Nanofluid flooding has become one of the advantageous enhanced oil recovery (EOR) techniques due to the transcendental property of nanoparticle. Nanoparticles with various morphology presents different effects and mechanisms of EOR. To provide some guidance on the selection of nanomaterials for flooding, 4 kinds of nanomaterials, including silicon dioxide (SiO2), magnesium oxide (MgO), graphene oxide (GO) and molybdenum disulfide (MoS2), are chosen to serve as research objects in the study. In lab, the morphology of four nanomaterials were systematically characterized by scanning electron microscope (SEM) tests. Moreover, nanofluid flooding experiments were further conducted to explore the relationship between the morphology of nanomaterials and EOR. Results show that the SiO2 was spherical structure, and the MgO was blocky structure, while the GO and MoS2 were both sheet structure. All the four nanomaterials have similar spatial dimensions. Flooding experiments revealed that MoS2 enable improves the oil recovery by approximately 11.53 %, and GO was followed with 10 %, twice as much as both SiO2 and MgO. From the results, it can be inferred that spherical materials only realize “point-to-surface” contact at multiphase interfaces, while sheet materials can achieve “surface-to-surface” contact with a higher interfacial activity. This paper is the first to focus morphology of nanomaterials on flooding, which contributes to the optimization of nanomaterials for high-efficiency oil displacement.

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The polyacrylamide weak gel is an effective system to block a high-permeability layer, realize water control, and enhance oil recovery. However, its application is limited by poor temperature resistance and high polymer dosage. In this paper, an inorganic–organic composite cross-linking agent was synthesized by using Cr(III) and phenolic resin. The composite cross-linking agent can cross-link low concentrations of polyacrylamide to obtain a high-temperature-resistant weak gel system in oilfield sewage. By adjusting the ratio of Cr(III), phenolic resin, and polyacrylamide, an optimum formula MF-7 can be obtained according to the gel strength. Results from evaluation experiments show that the strength of MF-7 can reach H grade even at polyacrylamide concentrations as low as 0.3%. The temperature resistance of the weak gel system is up to 100 °C, and no syneresis occurs after 330 h at 95 °C. Scanning electron microscopy (SEM) results show that MF-7 has a three-dimensional network structure with spherical nodes. The spherical node is composed of polyacrylamide, and its structure size is completely matched with the hydrodynamic radius of the used polyacrylamide. When combined with the network structure formed by Cr(III), the dense cross-linking network structure with nodes can greatly improve the strength and thermal stability of the gel system. The higher the molecular weight of the polyacrylamide used, the higher the strength of the gel obtained. Overall, the composite cross-linking agent can synergistically improve the mechanical properties of the gel, and this weak gel system formed by oilfield sewage is more economical and tolerant.

Nanomaterials can be used to emulsify and reduce the fluid viscosity, reduce pore pressure, and increase injection volume. Therefore, nanomaterials have a great potential in the enhanced oil recovery. However, the current research on nanooil flooding materials mostly focuses on the evaluation of the oil-displacing effect, and the lack of research on the size of the oil-displacing materials for tight oil is obvious. In this work, 1T phase molybdenum disulfide nanosheets were prepared by one-step hydrothermal method, which were further modified with CTAB powders to obtain MoS2-CTAB nanosheet powders. Combined with SEM, TEM, and AFM methods, the nanosheets were optimized based on the appearance and morphology. The stability, wetting reversal, and oil displacement capacity of selected nanosheets were tested. The results show that the best experimental condition to synthesize small-sized molybdenum disulfide is 200°C in the weak acid environment through 12 h. Due to the steric hindrance effect of the CATB molecule, the size and interlayer gap of MoS2 nanosheets increased slightly after modification. The layer gap reaches to 0.7 nm, and the number of stacked layers is 3~4 layers. Strong Raman bands are observed at 137 cm-1, 291 cm-1, and 391 cm-1, which indicates that the synthesized product is 1T MoS2. The modified MoS2 nanosheets in aqueous solution have better dispersion than the unmodified one. After the Zeta test, it was found that the absolute value after modification became lower, indicating that the modification of nano-MoS2 was effective. Moreover, the MoS2-CTAB can complete the wetting reversal within 4 h and make the interfacial tension reach 0.89 mN/m at 0.005 wt%, which greatly reduces the capillary pressure. The enhanced oil recovery effect of MoS2-CTAB nanosheets increased by 85.7% compared with that before modification and 62.5% higher compared with pure surfactant.