On deep and ultra-deep wells, high temperature stable additives are applied to control the properties of cement slurries. For fluid loss control, polymers based on 2-acrylamido-tertiary-butyl sulfonic acid (ATBS) are commonly used. However, at elevated temperatures the performance of such polymers can decrease. The mechanism behind this change is mostly attributed to temperature-induced decomposition of the polymer, e.g. via oxidation.

In our study, we investigated the effect of high temperature on four synthetic fluid loss additives (FLAs) containing ATBS monomer. All FLAs were dissolved in cement pore solution and exposed to 100 – 220 °C for 8 hours. The heat-treated polymer solutions were then added to cement slurries, and the respective fluid loss of these slurries was determined.

At increased ageing temperatures, a decrease in fluid loss performance was observed for the ATBS-NNDMA copolymer while two others, a humic acid-ATBS-NNDMA-AA graft copolymer and an ATBS-NNDMA-AHPS-AA forpolymer showed consistent performance over the entire temperature range of 100 – 220 °C. The performance of the fourth polymer, a lignite based ATBS-NNDMA graft copolymer, was most surprising: it decreased gradually until 200 °C, but improved very strongly after ageing at 220 °C.

To understand these different behaviors, all polymer samples were characterized regarding their molecular weight, anionic charge, radius of gyration, hydrodynamic radius and adsorbed conformation on cationic polystyrene nanoparticles as a model system for cement particles. For the ATBS-NNDMA copolymer significant coiling occurred, thus reducing its ability to plug the pores of the cement filter cake. Opposite to this, the ATBS-NNDMA-AHPS-AA forpolymer and the humic acid graft copolymer resisted to such coiling. Interestingly, the lignite based graft copolymer undergoes gradual temperature induced coiling up to 200 °C, but expands significantly after ageing at 220 °C. This effect greatly improves its fluid loss performance.

The results can help to better understand the high temperature behavior of FLAs and other polymeric additives in cement, drilling or completion fluids.

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