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Core-shell micro-spheres (MS) with both negative and positive charges in the core and only negative charges in the shell were developed as adaptive plugging agents for in-depth conformance control for enhanced oil recovery. The MS were designed to propagate deeply into the reservoir due to the small particle size and electrostatic repulsion between the MS and the sandstone at the initial stage of injection and form aggregates by electrostatic attraction between the cores with mixed charges when the shells degraded at a given time during transportation, leading to an effective plugging of the highly permeable layers with low residual oil saturation. The self-assembling and plugging behaviors of the MS have been studied by Monte Carlo simulation. The results show that charge density (Dcharge), fraction of positive charge (Fp), MS concentration, temperature, and salinity are the key factors influencing the self-assembling behaviors. The electrostatic interaction would become stronger with the increase in Dcharge when it is larger than 0.5. The MS are more likely to form aggregates when Fp approaches 0.5. The higher the concentration of the MS, the stronger the electrostatic interaction between the MS. In addition, electrostatic interactions between the MS become stronger with the increase in temperature and decrease in salinity. Simulation results prove that the MS with mixed charges can effectively and adaptively plug highly permeable layers with low residual oil saturation through self-assembling by combination of electrostatic interactions along with physical bridging, leading to the improvement of oil recovery. Furthermore, block charge distribution will be helpful for the MS with mixed charges to form larger aggregates than that of the random mode to effectively plug the highly permeable layers.

Summary The first pilot test of a surfactant/polymer (SP) flood in the world with mixtures of anionic/cationic surfactants (Sa/c) was performed in a high-temperature and low-permeability sandstone reservoir with a high content of clay to demonstrate the potential of this novel technique to improve oil recovery. Low critical micelle concentration of 2.78×10−6 mol/L, ultralow interfacial tension (IFT) of 10−3 to 10−4 mN/m when surfactant concentrations were greater than 0.025 wt%, and lower phase microemulsion with solubilization ratio of 22, as well as oil-washing rate of 61%, were obtained by using Sa/c. Carboxymethylcellulose sodium was adopted as an adsorption inhibitor (AI) to reduce the adsorption because of the high clay contained in the natural core. With the addition of AI, dynamic adsorption of Sa/c was approximately 0.30 mg/g sand, and the IFT remained nearly unchanged before and after adsorption. The viscosities of polyacrylamide with molecular weight (MW) of 6.16×106 daltons and SP were 3.0 and 4.1 mPa·s, respectively. Coreflooding results showed that more than 16% of original-oil-in-place (OOIP) crude oil was recovered by SP over waterflood. The pilot test of SP containing Sa/c was performed in a Sinopec reservoir with temperature of 83°C, salinity of 1.69%, permeability of 41.5 md, and clay content of 10 to 16%. A total of 0.40-pore volume (PV) chemicals, including 0.1-PV polymer preslug, 0.25-PV SP main slug, and 0.05-PV post-polymer drive, were injected from August 2012 to December 2017 with an injection rate of 0.08 PV/yr. Then, waterdrive was conducted. Maximal water cut decreased from 82.2 to 62.1%, and the peak daily oil production increased from 12.2 to 32.3 t. The oil recovery was increased by 8.0% of OOIP within 4 years.

The surfactant structure–performance relationship and application properties in enhanced oil recovery (EOR) for binary mixtures of anionic and cationic surfactants are presented and discussed. A polyoxyethylene ether carboxylate anionic surfactant was blended with a quaternary ammonium chloride cationic surfactant and tested for a high-temperature, low-salinity, and high-hardness condition as found in an oil reservoir. These mixtures were tailored by phase behavior tests to form optimal microemulsions with normal octane (n-C8) and crude oil having an API gravity of 48.05°. The ethoxy number of the polyoxyethylene carboxylate anionic surfactant and the chain length of the cationic surfactant were tuned to find an optimal surfactant blend. Interfacial tensions with n-C8 and with crude oil were measured. Synergism between anionic and cationic surfactants was indicated by surface tension measurement, CMC determination, calculation of surface excess concentrations and area per molecule of individual surfactants and their mixtures. Molecular interactions of anionic and cationic surfactants in mixed monolayers and aggregates were calculated by using regular solution theory to find molecular interaction parameters β σ and β M . Morphologies of surfactant solutions were studied by cryogenic TEM. The use of binary mixtures of anionic/cationic surfactants significantly broadens the scope of application for conventional chemical EOR methods.

Heat- and salt-resistant amphiprotic polyacrylamide microsphere (APMS) was synthesized using inverse microemulsion polymerization method. The microstructure, swellability, filtration, and core flooding performances were tested and characterized. FT-IR spectroscopy indicates that APMS was successfully prepared. Morphological analysis reveals that it is relatively uniform and spherical with an average diameter about 50 nm. Its average diameter increases up to 634 nm after aging at 90°C for 7 days in synthetic brine. After aging for 30 days, the particle size is decreased to 482 nm. But the swelling magnification is still 9.6 times its initial diameter. Compared with the traditional microspheres, APMS exhibits excellent thermal stability, salt resistance, and swelling properties. In addition, the plugging factor is in the range of 0.236 to 0.317, and the number of particles for bridge-plugging is slightly increased from 2 to 4 over aging time. Moreover, the plugging parameters show that APMS can favorably enter and effectively plug the artificial cores with permeability of 0.19 to 0.50 µm2 by deformation, breakthrough and migration, bridging and plugging mechanisms. Furthermore, APMS also shows well its resistance to water flushing and good plugging strength in continuous injection experiment. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 43366.