The synergetic effects of minerals and steam on the hydrocarbon group, viscosity, and element distribution of the heavy oils were studied in this paper. The results have shown that the mineral has a catalytic effect in the aquathermolysis of the heavy oils. When 10 wt % of mineral was added to the reaction system, the saturate and aromatic increase, the resin and asphaltene decrease. The VPO results show that the average molecular weight of heavy oil and asphaltene reduced after treatment, and the content of sulfur decreased. The viscosity of heavy oils used in our study was decreased to 23.4-25.6%, and decreased to 84.2-86.3% when the mineral and catalyst coexist in the reaction system under the steam.
The
emulsification of heavy oil and water into a W/O emulsion would
increase the viscosity of the W/O emulsion and reduce the mobility
of heavy oil. As we know, addition of a viscosity reducer such as
water-soluble surfactants could change the status of emulsion and
improve the mobility of heavy oil. Although up to 90% viscosity reduction
could be achieved in laboratory experiments, field improvement of
the heavy oil recovery is not satisfactory with addition of a viscosity
reducer. Actually, it is related to the difference of the mixing process
of water-soluble viscosity reducer, water, and heavy oil between lab
experiments and subsurface flow in the formation. The viscosity of
heavy oil is significantly affected by the temperature and emulsification.
As for the heavy oil sampled from Bohai Oilfield, its viscosity decreases
by 95% when the temperature increases from 50 to 100 °C. At 50
°C, the viscosity of the W/O emulsion with 70% water content
is up to 21.05 times that of the dehydrated heavy oil. In the emulsification
process of the W/O emulsion, water molecules disperse into heavy oil
and agglomerate together to form water droplets, and the number of
water droplets increases when the water content is increased. When
we add some amount of a water-soluble viscosity reducer to the W/O
emulsion, the re-emulsification process or the way of addition of
the water-soluble viscosity reducer has a great effect on the variation
of the state and viscosity. When heavy oil is mixed with 30% content
of water-soluble viscosity reducer solution (the reducer content is
1% of the emulsion), up to 98% reduction in the viscosity of heavy
oil could be obtained. However, when the W/O emulsion with 30% water
content is mixed with 1% water-soluble viscosity reducer, only 4%
reduction in the viscosity of emulsion is observed. In the re-emulsification
process of water in a W/O emulsion with addition of a reducer, pieces
of heavy oil containing a number of water droplets are separated by
the reducer solution and a W/O/W emulsion is generated. The pieces
of heavy oil containing water droplets in such a W/O/W emulsion are
much larger than the heavy oil droplets in an O/W emulsion. As the
heavy oil in the reservoir is in the state of a W/O emulsion, a W/O/W
emulsion with large heavy oil pieces containing water droplets will
be generated when a water-soluble viscosity reducer solution is injected,
and the resulted viscosity reduction is much lower than expected based
on conventional lab experiments. Consequently, the experiments related
to heavy oil with addition of a water-soluble viscosity reducer should
be carried out with the W/O emulsion.
In situ emulsion formation is an effective nonthermal method to improve conventional heavy-oil recovery. In this paper, a newly designed viscosity reducer (TPVR7) and a high-performance surfactant (dioctyl sodium sulfosuccinate, DSS) for enhanced conventional heavy-oil recovery have been introduced and evaluated. A dewatering rate test, emulsion droplet size measurement, multiple-light scattering, and interfacial tension measurement are carried out to evaluate the synergistic effect of the viscosity reducer and the surfactant on the emulsion stability and efficiency in enhancing heavy-oil recovery. The results show that adding a surfactant can significantly increase the emulsion stability by decreasing the size of the emulsion droplet and forming a tighter interfacial film. The optimum viscosity reducer−surfactant system is formulated as TPVR7-0.5% + DSS-0.5%. With the optimum system, the viscosity of heavy oil decreased from 350 to 9 mPa•s. The equilibrium interfacial tension of the oil/TPVR7-0.5% + DSS-0.5% solution is ∼0.092 mN/m, much lower than that of the oil/TPVR7 solution. The mechanisms of synergistic collaboration between the viscosity reducer and the surfactant include enhanced emulsion stability, viscosity reduction, and interfacial tension reduction. The sand-pack flooding experiments show that a viscosity reducer and surfactant could improve heavy-oil recovery by 21.89% after water flooding at the optimum concentration, which indicates that viscosity reducer and surfactant flooding has great potential to enhance heavy-oil recovery.
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