Low-molecular-weight composite contamination-resistant additive for well cementing

A composite additive for well cementing, comprising:

• 60-85 parts by mass of phosphonic acid polymer solution,
• 15-40 parts by mass of silicate solution;
• wherein a preparation method of the composite additive for well cementing comprises:
• stirring the phosphonic acid polymer solution and the silicate solution at a temperature of not higher than 40° C. at a speed of 200-400 r/min;
• a preparation method of phosphonic acid polymer in the phosphonic acid polymer solution comprises the following steps:
• Step S1:
• mixing and dissolving a phosphonic acid group-containing monomer, a rigid group-containing monomer and a carboxylic acid group-containing monomer in a molar ratio of 4-7:2-4:1-3 in deionized water to obtain a monomer solution with a mass fraction of 20%-50%;
• Step S2:
• adding a chain transfer agent accounting for 3%-20% of a total mass of the monomers into the monomer solution, stirring and dissolving at a speed of 200-300 r/min, and heating to 45° C. under the protection of inert gas in the stirring process;
• Step S3:
• dripping a solution of an initiator in deionized water in a concentration of 10%-20% into the monomer solution with stirring, and reacting for 3-8 h at 50-70° C. after dripping within 30-50 min, wherein the initiator in the solution of the initiator in deionized water accounts for 0.05%-1.5% of a total mass of the monomers;
• Step S4:
• distilling a reactant under reduced pressure at 30-80° C., cooling the reactant to room temperature, freezing, drying, and grinding into powder;
• the phosphonic acid group-containing monomer is at least one of vinylphosphonic acid, dimethyl-vinyl phosphonate, diethyl-vinyl phosphonate, and 2-acrylamide-2-methylpropane phosphonic acid;
• the rigid group-containing monomer is N-vinyl pyrrolidone or 4-acryloylmorpholine;
• and
• the carboxylic acid group-containing monomer is one of acrylic acid, maleic acid, methacrylic acid, and itaconic acid.