Influence of carboxyl group on filtration property of oil well cement paste at different temperatures investigated using molecular dynamics simulation

Abstract

Abstract As a porous material, the microstructure of cement-based material is complex and variable. The pores and cracks are randomly distributed in the cement paste and interconnected to form a fluid channel. To ensure the safety of cementing engineering, filtrate reducers (FR) usually are added to decrease the filtration loss. It was found that two filtrate reducers with different amounts of –COOH caused different filtration loss of oil well cement paste. The API filtration loss of cement paste containing 3% bwoc FR-I with less –COOH increased rapidly from 42.0 mL to 98.0 mL with the curing temperature rising from 30 °C to 90 °C. However, the increment of filtration loss of FR-II with more –COOH is less than 5 mL (from 56.0 mL to 58.0 mL) under the same experimental condition. The microstructure and component of filter cakes which were obtained from static filtration loss experiments were measured to compare the difference between two FRs. The experimental data show that the porosity and chemical composition of the cement phase directly affects the amount of filtration loss of cement paste, but it is not the main reason for the early filtration loss. Furthermore, the components of filtrates were measured and all-atom molecular-scale model was built to perform molecular dynamics simulation. The results show that, compared with FR-I, the advantage of FR-II is that the strong chelating of the “adjacent” carboxyl to Ca2+ ion, causing the local rigidity of the molecular segment, which in turn promotes the uniform distribution of the molecular chain throughout the cement paste. Thus, it shows a stronger binding capacity with water and controls the amount of filtration loss, regardless of the temperature.