by water. The proposed NMR wettability index requires the measurement of relaxation time distribution at four different saturation states. At the irreducible water saturation, we determine the dominant relaxation time of oil in the presence of a small amount of water, and at the oil residual saturation, we determine the dominant relaxation time of water in the presence of a small amount of oil. At 100% water and 100% oil saturation, we determine the surface relaxivity ratio. The interaction of oil with the surface is also evidenced by the comparison of the spin-lattice (T1) and spin-locking (T1rho) relaxation times. The new NMR index agrees with standard wettability measurements based on drainage-imbibition capillary pressure curves (USBM test) in the range [-0.3-1].     

Development of the Miniature NMR Apparatus for Edible Oil Quality Control

Edible oils are necessary in daily cooking; therefore, it is significantly important to find an efficient scheme for its quality control. In this work, a miniature nuclear magnetic resonance (NMR) apparatus was set up for the identification of edible oils with different qualities. Experimental results show that the total transverse relaxation time of the commercial refined edible oil is shorter than the poor quality edible oil. Further data processing was performed with inverse Laplace transformation for transverse relaxation time distribution analysis. The spectra of the distribution of transverse relaxation times of different qualities edible oils are also significantly different, among which, the refined edible oil has two peaks whereas the poor quality edible oil has three peaks. The experimental results are well agreed with the theoretical analysis. As the miniature NMR apparatus was employed to analyze three suspected illegal cooking oils which were seized at the scene of inspection, the three oils were all confirmed to be poor quality edible oils. The apparatus can also detect refined edible oils adulterated with illegal cooking oils. In conclusion, the presented miniature NMR apparatus could be a potential tool for edible oil quality control.     

NMR imaging of white button mushroom (Agaricus bisporis) at various magnetic fields

Nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) have been applied to visualize physiological phenomena in plants and agricultural crops. Imaging sequences that result in contrast of a combination of parameters (e.g., proton density, ) cannot be used for a correct and unique interpretation of the results. In this study multiecho imaging together with monoexponential T₂ decay fitting was applied to determine reliable proton density and T₂ distributions over a mushroom. This was done at three magnetic field strengths (9.4, 4.7, and 0.47 T) because susceptibility inhomogeneities were suspected to influence the T₂ relaxation times negatively, and because the inflences of susceptibility inhomogeneities increase with a rise in magnetic field strength. Electron microscopy was used to understand the different T₂'s for the various tissue types in mushrooms. Large influences of the tissue ultrastructure on the observed T₂ relaxation times were found and explained. Based on the results, it is concluded that imaging mushrooms at low fields (around or below 0.47T) and short echo times has strong advantages over its high-field counterpart, especially with respect to quantitative imaging of the water balance of mushrooms. These conclusions indicate general validity whenever NMR imaging contrast is influenced by susceptibility inhomogeneities.     

水驱油过程的核磁共振二维谱研究

岩心水驱油过程中油水分布状况是岩心多孔介质的重要性质. 水驱油过程的研究是进一步进行提高采收率研究的基础. 核磁共振扩散-弛豫二维谱提供了岩心中流体性质的多方面信息,与核磁共振一维弛豫谱相比极大地提高了区分油水的能力. 该文通过2组岩心水驱油实验,从不同含油饱和度的扩散-弛豫二维谱中提取出水的一维弛豫谱,在原油粘度比较高的情况下获得了驱替过程中油水在不同孔隙中的分布状况以及润湿性等信息, 解决了单独用一维弛豫谱方法难以区分油水的问题. 该文的研究方法对油田提高采收率的研究有比较大的参考价值.     

Proton NMR study of α-MnH0.06

Proton nuclear magnetic resonance (NMR) spectra and spin-lattice relaxation rates for the solid solution α-MnH_0.06 have been measured over the temperature range 11-297 K and the resonance frequency range 20-90 MHz. A considerable shift and broadening of the proton NMR line and a sharp peak of the spin-lattice relaxation rate are observed near 130 K. These effects are attributed to the onset of antiferromagnetic ordering below the Néel temperature T_N≈130 K. The proton NMR line does not disappear in the antiferromagnetic phase; this suggests a small magnitude of the local magnetic fields at H-sites in α-MnH_0.06. The spin-lattice relaxation rate in the paramagnetic phase is dominated by the effects of spin fluctuations.     

