Surfactants based on monounsaturated fatty acids for enhanced oil recovery
By Paul Berger
Editor's note: The following article is the first in a two-part series. Look for the second part-"New very long-chain fatty acid seed oils produced through introduction of strategic genes into Brassica carinata" by David C. Taylor-in the October issue of inform.
The increasing demand for petroleum-based fuels and petrochemicals, along with the diminishing supply of new sources for oil, has escalated interest in improving the extraction of oil from existing reservoirs. Among the most promising processes is enhanced oil recovery (EOR). In one version of the EOR process, various chemicals are injected into the reservoir to overcome capillary pressures and wettability and to remove residual oil that would otherwise remain within the microscopic pores. Chemicals that are used include polymers, surfactants, and alkali. In the alkaline surfactant polymer (ASP) process, an injection solution containing an alkali to reduce adsorption and to form in situ surfactant is used, along with one or more surfactants to lower the interfacial tension (IFT) between the injection fluid and the trapped oil, and a polymer to control the mobility ratio and sweep efficiency of the injected fluid.
In most cases, after primary oil recovery methods utilizing the internal pressure from within the reservoir to extract oil have been exhausted, a secondary process employing water or a gas is used to recover additional oil. Primary recovery generally recovers 15-20% of the original oil in place. Secondary recovery may recover 20-30% of the remaining oil after primary recovery. A point is reached where secondary recovery becomes inefficient or ineffective. At this point, tertiary recovery methods such as EOR must be employed.
Figure 1 shows the correlation between the capillary number (Nc) and the residual oil recovery obtained after water flooding. Nc is the ratio of the viscous forces to the interfacial forces and is defined in Equation 1,
Nc = (viscosity)(velocity)/(IFT)(cos contact angle) (1)
where viscosity refers to the viscosity of the injected fluid containing the surfactant, velocity is the rate at which the injected fluid moves into the reservoir, and contact angle is the angle between the injected fluid and the reservoir rock. The IFT between the injection water and the oil is usually between 10 and 35 mN/m before the addition of surfactants. Surfactants have been developed to give very low IFT values of less than 10-2 mN/m, and therefore they can increase the capillary number three to four orders of magnitude, allowing the capillary forces trapping the oil to be overcome.
Recent estimates concerning the amount of surfactant necessary to meet projected future EOR demand indicate that this volume would rival the current use of surfactants in detergents.
Surfactants can be manufactured from various raw materials that are ultimately derived from petroleum or agricultural feedstocks. Currently, most EOR surfactants are based on petrochemical feedstocks. In the future a vicious cycle would be created, as the demand for more petroleum-based surfactants would increase proportionally to the demand for more oil for fuel and petrochemicals. To address this challenge, research is currently under way to develop surfactants, suitable for EOR, that are partially or completely derived from renewable agricultural crops. These offer the advantage of providing green, biodegradable, nontoxic, environmentally friendly alternatives that are not dependent on the availability of petrochemicals.
Surfactants derived from unsaturated fatty acids are among the most effective for EOR applications. The fatty acids are derived from the glycerides present in many common seeds, nuts, and grasses. They can be isolated and further converted to many useful products including alcohols, alcohol ethers, alcohol ether carboxylates, alcohol ether sulfates, alcohol ether sulfonates, alkyl dimethyl betaines, alkyl dimethyl hydroxysultaines, alkyl dimethyl amine oxides, alkylamidopropyl dimethyl betaines, alkyl amidopropyl dimethyl hydroxysultaines, and so on.
The anionic derivatives of these unsaturated fatty acids and alcohols are especially effective in lowering IFT and also have been designed to overcome other obstacles such as high temperatures, high salinities, high divalent cation concentrations, and high adsorption onto the reservoir rock.
The limiting factor in the use of various unsaturated fatty acids has been their limited availability and their low concentrations in many agricultural sources. Table 1 shows some of the unsaturated fatty acids and their current sources. Researchers at the National Research Institute of Canada in Saskatoon, Saskatchewan, under the direction of David Taylor, have developed several strains giving extremely high yields of erucic (22:1) and nervonic (24:1) acids. They are looking to add new unsaturated acids containing 12 to 20 carbons, obtained either by genetic modification or yield enhancement techniques, to those already commercially available. Taylor's work will be described in detail in the October 2010 issue of inform.
Monounsaturated fatty acids have two functional groups that can be further derivatized to give useful surfactants. The double bond can be sulfonated to give strong anionic properties such as thermal stability. The carboxylate can be converted to an alcohol and subsequently alkoxylated and carboxylated to give electrolyte and divalent cation tolerance. The alcohol, glyceride, or fatty acid can be used to produce tertiary amines that can subsequently be converted to betaines, amine oxides, sultaines, and the like.
Figure 2 summarizes some of the many reactions that have been successfully carried out using unsaturated fatty acid derivatives.
Hybrid surfactants containing both petrochemical and agrichemical feedstocks have also been developed. Some of these make use of the reaction of a terminally sulfonated alkylaromatic. It has been found that this type of molecule readily reacts with unsaturated aliphatics such as unsaturated acids, unsaturated alcohols, and olefins to form surfactants that can be designed to have a wide variety of properties.
Unsaturated fatty acids provide a useful starting point for the synthesis of many surfactants. Because these surfactants are based on renewable resources, they can help meet the challenge of the future demand for EOR surfactants in large quantities. In certain cases, they offer alternatives that cannot be found using petrochemicals such as unsaturated alcohols and internal olefin ethers that can be converted to many useful surfactants for EOR and other applications. Current research shows promise in developing crops yielding sufficient quantities of various chain lengths of these acids to meet the anticipated future demand.
Paul Berger is the vice president and technical director for Oil Chem Technologies (Sugar Land, Texas, USA). Berger is a graduate of City College of New York and New York University, both located in New York City, and has over 30 years' experience in surfactants research and applications. Prior to joining Oil Chem Technologies, he was the research director for Witco Chemicals worldwide. He holds many patents, has authored several books, and has published numerous papers related to the use of surfactants for oil recovery, agriculture, and industrial applications. He was the recipient of Witco's first Scientist of the Year Award in 1990, and he received the 2001 Samuel Rosen Memorial Award from the American Oil Chemists' Society for pioneering work in discovering a new alkylation, sulfonation reaction. He is a longtime member of SPE (Society of Petroleum Engineers), American Chemical Society, AOCS, and is a Fellow in the American Institute of Chemists.
For more on enhanced oil recovery, see Catherine Watkins' article "Chemically enhanced oil recovery stages a comeback" (inform 20:682-685, 2009).