Engineering Rhamnolipid Biosurfactants as Agents for Microbial Enhanced Oil Recovery

Abstract

Abstract This investigation considered engineered rhamnolipid biosurfactants as EOR agents that potentially could be manufactured at low cost from renewable resources, and have lower toxicity than synthetic EOR surfactants. This particular biosurfactant comes mainly from the microbe Pseudomonas aeruginosa. Disadvantages of working with this strain include that the chemical structures of the produced rhamnolipids are not easily controlled, plus there is a preference to use instead a completely non-pathogenic microbe. Towards that end, the study took the approach to clone the genetic information from a P. aeruginosa strain into E. coli to manipulate systematically the structure of the created rhamnolipids and evaluate their EOR performance by themselves (no co-surfactant or viscosity chemical added). Six E.coli strains (ETRA, ETRAB, ERAC, ETRABC, ETRhl, ETRhl-RC) that carry different combinations of the genes involoved in rhamnolipid bio-synthesis were successfully engineered and tested for their rhamnolipid production. Sand-pack core flooding tests were run to evaluate and compare the effectiveness of these products as agents for enhanced oil recovery. The brine with optimized pH and salt concentration in which a given biosurfactant product has its lowest IFT was used to saturate the core, perform a waterflood, and prepare the surfactant solution. Injection of 6 PV of only a 250 ppm rhamnolipid biosurfactant solution and 4 PV of a brine chaser could recover as much as half of the waterflood residual hydrocarbon (n-octane). The engineered E. coli strains that include more of the implanted genetic code had the better performance in these oil displacement tests. The IFT, biosurfactant concentration and pH of effluents from core flooding were monitored to address EOR mechanisms and quantify the adsorption of each product in the sand pack. Introduction Biosurfactants are biologically produced by bacteria or yeast from various substrates including sugars, glycerol, oils, hydrocarbons and agricultural wastes1. They are classified as glycolipids, lipopeptides, phospholipids, fatty acids, neutral lipids, polymeric and particulate compounds2. Most of these compounds are either anionic or neutral. Only a few are cationic such as those containing amine groups. The hydrophobic portion of the molecule is based on long-chain fatty acids, hydroxyl fatty acids or a-alkyl-ß-hydroxy fatty acids. The hydrophilic moiety can be a carbohydrate, amino acid, cyclic peptide, phosphate, carboxylic acid or alcohol. Biosurfactants reduce surface tension and interfacial tension in both aqueous solutions and hydrocarbon mixtures. These properties lead to the formation of microemulsions, where hydrocarbons can solubilize in water, or water in hydrocarbons. Biosurfactants have been receiving increasing attention as Enhanced Oil Recovery (EOR) agents because of their unique properties (i.e., mild production conditions, lower toxicity, and higher biodegradability) compared to their synthetic chemical counterparts3. The chemical material cost is perhaps the main factor controlling the profitability of conventional surfactant flooding EOR4,5. An inherent potential advantage of using biosurfactants is that the cost of making biosurfactants is decoupled from the price of crude oil. The cost of producing biosurfactants can be greatly reduced when the nutrients and other raw materials to create biosurfactants come from renewable resources, are not petroleum based, and in fact potentially may come from waste streams.