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CO2 flooding has been widely applied in lots of low permeability reservoirs. After extensive CO2 injection, some reservoirs began to show serious gas channeling problems. CO2 foam had been successfully used to solve gas channeling problems due to its advantages of water selective plugging features (not plugging oil). In this paper, a novel CO2 foam system was developed for high temperature and high salinity(HTHS) (85 ℃, 60,000 mg/L) aiming at solving the gas channeling in Changqing Oilfield. Taking the foam half-life as the evaluation standard, the optimum foam system (0.5 wt% EC-1 + 1 wt% SiO2) for the target reservoir was determined. The influences of temperature, salinity and pressure on the CO2 foam performance were studied by high temperature and high pressure(HTHP)method. The ability and mechanism of anti-gas channeling were studied by experiments of sand packed tube and microscopic displacement, respectively. The results showed that the foam system possessed good foam properties at HTHS with pressure. As the concentration of SiO2 nanoparticles increased, the resistance factor of the foam system increased. However, temperature showed an adverse effect on foam stability, the resistance factor decreased with the increase of temperature. The Jamin superposition and emulsion plugging mechanism of foam system was revealed by microscopic displacement experiments.

High-frequency pulsed alternating current field (HFPACF) has attracted attentions owing to its excellent performances of eliminating short circuit, saving energy and reducing environmental pollution. To study the coalescence mechanism of aqueous drops in water-in-oil (W/O) emulsions under HFPACF, micro-behavior of dispersed drops for W/O emulsion was monitored by using a self-designed micro-visual model and coalescence mechanism was analyzed by discussing effects of pulse amplitude, pulse frequency, pulse width ratio, demulsifier concentration, aqueous mass ratio of emulsion, and different model oil emulsions. The results show that the dispersed drops coalesce under the action of electric field, and the particle size of dispersed drops increases with the increase of pulse frequency and amplitude. When the pulse frequency or amplitude reach to a critical value, aqueous drops are driven to form water string or water channel between two electrodes which hinders further drops coalescence in other areas. However, with the increase of pulse width ratio, the size of dispersed drops increases slowly. The optimum pulse frequency and pulse amplitude are obtained at 3.9 kV and 4.21 kHz, respectively, the optimal pulse width range is 54.9%-84.4%. When the concentration of demulsifier is 60 mg/L, the synergistic demulsification effect achieves the highest performance. When the aqueous mass ratio of emulsion reaches a high level (over 30%), which means the drops in emulsion have big size, the coalescence performance under the same electric field strength is great. Also, it is discovered that the dispersed drops in Yongping model oil emulsion have difficulty in forming water chains, since there are much asphaltenes and resins in the water-oil interface leading to high strength of film. For Shuanghe oil emulsion drops are prevented from coalescing, because it contains a mass of paraffin, which can stabilize emulsion at low temperature.

In polymer flooding process, the poor solubility of polymer and the wrap of incompletely dissolved polymer particles with heavy oil lead to the generation of complicated blockage, which seriously affects the normal production of wells. Therefore, how to fabricate an effective blockage removal agent is particularly important. Based on the blockage from Bohai Offshore Oilfield in China, a blockage removal agent composed of (0.5 wt% potassium persulfate + kerosene (the ratio of kerosene: blockage is 3:5)+0.3 wt% OP-10, the oil-water ratio is 1:9) was developed. The removal effect and mechanism were characterized by weight loss method and core test. Moreover, the emulsification property of the removal agent was also evaluated by microscopic appearance. The results showed that the blockage removal agent can be formulated as an O/W emulsion and had good stability for more than 72 h. Core test showed that the permeability had a significant improvement after injection of removal agent. The emulsion-type removal agent can demulsify after contact with the blockage, then the oil-soluble detergent as internal phase in emulsion can be released which contributes to the wash off of heavy oil. Accompanied with the polymer degradation by the oxidant, this agent realized the effective dissolution and removal of blockage.

