Geir Humborstad Sørland

Behavior of Asphaltene Model Compounds at W/O Interfaces

Asphaltenes, present in significant amounts in heavy crude oil, contains subfractions capable of stabilizing water-in-oil emulsions. Still, the composition of these subfractions is not known in detail, and the actual mechanism behind emulsion stability is dependent on perceived interfacial concentrations and compositions. This study aims at utilizing polyaromatic surfactants which contains an acidic moiety as model compounds for the surface-active subfraction of asphaltenes. A modified pulse-field gradient (PFG) NMR method has been used to study droplet sizes and stability of emulsions prepared with asphaltene model compounds. The method has been compared to the standard microscopy droplet counting method. Arithmetic and volumetric mean droplet sizes as a function of surfactant concentration and water content clearly showed that the interfacial area was dependent on the available surfactant at the emulsion interface. Adsorption of the model compounds onto hydrophilic silica has been investigated by UV depletion, and minor differences in the chemical structure of the model compounds caused significant differences in the affinity toward this highly polar surface. The cross-sectional areas obtained have been compared to areas from the surface-to-volume ratio found by NMR and gave similar results for one of the two model compounds. The mean molecular area for this compound suggested a tilted geometry of the aromatic core with respect to the interface, which has also been proposed for real asphaltenic samples. The film behavior was further investigated using a liquid-liquid Langmuir trough supporting the ability to form stable interfacial films. This study supports that acidic, or strong hydrogen-bonding fractions, can promote stable water-in-oil emulsion. The use of model compounds opens up for studying emulsion behavior and demulsifier efficiency based on true interfacial concentrations rather than perceived interfaces.

A Pulsed Field Gradient Spin-Echo Method for Diffusion Measurements in the Presence of Internal Gradients.

Over the past decade several pulsed field gradient stimulated-echo methods have been presented for diffusion measurements in heterogeneous media. These methods have reduced or eliminated the coupling between the applied magnetic field gradient and a constant internal magnetic field gradient caused by susceptibility changes throughout the sample. For many research purposes the z-storage delay between the second and third pi/2 RF pulse has been included in order to increase the decay of the echo attenuation to an appropriate level and to increase the signal-to-noise ratio by avoiding T2 relaxation of the magnetization in parts of the pulse sequence. For these reasons a stimulated-echo method has been applied instead of a spin-echo method. When studying systems where it is necessary to keep the duration of the pulse sequence at a minimum, and one is not dependent on using z-storage time to increase the echo attenuation or to study diffusion as a function of observation time, a spin-echo method should be chosen. Here we propose a bipolar pulsed field gradient spin-echo method which is well suited to this purpose, and preliminary diffusion measurements are presented as illustration. Copyright 1999 Academic Press.

Relaxation and diffusion studies of cyclohexane confined in MCM-41 by NMR

Abstract In this work the rotational and translational dynamics of cyclohexane confined in MCM-41 have been studied as a function of temperature by measuring intra-crystalline diffusivities and T 1 and T 2 relaxation times. The results are compared with values obtained for bulk cyclohexane and cyclohexane confined in 40 A silica. T 1 was measured for three different samples, with three different filling grades of cyclohexane. The T 1 curves are continuous over the studied temperature region, and the slope is practically unchanged. However, a marked increase in T 1 with the degree of pore filling is seen owing to an increasing contribution from the bulk-like molecules at the interior of the pores. Only one surface component, attributed to the molecules at the pore walls, was observed when measuring the T 2 relaxation. The relatively narrow spin-echo signal and the long T 2 indicate that the liquid adsorbed on the pore surface does not freeze at all, even at temperatures far below the plastic-brittle solid transition point of the bulk material. The true intra-crystalline diffusivity was obtained by using the short diffusion time model and extrapolating to zero observation time. A high diffusion rate was observable over a wide temperature region, and the measured diffusivity is about three orders of magnitude larger than in the plastic phase of bulk cyclohexane.

Separation profile of model water-in-oil emulsions followed by nuclear magnetic resonance (NMR) measurements: Application range and comparison with a multiple-light scattering based apparatus

The application range and validity of two new NMR sequences (hereafter called sequence 1 and sequence 2) for the study of water-in-oil emulsions (w/o) has been assessed using model emulsions and comparison with results obtained by a commercial apparatus (Turbiscan). These new NMR sequences allow to determine the brine profile i.e. the vertical variations of the dispersed phase content (brine) in the NMR tube. Measuring these parameters as a function of time allows to monitor the separation (sedimentation and coalescence rate) between oil and water. The results obtained on model water-in-oil emulsions with both NMR sequences are consistent and meaningful for both stable and coalescing emulsions and are similar, even if not strictly identical, to the ones obtained with the Turbiscan. It also appears that the second NMR sequence is faster (30s to obtain a profile compared with 3 min for the 1st one in the conditions used in this article) and has a broader application range. Indeed, for these two methods, the oil phase must have a viscosity higher or equal than values which is around 5 mPas for the sequence 2 and 20-25 mPas for the method 1.

Exploring the separate NMR responses from crude oil and water in rock cores

In analysis of transverse relaxation time (T2) curves in a Carr-Purcell-Meiboom-Gill (CPMG) experiment in a multicomponent system originating from measurements of oil and water in rock cores, where internal magnetic field gradients broaden the line widths significantly, there is very little direct information to be extracted of the different components contributing to the totalT2 relaxation time curve. From the study of rock cores saturated with different amounts of crude oil and water, we show that with an optimised experimental setup it is possible to extract information from the nuclear magnetic resonance response that is not resolved by any other methods. This setup combines pulsed field gradient methods with the CPMG experiment utilizing data from both rock cores and bulk oil and water. Then it becomes feasible to separate the signals from oil and water where the two-dimensional inverse Laplace transform ordinarily seems to fail.

