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For the past decades, long-term water flooding processes have to water channeling in mature reservoirs, which is a severe problem in oilfields. The development of better plugging ability and cost-effective polymer microspheres is a key aspect for the control of excess water production. Research on polymer microspheres 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 different types of the polymer microspheres and oil displacement efficiency. The related difficulties and future prospects of polymer microspheres are also covered. The review provides a basis to develop polymer microspheres for future applications in oilfields. It will assist the researchers to further develop polymer microspheres to improve the oil recovery from mature reservoirs under economic conditions to meet the requirements of future oilfields.

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.

Viscoelastic surfactant (VES) fracturing fluids, with the advantages of low residues, complete gel breaking, and low formation damage, have received considerable attention in hydraulic fracturing t...

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.

Responsive wormlike micelles (WLMs) consisted of cationic surfactants and organic-acids are fascinating due to their reversible molecular recognition properties. However, it is unknown how the structure of organic-acids alters the stimuli-responsiveness of WLMs systems. Herein, the peculiar nature of temperature-responsive behaviors in three WLMs systems were systematically investigated. These were manufactured by combining N-erucamidopropyl-N,N-dimethylamine (UC22AMPM) with isomers of organic-acids: o-phthalic acid (o-PA), m-phthalic acid (m-PA) and p-phthalic acid (p-PA) at molar ratio of 2:1 (named as o-EAPA, m-EAPA and p-EAPA respectively). The phase behaviors, macro- and micro-rheology, as well as the mechanism of temperature-responsiveness were explored by visual inspection, rheological and optical methods. The results showed that the three systems exhibited different responsiveness with increase of temperature. Among them, the viscosity and viscoelasticity of o-EAPA were gradually decreased with temperature increase from 30 °C to 90 °C. On the other hand, those of p-EAPA were firstly increased and subsequently decreased, exhibiting the highest viscosity during the heating process. This peculiar phenomenon was attributed to the hydrophilic difference of organic-acids isomers, leading to variations of micelle transitions upon temperature increase. This study is the first report of aromatic-acids isomers inducing different on temperature-responsiveness, and finding beneficial for the development of responsive WLMs for different applications.

In order to prepare materials with controllable properties, changeable microstructure, and high viscoelasticity solution with low polymer and surfactant concentration, a composite is constituted by adding surfactant (sodium dodecyl sulfate, SDS) to hydrophobically associated water-soluble polymer (abbreviated as PAAC) solution. The viscoelasticity, aggregate microstructure, and interaction mechanism of the composite are investigated by rheometery, Cryo-transmission electron microscopy (Cryo-TEM), and fluorescence spectrum. The results show that when the mass ratio of polymer to surfactant is 15:1, the viscosity of the composite reaches the maximum. The viscosity of the composite system increases hundredfold. The viscosity plateau under dynamic shear is generated. The composite has the properties of high viscoelasticity, strong shear thinning behavior, and good salt tolerance, and temperature resistance. The maximum viscosity of the composite is shown at the salinity of 20000 mg L-1 . In addition, there is no phase separation in the composite with the increase of polymer and surfactant concentration, which indicates the good stability of the system. It is proposed a method to obtain a high viscoelasticity solution by adding surfactants without wormlike micelles to a hydrophobically associated water-soluble polymer solution.

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.

Microemulsion, because of its excellent interfacial tension reduction and solubilization properties, has wide range of applications in the petroleum industry, especially in improved oil recovery (IOR). Herein, the concept, types and formation mechanism of microemulsion were primarily introduced. Then, the preparation and characterization methods were illustrated. Additionally, several effect factors were elaborated specifically based on the composition of microemulsion. Finally, the application of microemulsion in IOR was addressed, including IOR mechanism analysis based on sweep efficiency and displacement efficiency, injection method (microemulsion flooding, in-situ microemulsion formation) and field tests. Furthermore, the current challenges and prospects of microemulsion on IOR were analyzed.

