![Experimental decays of the transverse magnetization (FID and CPMG) of an oil sample containing 5.4% of asphaltenes and the fitting results of these data based on Equations 1 and 2 after taking into account the inhomogeneity of the magnetic field. Noisy lines from the top characterize the quality of the approximation, fivefold enhanced difference between the measurement results and the fitting curves are presented [9].](https://www.researchgate.net/profile/Vladimir-Volkov-8/publication/321780977/figure/fig2/AS:571192609382400@1513194338692/Experimental-decays-of-the-transverse-magnetization-FID-and-CPMG-of-an-oil-sample_Q320.jpg)
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LOW-FIELD NMR METHOD FOR ANALYSIS OF HEAVY OILS WITHOUT EXTRACTION OF ASPHALTENES
Abstract and Figures
For the analysis of heavy oil samples of the Ashalchinskoye deposit the combined nuclear magnetic resonance (NMR) method to measure the free-induction decay (FID) together with the Carr-Purcell-Meiboom-Gill (CPMG) echo signal’s decay was used. The measurements were carried out on a NMR analyzer Chromatek-Proton 20M, operating at a frequency of 20 MHz. Special control program was created on the NMR analyzer, which automatically tunes and measures full FID curve, then switches to measuring the echo amplitudes decay by the CPMG pulse sequence, and the study is completed by co-processing together all experimental data. This technique makes it possible to measure the amplitudes of the NMR signals and the relaxation times T2 of the protons of heavy oil components in situ, including asphaltenes, without introducing any perturbations into the analyzed system. Under the influence of paramagnetic centers included to the composition of asphaltene, mainly vanadyl complexes on EPR data, the amplitude-relaxation characteristics of protons in the samples are divided into 6 groups related to solid asphaltenes (in crystalline and amorphous states), resins (high and low density), aromatic and saturated compounds. The amplitudes of NMR signals characterize the content of these fractions in the sample, and the relaxation times are determined by the mobility of molecules and the dynamics of their exchange between fractions. The proposed NMR method is promising for on-line monitoring the technological processes of heavy oil mining, transportation and processing under real conditions on temperature, pressure and the presence of dissolved gases, provided that the design of NMR sensor is adapted to industrial applications.
Keywords SARA, LF-NMR, Vanadyl, Relaxation, Process Monitoring
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... Recently, experiments with magnetic resonance in the time domain (TD-NMR) have gained popularity [29][30][31][32]. Due to the low cost and sometimes high speed of a measurement, as well as the simplicity of samples preparation, TD-NMR is considered as one of the most suitable techniques for organizing quality control and quality analysis in industrial automation processes. ...
The role of cellulose in various applications is remarkably going up in recent decades as it is a renewable material for wide areas of applications that can be delivered in huge quantities. This drives permanently growing interest to development of sensors for processing industry, that can give fast and reliable information about physicochemical structure of cellulosic materials. In this framework, Time-Domain NMR with its capability to be miniaturized may become very attractive to measure such structural parameters as crystallinity, domain sizes, and sorption properties. This contribution is aimed to summarize existing NMR relaxometric methods and update them with the techniques of signal processing and pulse sequences like Magic Sandwich Echo or multiple quantum transitions, that are well established for synthetic polymers, but still have not been extensively used for polysaccharides. Here, we also demonstrate our own results of cellulose study trying to contribute to the promotion of Time-Domain NMR applicability to biopolymers.
The distribution of NMR relaxation times and diffusion coefficients has been employed for modeling the composition of typical crude oils both in bulk and in the reservoir rock. Although the relation of relaxation times and diffusion coefficients with viscosity and molecular size in a typical maltene composition has been established for bulk oils, it is known that the presence of asphaltene affects the relaxation times in a complex way not sufficiently understood so far. In particular, it is expected that the interaction of aromatic or aliphatic maltene molecules, respectively, with asphaltene aggregates leads to a different influence on the relaxation time. In this study, we have applied, for the first time, fluorine containing tracer molecules to natural oils of different asphaltene content and investigated the tracers’ relaxation time ratios T1/T2 and the magnetic field dependence of longitudinal relaxation times, T1(ω). This strategy has the advantage of specifically determining the behavior of individual tracer molecules, where molecular weight and aromaticity have been considered as variables. One main finding of this study is the remarkably strong change of T1/T2 between perfluorinated alkanes and aromatics in the presence of asphaltenes, whereas this ratio remains nearly constant for asphaltene-free oils. The results are interpreted in terms of selective maltene–asphaltene interaction on the basis of frequency dependent relaxation results. The strong contrast of relaxation times allows for a simplified quantification of either asphaltene concentration or aromaticity of maltenes in natural oils.