Combining PFG and CPMG NMR measurements for separate characterization of oil and water simultaneously present in a heterogeneous system

When analyzingT2 relaxation time curves from an ordinary Carr-Purcell-Meiboom-Gill (CPMG) experiment in a multicomponent system, where internal magnetic field gradients broaden the line widths significantly, there is very little direct information regarding the mobility of the components and on the type of environment experienced by each component. Compared to a standard CPMG experiment, a combination of pulsed field gradient (PFG) methods with the CPMG experiment will increase the amount of information that is obtainable from the nuclear magnetic resonance (NMR) experiment on a system of components differing significantly in molecular mobility. We propose a method for achieving separate measurements of theT2 attenuation of two components simultaneously present within a sample, and we believe it to be generally valid for any system in which the components differ significantly in molecular mobility. The two components could be oil and water in porous rock, or fat and water in a biological tissue, where a separation of theT2 attenuations for the two components will add insight to the study of the systems. In order to verify the method we made use of a sample containing a mixture of oil and water in two separate bulk phases, and compared the results with PFG-CPMG experiments performed on samples containing oil or water only, respectively. The method was applied to systems containing glass spheres immersed in water and oil, and it was possible to obtain information about the physical environment of the components which otherwise is not easily obtainable. The method presented here is therefore presumably applicable to whole rock cores or tissue samples.

Population Balance Model for Batch Gravity Separation of Crude Oil and Water Emulsions. Part II: Comparison to Experimental Crude Oil Separation Data

The mathematical model presented in Part I of this article is compared to experimental data obtained from low-field NMR experiments on a heavy crude oil undergoing gravity separation with two different concentrations of a chemical demulsifier. Experimentally measured parameters are used in the model and include a) the water cut and drop size distribution of the emulsion (obtained directly from the NMR measurements), b) the densities and viscosities of the bulk liquids, and c) the interfacial tension; the results obtained from the model were used to analyze the experimental data in terms of these parameters. The model was formulated based on first-principle physical mechanisms, and thus, not only are the fitting parameters are kept to a minimum, good agreement between experiment and simulation results were obtained. The model reasonably predicts both sets of data for the different demulsifier concentrations with no change to the so-called fitting constants, but by the parameter that represents the magnitude of the interfacial force in the film drainage equations. Such a change in this physical parameter can be reasonably inferred by the action of the increased demulsifier concentration. Both the model and experimental NMR data indicate that the degree of poly-dispersity is a key factor in the rate of coalescence and subsequent rate of separation by sedimentation. This analysis illustrates the link between poly-dispersity and the coupling of coalescence and sedimentation rates; this is crucial for determining how different droplet size fractions will affect the overall efficiency of the separation and how physical properties of the fluids and phase interface can amplify or negate these phenomena. The separation model is a very helpful complement to the NMR technique as the model can output direct comparisons to the NMR data.

Utilization of DSC, NIR, and NMR for wax appearance temperature and chemical additive performance characterization

Wax crystallization processes are investigated using differential scanning calorimetry, near-infrared spectroscopy, and nuclear magnetic resonance spectroscopy. The performance of a chemical additive is assessed using calorimetry and NMR. Heat flows of model waxy oils are obtained using differential scanning calorimetry, providing the wax appearance temperature and crystallization profiles. The effect of cooling rate, wax content, asphaltene, and chemical additive on the wax appearance temperature is investigated. The wax appearance temperature increases with increasing wax contents. The wax appearance temperature decreases in the presence of chemical additive, effectively increasing the instantaneous supersaturation. Furthermore, near-infrared spectroscopy and nuclear magnetic resonance spectroscopy are utilized to determine wax appearance temperature. The NMR experiments quantify liquid and solid fractions at thermal equilibrium conditions, effectively circumventing the need for dynamic thermal techniques.

Influence of HPAM on W/O emulsion separation properties

ABSTRACT The present study reports on the influence of partially hydrolyzed polyacrylamide (HPAM) on essential w/o emulsion properties. The characterization has been undertaken with low field NMR to follow droplet sizes and distributions, sedimentation and coalescence kinetic, bench-scale electrocoalescence (Ecrit) experiments to follow emulsion stability changes, and electrorheology to detect changes in the viscosity upon applying an external electric field. The result is that HPAM does not basically influence the droplet size distribution (DSD) and the stability level of the emulsions as can be expected of bulk polymers. However, there seems to be an interaction between added demulsifiers either through direct molecular interaction or via an interfacial complexation. GRAPHICAL ABSTRACT

The application of pulse field gradient (PFG) NMR methods to characterize the efficiency of separation of water-in-crude oil emulsions

Demulsification of water-in-crude oil emulsions is an essential and sometimes challenging procedure for crude oil processing facilities. Pulse field gradient (PFG) NMR techniques are known to monitor the dynamics of emulsion separation. This method has limitations that restrict its application to some crude oils. A comprehensive methodology applicable to all types of crude oil regardless of its viscosity, without assumptions, and providing a large number of data with fast measurements, is proposed in this paper. The coalescence and sedimentation of unstable emulsions was observed through simultaneous measurements of the evolution of the brine profile and droplet size distribution (DSD). Measurements of emulsions after stabilization, with and without the contribution of the free water layer, revealed the residual emulsified water quantity and location in the sample. A new, faster approach to separate the oil and water overlapping T2 relaxation signals was demonstrated on real water-in-crude oil emulsions, using the root mean square displacement (RMSD) measured with the spoiler recovery and a loop of 13-interval pulsed field gradient stimulated echo (PFGSTE) oneshot sequences. The residual water within the crude oils after separation was determined and used to quantify the efficiency of the demulsifier used.