Effects of functional groups in polymers on rate of acid–rock reaction in reservoir stimulation

Abstract

The focus of research on acid stimulation is to reduce the rate of the acid–rock reaction, which involves the infiltration of acid into deep formations. Gelled acids with high viscosity can decelerate the diffusion of hydrogen ions to decrease the rate of the acid–rock reaction. However, there have been few reports on the interactions between functional groups in polymers and rocks. In this study, three polymers (AA7, AA24, and AAS) with different functional groups were synthesized, and their effects on the acid–rock reaction were investigated. The results showed that the order of the abilities of the three polymers to delay the acid–rock reaction was as follows: AA24 > AA7 > AAS. The retardation rate for AA24, AA7, and AAS was 79.31%, 66.67%, and 40.23%, respectively. Adsorption experiments illustrated that AA7 and AA24 were adsorbed on the surface of calcite via hydrogen bonds formed by ethoxy groups, whereas AAS relied on the hydrophobic effect of the benzene ring. The anionic polymer AAS was subjected to a strong electrostatic effect in an acid solution, which caused the molecular chains to curl and impaired the retarding performance. This was proved by the fact that the viscosity dramatically decreased from 66.00 mPa·s in an aqueous solution to 7.50 mPa·s in an acid solution and the effective diameter decreased from 322.2 nm in an aqueous solution to 102.2 nm in an acid solution. In the cases of the nonionic polymers AA7 and AA24, there was little change in viscosity or effective diameter, whether in an acid solution or an aqueous solution. Scanning electron microscopy confirmed that the film formed by the adsorption of AA24 on the surface of calcite was denser than that formed by AA7 because more ethoxy groups were present in AA24, but the film exhibited some narrow cracks in the case of AAS. Atomic force microscopy demonstrated that the surface roughness of calcite etched by a retarding acid solution of AA24 was only 78 nm, whereas it was as high as 130 nm in the case of AAS, which was consistent with the retarding performance with regard to the acid–rock reaction. This research could contribute to improvements in carbonate acidizing for reservoir stimulation.