Nanosilicas as Accelerators in Oilwell Cementing at Low Temperatures

Accelerators are important cementing additives in deepwater wells where low temperatures can lengthen the wait-on-cement (WOC) time, potentially increasing the cost of operations. The cement-set accelerators traditionally used for shortening WOC times are inorganic salts, such as calcium chloride (CaCl2). These accelerators are known to have the potentially negative side effect of increasing the set-cement permeability. Nanosilicas, on the other hand, can be advantageous compared with conventional cement-set accelerators because they reduce the permeability and increase the mechanical strength of cement-based materials. For this reason, nanosilicas are known to be particularly good candidates as replacement materials for traditional salt accelerators. This study investigates the feasibility of the use of different sizes and aspect ratios of nanosilicas as cement hydration accelerators under low-temperature conditions of 59 degrees F (15 degrees C). The nanosilica activities are herein defined through their comparative advantages with respect to traditional accelerators, as well as through the advantages and disadvantages of the different nanosilicas resulting from their various sizes and shapes. Although hydration of oilwell cement is known to be accelerated by the addition of nanosilica, the effects of nanosilica particle shape on cement hydration kinetics has not been previously investigated. The isothermal calorimetry experiments conducted in this study reveal that just as smaller nanosilica particle sizes increase the cement-set acceleration, so do higher nanosilica aspect ratios. The effects of slurry density on the relative merits of CaCl2 and nanosilicas are also investigated. In regular-weight slurries, the effectiveness of nanosilica acceleration appears to be weaker than that of CaCl2, especially during early ages (<= 3 days). In lightweight slurries, the effectiveness of nanosilica acceleration can be much stronger than that of CaCl2, especially when mid-to long-term properties (>= 2 days) are considered. Smaller particle sizes and higher aspect ratios enhance the acceleration effect of nanosilicas. The compressive-strength development of lightweight oilwell cements with and without accelerators was also investigated. Lightweight cements accelerated with nanosilica displayed 7-day compressive strengths up to 136% higher than those accelerated with CaCl2.