Physical properties and chemical structure evolution mechanism of AMPS-based copolymer oil well cement retarder in ultra-high temperature alkaline solution environment

2-Acrylamido-2-methylpropanesulfonic acid (AMPS) based copolymer retarder is one of the most commonly used oil well cement additives in cementing engineering. However, its performance decline caused by the thermal degradation behavior in ultra-high temperature (UHT) environment is particularly prominent, and the underlying mechanism remains unclear. In view of this issue, the physical properties and chemical structure evolution mechanism of two kinds of representative oil well cement copolymer retarder, namely the anionic binary copolymer PAI [AMPS-co-itaconic acid (IA)] and the heterocyclic zwitterionic quaternary copolymer PAINM [AMPS-co-IA-co-N-vinylpyrrolidone (NVP)-co-methacrylamido propyl trimethylammonium chloride (MAPTAC)], were comparatively investigated in UHT alkaline solution environment, aiming to provide theoretical guidance for the optimization design strategy of improving the UHT efficiency of retarder in future research. The results indicated that copolymer retarders had significant thermal degradation behavior in UHT alkaline solution environment (T > 200 degrees C, pH approximate to 13), which was manifested in the decrease of apparent viscosity and viscosity- average molecular weight, the change of pH value, the variation of side groups and the pyrolysis of main chain (producing unsaturated alkenes). Both the hydrolysis of amide bond and the decarboxylation reaction in the copolymer side chains would lead to the configuration transformation of carboxyl from di-carboxyl to mono- carboxyl, and the thermal degradation behavior made the main chain configuration of the aged product severely deviate from the initial structure. The above two factors were the main reasons for the retarding efficiency reduction of copolymers at UHT. Furthermore, the introduction of rigid cyclic and cationic side groups could slightly delay the main chain rupture and the decarboxylation process, but was unable to reverse the thermal degradation progress of the copolymer retarder.