Selective saturation in strongly inhomogeneous magnetic fields

The possibility to produce selective saturation by nuclear magnetic resonance (NMR) sequences of low-power radio-frequency pulses in strongly inhomogeneous magnetic fields is explored. The saturation of parts of the sensitive volume is produced by a particular pulse sequence with reduced amplitude distribution and the spectrum of the recorded signal is compared with the simulated spectrum. The spectra of the recorded free induction decays and echo signals are in good agreement with the simulated spectra of the pulse sequence, which demonstrates the effect of the selective saturation. The results obtained are an important step towards the development of new mobile and lowpower NMR equipments operating with inhomogeneous magnetic fields.     

Two-dimensional nuclear magnetic resonance petrophysics

Two-dimensional nuclear magnetic resonance (2D NMR) opens a wide area for exploration in petrophysics and has significant impact to petroleum logging technology. When there are multiple fluids with different diffusion coefficients saturated in a porous medium, this information can be extracted and clearly delineated from CPMG measurements of such a system either using regular pulsing sequences or modified two window sequences. The 2D NMR plot with independent variables of T2 relaxation time and diffusion coefficient allows clear separation of oil and water signals in the rocks. This 2D concept can be extended to general studies of fluid-saturated porous media involving other combinations of two or more independent variables, such as chemical shift and T1/T2 relaxation time (reflecting pore size), proton population and diffusion contrast, etc.     

岩心孔隙介质中流体的核磁共振弛豫

弛豫时间是核磁共振研究中的一个重要参数,岩心孔隙介质流体的弛豫过程是自由流体弛豫机制、表面弛豫机制和流体的扩散弛豫机制共同作用的结果,它包含了丰富的孔隙和流体本身的信息. 弛豫时间和自扩散系数的测量及对弛豫时间的分析是核磁共振技术应用于岩心分析和石油勘测的重要内容.     

A New Approach of Two-Dimensional the NMR Relaxation Measurement in Flowing Fluid

Driven-equilibrium fast saturation recovery (DEFSR), as a new method for two-dimensional (2-D) nuclear magnetic resonance (NMR) relaxation measurement based on pulse sequence in flowing fluid, is proposed. The two-dimensional functional relationship between the ratio of transverse relaxation time to longitudinal relaxation time of fluid (T ₁/T ₂) and T ₁ distribution is obtained by means of DEFSR with only two one-dimensional measurements. The rapid measurement of relaxation characteristics for flowing fluid is achieved. A set of the down-hole NMR fluid analysis system is independently designed and developed for the fluid measurement. The accuracy and practicability of DEFSR are demonstrated.     

Nuclear magnetic resonance studies of vortices in high temperature superconductors

The distinct distribution of local magnetic fields due to superconducting vortices can be detected with nuclear magnetic resonance (NMR) and used to investigate vortices and related physical properties of extreme type II superconductivity. This review summarizes work on high temperature superconductors (HTS) including cuprates and pnictide materials. Recent experimental results are presented which reveal the nature of vortex matter and novel electronic states. For example, the NMR spectrum has been found to provide a sharp indication of the vortex melting transition. In the vortex solid a frequency dependent spin–lattice relaxation has been reported in cuprates, including YBa₂Cu₃O_7-x, Bi₂SrCa₂Cu₂O_8+δ, and Tl₂Ba₂CuO_6+δ. These results have initiated a new spectroscopy via Doppler shifted nodal quasiparticles for the investigation of vortices. At very high magnetic fields this approach is a promising method for the study of vortex core excitations. These measurements have been used to quantify an induced spin density wave near the vortex cores in Bi₂SrCa₂Cu₂O_8+δ. Although the cuprates have a different superconducting order parameter than the iron arsenide superconductors there are, nonetheless, some striking similarities between them regarding vortex dynamics and frequency dependent relaxation.     

