Action Mechanism Of Copolymer Additives On The Hydration Process Of Cement

Copolymer additives,one of important oil well cement additives,used to improve the combination property of cement slurries to adapt the complex downhole environment of oil and gas wells.The interaction and the chemical mechanism between the anionic fluid loss additive and the cement particles were analyzed through the combination of experimental methods and molecular dynamics simulation.The anionic fluid loss additive(FLA)was synthesized by free radical polymerization using 2-acrylamide-2-methylpropanesulfonic acid(AMPS),N,N-dimethylacrylamide(NNDMA)and itaconic acid(IA).It was found that FLA significantly prolonged the induction period of cement hydration through the hydration kinetics and XRD characterization of the FLA cement slurry system.The adsorption kinetics and isothermal adsorption of FLA on the surface of cement particles were studied under the condition of cement to water ratio of 1/10.The all-atomic models of FLA sodium salt at different Ca²⁺concentrations were established to calculate the molecular dynamics by Materials Studio simulation software.The adsorption mechanism of FLA on the surface of cement particles was clarified.Ion exchange occurred between Na⁺which was combined with FLA in solution and Ca²⁺which was on the cement surface.FLA adsorbed on the surface of cement particles through the interaction of anionic functional groups and Ca²⁺.The content of carboxyl groups(-COOH)in the anionic fluid loss additives would affect the fluid loss of the cement slurry and the strength development of the cement stone.Two kinds of fluid loss additives with different amount of-COOH,FLA-I and FLA-II,were synthesized by using acrylic acid(AA)and IA as functional monomers,with AMPS and NNDMA.The fluid loss amount of the cement slurries with FLA-I and FLA-II were measured by static fluid loss test.The filter cake and filtrate,which were obtained from the static fluid loss test,were characterized to analyze the mineral composition and pore structure in the filter cake,and the polymer and ion content in the filtrate.Based on the dynamic simulation of the all-atom model,it is clarified that the key factor of fluid loss is the synergy of chelation,hydrogen bonding and electrostatic interaction between fluid loss additives,cement particles and water.The hydration kinetics of cement and the mineral composition and structure of cement stone were measured.The results showed that the low initial strength of cement stone is due to the strong retardation of FLA-II at low temperature.When the cement slurry contains FLA-I,the strength of the cement stone is low at high temperature because it affected the structure of the cement hydration product.The reasons for the abnormal gelation of cement slurry caused by polycarboxylate additives was analyzed by characterizing the composition of gelation and non-gelation part of cement slurry.The strong chelation between carboxyl group and Ca²⁺caused the cross-linking and entanglement between polymers,mineral and cement hydration products,leading to the uneven condensation of the cement slurry.The cationic monomer or long side chain group was added in the molecular structure of anionic fluid loss additives(FLA1)to obtain FLA2 and FLA3.The application performance of the modified two fluid loss additives was compared with FLA1.The results show that FLA3 with long side chain structure can relieve the abnormal gelation of cement slurry.It was found that the fluid loss additives with long side chain structure can effectively weaken the chelation between carboxyl groups and Ca²⁺due to the steric hindrance effect,through the molecular dynamics simulation and the characterization of cement pore solution.The stability of cement slurry was improved.