Mechanisms of Cement Slurry Contamination by Drilling Fluid Filtration Agents and Measures of Preventing the Contamination

The understanding of the effects of drilling fluid additives on the microstructure and agglomeration stability of cement slurries is necessary to combat the contamination of low-density cement slurries by drilling fluids. Using single-factor analysis, the effects of LS-2A, a drilling fluid filtration reducer, on the properties of a cement slurry are studied. Experimental results show that the functional groups in the molecules of LS-2A, such as the carboxyl, the hydroxyl and the sulfonic group, can react with the Ca²⁺ ions in the cement slurry through crosslinking to form flocs, resulting in the formation of a gel network structure in the set cement. The gel structure wraps up and absorbs the free water in the cement slurry, reducing the fluidity of the cement slurry, inhibiting the speed of the early stage hydration of the cement slurry, and reducing the compressive strength of the set cement. To deal with the contamination of LS-2A to the cement slurry, a contamination inhibitor is developed by mixing bis (hexamethylene triamine penta (methylene phosphonic acid)) (BHMTPMPA) and zinc oxide (ZnO) in a mass ratio of 3∶1. By adding 5% of the contamination inhibitor into the cement slurry, the fluidity of the cement slurry is increased from 14 cm to 24 cm. At conditions of 205 ℃ × 130 MPa × 110 min, the thickening time of the mixture of the cement slurry and the drilling fluid (7∶3) is only 51 min, unable to satisfy the needs of well cementing. After using the contamination inhibitor, the thickening time of the mixture of the cement slurry, the drilling fluid and a spacer (7∶2∶1) is longer than 300 min. The contamination inhibitor BHMTPMPA and ZnO, by reacting with the Ca²⁺ ions in the cement slurry, generate a protective film on the surfaces of the cement particles and produce an electric repulsion, thereby successfully prohibiting the contamination to the cement slurry by LS-2A. The contamination inhibitor has been successfully used in running cement plug and cementing the well Pengshen-6.