Effects of calcium sulfate in oil well cement on the property of AMPS based fluid loss additive

Calcium sulfate, generally in the form of gypsum or bassanite, is a significant component in oil well cement. The impact of gypsum/bassanite on the fluid loss of cement slurry was thoroughly examined with varied concentrations of a 2-Acrylamido-2-methylpropane sulfonic acid based fluid loss additive (AMPS-FLA). It was found that bassanite in cement was more conducive to decreasing the fluid loss than gypsum. The results showed that the dissolution rate of bassanite in cement slurry was faster than that of gypsum, generating more Ca2+ and resulting in a reduction in the adsorption capacity of the AMPS-FLA on cement particle surfaces. The adsorption mechanism of FLAs cannot explain the result. This study discovered that the equilibrium concentration of the AMPS-FLA in the pore solution had a good functional relationship with the fluid loss, which was not impacted by cement components. Increasing the equilibrium concentration helped reduce the fluid loss.