Search

Modern state of main oil fields all over the world is characterized by passage to finishing stage of exploitation with getting worse as structure and quality of oil reservoirs. Under these conditions, the intensification of oil recovery process for the oil fields in working up stands more and more actual and the oil recovery enhancement methods play very important role. In this work, the scientific base of the technology and the results of electro treatment using at the row of the oil fields are presented. The technology allows to realize the combined influence on the wellbore and nearest area of oil reservoir. As a result, the permeability in this area is increasing and water content in total fluid production is decreasing.

The original technique with the use of longitudinal electron paramagnetic resonance detection was applied to measure longitudinal and transversal electron relaxation times (T 1 andT 2) in specially prepared carbon chars containing paramagnetic centers (PCs) on the surface. The results not only confirmed the conclusions of our previous studies but enabled us to determine the exchange integral and clarify the mechanism of interaction between the PCs and adsorbed molecular oxygen O2. A spin relaxation mechanism was also suggested which takes into account both the exchange and dipolar interactions in the systems combining PC and O2. The value of the exchange integral between PCs as well as its functional dependence on the oxygen content were estimated.

With original modulation technique, the longitudinal electron spin-relaxation timeT 1 has been measured in the La1-xCaxMnO3 manganite (x = 0.1) both in the paramagnetic state and around the temperature (T c) of the ferromagnetic ordering. The data are compared with the evolution of the transverse relaxation time T2 as determined from the electron spin resonance (ESR) linewidth. Well above Tc, theT 1 =T 2 equality was confirmed, whereas a steep slowing down ofT 2 was observed asT c was approached (theT 1/T2 ratio increased by two orders of magnitude). The temperature dependence ofT 1 within the whole temperature range was found to be consistent with that ofT · χ(T), where χ(T) is the electron-spin susceptibility obtained from the ESR absorption area. The interpretation suggests that both the longitudinal and transverse electron-spin relaxation rates are governed by strong exchange interaction between the Mn ions, the ESR linewidth being inhomogeneously broadened in the vicinity of the phase transition.

A modulation technique for direct measurement of extremely fast electron spin−lattice relaxation is described. The method is based on longitudinal magnetization detection. It is applied here to study the spin−lattice relaxation times T1e in aqueous solutions of perdeuterated nitroxide radical (T1e ≈ 200 ns) with various concentrations as well as to two Gd DOTA complexes (T1e ≈ 1.8−3.2 ns), which can be employed as paramagnetic contrast agents in magnetic resonance imaging.

The mechanism of oxygen response in several newly synthesized oxygen-sensitive chars was studied with the use of EPR spectroscopy. The results suggest that the compounds contain two basic types of paramagnetic centers (PC). The change in oxygen concentration leads to a mutual and reversible transformation of PCs in chars, which is reflected in EPR parameters. The adsorbed molecular oxygen progressively disturbs the wave functions of the PCs and so breaks the Heisenberg exchange between them. At high oxygen concentration, the 2D dipole–dipole interaction between PCs at the surface comes into play and determines the EPR lineshape. A suggested model quantitatively describes the evolution of the basic EPR parameters of each PC as a function of oxygen concentration.

Experiments with char samples provide support for the theory of EPR line shape in a system of surface paramagnetic centers with dipole-dipole interactions. It is also demonstrated that, if a strong Heisenberg exchange dominates over the dipole-dipole interactions, the EPR line of a two-dimensional system narrows and acquires Lorentzian shape, similar to the three-dimensional case.

With an original modulation technique, the Gd3+ electron spin-lattice relaxation has been investigated in normal and superconducting states of YBa2Cu3O6+x (123) and YBa2Cu4O8 (124) compounds doped with 1% Gd. In the 123 sample withx = 0.9T c = 90 K), theT 1 behavior within 50

Using an original modulation technique, the longitudinal (spin-lattice) relaxation of the impurity ${\mathrm{Gd}}^{3+}$ ions has been investigated in the superconducting state of ${\mathrm{Y}}_{0.99}{\mathrm{Gd}}_{0.01}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{4}{\mathrm{O}}_{8}$ magnetically aligned powders. The results agree well with the linewidth analysis performed on a single crystal. At low temperatures $(Tl60\mathrm{K})$ the relaxation rate ${T}_{1}^{\ensuremath{-}1}$ is found to be proportional to ${T}^{3},$ suggesting d-wave superconducting pairing. The data are consistent with previous Knight shift ${(}^{\mathrm{Gd}}K)$ measurements that revealed a linear temperature dependence of the uniform spin susceptibility. The Korringa relation between ${T}_{1}$ and ${}^{\mathrm{Gd}}K$ is confirmed indicating the validity of the Fermi-liquid model.