For the past decades, long-term water flooding processes have led to water channeling in mature reservoirs, which is a severe problem in oilfields. The development of better plugging ability and cost-effective polymer gel is a key aspect for the control of excess water production. Research on polymer gel applicable in a heterogeneous reservoir to plug high permeable channels has been growing significantly as revealed by numerous published scientific papers. This review intends to discuss the polymer gel techniques from innovations to applications. The related difficulties and future prospects of polymer gels are also covered. Developments of polymer gels to resist temperature, early gel formation, synergistic mechanisms and influence of pH, high salinity are systematically emphasized. The review provides a basis to develop polymer gels for future applications in oilfields to meet harsh reservoir conditions. It will assist the researchers to further develop polymer gels to improve the oil recovery from mature reservoirs under economic conditions to meet the requirements of future oilfields.

Chemical demulsifiers were widely applied for demulsification of crude oil emulsions which were generally formed with natural stabilizers during the extraction. Based on our previous study of the light crude oil emulsions stability, the demulsification performance of four classes of nonionic poly(ethylene oxide) poly(propylene oxide) copolymer demulsifiers and sodium dodecyl sulfate (SDS) as well as their dynamic demulsification process on the W/O light crude oil emulsion were analyzed by the multiple light scattering method. The demulsification mechanism of these demulsifiers were illuminated by the measurement of dynamic interfacial tension (IFT), dynamic interfacial viscoelasticity, rupture rate and microscopic changes of oil film by single droplet method. The results have shown that demulsification performance doesn’t depend on IFT, though effective demulsifiers could have a lower IFT around 1 mN/m. Multiple light scattering data and dynamic IFT clarified the demulsification process of efficient demulsifiers, quick diffusion and adsorption of demulsifiers, palpable change in interfacial property, droplet coalescence and sedimentation. The capacities of demulsifiers to penetrate into the interface and to decrease the interfacial viscoelasticity were the dominating demulsification mechanisms. The quantitative interpretation of multiple light scattering data associated with the dynamic IFT and interfacial viscoelasticity illuminated the demulsification mechanism of different demulsifiers. The rupture rate and dynamic rupture process of the oil film between the aqueous phases also vividly showed the differences in demulsification process. The nonionic demulsifiers AR902 and PR929 with aromatic groups has the highest speed and best performance due to the similar aromatic groups with those in asphaltenes, which produced the least stable oil film. This study provides a valuable guideline for the selection of demulsifier for light crude oil emulsions and enhanced demulsification mechanism studies.

In order to study the interaction between the sodium dodecyl sulfate (SDS)/ n-hexylamine (NHA) composite aqueous solution, the supramolecular system had been constructed at the optimal molar ratio of 1:1 at high mass concentration. Based on the experimental results, supramolecular system formation mechanism was proposed according to the determination of its rheology, Cryo-TEM, Dynamic light scattering (DLS). SDS and NHA form vesicles via hydrogen bonding and electrostatic repulsion. The phase behavior of the system transforms from unilamellar vesicles under relative low mass concentration to multilamellar vesicles when the mass concentration increases to 200 mM. Meanwhile, temperature and inorganic salts were used to observe the effects of hydrogen bonding and electrostatic repulsion on vesicles. When temperature rises to 55℃, the vesicle basically disappears. When inorganic salt concentration reaches 150 mM, the vesicle gradually transforms into fibrous structure. From the perspective of fluidity state, the influence of temperature on vesicles is more than that of inorganic salts.

Dynamic covalent bonds are reversible and responsive, the external environment can cause change of molecular configuration of the compound system, leading to changes in the microscopic morphology of the micelles, thereby achieving intelligent control of the fluid state. Therefore, this work used cationic surfactant cetyltrimethylammonium bromide (CTAB), 4-carboxybenzaldehyde (CB) and aniline (NA) to develop a ternary composite fluid state transition system with a molar ratio of 60:40:40 (CTAB/CB/NA). The bottle test method, rheology, 1H NMR, infrared spectroscopy, and Cryo-TEM were used to test and analyze the prepared ternary composite system to explore the effects of different dosages, temperatures and pH values on the rheological behavior of the system. The results show that when the pH increased from 3.13 to 7.25, the CTAB/CB/NA solution changed from a low-viscosity fluid to a transparent gel-like fluid, and the viscosity increased from 1 to 4.6 × 104 mPa·s, and the viscosity recovered when the pH decreased again. Therefore, the reversible changes in the viscoelasticity of the system can be triggered by changing the pH of the CTAB/CB/NA solution. This drastic change in rheological properties is attributable to the pH dependent ionization and formation of dynamic covalent bond CBNA, which is shown microscopically as the morphology of the system micelles changing from spherical micelles to worm-like micelles. The viscosity of the system decreases when the temperature increases, and the viscosity is almost zero when it reaches 50 °C. In addition, by adjusting the pH value, the CTAB/CB/NA solution can obtain more than 3 cycles of morphological changes from spherical micelles to worm-like micelles. This pH-responsive fluid state transition system can meet the viscosity requirements of the oil-displacing agent, and can also reduce the adsorption loss in the formation, and has more valuable research in the field of enhanced oil recovery.

