Monitoring Bitumen−Solvent Interactions with Low-Field Nuclear Magnetic Resonance and X-ray Computer-Assisted Tomography

Abstract

This work involves the detection and monitoring of solvent interactions with heavy oil and bitumen. Two nondestructive methodslow-field nuclear magnetic resonance (NMR) and X-ray computer-assisted tomography (CAT)were used. It is shown that low-field NMR can be a very useful tool in understanding the relationship of viscosity, density, and asphaltene precipitation in bitumen−solvent mixtures. Such mixtures are present in solvent-related heavy oil and bitumen recovery processes, such as vapor extraction (VAPEX). As a solvent comes into contact with a heavy oil or bitumen sample, the mobility of hydrogen-bearing molecules of both solvent and oil changes. These changes are detectable through changes in the NMR relaxation characteristics of both the solvent and the oil and can be correlated to mass flux and concentration changes. Based on Fick's second law, diffusion coefficients were calculated for combinations of three oils and six solvents. X-ray CAT scanning was also used in parallel for analysis of solvent diffusion into the bitumen. As the solvent was diffusing into the bitumen, a concentration gradient was obtained. Concentration values at certain times were used to calculate diffusion coefficients, which were compared with results obtained from NMR data, using both an analytical method and a numerical method. The diffusion coefficients were considered either as constants or as functions of solvent concentration in two models that have been developed during this research. The overall diffusion coefficients calculated for several pairs of oils and solvents at different ratios, both by NMR data and X-ray tomography, were on the order of 10-6 cm2/s.

Investigation of concentration‐dependent diffusion on frontal instabilities and mass transfer in homogeneous porous media

“…[12] However, recent experimental studies have showed that the diffusion coefficient is actually a variable that is dependent on the concentration of fluids. [16][17][18] Neglecting the dependence of it on concentration may result in unrealistic results. [30] In the present study, the CDDC derived from the generalized Stokes-Einstein relation [20,21] is used in the form of Equation (8).…”

“…Yet, recently, both experimental measurements and mathematical modelling have indicated that the use of the CDDC in miscible processes would be more realistic. [17,30] In this section, the difference between the CDC and the CDDC in miscible displacements will be examined qualitatively by comparing the concentration fields. It is worth noting that the time sequences are not necessarily the same for different cases and only those that can better show the different frontal instabilities and mass distributions will be displayed.…”

NMR relaxometry and diffusometry in characterizing structural, interfacial and colloidal properties of heavy oils and oil sands

“…The use of NMR to study solvent-heavy oil interactions was also extended to the study of the effects of different solvent concentrations on heavy oils and bitumens [88][89][90][91]. T 2 relaxation time distributions for different oil-solvent mixtures were measured.…”

“…This work was continued, and NMR was used to monitor the oil-solvent mixture properties as a function of time, and diffusivity coefficients predicted [89,90]. The diffusivity is important as it is a major factor in determining how fast a solvent dilutes into the oil.…”

“…Changes in the T 2 distribution were then monitored over four weeks as the solvent diffused into the oil [89]. These measurements clearly show the two distinct solvent and oil peaks in the T 2 distribution, which become one continuous distribution.…”