Achieving quality cement performance in deep water wells with large amounts of salt present requires fundamental understanding of the salt effects on the interactions of cement additives with cement on the slurry performance. Many problems encountered later in the life of the well can be traced to slurry performance issues during placement and the initial setting process. Cement slurry designs for an oilwell can range from simple to highly complex depending on the geology, lithology, placement logistics, wellbore conditions and long-term performance. Logically designed cement slurry formulations and dependable additive behavior are critical to meeting cement performance requirements. Formulations can include mineral admixtures added in substantial quantities and additives in smaller (<2% by weight of cement) quantities. The performance of the additives in cement slurries depends strongly on competitive adsorption on cement and mineral surfaces. Adsorption interactions are directly influenced by downhole temperature, the nature of the formation, and the mix water composition. These aspects underline the need for understanding additive interactions with cement under wellbore conditions.
Of particular relevance to deep water cementing is the performance of additives in sea water containing monovalent and divalent salts with chloride and various other anions present in amounts close to 4% by weight. The problem of additive performance becomes even more of a concern when cement is placed against salt formations. High ionic strength of sea water or of a sea/fresh water slurry placed against a salt formation, can compress the electrical double layer of cement clinker particles, alter chain conformations of ionically charged or hydrophobically modified polymeric additives, or change the solubility of additives. Additives may perform extremely well in fresh water slurries, but may perform poorly in slurries with high salt content. Additionally, the salt itself can affect cement performance. For example, sodium chloride is a set accelerator in small quantities (≤13% by weight of cement) while it functions as a retarder in large quantities.
The objective of this presentation is to gain molecular level understanding of relationships among additives based on chemical structures, adsorption onto cement surfaces, and the mix water ionic strength.
In this study, typical additives for dispersion and retardation are contacted with Portland cement in deionized water, synthetic sea water, water containing 2% to saturation level of NaCl or divalent salts in a formation brine. The reactions were analyzed by isothermal calorimetry, UV/Visible spectroscopy and rheology. The interpretation and implications of laboratory results are presented.