Polymer flooding is one of the most effective processes to enhance oil recovery (EOR), but the poor thermo-stability and salt tolerance of the currently used water-soluble polymers challenge their usage in presence of high temperatures and salinity within the oil reservoirs. Thermo-viscosifying polymers (TVP’s) are a novel class of materials developed for EOR applications in high-temperature and high-salinity oil reservoirs. In this study, the interactions of novel TVP with different salts and its temperature and rheological properties were evaluated. The thermal and rheological properties of this polymer were studied as a function of polymer concentration, and salinity at different temperatures. It was observed that by increasing temperature, the aqueous solution viscosities first smoothly decreased, but then increased at above the critical association temperature (T cass ). T cass was also found to be a function of the TVP’s concentration and temperature. Although, the shear effect demonstrated shear-thinning, with the increase of temperature, the shear viscosities showed shear thickening at all concentrations of polymer. Furthermore, the novel TVP with low concentration showed good viscoelastic properties compared with high concentration of HPAM. The introduced novel TVP’s thermo-thickening tendency is enhanced by increasing temperature and salinity which makes it more promising for EOR applications.

The influence of salt and temperature on the viscosity of a hydrophobically modified polyacrylamide-based (PAM-based) novel functional polymer (RH-4) was investigated. To study the viscosifying mechanism in brine solution, scanning electron microscope (SEM) was applied to observe the network microstructures of polymer solution. The solution apparent viscosity behaviors of the RH-4 polymer are studied as functions of concentrations, pH value, salt types and amount, microscopic morphology. The viscoelasticity of aqueous solutions of RH-4 polymer changed with shear rate under high salinity condition. Additionally, the influence of temperature on the apparent viscosity of RH-4 polymer was investigated. The results showed that the viscosity decreased at lower salt concentrations and increased at higher salt concentrations. The SEM results show that the network structures of RH-4 polymer become much more condense at high salinity, which lead to the increasing its apparent viscosity. When the salinity reaches 80,000 mg/L, thickening ability of the system is the strongest. With the rise in temperature, the viscosity of polymer solutions with various concentrations decreases slightly except for the two concentrations of 1000 mg/L and 1100 mg/L, their viscosity increases with the increasing of temperature. The thickening tendency is enhanced by high salinity and high temperature, which makes it more promising for EOR.

In order to improve the shear resistance of amphiphilic polymer gel, cyclodextrin polymer is selected to construct inclusion polymer gel named as CADC inclusion gel. The critical shear condition was determined to be 4000 rpm and 7 min through rheological method. Under this condition, variety of instruments such as rheometer, scanning electron microscope (SEM) were used to prove CADC inclusion gel has more excellent shear stability than amphiphilic polymer gel. This is because the cyclodextrin structure in CADC inclusion gel can reorganize amphiphilic polymer which has been broken via host-guest inclusion effect. At the same time, the sand pack tube was used to test plugging performance of the sheared CADC inclusion gel. The formation of such host-guest inclusion gel solves the problem of poor shear resistance ability of amphiphilic polymer gel. It has great potentials to replace the amphiphilic polymer gel for water shutoff applications in oilfield.

The thickening behavior and synergistic mechanism of the mixed system of the hydrophobically associating polymer (HAP-S) (acrylamide/acrylate-EOm-POn/sodium acrylate terpolymer) with small molecular weight and the acrylamide/N-isooctyl acrylamide/sodium acrylate terpolymer (HAP-L) with large molecular weight were investigated via the rheology, inclusion action of β-cyclodextrin (β-CD), cohesive energy, dynamic light scattering (DLS), and scanning electron microscope (SEM). The apparent viscosity of the mixed-system solution was higher than that of the HAP solution alone in a wide range of mass ratios and concentrations of the two polymers, ascribing to the enhancement of associated viscosity and strength of the network structure by forming mixed aggregates and bridging among them. Furthermore, the hydrodynamic radius of the mixed-system aggregates increased due to the improved network structure and the electrostatic repulsion effect among the different kinds of HAP molecules. Under the synthetical...

