Abstract
Chemical enhanced oil recovery (EOR) is surely a topic of interest, as conventional oil resources become more scarce and the necessity of exploiting heavy and unconventional oils increases. EOR methods based on polymer flooding, surfactant-polymer flooding and alkali-surfactant-polymer flooding are well established, but new challenges always emerge, which give impulse to the search for new solutions. Polymeric surfactants represent a very attractive alternative to these techniques, because they can provide simultaneously increase in water viscosity and decrease in interfacial tension, both beneficial for the efficiency of the process. The analysis of the literature shows that the use of polymeric surfactants as displacing fluid has the potential to improve the performances of EOR in some cases. However, the synthesis are often challenging and costly and the available data about the real performances of such systems in oil recovery are still sparse. This holds back the possibility of a significant use of polymeric surfactants for EOR. This review collects the relevant work done in the last decades in developing and testing polymeric surfactants for EOR, with a particular emphasis on the chemical aspects, the patent literature and bio-based systems.
Highlights
Traditional HPAM and hydrophobically modified ones (HMPAM) have been compared in core flood experiments (Lai et al, 2013; Maia et al, 2009). In those experiments, the viscosity of the hydrophobically modified polyacrylamide (HMPAM) solutions are higher than the HPAM ones, which makes impossible to determine if the lowered interfacial tension due to the presence of hydrophobic groups plays a role in the improved recovery
Patents covering a very broad range of amphiphilic copolymers for enhanced oil recovery (EOR) can be found: a recent patent (Zhang and Tang, 2010) includes a broad range of partially hydrolyzed acrylamide based polymers, containing charged or neutral hydrophobic monomers, having interfacial tension between the two phases (IFT) values included in the range 0.1–15 mN/m
From a theoretical point of view, the use of polymeric surfactants for chemical enhanced oil recovery could represent a valid alternative to the commonly used systems, since they potentially combine desired rheological and interfacial properties in only one component, while in general mixtures of different chemicals are required. This would present the advantage to avoid separation and loss of some components during the flooding processes or unwanted interactions (Sheng, 2010). This potential is confirmed by the recent appearance in the patent literature of examples of polymeric surfactants for EOR
Summary
Polymeric surfactants are macromolecules which contain both hydrophilic and hydrophobic parts in their structure. A very important feature of some polymeric surfactants is the possibility to tune their hydrophobicity, and their aggregation behavior, by changing external parameters, such as pH, temperature or electrolytes concentration These kinds of polymers are important building blocks for smart materials. The importance of solution rheology for EOR is mostly connected to the need of adjusting the mobility ratio by increasing the water phase viscosity, as it has been already emphasized in the introduction This applies in a general way for polymer flooding. For EOR it is convenient to base the calculation on the force balance on a drop of oil squeezed through a pore throat Reed and Healy (1977) In this case we have: Ca = ∇P⋅k γ where ∇P is the gradient pressure, γ is the interfacial tension between the two immiscible phases (often indicated with IFT) and k is a constant. Other hardly quantifiable characteristics that a suitable polymeric surfactant should possess, are the same as for traditional polymers for EOR: low adsorption on the rocks, high thermal stability and salt tolerance
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