A modified pressure decay method has been designed and tested for more reliable measurements of molecular diffusion coefficients of gases into liquids. Unlike the conventional pressure decay method, the experimental setup has been designed such that the interface pressure and consequently the dissolved gas concentration at the interface are kept constant. This is accomplished by continuously injecting the required amount of gas into the gas cap from a secondary supply cell to maintain the pressure constant at the gas−liquid interface. The pressure decay is measured in the supply cell. The advantage of the new technique is that, assuming the diffusion coefficient to be constant, a simple analysis allows determination of the equilibrium concentration and diffusion coefficient.
Summary Steam foams have been considered effective additives for unconventional oil-recovery processes. Conventionally, for steam-foam applications, chemical additives are injected with steam. However, this procedure can have serious challenges because of poor thermal stability of additives and high volume of additives loss caused by adsorption to the rock surface. To overcome these limitations, nanoparticles can be used as novel additives to improve generation and stabilization of the foams for steam-foam applications. In this study, silica nanoparticles in synergy with surfactants have been used as steam additives. Dynamic light scattering (DLS), a foam-height test using N2 at reservoir conditions, and thermal-stability analysis were designed to measure nanoparticle size distribution in brine, foamability, and thermal stability of the additive solutions, respectively. Subsequently, coreflooding tests were performed to evaluate the synergistic effect of nanoparticles and surfactants on the foam performance and oil recovery. We observed an optimal ratio of nanoparticle and surfactant that yields the best foam-generation performance in bulk medium. Herein, surface-treated silica nanoparticles have been tested with two of our candidate surfactants. The nanoparticles alone generate a small amount of foam, whereas each surfactant generates a small-to-moderate amount of foam. Synergy is demonstrated by the system that contains 0.1-wt% nanoparticles (the optimal concentration) and 0.5-wt% surfactant solution at neutral pH (≈7), as it leads to approximately 67 and 50% greater foam height, respectively, for Surfactants A and B than foam height observed in tests with surfactants only, in bulk medium. Corefloods with coinjected steam and water containing nanoparticles and surfactant confirm the synergy, exhibiting measurable improvement in mobility-reduction factor (MRF) and steam control, compared with coinjection of steam and water containing only surfactant.
Molecular diffusion coefficient is an important parameter in modeling mass-transfer based reservoir processes. However, experimentally measured diffusivities for heavy oil systems are relatively scarce and no standardized method exists for measurements of this important parameter. The available measurement techniques are tedious, expensive and often not very reliable. There is an obvious need for developing improved methods for measuring diffusivity of gases in heavy oils. It is well known that as a gas dissolves into heavy oil, the oil viscosity drops considerably and this affects the diffusivity. The objective of this work is to measure the diffusivity of highly soluble gaseous solvents in heavy oils at different concentration levels. We have developed a modified pressure decay method that maintains constant concentration at the gas-liquid interface and measures the amount of gas transferred to the liquid as a function of time. An analytical solution has been developed for finding diffusion coefficient and the equilibrium solubility of gas in the oil at the test pressure. To study the concentration dependence of diffusivity, a stepwise increase in pressure is used starting from a low pressure. Through this stepwise procedure, a diffusion coefficient is measured for each gas saturation pressure (concentration), going from low pressure to near gas dew point pressure in 5 to 6 steps. The bitumen height in our cell is updated at each pressure to account for bitumen swelling. Propane was used as vapor solvents and diffusion cell was kept at constant temperature. Introduction Efficient recovery of heavy oil and bitumen is still very challenging and remains an issue in ongoing researches all around the world. Thermal recovery methods, which rely on heat for viscosity reduction, are generally accepted as viable and several steam based projects have been successful, especially in Canada. Using light hydrocarbon solvents can provide similar viscosity reduction and is potentially more efficient in so-called challenging reservoirs where thermal methods do not work. In comparison with thermal methods, solvent based processes are more environmentally friendly and require no fresh water resources. Moreover, using CO2 as a major component in the injected solvent helps in reduction of Green House Gases by sequestering CO2 emissions. Keeping that in mind, reliable measurements of molecular diffusion coefficient are required for designing solvent-based recovery processes in heavy oil reservoirs. Accurately knowing the diffusion coefficient and its variation with process conditions would also improve the predictions of compositional reservoir simulators. Unlike heat conductivity or viscosity in the analogous transport phenomena, measurements of mass diffusivity of gases in liquids are more difficult and no standardized procedures for such measurements exist. Several experimental and mathematical methods have been introduced to characterize this key parameter. The ratio between the local flux and the local gradient in the concentration is defined as diffusion coefficient 1 which controls the rate of dissolution in a diffusion process. There are two other parameters, which are equally important in controlling the overall rate of diffusion process. First is the solubility of the gas in the liquid and the second is the mass transfer coefficient on the gas side of the gas-liquid interface. Solubility value or saturation concentration is the maximum or eventual amount of gas that can be dissolved into the liquid phase. The mass transfer coefficient is defined as a proportionality constant that relates the rate of mass transfer to difference between the bulk gas phase concentration and the concentration at the interface 2. When the gas phase is a pure component, this constant reflects the existence of a resistance to mass-transfer at the interface which makes the interface concentration smaller than the equilibrium concentration. In absence of such resistance, the concentration at the interface exposed to a pure component gas would be equilibrium concentration determined by the solubility of the gas at the test conditions.
Steam foams have been considered as effective additives for unconventional oil recovery processes. Conventionally for steam-foam applications, chemical additives are injected with steam. However, this operation has serious drawbacks owing to poor thermal stability of additives and high volume of additives loss, due to adsorption to rocks surface. To overcome these limitations, nanoparticles can be used as novel additives to improve generation and stabilization of the foams for steam-foam applications. In this study, silica nanoparticles in synergy with surfactants have been used as steam additives. Dynamic light scattering (DLS), foam height test using N2 at reservoir conditions, and thermal stability analysis were designed to measure nanoparticles size distribution in brine, foam-ability and thermal stability of the additive solutions, respectively. Subsequently, core-flooding tests were performed to evaluate the synergistic effect of nanoparticles and surfactants on the foam quality and oil recovery. We observe an optimum ratio of nanoparticle and surfactant that yields the best foam generation performance (maximum foam height). Herein, surface treated silica nanoparticles have been tested with two of our candidate surfactants. The nanoparticles alone generate small amount of foam, while each surfactant generates a small to moderate amount of foam. Synergy is demonstrated by the system which contains 0.1 wt% nanoparticles (the optmum concentration) and 0.5 wt% of surfactant solution at neutral pH (~7), as it leads to about 67% and 50% greater foam height respectively for Surfactants A and B than observed in tests with surfactants only. Core-floods with co-injected steam and water containing nanoparticles and surfactant confirm the synergy, exhibiting significant improvement in mobility factor reduction (MRF) and steam control, compared to co-injection of steam and water containing only surfactant.
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