Structural, static and dynamic magnetic properties of dextran coated γ-Fe(2)O(3) nanoparticles studied by (57)Fe NMR, M?ssbauer, TEM and magnetization measurements

The structural and magnetic properties and spin dynamics of dextran coated and uncoated γ-Fe(2)O(3) (maghemite) nanoparticles have been investigated using high resolution transmission electron microscopy (HRTEM), (57)Fe nuclear magnetic resonance (NMR), M?ssbauer spectroscopy and dc magnetization measurements. The HRTEM observations indicated a well-crystallized system of ellipsoid-shaped nanoparticles, with an average size of 10 nm. The combined M?ssbauer and magnetic study suggested the existence of significant interparticle interactions not only in the uncoated but also in the dextran coated nanoparticle assemblies. The zero-field NMR spectra of the nanoparticles at low temperatures are very similar to those of the bulk material, indicating the same hyperfine field values at saturation in accord with the performed M?ssbauer measurements. The T(2) NMR spin-spin relaxation time of the nanoparticles has also been measured as a function of temperature and found to be two orders of magnitude shorter than that of the bulk material. It is shown that the thermal fluctuations in the longitudinal magnetization of the nanoparticles in the low temperature limit may account for the shortening and the temperature dependence of the T(2) relaxation time. Thus, the low temperature NMR results are in accord with the mechanism of collective magnetic excitations, due to the precession of the magnetization around the easy direction of the magnetization at an energy minimum, a mechanism originally proposed to interpret M?ssbauer experiments in magnetic nanoparticles. The effect of the surface spins on the NMR relaxation mechanisms is also discussed.     

Hall constants and magnetoresistance in aluminium single crystals,studied by means of helicon resonances

The Hall coefficient and the transverse magnetoresistance in aluminium single crystals were determined from measurements on standing helicon waves, in the three main crystallographic directions [100], [110] and [111]. The magnetic fields ranged from 1 to 7T and the temperature from 4.2 to 20 K. A Legendy-type theory with parameters adapted to the separation of the satellites as measured at high fields gave good fittings over the whole field range. The Hall coefficients, determined in this work, decrease with magnetic field as mentioned in earlier publications, but are generally lower than those published previously for comparable field ranges.The transverse magnetoresistance as a function of temperature was determined from a continuous recording of the helicon response at resonance. The temperature dependent part of the resistivity varies according to a T³ law for all fields. The field dependence is close to linear, but indications of quadratic and saturation behaviour are also present.     

基于高温超导SQUID的低场核磁共振研究

采用一高温超导射频量子干涉器(HTS rf-SQUID)作为信号探测器件,研究了多种液体样品的低场核磁共振信号。通过改变测量场(简称Bm)的大小,可以探测到质子拉莫频率(简称fL)从2Hz到40kHz的信号。由于在低场核磁共振中,Bm的均匀性能很好的得到满足,因而可能得到很窄的谱线宽度。实验发现,对自来水样品,在7μT以下均可接近谱线的自然宽度。同时,在低场核磁共振条件下,样品的化学位移很小以至于消失,因而可以研究"纯"的异核间的自旋耦合谱。作者研究了低场下2,2,2-三氟乙醇的低场自旋耦合谱。另外,作者首次采用SQUID在户外探测到地球磁场下的核磁共振现象,并研究了地球磁场的涨落对测量的影响,为SQUID的低场核磁共振研究开辟了一个新的研究方向。     

Nuclear magnetic resonance of some oils

The author subjected some oils to nuclear magnetic resonance in order to obtain information on their structure and to compare their behaviour at NMR. Saturation curves were obtained and the longitudinal and transverse relaxation times of the resonance of these oils were investigated. Relations were derived for calculating the number of resonating hydrogen nuclei per unit volume of oil, both by direct study of the signals on a cathode ray oscilloscope and by recording the differential of the absorption curve of NMR. The number of hydrogen nuclei per unit volume and mass of the oils was calculated. Equations were derived for the longitudinal relaxation time of glycerine and castor oil. Relations were also found from which qualitative conclusions could be drawn as to the average molecule mass of the oils and the mean distance of the resonating hydrogen nuclei of the different oils. Castor oil, bearing oil 207, paraffin oil, transformer oil B, one lubricating oil of commercial quality and methyl silicon oil were investigated and glycerine was used as the normal of the relaxation time and signal intensity. In addition, the intensities of the NMR signals of some naphtha oils, immersion oil and some polymer oils were measured.The author used an arrangement for observing and recording the NMR signal with an autodyne detector, which has been described in another paper. The sensitivity curve of the autodyne detector was measured and respected in the calculations.     

