Energy Fuels 2010, 24, 1829–1836 Published on Web 03/04/2010
: DOI:10.1021/ef901310v
Which One Is More Important in Chemical Flooding for Enhanced Court Heavy Oil Recovery, Lowering Interfacial Tension or Reducing Water Mobility? Haiyan Zhang,†,‡ Mingzhe Dong,*,‡ and Suoqi Zhao† †
State Key Laboratory of Heavy Oil Process, China University of Petroleum, Beijing 102249, China, and ‡Department of Chemical and Petroleum Engineering, University of Calgary, Calgary, Alberta T2N 1N4, Canada Received November 7, 2009. Revised Manuscript Received February 5, 2010
A total of 33 sandpack flood tests were carried out to investigate the effects of interfacial tension (IFT) and water-phase viscosity on enhanced heavy oil recovery by chemical flooding. The amount of oil recovered by alkaline-only flooding increased sharply with the NaOH concentration in the range of 0.3-0.5 wt %. The oil recovery only varied slightly with the changing alkaline concentration outside the range. The coexistence of the surfactant and NaOH reduced the IFT between the oil and aqueous phase to an ultralow level. However, the amount of oil recovered by alkaline/surfactant flooding only increased slightly with an increasing NaOH concentration up to a threshold value of 0.5 wt %. Beyond this threshold value, the recovery efficiency stopped increasing with the alkaline concentration and its value was lower than that of the alkaline-only displacing process. The addition of a polymer improved the tertiary oil recovery by increasing the viscosity of the water phase, although it also increased the IFT slightly. The combination of alkaline with polymer was more effective than polymer only upon enhancing the tertiary oil recovery. Comparing the results of tertiary oil recovery shows that the tertiary oil recovery of Court oil is correlated better with water-phase viscosity than IFT; i.e., increasing the viscosity of the water phase is more effective than lowering IFT in improving the tertiary oil recovery.
species formed by the alkalis and acids in the oil reaction were more effective in lowering the IFT, while the presence of the prepared surfactant was actually detrimental to the IFT reduction achieved by the in situ surfactant. Several researchers1,4-6 have studied the effect of the surfactant-combined alkaline enhanced oil recovery (EOR) on model oil or light oil, with viscosity ranging from 2 to 76.8 mPa s. The results showed that a small amount of the surfactant ( A (5%). This shows that the EOR is correlated better with the pressure drop during the chemical flood than with IFT. The
higher the pressure drop, the higher the tertiary oil recovery. On the other hand, the results also demonstrate that controlling mobility is more effective than lowering IFT upon enhancing the recovery efficiency of Court heavy oil. The effect of different chemicals on the pressure drop and oil recovery can be explained as follows: The reaction between NaOH and oil not only reduces IFT but also generates (unstable) emulsions that assist in blocking the water channel. As a result, more displacing fluid is diverted to the unswept zone to displace more oil. At lower alkaline concentrations, the alkalinity is incapable of clogging the shortcut of the water phase, and the oil recovery enhancement only occurs at higher alkaline concentrations. Adsorption of the surfactant onto the oil drop surface is detrimental to the blocking of the water channel because ultra-low IFT makes the oil drops more flexible and easy to flow in porous medium. Adsorption of the surfactant on the oil-water interface is also disadvantageous to the reaction between alkaline and oil. That is why the pressure drop decreases and recovery efficiency reduces after the surfactant is added to the alkaline solution. The polymer has two effects: the addition of polymer increases the viscosity of the water phase and reduces the mobility of the water phase; therefore, more oil is displaced at lower alkaline concentrations, while the alkaline is ineffective in increasing the oil recovery efficiency. On the other hand, the addition of polymer is detrimental to the diffusion of NaOH. The reaction between NaOH and oil is inhibited under the same alkaline concentration; therefore, the recovery efficiency of AP at higher alkaline concentrations is lower than that of alkaline-only flooding with the same alkaline concentration. With the increase in the polymer concentration, this inhibition is stronger and the difference of the recovery efficiency between different alkaline concentrations (0.1 and 0.5 wt %) is reduced with an increasing polymer concentration. The results of this study show that the enhancement of oil recovery is well-correlated with the pressure drop of the flooding process rather than IFT. This denotes that the EOR is mainly due to the plugging of the water channel and not the lowering of IFT. For screening chemical formulas for enhanced heavy oil recovery, more effort should be made to improve sweep efficiency by mobility control or plugging water channels. In this mechanism of EOR, ultralow oil/water IFT is not necessary. Conclusions From the results of IFT between different chemical solutions with Court oil, the following conclusions are drawn: (1) IFT of Court oil/NaOH solution can be reduced to 0.2 dyne/cm 1835
Energy Fuels 2010, 24, 1829–1836
: DOI:10.1021/ef901310v
Zhang et al.
in the NaOH concentration range of 0.1-0.8 wt %. (2) The synergy between alkaline and selected surfactant can reduce the IFT to around 0.001 dyne/cm, despite the presence of the polymer. (3) Addition of the polymer can increase the IFT at lower alkaline concentrations because of the effect of increased viscosity on the diffusion of alkaline. The effect becomes insignificant with an increasing alkaline concentration. Different chemical flooding systems are applied on Court oil: alkaline-only, polymer-only, AS, AP, and ASP systems. The results indicate the following: (1) NaOH can enhance the tertiary oil recovery prominently when the alkaline concentration is in the range of 0.5-0.8 wt %. (2) The ability of the polymer-only system upon increasing the oil recovery is limited. There is a concentration limit where the oil recovery does not increase with a further increase in the polymer concentration. (3) The recovery efficiency of AP flooding
increases with the increase of the polymer concentration, which is higher than that of the polymer-only system. (4) When the polymer is combined with the A or AS system at different alkaline concentrations, the oil recovery only increases noticeably at lower alkaline concentrations. (5) Adding surfactant to the A or AP system at different alkaline concentrations does not increase the oil recovery effectively. (6) The enhancement of oil recovery is well-correlated with the pressure drop of the flooding process rather than IFT. This denotes that the EOR is mainly due to the plugging of the water channel and not the lowering of IFT. Acknowledgment. Acknowledgement is gratefully extended to the Petroleum Technology Research Centre, Regina, Saskatchewan, Canada, for the financial support of the project and the China Scholarship Council for the financial support for Haiyan Zhang.
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