Asphaltene precipitation and deposition occur in petroleum reservoirs as a change in pressure, temperature and liquid phase composition and reduce the oil recovery considerably. In addition to these, asphaltene precipitates may deposit in the pore spaces of reservoir rock and form plugging, which is referred to as a type of formation damage, i.e. permeability reduction. In all cases above, it is of great importance to know under which conditions the asphaltenes precipitate and to what extent precipitated asphaltenes can be re-dissolved. In other words, to what extent the process of asphaltene precipitation is reversible with respect to change in thermodynamic conditions. In present work, a series of experiments was designed and carried out to quantitatively distinguish the reversibility of asphaltene precipitation upon the change in pressure, temperature and liquid composition. Experiments were conducted in non-porous media. Generally it was observed that the asphaltene precipitation is a partial reversible process for oil under study upon temperature change with hysteresis. However, the precipitation of asphaltene as a function of mixture composition and pressure is nearly reversible with a little hysteresis.
A fast and accurate method for the group analysis of crude oils in porous media that describes petroleum components (especially heavy fractions) has been developed. NMR structure group analysis is used as the tool for the characterization of crude oils. This method is proposed as an alternative to existing complicated and laborious methods of characterization of extracted oil samples. Currently, group analysis of heavy fractions of crude oils is being investigated by means of chromatographic methods, such as the SARA (saturates-aromatics-resinsasphaltenes) test. These methods are usually expensive, and require considerable work from qualified personnel. Furthermore, these methods cannot be used for estimating the oil components in situ.
The basis of determining crude oil components using NMR is the difference of the nuclei mobility in the different hydrocarbons during the NMR testing period. A combination of solvent extraction, NMR testing and data processing gives a series of NMR terms that are then used to specify hydrocarbon mixtures and their components both in the bulk phase and in unconsolidated porous media.
Introduction
NMR logging tools are currently used for determining reservoir properties such as porosity(1, 2), permeability(1-4), as well as mobile and immobile fluids(5-7). Recent developments in NMR research offer tools for separating water, oil, and gas from the combined NMR signal(7, 8). Very little is known about the use of NMR logging tools for the in situ characterization of crude oils(1). With respect to heavy oil and bitumen formations, NMR logging has not been very successful in characterizing crude oil. The reason for the lack of such success is the fact that the NMR logging tools cannot detect the spectra from most heavy oil and bitumen formations. It should be noted that high field NMR technology has solved such problems in the past, but such technology cannot be used downhole.
A fundamental objective of the research performed in our laboratory is to extend the use of NMR logging tools to all heavy oil and bitumen formations. To this end, the NMR characteristics of heavy oils in porous media were investigated(9, 10). The objective of our work is to isolate the oil signal from the combined NMR spectrum of the formation, and then shift it towards the relaxation time range that can be detected by the conventional NMR logging tools. Once this goal is achieved, NMR logs of heavy oil and bitumen formations can become successful in the analysis of oil downhole.
This objective was accomplished in the material presented in this paper through a series of experiments that addressed the following issues:Relaxation interactions in mixtures of simple organic liquids with water.Relaxation interactions in mixtures of simple organic liquids. Relaxation interactions in mixtures of simple organic liquids with solvents.Relaxation times of conventional crude oils with and without solvents present.Relaxation times of heavy crude oils and bitumen with and without solvents present.Relaxation times of heavy crude oils and bitumen with and without solvents in unconsolidated sands and with variable connate water saturation.
The nuclear magnetic résonance (NMR) signal obtained from conventional oil, heavy oil, and bitumen formations can consist of both hydrocarbon and water signals. Each NMR signal can further characterize both mobile and immobile fluids in the porous media. However, as the viscosity of the hydrocarbon phase increases and the NMR signal shifts toward lower relaxation times, the composite NMR signal for the hydrocarbon-bearing formation becomes very complicated. As the viscosity of the bitumen exceeds 100,000 cp (at natural conditions), the relaxation characteristics of bitumen become partially nondetectable by both the logging and laboratory NMR tools. As a result, the conventional methods of NMR detection fail to precisely recognize the hydrocarbon components.
Laboratory NMR measurements of bitumen-bearing porous media under different temperatures were performed. This method delivered new information about bitumen reserves in situ. The results show that low-field NMR holds promise for the characterization of recoverable heavy oil and bitumen reserves. This new approach can be applicable for real-time monitoring of thermal extraction, including monitoring the efficiency of thermal recovery methods.