In Enhanced oil recovery methods, CO2 is an excellent injecting fluid than water because of its prominent properties which includes non-flammability, non-polarity and non-toxicity. The reservoir pressure is over minimum miscibility pressure (MMP), it also provides for miscible displacement process of multi-contact. The CO2 injection process has been widely practiced in oil industry in the last decades. In the fractured or heterogeneous reservoirs, the injected gas will mostly passage through the fractures or high permeability layers which present the minimum flow resistance, leaving much of crude oil unrecovered in the low permeability rock-matrix. In this research, a composite polymer gel was proposed for harsh reservoir conditions, such as high salinity (NaCl 15.0 wt%, Ca2+ 0.8wt% and Mg2+ 0.8wt %), acid conditions (pH 3.25) and analyzed systematically at laboratory scale, consisting of polymer, cross linker and nanoparticles (silica dioxide). The gel system was characterized by different parameters like viscosity (μ), loss modulus (G″) and elastic modulus (G′) by varying polymer concentration, cross-linker concentration, silica dioxide (SiO2) concentration by using macro-rheology. The micro-rheology was used to calculate mean square displacement (MSD) and elasticity index (EI) of composite gel system. The bottle testing method was used for characterizing the gelation time of composite, gelation code and synergistic mechanism. The macro-rheological results of composite polymer gel proved that elastic modulus (G′) and loss modulus (G″) of composite gel system tends to increase with increment in silica dioxide (SiO2), polymer, cross linker and salinity concentrations. The results of G′ and G″ showed that composite gel have good elasticity at high salinity and acidic conditions. The gelation time observed was 6 days, which is sufficient to pump the gel into targeted zone. The gel code obtained was G at the temperature of 60°C. The elasticity index (EI) in micro-rheological results showed that gel has good elasticity, and MSD curve showed gelling fluid is elastic fluid and bears good stability. The composite polymer gel bears very low syneresis in acid conditions for 60 days at the temperature of 60°C, and having good thermal stability. The laboratory scale research indicates that newly formulated composite polymer gel can be efficiently applied to heterogeneous reservoirs for harsh reservoir conditions and CO2 shut-off problems, and can decrease the water or gas breakthrough problems by plugging the highly permeable channels and increasing the sweep efficiency from low permeable reservoir to produce the residual oil and ultimately increase the production of oil.

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.

Surfactants have been widely used in enhanced oil recovery. The zwitterionic surfactant possesses a high critical micelle concentration, that its use requires an increased concentration and thus it is uneconomical. In this paper the use of erucic acid amidopropyl- betaine (EDAB, C22) and sodium alkyl sulfate (SDS) is proposed to construct a worm-like micelle with mass ratio of 1.5:1. The viscoelasticity and the structural morphology of worm-like micelle systems were studied using cryo-transmission electron microscopy, dynamic light scattering (DLS) and rheology. The results showed that EDAB/SDS with a mass concentration of 0.3% (0.18% EDAB + 0.12% SDS) exhibited viscoelastic fluid behavior when the pH value was 9.15 at 43 °C. The shear viscosity of the EDAB/SDS system is greater than that of the EDAB system. It shows that longer linear worm-like micelle and larger entangled three-dimensional network structure are formed in the EDAB/SDS system. The rheological properties EDAB/SDS system are affected by pH, temperature and inorganic salts. The viscoelasticity of the EDAB/SDS system increases first and then decreases with the increase of temperature and NaCl concentration. The electrostatic interaction between EDAB and SDS can be adjusted by changing the pH value to affect the aggregation behavior of worm-like micelle molecules. Worm-like micelles continue to grow longitudinally with the increase of pH value and form larger aggregate structures, which are manifested by an increase in viscosity.

