Comparative Study of the Working Mechanisms of Different Cement Fluid Loss Polymers

Abstract

Abstract 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.