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A polymer comprising of 2-acrylamido-2-methyl propane sulfonic acid, N, N-dimethyl acrylamide, allyloxy-2-hydroxy propane sulfonic acid (AHPS), acrylic acid, and N, N-methylene bisacrylamide was synthesized by aqueous free radical copolymerization and tested as high temperature performing fluid loss additive (FLA) in oil well cement. Successful incorporation of AHPS was confirmed and characteristic properties of the copolymer were determined using size exclusion chromatography. The FLA showed excellent water retention in cement at 200 C/70 bar. At this temperature, polymer structure changed from branched to linear and hydrodynamic size decreased by $50%, thus indicating potential fragmentation, while performance remained unaffected by these alterations. The FLA copolymer does not viscosify cement slurries which is advantageous in high temperature well cementing. The working mechanism of the AHPS-based copolymer was found to rely on reduction of filtercake permeability which is caused by a voluminous coprecipitate of the FLA with tartaric acid retarder, mediated by Ca 2þ ions.

A copolymer comprising of 2-acrylamido-2methyl propane sulfonic acid (AMPS V R ) and itaconic acid (molar ratio 1 : 0.32) was synthesized by aqueous free radical polymerization and probed as high temperature retarder for oil well cement. Characteristic properties of the copolymer including molar masses (M w and M n ), polydispersity index and anionic charge amount were determined. The copolymer possesses a M w of $ 2 Â 10 5 g/mol and is highly anionic. HT/HP consistometer tests confirmed effectiveness of the retarder at temperatures up to 200 C. The working mechanism of NaAMPS V R -co-itaconic acid was found to rely exclusively on its huge calcium binding capacity (5 g calcium/g copolymer). It reduces the amount of freely dissolved, nonbound calcium ions present in cement pore solution and thus hinders the growth of cement hydrates because of lack of calcium. The value for the calcium binding capability is 46 times higher than the stoichiometric amount per ACOO À functionality. Con-sequently, calcium also coordinates to other donor atoms present in the retarder. NaAMPS V R -co-itaconic acid also adsorbs onto cement, as was evidenced by TOC analysis of cement filtrates, zeta potential measurement and decreased rheology of cement pastes. However, adsorption plays no role in the retarding mechanism of this copolymer. Combination of NaAMPS V R -co-itaconic acid retarder with a common CaAMPS V R -co-NNDMA fluid loss additive (FLA) revealed that competitive adsorption on cement between these two admixtures occurs. The retarder fills interstitial adsorption sites on cement located between those occupied by the larger FLA molecules. In consequence, fewer amounts of CaAMPS V R -co-NNDMA can adsorb and its effectiveness is reduced.

Oilwell-cement slurries commonly incorporate several admixtures such as retarder, dispersant, fluid-loss additive (FLA), antifreewater agent, and defoamer. Between them, additive/additive interactions may occur that can result in incompatibilities and reduced performances (the most frequent case) or, oppositely, in improved effectiveness. Here, an overview of some synergistic and antagonistic effects between selected cement additives is presented. Four combinations of additives were tested and studied.First, the interaction between 2-Acrylamido-tertiary-butyl sulfonic acid-co-N,N-dimethylacrylamide (CaATBS-co-NNDMA) FLA and an NaATBS-co-itaconic acid retarder as well as welan gum, an anionic biopolymer applied as an antifree-water additive, was investigated. It was found that the retarder, which possesses a particularly high-anionic charge, reduces the effectiveness of the CaATBS-co-NNDMA FLA by decreasing its amount adsorbed on cement. Similarly, the anionic biopolymer can also negatively affect the effectiveness of the FLA through competitive adsorption, in which the biopolymer hinders the sufficient adsorption of the FLA on cement. The incorporation of stronger anchor groups (e.g., dicarboxylates or phosphonates) into the CaATBS-co-NNDMA FLA enhances its affinity for the surface of cement and thus renders it more robust against the negative impact from other admixtures.Second, the compatibility between an Na þ lignosulfonate (Na-LS) retarder and the CaATBS-co-NNDMA FLA was investigated. Here, surprisingly, a dual synergistic effect was found. Na-LS improves the fluid-loss performance of CaATBS-co-NNDMA, whereas the latter greatly enhances the retarding effectiveness of lignosulfonate. The experiments demonstrate exceptionally high compatibility of both admixtures. The positive effect is based on coprecipitation of both polymers, which enhances FLA adsorption on cement. At the same time, because of the thick adsorbed polymer layer, the dissolution of the clinker phases is hindered, resulting in the retardation of cement hydration.Finally, it was found that hydroxyethyl cellulose (HEC) and sulfonated formaldehyde polycondensate-based dispersants -such as poly melamine sulfonate (PMS) or acetone formaldehyde sulfite (AFS) -act synergistically; thus, the fluid-loss control provided by HEC is considerably improved. Dynamic light-scattering measurements revealed that, in the presence of those dispersants, the association of HEC molecules into large hydrocolloidal assemblies was greatly enhanced. Obviously, the increased ionic strength resulting from the polycondensate dispersants renders the nonionic HEC molecules less water-soluble and initiates their aggregation at an earlier stage. The larger hydrocolloidal polymer associates can plug filter-cake pores more effectively, thus reducing cement fluid loss.The study suggests that multiple additive/additive interactions can occur in oilwell cement. Understanding the underlying mechanisms can help both to avoid unwanted incompatibilities and to develop mitigation strate...

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