Using an original modulation technique, the electron spin-lattice relaxation have been investigated in two noncommon metals: YBa2Cu3Ox, high-Tc material doped with 1% Gd, and Rb1C60, linear polymer phase fulleride. In the first case, the Korringa-like temperature dependence of the Gd3+ longitudinal relaxation time T1, is found forx = 6.59 in a wide temperature range 4.2

Using the original technique of measuring very short spin-lattice relaxation times T1, the electron-spin relaxation has been investigated in a number of HTSC materials and related phases. The relaxation of Gd3+ ions in GdBa2Cu3O6+x is shown to be affected by both quasiparticle and antiferromagnetic fluctuations, the latter being increased at lower oxigen content. The pseudo-spin gap of about 200 K is found for both contributions. For Cu2+ EPR in YBaCuO, the T1 values reveal several types of paramagnetic centers including the “green phase” Y2BaCuO5. In the fulleride RbC60, the measuring of T1 and T2 yielded additional support for 1D conductivity and allowed to monitor the metal-insulator phase.

We have used an original modulation technique to measure longitudinal (spin-lattice) relaxation time T 1 in stable and metastable phases of RbC 60 at 77 – 450K. The data are correlated with transverse relaxation time T 2 and spin susceptibility measured in the wide temperature range. The equality T 1 = T 2 is found to be valid both in the stable polymer (ortho-I) and fcc phases. In quenched samples, two different T 1 values are found below 140K, one of them being much longer than T 2 . Interpretation is suggested implying quasi 1D metal behavior in the polymer phase and localized paramagnetic centers with exchange interaction in the others.

A method of directly measuring electron-spin-lattice relaxation times ${\mathit{T}}_{1}$ as short as ${10}^{\mathrm{\ensuremath{-}}9}$--${10}^{\mathrm{\ensuremath{-}}10}$ s is developed. The technique is based on the relation between the EPR absorption magnitude and the response of longitudinal spin magnetization to a radio-frequency modulation of microwave power. By means of this method, the electron-spin-lattice relaxation of ${\mathrm{Gd}}^{3+}$ ions was studied in high-${\mathit{T}}_{\mathit{c}}$ superconductor ${\mathrm{GdBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6+\mathit{x}}$ at 0.4\ensuremath{\le}x\ensuremath{\le}0.84 in the temperature range 77--300 K. It was found that spin-spin interactions between ${\mathrm{Gd}}^{3+}$ ions have no influence on the ${\mathit{T}}_{1}$ value, so ${\mathrm{Gd}}^{3+}$ can be used as a spin probe even in concentrated substances. The relaxation rate W=(${\mathit{TT}}_{1}$${)}^{\mathrm{\ensuremath{-}}1}$ is shown to consist of two parts, ${\mathit{W}}_{0}$ and ${\mathit{W}}_{\mathrm{AF}}$, resulting from the quasiparticle contribution and the antiferromagnetic fluctuations, respectively. Both contributions reveal a spin gap opening with \ensuremath{\Delta}/${\mathit{k}}_{\mathit{B}}$=200 K. The temperature variation of ${\mathit{W}}_{0}$ is in agreement with published $^{89}\mathrm{Y}$ NMR data in ${\mathrm{YB}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6+\mathit{x}}$, the ${\mathit{W}}_{0}$ values being proportional to the $^{89}\mathrm{Y}$ Knight shifts. The ratio ${\mathit{W}}_{\mathrm{AF}}$/${\mathit{W}}_{0}$ shows Curie-Weiss behavior and increases strongly as x decreases. This implies incomplete filtering of the antiferromagnetic fluctuations over the finite-sized ${\mathrm{Gd}}^{3+}$ 4f orbitals and symmetry breaking due to oxygen vacancies.

