Anionic Surfactant Adsorption: Insight for Enhanced Oil Recovery

Oil is the most used nonrenewable source of energy in the world and this has led to the study of new processes for exploration and production of oil reservoirs. Oil and gas industry has invested heavily in increasing the recovery and productivity of reservoirs, mainly in processes of enhanced oil recovery (EOR) [1,2]. The enhanced recovery methods are processes that seek an additional recovery from the reservoirs, after their natural energy is depleted [3,4]. These methods have been developed with the objectives of producting more hydrocarbon than that provided just by the natural energy of the reservoir. It consist of the of thermal, microbial, hydrocarbon gas injection, and chemical injection, which consist mainly of the injection of fluids such as; alkali, surfactants and polymer into the reservoir to move the oil the oil to the well bore [5].


Introduction
Oil is the most used nonrenewable source of energy in the world and this has led to the study of new processes for exploration and production of oil reservoirs. Oil and gas industry has invested heavily in increasing the recovery and productivity of reservoirs, mainly in processes of enhanced oil recovery (EOR) [1,2]. The enhanced recovery methods are processes that seek an additional recovery from the reservoirs, after their natural energy is depleted [3,4]. These methods have been developed with the objectives of producting more hydrocarbon than that provided just by the natural energy of the reservoir. It consist of the of thermal, microbial, hydrocarbon gas injection, and chemical injection, which consist mainly of the injection of fluids such as; alkali, surfactants and polymer into the reservoir to move the oil the oil to the well bore [5].
Surfactant flooding is a CEOR process that involves the injection of one or more Surfactant into the reservoir with the aim of lowering the interfacial tension between the oil and the water present in the reservoir [6][7][8][9]. These will lead to the pressure gradient across the oil droplets to exceed the retaining capillary force. The presence of surfactant between water and oil tend to re arrange the distribution between the two phases [10][11][12]. Surfactant system used for sandstone reservoirs comprise mainly of anionic surfactants [13], the use of Surfactnt in CEOR practice is quit challenging, if not well design could lead to the following; transport failure (Filtration and plugging), Chemical instability, Surfactant retention, and Adsorption on rock [14][15][16][17][18][19][20].
Adsorption phenomena refers to the collection of molecules on the external surface or internal surface of solids or liquids [21,22]. During the flooding process, the surfactant molecules tend to aggregate on the surface of rock which lower the initial concentration needed for lowering the interfacial tension (IFT). The amount of surfactant adsorbed depends on various parameters such as; types of surfactant used, the mineralogy and the morphological characteristics of the rock, presence of co-surfactants and alcohol, impurities, the nature of electrolytes existing in the solution , reservoir temperature and the pH of the reservoir [23][24][25][26].
Chemical EOR (CEOR) has been under the spotlight for decades and has attracted academic researcher attention particularly in the area of reservoir applications [5,6]. Several studies have in the past attempted to address the chemical flood challenges by studying these mechanisms responsible for the adsorption of the chemicals in order to preserve the concentration of effective groups during flooding [7].
This paper therefore, is to review the basic concepts of the aforementioned parameter for future guide line in addressing the surfactant losses due to the adsorption. The focus is on

Effect of anionic surfactant nature
The purity of anionic surfactants, hydrocarbon chain and functional groups all these conditions play a rule in surfactant adsorption. Anionic surfactant purity is not much of a concern to the oil and gas industry but however, it is a concern for colloids. Several reports states that the effect of the functional groups play an important role in the structure of the adsorbed layers depending on the packing of the molecules, which in turn depends on the mutual repulsion. For isomeric surfactants with the same functional group, the comparison of adsorption behaviour remains the same and strongly depends on the charge of the functional group [27][28][29]. As a result, surfactants with longer hydrocarbon chains has a much greater driving force for the aggregation, it shows that anionic surfactant with longer chain has direct relationship with aggregate on the rock surface [28].

Effect of mineralogy
Sandstone reservoir consists of different minerals the dominant are Sand-quartz, feldspar and clay minerals. The clay minerals vary based on the sandstone depositional environment, they include; kaolinite, illite, montmorillonite and chlorite. Several authors have attempted to address the effect of clay minerals presence on the adsorption of anionic surfactant with single adsorbent [30,31] or clay fraction effect [20]. They reported that the kaolinite mineral has the highest adsorption [32], the impact of mineral surface area also affect adsorption, even with the negative ions [33]. Sanchez et al. [33], investigated the effect of clay minerals structure and surfactant nature on adsorption, very few researchers have studied the effect of montmorillonite and illite as single adsorbent on the anionic surfactant [34]. All the results show that electrostatic bond between the kaolinite mineral and the anionic surfactant as the dominant adsorption mechanism and very weak hydrogen bond with the other clay mineral.

Effect of Salinity
The presence of free salt ions is one of the reasons for increased adsorption. The presence of sodium chloride (NaCl) and calcium chloride (CaCl2) in the solutions of anionic surfactants (NP4S, NP10S, NP25S, oxyethylenic) affects the adsorption of anionic surfactants on quartz and kaolin sample. Addition of salts increases the amount of anionic surfactants adsorbed on the solid surfaces [35]. Bera et al. [23] studied the Anionic surfactant, Sodium dodecylsulphate (SDS) (with 98% purity), they reported that, increasing the salinity of the solution, increased the adsorption of SDS on the sand surface, this is due to low electrostatic repulsion between the adsorbed surfactant species. Also, increase in the surfactant concentration, led to increase in adsorption on the surface of sand particles until it reaches the saturation point. Just recently, Yekeen et al. [36] investigated the adsorption of sodium dodecyl sulfate (SDS) on kaolinite at various surfactant concentration and added electrolyte (NaCl, CaCl2 and AlCl3), using the surface tension technique and two phase titration method. They concluded that, the adsorption of SDS by kaolinite increases with increasing concentration of NaCl and CaCl2. Indicating that the effect of monovalent ions increases the adsorption. However, the presence of divalent ions and trivalent ions make the surfactant solubilisation less and causes surfactant precipitation.

Effect of Temperature
Early investigations of the temperature dependence of adsorption from solution indicated that adsorption decreases with increase in temperature [37,38]. Since adsorption is an exothermic process, the lower the temperature, higher the adsorption. However, interference from surfactant thermal decomposition should be eliminated for successful interpretation of the effect of temperature on adsorption. Mineral dissolution at elevated temperatures has caused sulfonate precipitation. Use of sulfonates as additives may require the addition of co surfactants and sequestering agents to improve surfactant solubility [39,40].

Effect of pH
The effect of pH has a major role to play in adsorption, the charge of the rock surface is dependent on the pH. When the pH of an aqueous solution is lower than 6, the solid surface become more positive due to the adsorption of protons from the solution on to charged sites. The low pH will tend to increase the adsorption of anionic surfactants which is carrying the negative sign [41][42][43].

Conclusion
Considering the great variability of different surfactant types, anionic surfactant remains favorable for CEOR especially for sand stone reservoirs. The soil mineralogical composition, formation water total dissolved ions and reservoir temperature reported may affect the surfactant stability and adsorption in the practical field. The reported consideration need to be understood under different physicochemical conditions to avoid the surfactant losses and enabling the project to be economically feasible. This losses need to be eliminate which will led to open door for developing efficient anionic surfactant and processing schemes for oil recovery.