Monitoring of Diffusion of Heavy Oils With Hydrocarbon Solvents in the Presence of Sand

Abstract

Mass transfer of solvents in bulk heavy oil and bitumen was monitored successfully using X-ray Computer Assisted Tomography (CAT). Concentration gradients were obtained from analysis of bulk density maps as a function of location and time. Analysis of the data using Fick's Law provided apparent diffusion coefficients, which were consistent with previously published data using different solvents and different oils. A second series of experiments was then performed whereby the heavy oil was mixed with sand at a level that is similar to what expected in heavy oil reservoirs. There was no connate water in the system. Different solvents were then placed on top of the oil/sand mixtures and the diffusion of the solvent into the sand pack was observed.X-ray CAT was used to monitor density and concentration profiles. Oil concentration profiles in solvent were clearly observed but the solvent concentration in the sand pack proved to be more challenging. The same Fickian approach that was used in bulk oil systems was applied to predict apparent diffusion coefficients for the liquid volume portion of the experiment. Preliminary results showed that the apparent diffusion coefficients are of the same order of magnitude as in the bulk oil systems. Introduction The increased interest in secondary production (or post-cold production) of heavy oil has renewed interest in solvent-based methods for heavy oil recovery such as Vapor Extraction (VAPEX). Butler and Mokrys (1), as an alternative to Steam Assisted Gravity Drainage (SAGD), proposed the VAPEX process for reservoirs where heat loss in the formation would make SAGD uneconomical. In VAPEX, vapor solvent instead of steam is injected, and the solvent mixes with the heavy oil to reduce its viscosity. The diluted oil then drains by gravity to the production well. Several experiments have been published using Hele-Shaw cells (2, 3), pore network glass micromodels (4), magnetic resonance imaging (MRI) (5) and pressure-volume-temperature (PVT) experiments (6). The mechanisms for solvent-based processes involve diffusion and dispersion of the solvent into the oil sands. Dispersion coefficients are related to the diffusion coefficients, making it important to evaluate the diffusion coefficients in both cases. X-ray CAT has potential as an attractive tool for measuring the properties of reservoir fluids in a non-intrusive way. CAT can non-destructively image a host of physical and chemical properties of porous rocks and multi phase fluids contained within their pores. The images are taken within seconds to minutes, at reservoir temperature and pressure (7). Employing x-ray CAT techniques to generate solvent and bitumen concentration profiles, allows for the calculation and comparison of the diffusion coefficients of hydrocarbon solvents in bulk heavy oil and oil sands to be calculated and compared. In solvent-based EOR techniques, mass transfer, viscous forces and gravity drainage affect the rate of oil recovery, given that accessibility is provided. The solvent diffuses and/or disperses into the heavy oil reducing its viscosity, therefore the solvent-oil mixture then drains and is recovered from the production well. Background If two miscible fluids are in contact, with an initially sharp interface, they will slowly diffuse into one another. As time passes, the sharp interface between the two fluids will become a diffused mixed zone grading from one pure fluid to the other. This diffusion arises because of the random motion of the molecules (8). Therefore diffusion flux can be defined as a flow due to the concentration difference between two fluids. Even if the two fluids are stable, there will be mixing due to the difference in concentration.

Diffusion Properties for CO2–Brine System under Sequestration-Related Pressures with Consideration of the Swelling Effect and Interfacial Area

“…Tang and Kovscek presented an experimental investigation of forced imbibition in a linear homogeneous sandstone at various injection rates. They constructed 3D images of the stable displacement and the growth of unstable water fingers at the water/oil interface by using X-ray CT. Salama and Kantzas applied calculation and comparison techniques to the diffusion coefficients of hydrocarbon solvents in bulk heavy oil. We and Kantzas performed detection and monitoring of solvent interactions with heavy oil and found that the diffusion coefficient was a strong function of the concentration over wider distance and concentration ranges, while in a narrow range of distance and concentration, the diffusion coefficient could be considered constant.…”

Evaluating Diffusivity of Toluene in Heavy Oil Using Nuclear Magnetic Resonance Imaging

“…X-ray computer assisted tomography (CAT) has been used by several researchers to obtain concentration profiles. ,,− CAT distinguishes multicomponent systems based on atomic density differences. While this enables detection of both the rock matrix and contained fluids, the low density difference between the solvent, bitumen, and their mixtures can be a restricting factor in application of X-ray CAT imaging for tracking the diffusion process, in other words, density contrast is low and comparable to the noise level.…”

“…While this enables detection of both the rock matrix and contained fluids, the low density difference between the solvent, bitumen, and their mixtures can be a restricting factor in application of X-ray CAT imaging for tracking the diffusion process, in other words, density contrast is low and comparable to the noise level. Salama and Kantzas applied X-ray computer assisted tomography to monitor mass transfer of solvents in bulk heavy oil and bitumen. They also did a series of experiments to observe the diffusion of hydrocarbon solvents with heavy oils in the presence of sand by placing different solvents on top of oil/sand mixtures.…”

Concentration Dependence of Mutual Diffusivity of Liquid Hydrocarbons and Bitumen

“…Diffusivities are determined from an analytical or numerical model of the mass transfer data. Diffusion of liquid hydrocarbons into bitumen has often been modeled assuming a constant diffusivity. ,,− However, the constant diffusivity assumption is only valid for liquid–liquid diffusion when the solute concentrations are low, that is, approaching the infinite dilution condition. This assumption is usually justified based on the narrow distance, small concentration range, and short diffusion times used in the experiments.…”

“…The most commonly used method for interpreting the concentration profiles in mixtures of bitumen or heavy oil and a solvent is the Boltzmann-transformation approach, an analytical solution for linear mass transfer. ,− , These studies all reported a maximum in the diffusivity at a solvent concentration of approximately 50 vol %. Zhang and Shaw noted that the Boltzmann-transformation approach considers spatial density gradients to be negligible or zero.…”