Emulsions stabilized by the supramolecular systems with sodium polyacrylic acid and amphiphilic polymers with different hydrophobic monomer contents were investigated. Multiple-light scattering was used to characterize the stability of emulsions. Optical microscopy and particle size analyzer were utilized to determine the droplet size distributions. The glass-etching micro model and microscopy were used to observe the emulsion stability in the porous media. To explore the stability mechanisms of emulsions, the rheology of the external phase and the oil/water interface were determined with the Physica MCR301 Rheometer. Emulsions formed by the supramolecular systems exhibit a better stability than the individual amphiphilic polymer systems. Moreover, the stability of emulsions formed by the supramolecular systems increases with the hydrophobic monomer content of amphiphilic polymers. The results can be attributed to the large viscous effect of the external phase and the high strength of the oil/water interfacial film in the emulsions. The results provide a theoretical foundation for future studies of the oil displacement efficiency by amphiphilic polymer supramolecular systems in tertiary oil recovery.

In recent years, with the further development of old oilfields, in order to further improve the oil recovery, they must be conformance controlled. Among various types of conformance control methods, polymer gels composed of polymers and crosslinkers have attracted widespread attention because of their efficiency and low costs. Among them, organic chromium gels with their good formation adaptability and high stability have been fully developed in recent decades. This review introduces the different types of polymers and crosslinkers used in the preparation of organic chromium gels, and the mechanisms of affecting their performance are analyzed. On this basis, the organic chromium gels for different formation conditions are introduced, including nanoparticle-reinforced and compound organic chromium gels. At the same time, evaluation methods of organic chromium gels are introduced, while the focus is on the in-situ measurement method (mirco-rheology) of gel formation time developed in recent decades. Based on the currently developed organic chromium gel and the analysis of the development status in oilfields, future directions like the use of supramolecular organic chromium gel and shear organic chromium gel are suggested.

Emulsion formation is inevitable during polymer flooding. In order to study the mechanism of the emulsion stability formed by inclusion complex and crude oil, the optimal inclusion ratio of inclusion complex was determined, and then, the properties of the emulsion such as particle size, stability, and interface film strength were tested by microscope, laser particle size analyzer, rheometer, and stability analyzer. Test results show that the addition of cyclodextrin polymer in inclusion complex have a positive effect on the stability of the emulsion. There are two main mechanisms which cyclodextrin polymers enhance the stability of emulsions. Firstly, with the addition of the cyclodextrin polymer, the viscosity of the external phase is increased, making it difficult for emulsion to coalesce and demulsify. Secondly, the cyclodextrin polymer and the hydrophobic chain of the amphiphilic polymer are adsorbed at the oil-water interface together, which enhances the interface film strength. The stability mechanism of the emulsion formed by the inclusion complex and crude oil has laid a foundation for its application in oil displacement.

The betaine-type amphiphilic polymer exhibits excellent salt-thickening properties due to the presence of betaine groups, but it also has some defects, such as poor water solubility at high betaine groups content, the viscosity-thickening ability is poor under the low betaine groups content. In order to solve the problem of poor viscosity-increasing ability of betaine-type amphiphilic polymers at low betaine group content, a cyclodextrin polymer was synthesized through free radical copolymerization. Characterization of the synthesized product was conducted by means of FT-IR and 1H_NMR. The two polymers were subjected to host-guest inclusion. Changes of viscosity, viscoelasticity, shear resistance and micro-morphology of the host-guest inclusion complex were measured by viscometer, rheometer, scanning electron microscope, and atomic force microscopy. The results showed that the optimal inclusion ratio for the complex was cyclodextrin polymer to betaine-type amphiphilic polymer ratio of 1:1. When the concentration of the host-guest inclusion complex reached 3000 mg/L, its viscosity increased twice compared to the betaine-type amphiphilic polymer with the same concentration. The host-guest inclusion complex was also more shear-resistant, which poses great potential application in chemical enhanced oil recovery.