Abstract：

CO2 foam flooding is an important method at the stage of tertiary oil recovery. It is characterized by dissolving CO2 into formation water as a dispersed phase, which can effectively control the mobility of CO2, thus increasing the sweep efficiency of displacing phase. Meanwhile, CO2 can be stored in the geological formations via the method, mitigating the “Global Warming Effect”. Surfactants can reduce the interfacial tension (IFT) between the CO2 bubble and foam liquid film (i.e., water film) hence increasing the resistance to the Laplace capillary suction. Adding polymer can increase the viscosity of the foam liquid film hence retarding the liquid drainage and reducing the bubble coalescence. The two kinds of chemicals are always simultaneously used to stabilize CO2 foam in the oilfields. However, surfactants would have different interfacial behaviors due to the different properties of headgroups, and the microscopic mechanisms of interactions between surfactant and polymer molecules at the CO2-water interface are poorly understood. Here, classical molecular dynamics simulations were employed to investigate the influences of charge property (i.e. positively/negatively charged) on the interactions between surfactant and polymer molecules. Sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CTAB) were selected to represent anionic and cationic surfactant molecules, respectively. Hydrolyzed polyacrylamide (HPAM) with a hydrolysis degree of 25% was chosen to represent polymer molecules. The features of the spatial distribution of the simulated components and molecular arrangements at the CO2-water interface were analyzed, and the relationships between macroscopic properties (such as IFT and viscosity of the foam liquid film) and detailed molecular structure were discussed. The results showed that CTAB is more powerful than SDS in terms of reducing IFT at the CO2-water interface, the variation of IFT values is in good accordance with the changes in interfacial width and interfacial coverage. For the simulated system consisting of CTAB and HPAM, the interactions are mainly controlled by Coulomb force (attractive force). HPAM molecules are apt to be adsorbed at the interface, and are subjected to competitive adsorption with the Br– counterions. The morphology can facilitate reducing the IFT values and increasing the water film thickness. For the simulated system consisting of SDS and HPAM, the Na+ counterions form cationic bridges between SDS and HPAM molecules at the interface. HPAM molecules distribute at the interface as well as in the bulk water phase. The morphology is not beneficial to the reduction of IFT values. The two patterns of interactions between ionic surfactant and polymer are comparable in terms of increasing the viscosity of the foam liquid film. Based on the simulation results, the synergistic effect of hydrolyzed polyacrylamide and ionic surfactant for improving CO2 foam stability is revealed.

Key words：CO2 foam stability; foam liquid film; surfactant; polymer; interfacial tension; viscosity; molecular dynamics simulations

Received： 2022-06-01

Corresponding Authors： jiajh@cup.edu.cn

Cite this article：贾冀辉, 蔡杭, 梁云峰, 辻健, 林梅钦, 彭勃. HPAM聚合物与离子型表面活性剂协同稳定CO2 泡沫的分子模拟研究. 石油科学通报, 2023, 01: 69-86 JIA Jihui, CAI Hang, LIANG Yunfeng, TSUJI Takeshi, LIN Meiqin, PENG Bo. Synergistic effect of hydrolyzed polyacrylamide and ionic surfactant to enhance the stability of CO2 foam: A molecular dynamics study. Petroleum Science Bulletin, 2023, 01: 69-86.

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