Research On Surfactant-Enhanced Water Injection For Carbonate Reservoirs

Replacement oil by water injection of single well is an effective technology for the development of carbonate reservoirs, and this method has been widely applied to CRIS oilfield. But the effect is descending with the multi-round injecting of water. Surfactants can change the wettability of rock surface and reduce interfacial tension between oil and water to improve development performance. In this paper, surfactant used to enhance water injection for carbonate reservoirs was studied.Influences of salt content, pH, adsorptions of polar substances and surfactants on the interfacial electric property of carbonate powders surfaces were investigated through measuring Zeta potential. And variations of rock wettability before and after surfactant adsorption at different temperatures were also studied. The results show that carbonate powders are positively charged and Zeta potentials are not influenced by pH and adsorptions of polar substances under the condition of formation water. As salt content reduces, however, Zeta potentials decrease; The charge of powders do not affected by salt content after cationic surfactant 1231 was adsorbed, powders have positive charge after anionic surfactant D161 S was adsorbed with high salt content, while, negative charge can be displayed at low levels of salt. Wetting angle determination experiments indicate that the adsorptions of cationic surfactant on rock surface were poor and rock wettability were still water wet at room temperature, 80℃ and 120℃; Wettability was changed from oil-wet to water-wet after the stronger adsorptions of anionic surfactant at room temperature, however, the adsorption capacity were relatively reduced at 80℃ and 120℃, and wettability became weakly water-wet.Laboratory static evaluation method of surfactants used to enhance water injection effect has been determined, and surfactants D161 S and D233 S, which have better washing oil ability and high washing oil speed were selected, respectively. In addition, the optimal formula was confirmed through evaluating washing oil effects of the compounded system containing D161 S and D233 S. It has been found that the washing oil efficiency was optimum when the ratio of D161 S and D233 S is 3:1, and the higher the mass fraction of the system, the better the washing oil power.The effects of the particle specific surface area, size, wettability reversal and oil-water interfacial tension on washing oil efficiency were carried out through washing oil experiment combined with spinning drop method. Results show that washing oil efficiency were reduced when particle specific surface area is great and the size is small; the washing oil ability of surfactant is not directly linked to its ability of wettability reversal; and surfactant with excellent washing oil ability has better ability of reducing oil-water interfacial tension, but this relationship does not completely one to one. Therefore, it should not be limited to pursue reducing interfacial tension solely when selecting and evaluating surfactant used in the water injection process.