The working mechanism of poly(vinyl alcohol) (PVA, M w $ 200,000 g mol À1 ), a fluid loss control additive (FLA) applied in oil well cementing, was investigated. First, characteristic properties of PVA such as solubility and particle size in cold and hot water, minimum film forming temperature, adsorption on cement, viscosity of cement pore solution and static filtration properties of cement slurries treated with PVA were determined. It was found that the working mechanism of PVA relies on hydrated, but water-insoluble PVA particles (d 50 $ 2.4 lm). During cement slurry filtration, they coalesce into a polymer film. This film effectively plugs the pores of the cement filter cake. The sample studied here becomes water-soluble at temperatures > 40 C (d 50 decreases to $50 nm) and looses its effectiveness. Addition of highly anionic dispersants such as ß-naphthalenesulfonate formaldehyde (BNS) or acetone formaldehyde sulfite (AFS) polycondensate extends the temperature range at which PVA works from 40 C to $60 C. This effect is ascribed to lower solubility of PVA in the presence of these dispersants. The study reveals that decreased performance of PVA caused by higher temperatures is not the result of thermal degradation of the polymer, but is owed to its increasing watersolubility.

Water-soluble 2-acrylamido-2-methylpropane sulfonic acid (AMPS V R )-based copolymers are commonly used to provide water retention (fluid loss control) for oil well cement slurries. Here, the fluid loss performance of a CaAMPS V R -N,N-dimethylacrylamide copolymer (CaAMPS V R -co-NNDMA) in the presence of Welan gum, an anionic microbial biopolymer produced by anaerobic fermentation using Alcaligenes ATCC 31555 bacteria was investigated at 80 C. Welan gum is used to control unwanted free water development at the surface of the cement slurry. The effectiveness of CaAMPS V R -co-NNDMA fluid loss additive (FLA) solely relies on its high adsorption onto the positively charged surfaces of cement hydrates. Adsorption of the FLA is, however, perturbed by Welan gum. This anionic polysaccharide competes with CaAMPS V R -co-NNDMA for adsorption sites on the cement surface. This effect is surprising because in cement pore solution, Welan gum exhibits a much lower specific anionic charge amount than CaAMPS V R -co-NNDMA. The reason is that Welan gum possesses carboxylate functionalities, which are much stronger anchor groups than the sulfonate groups present in CaAMPS V R -co-NNDMA. The superiority of the carboxylate groups regarding their affinity to the mineral surface, which possesses insufficiently coordinated Ca atoms is confirmed by a higher calcium binding capability for Welan gum than for the FLA. Thus, Welan gum can reduce effectiveness of CaAMPS V R -co-NNDMA as fluid loss agent by preventing its adsorption or through displacement of already adsorbed FLA molecules from the surface of cement. In multiadmixture systems, which are commonly used in oil well cement, concrete or mortars, competitive adsorption between different additives for surface sites can negatively impact the performance of these additives. Understanding the reasons behind can help to develop more effective admixture systems.

Oil well cementing uses a variety of chemically different fluid loss polymers, depending on the specific well conditions. In this study, the working mechanisms of polyvinyl alcohol (PVA), polyethylene imine (PEI) and CaATBS-co-NNDMA cement fluid loss additives were investigated. These polymers were selected due to their distinctively different chemical structures. PVA works by reducing filter-cake permeability through coalescence of hydrated PVA microgel particles which then form a polymer film. At temperatures > 38°C, non-crosslinked PVA starts to dissolve in water. Thus, above this temperature, film formation is no longer possible and fluid loss control is not achieved. Addition of AFS dispersant extends the temperature range at which PVA works from 38°C to about 60°C. The reason behind is that the more anionic and higher soluble AFS prevents PVA from dissolving. PEI, when used on its own, provides only poor fluid loss control. Its effectiveness is greatly enhanced by the addition of a minor amount of AFS dispersant. Charge titration experiments demonstrate that, when combined in the proper ratio, PEI and the AFS dispersant form a insoluble polyelectrolyte complex which effectively plugs the pores of the filter cake. To achieve optimum performance, PEI and AFS need to be pre-dissolved in the mixing water of the cement slurry to allow the complex to form quantitatively. In contrast to PVA and PEI, CaATBS-co-NNDMA works by adsorption onto the positively charged surfaces of cement hydrate phases, especially ettringite. Other anionic compounds, e.g. AFS dispersant or even sulfate released from the cement, can interfere with its adsorption and thus reduce its effectiveness through a competitive adsorption mechanism. Therefore, CaATBS-co-NNDMA should not be combined with admixtures which possess high anionic charge density.

Sulfonated aldol polycondensates were synthesized from acetone, formaldehyde, and different amounts of sodium sulfite, resulting in polymers with varying degrees of sulfonation (DS). The anionic charge amount of these macromolecules measured by polyelectrolyte titration decreased with lower DS. The effectiveness of the acetone-formaldehyde-sulfite (AFS) polycondensates as cement dispersant was found to depend on the amount of polymer adsorbed on cement. AFS adsorption decreases with lower DS. Interaction and compatibility between AFS and CaAMPS V V R -co-NNDMA fluid loss additive was studied by formulating binary additive systems composed of one of the modified AFS polymers and CaAMPS-co-NNDMA. At high DS, AFS adsorbs strongly and prevents CaAMPS-co-NNDMA from adsorbing in sufficient amounts on the cement surface. The result is poor fluid loss control of the cement slurry. AFS polymers with lower DS, however, allow simultaneous adsorption of both polymers in sufficient quantities to provide good fluid loss control and low rheology at the same time. Thus, effectiveness of both additives was retained. Obviously, effectiveness of such multiadmixture systems depends on the adjustment of the adsorption behavior of the individual components relative to each other. Molar anionic charge density of the polymers was found to be a major parameter influencing their relative adsorption behavior. The AFS polymer with DS ¼ 0.2 possesses a molar anionic charge density comparable to CaAMPS-co-NNDMA. Thus, when admixtures with similar molar anionic charge densities are used, the performance of one component is not negatively influenced by the other.

The fluid loss control performance of 2-acrylamido-2-methylpropane sulfonic acid (AMPS V R )-based copolymers added to cement slurries was studied at 27 and 100 C, respectively. It was found that effectiveness of these fluid loss additives solely relies on achievement of a high adsorbed amount on the surface of cement. At elevated temperature (100 C), CaAMPS V R -N,Ndimethyl acrylamide copolymer (CaAMPS V R -co-NNDMA) exhibits reduced adsorption and hence decreased fluid loss control of the cement slurry. The reason behind this behavior is poor calcium binding capability of the sulfonate anchor groups, which coordinate with calcium atoms present on the mineral surface. Whereas, an increase in the sulfate concentration present in cement pore solution instigates partial coiling of CaAMPS V R -co-NNDMA and causes only a slight influence on the performance of this copolymer. The elevated sulfate content results from thermal degradation of ettringite, a cement hydrate mineral produced during the early stages of cement hydration.Incorporation of minor amounts ($ 1.3 mol %) of maleic anhydride into this copolymer produces a terpolymer, which exhibits higher and more stable adsorption, even at high temperature. This effect is owed to the presence of homopolymer blocks of polycarboxylates distributed along the polymer trunk. On mineral surfaces, they present much stronger anchor groups than sulfonate functionalities, as evidenced by their higher calcium binding capability. Consequently, fluid loss performance of CaAMPS V R -co-NNDMA-co-MA is little affected by temperature. Understanding the influence of temperature on the physicochemical interactions occurring between additives and the mineral surface can help to design more effective admixtures suitable for high temperature applications.