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 produced fluid from reservoirs are generally in the form of emulsion with oil and water, in which indigenous components such as asphaltene, resin and wax can act as natural emulsifiers. In this study, water-in-oil emulsions were prepared with five light crude oils from CQ Oilfield respectively, and investigated to identify the indigenous stabilization mechanism based on a novel visualized method by in situ solvent-dissolution (ISV). The emulsion stability was studied by dehydration rate and droplet size via bottle test and microscope. Saturate-Aromatic-Resin-Asphaltene (SARA) separation, wax appearance temperature (WAT) measurement, and gas chromatography were applied to characterize the indigenous components in the crude oil. Colloid instability index was introduced to analyze the solubility of asphaltene. Furthermore, the stabilization mechanism of the oil emulsion was investigated by ISV method in both macroscopic and microscopic scales via dilution by n-heptane and toluene. The viscosity of oil, interfacial viscosity, interfacial storage and loss modulus were measured by rheometer to evaluate the contribution of the stabilizer quantitatively. The results showed that emulsion stability increased with the concentration of asphaltene, especially over a critical concentration. This ISV method provides a convincing approach to understand the indigenous stabilization mechanism of original crude emulsion vividly rather than that in the model emulsion. The interfacial viscosity, viscoelasticity or oil viscosity increased with asphaltene concentration in the crude oil. The strong elastic interface film formed with asphaltene restricted the droplet coalescence and controlled the stability of emulsions, which was found to be the crucial stabilization mechanism. Asphaltene and its concentration are the dominating factors of stability of these light crude oil emulsions. This work not only clearly clarifies the significant role of asphaltene in the stability of light crude oil emulsions, but also proposes a convenient in situ method to study the emulsion stabilization mechanism.

Blockage is a common problem in the development of water-flooding reservoirs. The key to solve this problem is to determine the composition and distribution rules of the blockage. In this work, the location distribution and components of blockage were analyzed by one-dimensional (1D) and two-dimensional (2D) cores, and x-ray diffraction and differential weight methods. The blockage formation mechanism was also studied. The 1D experimental results show that the blocking locations of the inlet and outlet ends are 3/10 and 8/10, respectively (1D core is divided into 10 equidistant sections; “3/10” refers to the three sections from the inlet end and “8/10” refers to the two sections from the outlet end). The blockage near the inlet is mainly composed of inorganic scale, which is formed by a combination of the carbonate and coarse sand particles. However, the blockage near the outlet is mainly composite scale, which is formed by the fine particles and inorganic scale with heavy oil. In addition, the water sensitivity with clay leads to the decrease in permeability. This work provides a theoretical and scientific basis for the development of technology to effectively prevent reservoir blockage.

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.

Nanoemulsions consist of small particle size, uniform dispersion, and long-term stability, making them suitable for use in enhanced oil recovery for low-permeability reservoirs. In this paper, an ultrastable oil-in-water nanoemulsion was prepared using the emulsification inverse phase method at a constant temperature. Rheology, multiple-light scattering, laser particle size analyzer, and optical microscope were used to determine the effects of the emulsifier and water content, salinity, and reservoir temperature on the stability of the prepared nanoemulsion. The oil washing performance of nanoemulsion was tested by the weight-loss method. Results show that the nanoemulsions tend to be stable with an increase in the emulsifier content (≥8 wt. %). As the water content increases, the viscosity of nanoemulsions gradually decreases as well as its elasticity. The salinity and temperature are found to have minor effects on the stability of nanoemulsions. At the same time, the oil washing efficiency of nanoemulsion can reach as high as 83%. Hence, this study will provide theoretical supports and guidelines for the preparation of highly efficient nanoemulsion flooding systems for low-permeability reservoirs.

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.

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.