The report consists of two parts. 1. The method is described to measure directly the electron spin-lattice relaxation timeT 1 in high-T c superconductors (HTSC). The technique is based on registering the oscillating longitudinal spin magnetization while saturating the EPR line by an amplitude-modulated microwave field. TheT 1 values of the Cu2+ centers in YBa2Cu3O6+x (x=0.24−0.9) and Y2BaCuO5 materials are measured and found to be about 1.4·10−9 s independently ofx and temperature in the range 77–300 K. Besides much longer relaxation is displayed in degradated materials. The data obtained can be considered as an argument in favor of the “green-phase” origin of the “high-temperature” EPR spectra in the YBaCuO HTSCs. 2. Non-linear responses of the microwave (1010 Hz) and rf (105–107 Hz) absorption in HTSC materials to low-frequency magnetic modulation are studied and found to be quite different in the two frequency ranges. It is shown that at microwaves the effect is caused by non-linear interaction with shielding supercurrents whereas at lower frequencies it is due to jumps of fluxons over potential barriers. The models developed take into account the Josephson weak links and the thermo-assisted flux creep.

The experimental works described are performed by the authors over last two decades by means of the LFD EPR technique. The essence of this method is low-frequency detection of the longitudinal spin magnetization while the magnetic resonance is excited by a strong microwave field. The first kind of LFD EPR is the enchanced longitudinal susceptibility effect (ELSE) which has been elaborated and applied to study spin thermodynamics in solids since 1972. Various applications of ELSE are described such as direct measuring of the spin-spin interaction temperatureTss in the course of resonance saturation, spin-lattice and cross relaxation, dynamic nuclear polarization etc. Another version of LFD EPR was employed to study electron spin-lattice relaxation of paramagnetic centers in high-temperature superconductors (HTSC). Recent experimental data are presented on the electron spin-lattice relaxation of Cu2+ ions in YBa2Cu3O6+x at various temperatures andx values.

AC losses as a function of the amplitude of the AC transverse magnetic field are measured on both poly- and monocrystalline YBa2Cu3O7- delta superconducting films. The data are consistent with the calculation based on the Bean critical state model adapted to a thin film perpendicular to a magnetic field. An influence of the 'lower critical field' Hc1' (which first penetrates the sample) is observed on the monocrystalline films, the value of Hc1' being about 0.1 times the 'true' Hc1 as known for bulk crystals. The technique described provides a simple method of determining critical current density and penetration fields in high-Tc films.

The effect of low-frequency (0.03–3500 MHz) ac magnetic modulation and transport current on rf (10 MHz) losses have been investigated in Y- and Bi-based high- T c superconducting ceramics, films and crystals. The additional rf absorption Δ χ ″ was observed and found to be proportional to the absolute value of the ac shielding current flowing through the dissipative weak-link medium. The data are consistent with the thermally activated flux flow model with allowance made for a wide, close to uniform, distribution of the pinning energies.

Additional rf and microwave absorption is observed on YBaCuO superconducting ceramics subjected to a small step-like change of the external magnetic field. The detected response Δχ″(t) is shown to be caused by transient shielding currents and reflects the process of re-distribution of magnetic flux approaching equilibrium in the time scale 0.1−1 ms. The mechanisms of the current decay and its influence on rf absorption are discussed in terms of the thermal assisted flux flow and fluxon diffusion in the intergranular system, assuming a broad distribution of activation energies.

A new technique of measuring extremely short electron spin-lattice relaxation times T1∼10−6–10−10 sec is developed and applied to study some oxide high-Tc materials, related compounds and fullerides RbC60.

Strong enhancement of the low frequency longitudinal susceptibility of paramagnetic solids (ELSE), caused by asymmetrical saturation of the magnetic resonance spectrum, has been theoretically predicted and experimentally studied. ELSE is due to cooling of the spin-spin interaction reservoir and can be used for direct measurement of its temperature Tss. Some applications of the ELSE to various magnetic resonance and spin dynamics problems are also described.