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We show that low-field proton nuclear magnetic resonance (NMR) relaxation and diffusion experiments can be used to study asphaltene aggregation directly in crude oils. Relaxation was found to be multiexponential, reflecting the composition of a complex fluid. Remarkably, the relaxation data for samples with different asphaltene concentrations can be collapsed onto each other by a simple rescaling of the time dimension with a concentration-dependent factor xi, whereas the observed diffusion behavior is unaffected by asphaltene concentration. We interpret this finding in terms of a theoretical model that explains the enhanced relaxation by the transitory entanglement of solvent hydrocarbons within asphaltene clusters and their subsequent slowed motion and diffusion within the cluster. We relate the measured scaling parameters xi to cluster sizes, which we find to be on the order of 2.2-4.4 nm for an effective sphere diameter. These sizes are in agreement with the typical values reported in the literature as well as with the small-angle X-ray scattering (SAXS) experiments performed on our samples.
In food science and technology, assessment of phase-compositional behaviour of lipids is critical for understanding many product properties and for effective process control. Time Domain NMR is a rapid and easy-to-handle technique, and is already well appreciated as a tool for phase-compositional assessment in foods. The phase-compositional detail that can be obtained with the established methodology is limited, however. In this work, we set out to obtain more phase-compositional details of lipids as currently feasible with the already established 'classical' NMR methods. We deployed a combined FID-CPMG experiment, and analyzed the Transversal Relaxation Decays by Deconvolution (TRDD) with semi-empirical mathematical functions for solid, semi-solid and liquid components. Within the solid component, different lipid crystal polymorphs can be discerned in a quantitative manner, i.e. alpha, beta and beta'. The TRDD method was validated against established NMR SFC methods, in terms of accuracy ('trueness') and precision. The solid fat content (SFC) of a large collection of fat blends was measured by the commonly employed NMR Direct and Indirect SFC methods and a good correlation with the results of the TRDD was observed, thereby demonstrating accuracy. Furthermore, TRDD was found to be equally precise as the established NMR SFC methods.
Heavy oil (bitumen) is characterized by its high viscosity and density, which is a major obstacle to both well logging and recovery. Due to the lost information of T2 relaxation time shorter than echo spacing (TE) and interference of water signal, estimation of heavy oil properties from NMR T2 measurements is usually problematic. In this work, a new method has been developed to overcome the echo spacing restriction of NMR spectrometer during the application to heavy oil (bitumen). A FID measurement supplemented the start of CPMG. Constrained by its initial magnetization (M0) estimated from the FID and assuming log normal distribution for bitumen, the corrected T2 relaxation time of bitumen sample can be obtained from the interpretation of CPMG data. This new method successfully overcomes the TE restriction of the NMR spectrometer and is nearly independent on the TE applied in the measurement. This method was applied to the measurement at elevated temperatures (8-90 degrees C). Due to the significant signal-loss within the dead time of FID, the directly extrapolated M0 of bitumen at relatively lower temperatures (<60 degrees C) was found to be underestimated. However, resulting from the remarkably lowered viscosity, the extrapolated M0 of bitumen at over 60 degrees C can be reasonably assumed to be the real value. In this manner, based on the extrapolation at higher temperatures (> or = 60 degrees C), the M0 value of bitumen at lower temperatures (<60 degrees C) can be corrected by Curie's Law. Consequently, some important petrophysical properties of bitumen, such as hydrogen index (HI), fluid content and viscosity were evaluated by using corrected T2.
Joint measurement and combined processing of FID and echo signals in the CPMG series for the evaluation of the component composition of heavy oils [Sovmestnoye izmereniye i kombinirovannaya obrabotka signalov SI i ekha v serii KPMG dlya otsenki komponentnogo sostava tyazholykh neftey
- B V Sakharov
- N M Khasanova
- V Volkov
- Ya
Sakharov B.V., Khasanova N.M., Volkov V.Ya., Joint measurement and combined
processing of FID and echo signals in the CPMG series for the evaluation of the
component composition of heavy oils [Sovmestnoye izmereniye i kombinirovannaya
obrabotka signalov SI i ekha v serii KPMG dlya otsenki komponentnogo sostava
tyazholykh neftey], «Novyye dostizheniya YAMR v strukturnykh issledovaniyakh».
Sbornik tezisov VI Vserossiyskoy konferentsii, Kazan, 6-9 April 2015, Russia, pp. 67-68, 2015.