Displacement front behavior of near miscible CO2 flooding in decane saturated synthetic sandstone cores revealed by magnetic resonance imaging

It is of great importance to study the CO₂-oil two-phase flow characteristic and displacement front behavior in porous media, for understanding the mechanisms of CO₂ enhanced oil recovery. In this work, we carried out near miscible CO₂ flooding experiments in decane saturated synthetic sandstone cores to investigate the displacement front characteristic by using magnetic resonance imaging technique. Experiments were done in three consolidated sandstone cores with the permeabilities ranging from 80 to 450 mD. The oil saturation maps and the overall oil saturation during CO₂ injections were obtained from the intensity of magnetic resonance imaging. Finally the parameters of the piston-like displacement fronts, including the front velocity and the front geometry factor (the length to width ratio) were analyzed. Experimental results showed that the near miscible vertical upward displacement is instable above the minimum miscible pressure in the synthetic sandstone cores. However, low permeability can restrain the instability to some extent.     

Investigation of the parametric excitation and relaxation of phonons in antiferromagnetic α-Fe2O3

Parametric excitation of magnetoelastic waves was investigated in the easy-plane antiferromagnet α-Fe₂O₃ by parallel and perpendicular microwave pumping over a wide range of frequencies, magnetic fields, and temperatures, and the parametric resonance thresholds were measured. The frequencies of the natural magnetoelastic vibrations of the sample were investigated as a function of the magnetic field and temperature. The results of the measurements were used to calculate the parameters of the magnetoelastic wave spectrum and the rate of relaxation of the excited quasi-phonons. Possible mechanisms for quasi-phonon damping were analyzed.     

Temperature dependence of the spin-lattice relaxation time for quadrupole nuclei under conditions of NMR line saturation

The contributions of different mechanisms of nuclear spin-lattice relaxation are experimentally separated for ⁶⁹Ga and ⁷¹Ga nuclei in GaAs crystals (nominally pure and doped with copper and chromium), ²³Na nuclei in a nominally pure NaCl crystal, and ²⁷Al nuclei in nominally pure and lightly chromium-doped Al₂O₃ crystals in the temperature range 80–300 K. The contribution of impurities to spin-lattice relaxation is separated under the condition of additional stationary saturation of the nuclear magnetic resonance (NMR) line in magnetic and electric resonance fields. It is demonstrated that, upon suppression of the impurity mechanism of spin-lattice relaxation, the temperature dependence of the spin-lattice relaxation time T₁ for GaAs and NaCl crystals is described within the model of two-phonon Raman processes in the Debye approximation, whereas the temperature dependence of T₁ for corundum crystals deviates from the theoretical curve for relaxation due to the spin-phonon interaction.     

Proton NMR and susceptibility measurements on the magnetic core of ferritin

We report an investigation of the magnetic core of the biomolecule ferritin by means of proton nuclear magnetic resonance (NMR) and relaxation, magnetic susceptibility and scanning electron microscope (SEM) measurements. SEM images show that the outer protein shell is taken out completely by an appropriate chemical treatment and indicate particle sizes ranging from 10² to 10⁴ nm. Susceptibility measurements show a maximum in the zero-field-cooled data which is strongly field-dependent and can be ascribed to superparamagnetic behavior, whereas the hysteresis curve is different from normal ferritin. Proton NMR and spin-lattice relaxation data as a function of temperature at 4.7 T suggest the presence of an antiferromagnetic transition around 100 K.     

Accelerating multi-echo T2 weighted MR imaging: analysis prior group-sparse optimization

High pressure measurements in most scientific fields rely on metal vessels given the superior tensile strength of metals. We introduce high pressure magnetic resonance imaging (MRI) measurements with metallic vessels. The developed MRI compatible metallic pressure vessel concept is very general in application. Macroscopic physical systems are now amenable to spatially resolved nuclear magnetic resonance (NMR) study at variable pressure and temperature. Metallic pressure vessels not only provide inherently high tensile strengths and efficient temperature control, they also permit optimization of the MRI RF probe sensitivity. An MRI compatible pressure vessel is demonstrated with a rock core holder fabricated using non-magnetic stainless steel. Water flooding through a porous rock under pressure is shown as an example of its applications. High pressure NMR spectroscopy plays an indispensable role in several science fields. This work will open new vistas of study for high pressure material science MRI and MR.