The Effect of Gypsum/Bassanite on the Retardation of Ethylene Diamine Tetra(Methylene Phosphonic Acid) Sodium in Oil Well Cement Slurry

Organophosphonates are commonly used retarders to prolong the thickening time of oilwell cement slurry at medium and high temperatures. In this paper, the impact of calcium sulfate in cement on the retarding effect of ethylene diamine tetra(methylene phosphonic acid) sodium (EDTMPS) was explored. First, the thickening properties of cements from four different factories were studied in detail with varying additions of EDTMPS. The study revealed diverse thickening phenomena, including retarding, accelerating, and increasing the initial consistency of cement slurries. The heat flow of cement hydration was detected, and the mineral changes of cement slurries at the early stage (1–3 hours) were analyzed. Additionally, the effect of EDTMPS on the hydration of tricalcium aluminate (C3A) and gypsum (GP)/bassanite (BS) slurry was investigated through X-ray diffraction (XRD) and ion concentration test. Finally, two clinkers from the same cement factory were mixed with GP/BS of different dosages to study the effect of calcium sulfate type on the thickening properties of cement slurry with EDTMPS. The results revealed that EDTMPS slowed down the dissolution of GP while promoting the dissolution of C3A. The rapid hydration of C3A increased the consistency of cement slurry without the retarding effect of GP. However, EDTMPS promoted the dissolution of BS, which can retard the hydration of C3A. Therefore, EDTMPS is appropriate for cements containing BS.