When cementing liners, the cement must develop compressive strength at the top of the liner before drilling is resumed. If drilling is delayed in order to acquire the compressive strength at top of the liner, it can result in excessive delays on cement (WOC) time which can exceed 24 hours. Cement slurries with conventional retarders often achieve acceptable thickening times under dynamic conditions. Sometimes these cement slurries do not produce rapid compressive strength under static conditions. The aim of this study is to develop optimized retarder systems for cements that are exposed to BHST of 400 ºF at the bottom of liner (BOL) and 330 ºF at the top of the liner (TOL).
More than ten retarders including lignosulfonate, ethylene glycol, and aromatic polymer derivatives were evaluated to provide extended thickening times for extreme high temperature cement slurries, while having minimal effect on sonic strength development. The lab studies included comparison between various retarders and their performances of thickening time, sonic strength development, free water, fluid loss, rheology, and gas migration control.
Two new retarded systems were developed. The first system is used for non-latex cements for wells that do not show indications of gas or fluid flow. The second cement system includes latex and is recommended for cases where there is potential for gas migration. The new retarder systems were effectively applied in the field. The paper will address lab studies that led to the development of new retarders.
In general, cement is composed of four main components: tricalcium silicate (C3S) responsible for the early strength development, dicalcium silicate (C2S) responsible for the final strength, tricalcium aluminate (C3A) contributing to the early strength and tetracalcium aluminaferrite (C4AF). In addition, gypsum is added to control the setting time of cement.
Retarders are cement additives whose function is to delay the setting of cement slurries. For a well whose temperature is about 125 ºF or less, no retarder is needed to be added when API class H or G is used.1 However, as temperature increases, the hydration process of C3S increases and, hence, the thickening time decreases.2
Some types of retarders tend to reduce the compressive strength of set cement.3 Hence, a well designed retarder needs to increase the thickening time without having a significant effect on the compressive strength. Similarly, fluid loss control can be affected by the addition of a retarder, especially at high temperatures.4
Typical examples of compounds used as retarders include: calcium lignosulfonate, sodium lignosulfonate, sodium tetraborate decahydrate (borax), starch derivatives, hydroxyethyl cellulose and weak organic acids. Selection of retarder depends on the type of cement and well conditions.1 Lignosulfonates are the most commonly used retarders. However, this retarder is not applicable at high temperatures because of the carbohydrate content and chemical structure. Instead, organic acids and water-soluble borates are the retarders of choice at these conditions.4
To explain the mechanism of retardation in Portland cement, the following four theories were proposed: adsorption, precipitation, nucleation and complexation.5
According to the adsorption theory, the retarder is adsorbed on the surface of the hydration products inhibiting contact with water. This theory suggests that retardation is due in part to the adsorption onto the surface of the C-S-H gel hydration product formed around the grains of C3S rendering it hydrophobic.5
The precipitation theory suggests that the retarder reacts with calcium and/or hydroxyl ions and form an impermeable sheet covering cement grains. 5
In the nucleation theory, it is suggested that the retarder slows down the growth rate of hydration products by adsorbing on their nuclei. Finally, the complexation theory states that calcium ions are chelated by the retarder preventing the formation of the nuclei. It is possible that all of the above mechanisms are involved to some extent in the retardation process. The predominant factor depends on the type of retarder used and the cement phases upon which the retarder acts. 5