Polymer microsphere profile control technology has been a promising new profile control technology in heterogeneous reservoirs. The matching between polymer microsphere and pore throat of the reservoirs is crucial for profile control treatment. In this paper, the matching mechanism between polymer microsphere and pore throat had been systematically studied by sand pack models and micro models. The optical microscope and laser particle size analyzer were used to analyze the morphology and particle size of polymer microspheres before and after injection. The matching factor was proposed to evaluate the compatibility. The long sand pack model was used to evaluate the migration rule of polymer microspheres. Three modes including the migration mode, blockage mode and seepage mode were concluded and five passing through patterns, which were deformable passing through, partition passing through, blockage, adhesion and direct passing through, were observed. The specific corresponding statistical data of matching factors and passing through patterns in different storage modulus was established. The long sand pack displacement experiment showed that polymer microspheres could migrate into the deep reservoir. The comprehensive correlation equation pin = 3.3072 × 10− 7G′2.6492R2.3882G′−0.0568 was derived according to the laboratory physical simulation data and the stable injection pressure of polymer microsphere with different storage modulus in different matching factors was predicted. This research will provide theoretical support for the further EOR research and field application of polymer microspheres profile control system.

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.

Both amphiphilic polymer and surfactant can be used to enhance oil recovery (EOR) by emulsifying crude oil. In this work, the emulsifying ability and emulsion stability of combination system constituted by amphiphilic polymer (APC16) and anionic surfactant (erucyl dimethyl amidopropyl betaine, EDAB) were studied. The bottle test was selected to evaluate the emulsifying ability. The TURBISCAN Lab and laser particle size analyzer were applied to evaluate emulsion stability. The rheometer, interfacial tensiometer and microscope were used to explore the stability mechanisms of emulsions. The results showed that the combination system exhibited better emulsifying ability than APC16 and emulsion formed by the combination system also performed greater stability than those formed by either EDAB or APC16 system. The better emulsifying ability of the combination system depended on low interfacial tension (IFT) due to the addition of surfactant. The greater emulsion stability of the combination system can attribute to three perspectives. Firstly, emulsion formed by the combination system exhibited higher external viscosity, which slowed down the migration velocity of the emulsion droplets. Secondly, the excellent interfacial film strength prevented the emulsion droplets from coalescence. Finally, the small emulsion droplet performed lower floating and coalescence velocity so as to increase emulsion stability.

For investigating the types and formation mechanisms of residual oil after surfactant imbibition, the easy-emulsifying surfactant Sodium Dodecylbenzene Sulfonate (SDBS) and ultra-low interfacial tension surfactant Sodium Oleate (SOL) were selected. The visual micromodel was adopted to observe the oil morphologies, while the rheometer and the emulsion stability analyzer were applied to analyze formation mechanisms. According to micro experiments, the main types of residual oil after SDBS imbibition are columnar, cluster, blind-end and small drop residual oil whose structure was small fragmented droplet. As for SOL solution, they are mainly blind-end and island residual oil. The formation mechanism of columnar and cluster residual oil is capillary force offsetting, the direction of capillary force forms the blind-end residual oil, while the Marangoni effect and snap-off effect are responsible for island residual oil. The small drop residual oil is from emulsification, which always appears at the intersection of multiple flows. The aromatic groups of SDBS can promote the emulsion more stable by increasing the strength of interfacial film, hence the small drop residual oil only exists after SDBS imbibition.

The blockage produced during polymer flooding can lead to a significant decline in reservoir productivity. The composition of the oilfield scale is very complicated, and formation mechanism and location distribution of blockage are unclear. In this work, the composition of the oilfield scale produced during polymer flooding was analyzed by thermal gravimetric analysis (TGA), infrared spectrum analysis (FI-IR), X-ray diffraction analysis (XRD), and scanning electron microscopy observation (SEM). The formation mechanism of the blockage was studied by means of water compatibility experiment, core flooding experiment, swelling experiment, and polymer retention experiment. The location distribution of blockage was explored by physical simulation displacement experiment using the long sand pack tube. The results show that the oilfield scale is composed of polymer, inorganic substances, water, and crude oil. Various components are interdependent and entangled in the scale. After the polymer solution is injected into the formation, the “fish eye” scale and inorganic salt crystal/polymer scale are produced in the near-injection well zone. Through scale migration and fine particle exfoliation, there occurs the complex composite scale combined by migrated scale and fine particles as well as the newly-generated fine particle/polymer flocculent scale. When the scale encounters with crude oil, the asphaltene from crude oil can be adsorbed and precipitated on the scale surface, leading to the oil-wrapped composite scale. Besides, the injected polymer can be adsorbed and captured in the oil layer, causing the decrease of the layer permeability. With decrease of oil layer permeability, various scale is deposited and bridge-blocked in the oil layer, causing the blockage during the polymer flooding process. After three displacement stages of water flooding, polymer flooding, and subsequent water flooding, blockage is produced in all zones of oil layer, with the most severe blockage in the middle zone near the injection well. The study can provide a theoretical basis for the subsequent research on the blockage-removing fluid systems suitable for polymer flooding reservoirs.

