Full Length ArticleUltralow concentration of molybdenum disulfide nanosheets for enhanced oil recovery
Graphical abstract
Introduction
Oil is recovered from reservoirs through three stages: primary, secondary, and tertiary. The primary and the secondary stages (water flooding) contribute to recovery of approximately 40–50% of oil from the reservoir as a result of sweep and displacement [1]. The remaining oil is recovered through enhanced oil recovery (EOR) or tertiary oil recovery techniques [2]. The chemical EOR (C-EOR) process involves injection of chemicals like polymer, surfactant, or foam into the reservoir [3], [4], [5], [6], [7]. This process is used to recover approximately 20–60% of residual oil from the reservoir. Generally, surfactants are used to alter the wettability of reservoir rocks and decrease the interfacial tension (IFT) of oil/water, which promotes the recovery of oil from the reservoir [8], [9]. The IFT depends on the surfactant critical micelle concentration (CMC) which promotes the adsorption/desorption of the surfactant at the oil/water interface and stable emulsion formation. So far numerous types of surfactants were used in the application of C-EOR. Gao et al. investigated the effect of IFT on the gemini surfactant [10]. They found the gemini surfactant could reduce the IFT to as low as 10−3 mN/m at the concentration of 0.02 wt%. Wu et al. reported an anionic surfactant (alkyl alcohol propoxylated sulfate) as an efficient candidate for C-EOR [11]. It could reduce the IFT at lower concentration under high saline condition and could recover 50% of residual oil. Zhang et al. developed a novel zwitterionic surfactant derived from the castor oil [12]. These types of surfactant could reduce IFT to ultra-low value of 5.4 × 10−3 mN/m at the concentration of 10 g/L. Another main mechanism is wettability alteration, where cationic surfactants like cetyl trimethyl ammonium bromide (CTAB) plays a vital role [13]. The negatively charge group in the crude oil are normally absorbed over the positively charged minerals in the carbonate reservoir [14]. Thus, the monomer of the cationic surfactant forms an ion pair with anionic group (carboxyl) adsorbed on the rock surface from the crude oil. This phenomenon leads to alter wettability of the rock and desorb oil from the pore surface [15]. However, the surfactants are often limited due to its high cost, possible formation damages and chemical loss. Thus, the researchers have investigated the cheaper and effective methods by using the metal-oxide based nanoparticles for oil recovery [16], [17]. These nanoparticles are not efficient because of their low recovery rate, <5% [18]. Hence, an alternative approach of modifying the particle with a hydrophobic and a hydrophilic group i.e., amphiphilic particle, similar to that in a surfactant, has been widely explored [19], [20], [21]. Compared to zero-dimensional (silica) [22] or one-dimensional (carbon nanotubes) [23], Gao et at. found that variation of particle shape and geometry have a better emulsion and stability with oil [24]. Creigton et al. presented a thermodynamic model to showcase the change in gibbs free energy during emulsion stabilization of graphene oxide in oil/water interface [25]. This is due to hydrophobic-hydrophilic balance of the nanosheets in the oil/water system. Further, Mejia et al. synthesized an amphiphilic lamellar sheet through pickering emulsion and verified its emulsion property [26]. Luo et al. used janus graphene oxide as a C-EOR agent and achieved approximately 15% oil recovery [27]. Yin et al. incorporated silica over janus graphene oxide to improve the recovery rate to 18% at 0.005 wt% [28]. Radina et al. enhanced the stability of the graphene oxide nanofluid through sulfonating the graphene nanosheets [29]. They achieved the recovery rate of 16% at 0.005 wt%. However, cost and issues related to large-scale production limit the practical use of graphene oxide [30]. Thus, an alternative material through simple methods are required to overcome the challenges and have a huge impact on the production and profit of oil and gas industries.
In this work, we demonstrate the potential use of molybdenum disulfide (MoS2) nanosheets as an alternative two-dimensional material for oil displacement. We synthesized metallic MoS2 through a simple one step hydrothermal process which is suitable for large-scale production. MoS2 is one of the family members of two-dimensional (2D) transition metal dichalcogenides (TMDs) and has many fascinating properties such as high carrier mobility [31], bandgap tunability [32], and photoconductivity [33]. These unique properties have made MoS2 a leading material in various industrial applications such as transistors [34], photovoltaics [35], catalysis [36], and batteries [37]. MoS2 is hydrophobic in the semiconductor state and exhibits hydrophilic characteristics in the metallic state [38]. The synthesized MoS2 nanosheets exhibited amphiphilic characteristics upon modification with an alkylamine chain (Movie S1, Supplementary material). This behavior enables the modified MoS2 nanosheets to alter the wettability of a substrate from oil-wet to water-wet. We achieved wettability alteration at a lower concentration (0.005 wt%) than conventional surfactants. Moreover, nanofluid was prepared under saline condition, which helps in decreasing the water usage. A residual oil recovery of approximately 21.18% was achieved from low-permeability (25 mD) cores compared to previous nanosheet based nanofluids (Table S1, Supplementary material).
Section snippets
Materials used
Molybdenum trioxide, thiourea, thioacetamide, octa decyl amine (ODA), liquid paraffine with 110–230 mPas of dynamic viscosity and ethanol (99.9% purity) were purchased from sigma aldrich, China. Aviation kerosene was purchased from Beijing aviation kerosene limited, China. The two types of oil used in this work are model oil and crude oil. The model oil was prepared by diluting liquid paraffine with kerosene at the volume ratio of 20:1 which represents the light oil with a viscosity of 25 cP
Morphological and structural analysis of nanosheets
Raman spectroscopy was utilized to determine the polymorphic nature of as-synthesized MoS2. Fig. 4a shows the Raman spectra of as-synthesized MoS2 nanosheets. The E1g and A1g vibrational modes are observed at 284 and 407 cm−1, respectively; this indicates the octahedral coordination of MoS2 [39]. The other Raman modes at 146 (J1), 226 (J2), and 333 cm−1 (J3) are attributed to formation of superlattice or distortion on the basal plane of single layer MoS2 [40]. The appearance of these modes
Conclusions
In conclusion, we have demonstrated that the 2D TMD material MoS2 as a promising C-EOR agent. The nanofluid was prepared using seawater condition and can be used at a lower concentration than surfactants. The nanofluid was employed as tertiary flooding during core flooding experiments. The as-prepared nanofluid recovers 21.18% and 18.25% of residual oil in 25 mD core samples saturated with model and crude oil respectively. We observed that the recovery of oil from the core samples was governed
Acknowledgments
This work was supported by the National Natural Science Foundation of China (Grant No. 21427812, 21776308 and 21576289), National “111 Project” (B13010), and Important National Science and Technology Specific Projects of China (Grant No. 2016ZX05014-004-004).
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