More than half of the global oil reserves are in carbonate reservoirs. Carbonate rocks, however, in most cases tend to be mixed-wet or oil-wet. Wettability alteration of carbonate reservoir rock has been proven to increase oil recovery significantly. Several chemicals have shown their effect on wettability; however, selection of an appropriate wettability modifier should be made on the basis of the underlying mechanisms and their behavior at reservoir conditions. This review discusses techniques that can help in assessing wettability alteration or reflect on the underlying mechanism and describes several categories of wettability modifiers focusing on their structure−property relationship and factors affecting their performance at reservoir conditions. Surfactants, nanoparticles, salts, and alkalis are four major categories of wettability modifiers that are discussed in this review. Among surfactants, gemini surfactants have great potential and could be a major focus of future research in this area. Nanoparticles are relatively novel materials for wettability alteration with the capability to reduce contact angle significantly at low cost. This review also identifies the current and future challenges related to the performance of various wettability modifiers at high-temperature and high-salinity conditions.

Two aspects are always considered in the design and development of new surfactants for oilfield application. One of them is that surfactant must be sufficiently stable at reservoir temperature and the other is the solubility of the surfactant in the injection water (usually seawater) and the formation brine. Most industrially applied surfactants undergo hydrolysis at elevated temperature and the presence of reservoir ions causes surfactant precipitation. In relevance to this, a novel series of quaternary ammonium gemini surfactants with different length of spacer group (C8, C10, and C12) was synthesized and characterized using FT-IR, 13C NMR, 1H NMR, and MALDI-TOF MS. The gemini surfactants were prepared by solvent-free amidation of glycolic acid ethoxylate lauryl ether with 3-(dimethylamino)-1-propylamine followed by reaction with dibromoalkane to obtain quaternary ammonium gemini surfactants. The gemini surfactants were examined by means of surface properties and thermal stabilities. The synthesized gemini surfactants showed excellent solubility in the formation brine, seawater, and deionized water without any precipitation for up to three months at 90 °C. Thermal gravimetric data revealed that all the gemini surfactants were decomposed above 227 °C, which is higher than the oilfield temperature (≥90 °C). The decrease in critical micelle concentration (CMC) and surface tension at CMC (γcmc) was detected by enhancing spacer length in the order C8 ˃ C10 ˃ C12 which suggested that the larger the spacer, the better the surface properties. Moreover, a further decrease in CMC and γcmc was noticed by enhancing temperature (30 °C ˃ 60 °C) and salinity (deionized water ˃ seawater). The current study provides a comprehensive investigation of quaternary ammonium gemini surfactants that can be further extended potentially to use as a suitable material for oilfield application.

High-temperature/high-salinity (HTHS) reservoirs contain a significant fraction of the world's remaining oil in place and are potential candidates for enhanced oil recovery (EOR). Selection of suitable surfactants for such reservoirs is a challenging task. In this work, two synthesized zwitterionic surfactants bearing a saturated and an unsaturated tail, namely 3-(N-stearamidopropyl-N,N-dimethyl ammonium) propanesulfonate and 3-(N-oleamidopropyl-N,N-dimethyl ammonium) propanesulfonate, respectively, were evaluated. The surfactant with the unsaturated tail showed excellent solubility in synthetic seawater (57,643 ppm) and in formation brine (213,734 ppm). However, the unsaturated surfactant with a saturated tail showed poor solubility, and therefore it was not evaluated further. The thermal stability of the synthesized unsaturated surfactant solution in seawater was evaluated by heating the solution at 90 C in a sealed aging tube for 2 weeks. The thermal stability of the unsaturated surfactant was confirmed by FTIR and NMR analysis of the aged samples at such harsh conditions. The critical micelle concentration (CMC) of the synthesized unsaturated surfactant in seawater was 1.02 × 10 −4 mol L −1 , while the surface tension at CMC was 30 mN m −1 . The synthesized unsaturated surfactant was able to reduce the oil-water interfacial tension tõ 10 −1 mN m −1 at different conditions. A commercial copolymer of acrylamide and 2-acrylamido-2-methylpropane sulfonic acid (AM-AMPS) was tested for EOR applications in HTHS conditions. The addition of the synthesized unsaturated surfactant to the AM-AMPS copolymer increased the viscosity of the system. The increase in oil recovery by injecting the unsaturated surfactant solution and the surfactant-polymer mixture in solution was 8 and 21%, respectively. The excellent properties of the synthesized unsaturated surfactant show that surfactants with an unsaturated tail can be an excellent choice for HTHS reservoirs.

The need to reduce surfactant adsorption on rock surfaces has been a difficult task in chemical enhanced oil recovery, as it directly impacts the economics of the project. This requires a comprehensive insight into the adsorption mechanism on rocks. The adsorption mechanism of an in-house cationic gemini surfactant on different rocks has been studied in this work. The novel surfactant is compatible with high temperature and high salinity environments. All experiments were conducted at room temperature and in deionized water. Adsorption in sandstone rocks was found to be significantly higher than that in carbonates because of the high density of negative charges. The differences in the quantity of the adsorbed surfactant can be explained by different layer structures. It is proposed that an interdigitated bilayer is formed on carbonate rocks, whereas a noninterdigitated bilayer is formed in sandstone rock samples. The maximum and minimum adsorption values were found to be around 9 and 1 mg/g-rock in sandstone and carbonates, respectively. Scanning electron microscopy showed that the surface of sandstone rocks was rougher after the adsorption of the gemini surfactant, whereas no substantial variation in the morphology of carbonates was found. Similarly, Fourier transform infrared spectroscopy showed the symmetric and asymmetric vibration of the CH2 groups in the post-adsorption analysis of sandstone but not in carbonates. Adsorption isotherm modeling was also conducted to investigate the adsorption mechanism of the gemini surfactant on different rocks. All rocks follow a Hill isotherm, showing that the adsorption process is cooperative. However, better curve fitting was obtained using a Redlich–Peterson isotherm in sandstone, whereas both the Langmuir and Redlich–Peterson isotherm performed better for carbonates. The experimental results confirm the formation of interdigitated and noninterdigitated bilayer of the employed surfactant, which explains its adsorption behavior in different rocks and how this adsorption follows different adsorption models in different rocks.