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.

Amphiphilic polymers have been widely studied and applied in oil fields as effective enhanced oil recovery (EOR) agents. However, the viscosity of their aqueous solution is low at low concentration. In order to improve their poor viscosity-thickening ability at low concentrations, a combined system was used by mixing a zwitterionic surfactant (erucyl dimethyl amidopropyl betaine, EDAB) and an amphiphilic polymer (APC16) at the mass ratio of 2:1. The rheological properties and aggregate microstructure of the EDAB–APC16 combination system were investigated by rheometry, scanning electron microscopy, transmission electron microscopy, and fluorescence spectroscopy, and the EOR efficiency was measured using core flood tests. The results showed that EDAB can significantly increase the viscosity of the APC16 solution, even though the polymer concentration is lower than its critical aggregation concentration. In the EDAB–APC16 combination system, the wormlike micelles of EDAB can synergistically interact with APC16 through hydrophobic association and electrostatic attraction to achieve a thickening effect. Furthermore, the effects of temperature, pH, and the inorganic salts on the viscoelasticity of the EDAB–APC16 system were investigated. An optimized EDAB–APC16 system at 1500 mg/l [pH = 7.5 and c (NaCl) = 5000 mg/l] was selected to be the oil displacing agent, which achieved an EOR of 18.4% after the initial water flood. The polymer–surfactant composite system under development exhibited great potential as an effective chemical EOR agent.

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 subject of the imbibition enhancing oil recovery (IEOR) mechanism by the use of ultra-low interfacial tension (IFT) and emulsifying surfactants remains controversial. In this study, an ultra-low IFT surfactant (SOL) and an easy-emulsifying surfactant (SDBS) were selected on the basis of their interfacial tension and emulsion stability to elucidate the IEOR mechanism. The cell test method was used to measure the imbibition oil recovery. The micro-IEOR mechanisms were analyzed by using a new device making use of a visual micromodel. The results show that the imbibition oil recovery was 8% with SDBS and 17% with SOL, with the two surfactants having a similar variation of imbibition speed, while the ultra-low IFT made the water front more uniform. The emulsion with low coalescence stability formed by SDBS improves the displacement efficiency and diverts flow, improving the sweep efficiency as well. This work proposes a new method for analyzing the micro-IEOR mechanism and provides new fundamental information to the selection of an efficient IEOR surfactant.

An amphiphilic polymer-based supramolecular system was formulated by exploring the electrostatic interactions between the cationic amphiphilic polymer and polyacid. The effect of aging time on the viscoelastic properties, salt and temperature tolerance, shear resistance, and viscosity recovery of the system was explored by means of rheological measurements. The intrinsic relationship between the viscoelasticity of the system at different aging times and their dynamic growth was studied by macro-rheological and optical micro-rheological methods. The results show that under the influence of various factors, the viscoelastic properties of the supramolecular system increase significantly with aging. Optical micro-rheological studies demonstrated that the viscoelasticity of the system continues to increase over a period of 30 days. After aging, the viscoelastic results indicate the formation of a strong gel. This study clearly demonstrates that the aging time is a key factor that should be considered in the formulation of supramolecular systems formed by the electrostatic interaction between the cationic amphiphilic polymer and polyacid.

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.