Extraction of the indigenous crude oil dissolved biosurfactants and their potential in enhanced oil recovery

Abstract

The presence of indigenous microorganisms in the petroleum reservoirs with the capability of biosurfactants production implies the existence of significant amounts of biosurfactants dissolved in the reservoir crude oil. The extraction of these biosurfactants to the aqueous phase can be considered as a promising microbial enhanced oil recovery process in a lower cost without the common limitations and risks related to the microbial systems. Ethylenediamine (EDA) was used to extract the crude oil dissolved biosurfactants (as anionic surfactants) of a crude oil sample to the aqueous phase and form a cat-ionic surfactant complex. The biosurfactant was isolated from the crude oil and preliminarily characterized which was capable of reducing the surface tension to 48 mN/m. Various concentrations of EDA were utilized to extract the crude oil biosurfactants and their effectiveness on the oil recovery was studied in a glass micro-model. Results indicated that an incremental 22 % oil production was obtained by flooding the 10 mM EDA solution. The incremental oil production was related to the formation of the cat-ionic complex surfactant which believed to cause a significant IFT reduction and simultaneously, have a higher efficiency in the wettability alteration than the anionic biosurfactants. It can be concluded that the extraction of the indigenous biosurfactant content of the crude oil and their interaction by EDA can be considered as a low-cost low-risk potential to the enhanced oil recovery processes.

Introduction

Petroleum hydrocarbons are still one of the main sources of energy and raw chemicals all over the world. Despite the increasing global demand, there is a competitive market in the production side between the conventional and shale producers. Moreover, high prices would be encouraging the development of alternative resources which can be considered as a treat to the crude oil producers. On the other hand, a large portion of the crude oil has remained in reservoirs after the first and second stages of production [[1], [2], [3]]. Therefore, developing a low cost, effective tertiary recovery method is highly advantageous [4,5]. To this end, effective mechanisms should be designed to conquer the unfavorable conditions such as the oil wet systems, low permeability formations, highly viscose crudes, and high oil/water interfacial tension resulting in strong unfavorable capillary forces [[6], [7], [8], [9]].

Microbial enhanced oil recovery (MEOR) is one of the recovery methods that can be very cost effective because of its low operational costs. In MEOR, microorganisms and their metabolites (i.e., biosurfactants, organic acids, biopolymers, and biogas) are used to mobilize the residual oil in reservoirs [10,11]. The metabolites can be produced either anaerobically within the reservoir by injecting the microorganisms and nutrients into the wells or on the ground surface facilities. Despite the lower costs of the in-situ operations, injection of microorganisms and nutrients into the reservoir would raise a few problems such as stimulating the growth of harmful indigenous strains (e.g., sulfate-reducing bacteria), incompatibility with the reservoir conditions, and plugging the reservoir, especially in the low permeability carbonated fractured systems. Therefore, the practical application of biomass-based in-situ systems has been limited [2,[12], [13], [14]]. Among the microbial metabolites, biosurfactants are of great importance because of their capability in the reduction of the interfacial tension and alteration the reservoir rock wettability [2,11,15]. In fact, the interfacial tension reduction facilitates the dissolution of oil in water through emulsion formation and improves the oil recovery from the reservoir [2,16]. Alteration of wettability from oil wet to water wet decreases the relative permeability of water and intensifies the simultaneous imbibitions mechanism [[17], [18], [19], [20]]. Surfactants can change the rock surface wettability by washing the oil wetting layer from the surface or by coating the surface by a water wet layer through adsorption [21,22]. It is revealed that cationic surfactants usually affect the surface wettability by the washing mechanism while the wettability alteration by anionic surfactants takes place by the adsorption on the surface as a coating layer which has a lower efficiency [22,23]. Unfortunately, almost all of the biosurfactants are anionic or non-ionic molecules acting through the second mechanism [24,25].

There are many reports on the isolation and characterization of biosurfactant producing microorganisms from the oil field fluids throughout the world which confirms the presence of such microorganisms in the reservoirs [25,26]. In fact, the indigenous microorganisms have to produce biosurfactants to enhance the solubility of hydrocarbons as the sole carbon source into the aqueous phase and assimilate them [27,28]. Over the centuries, the produced biosurfactants (such as alkyl carboxylic acids) have been accumulated in the reservoirs and because of their chemical structure (having a large hydrophobic tail leading to low HLB values) tend to dissolve in the oily phase and produce stable water in oil emulsions [[29], [30], [31]]. Theoretically, addition of a chemical with proper characteristics (co-surfactant) would extract the oil dissolved biosurfactants into the aqueous phase where they can be employed in an enhanced oil recovery process. The co-surfactant should have adequate solubility in the aqueous phase, have a good affinity towards the biosurfactant, remain stable in the reservoir condition and improve the water solubility of the biosurfactant.

The organic alkaline amines such as Ethylenediamine (EDA) and Ethanolamine have been previously utilized as organic alkaline materials to alkali-surfactant polymer (ASP) flooding and showed promising results [32,33]. In fact, a diamine compound like EDA can extract the biosurfactant molecules through the formation of hydrogen bonds or reaction with the polar functional groups of the surfactant hydrophilic head and improve their water solubility. Moreover, EDA can produce two positive charges which can attract the anionic parts of the biosurfactants and produce a cat-anionic surfactant system with a net positive charge. The resultant cat-anionic surfactant would be more effective than the native biosurfactant in wettability alteration as it acts through the washing mechanism [23,34,35].

On the basis of the above, the presence of sufficient amounts of biosurfactants in the crude oil composition, the capability of EDA (as a co-surfactant) to extract these biosurfactants to the aqueous phase, and their impact on the wettability alteration and oil recovery should be studied in detail. To the best of our knowledge, the extraction and use of the indigenous crude oil dissolved biosurfactants in the EOR processes has been hardly mentioned in the literature. Therefore, in the present study, an attempt is made to investigate the possibility of the extraction of biosurfactants from a crude oil sample to the brine and examine their capability to enhance the oil recovery in a glass micro-model. The EDA interactions with the crude oil biosurfactant as well as an anionic biosurfactant (rhamnolipid) to form a novel cat-ionic surfactant complex were studied and the efficiency of the resultant surfactant system in wettability alteration and oil recovery was measured.