Versatile Non-Lignin Cement Set Retarder Offers Highly Consistent Performance and Worldwide Environmental Applicability
Cement set retarders are chemical additives that, when placed in cement slurries, purposely extend the pump time of the cement without affecting its long-term mechanical properties. By extending its pump time, the slurry can be placed properly downhole before it thickens and become unworkable.
Also, cement set retarders can be used to offset the effects of high downhole temperatures that dramatically decrease the setting time of a slurry or to avoid complications that result when unavoidable delays occur between mixing and placement.
Most retarders are very similar in that they perform the same functions in the same way. Yet, many different retarders exist in order to meet varying geographical, regulatory, and technical requirements. The result is a multiplicity of retarders (and other additives) that must be developed, stocked, and managed. Additionally, logistical problems resulting from complex product portfolios and the maintenance of chemical compliance requirements for each portfolio must be handled.
Currently, retarder technology is largely based on three classes--lignins, sugars, and synthetic polymers. Lignins are the most common cement retarders. Sulfonated lignin (or lignosulfonate) is by far the most common choice. Lignins are structurally very complex but are essentially polyphenols with various hydroxyl groups associated with the structure. Lignosulfonates have wide availability and low cost but can also be highly variable in consistency over time. They also have a tendency to disperse the cement slurry (which may be beneficial or detrimental) as well as to inhibit the development of compressive strength in the set cement.
Common sugars are the oldest known cement retarders. They have wide availability and relatively low cost. Sugars are well known to be very erratic and very powerful retarders at temperatures below 200°F. Their erratic and inconsistent nature has led to very limited use in oilwell cements. Additionally, sugars will simply degrade at temperatures above the boiling point of water (212°F) becoming ineffective and often causing side effects in the slurry such as gelation.
The most recent developments in retarder technology are synthetic polymers based on acylic acid and/or acrlyamide technologies. These products may have variable properties such as a higher temperature range than lignins or sugars, or linear retardation with respect to concentration. They also display less sensitivity to temperature fluctuations and do not cause significant changes in pumping time due to relatively small changes in temperature. Disadvantages include high cost and the fact that they are highly hygroscopic which causes numerous problems with handling.
Recently, a new retarder technology was introduced by Halliburton for use by operators who desire an alternative to commonly used lignin products. HR-800™ is a high-performance, non-lignin, environmentally approved retarder capable of outperforming most of the currently available retarder technologies while maintaining wide availability and a highly consistent supply.
While neither a lignin nor a synthetic, the retarder displays many of the advantages of both. The material is food grade quality and meets even the most stringent requirements of the North Sea for environmental compliance. It is currently ranked as 'yellow' in Norway, 'gold' in the UK (with no label code), and 'R' in the Netherlands. These are the highest environmental ratings possible for such material.
Because of its good strength development, this retarder technology may be applied in a variety of situations including water-extended slurries, long cement columns, or cooler temperatures. The technology also has shown a predictable response to changes in concentration along with repeatable results which may aid in slurry design, lab time, and smoother operations. Also, it has been proven effective in several API Class cements including Class G, Class H, Class A, and Class C.
The HR-800™ retarder material exhibits many advantageous qualities in the cement slurry form. It is an effective cement set retarder to temperatures up to ~220°F and can be extended to >350°F with retarder enhancers. Also, it has a much better response to concentration than lignin retarders, being closer to the linear response desired in synthetic retarders. It has a very stable effect on pumping time across temperatures therefore; minor fluctuations in temperature conditions do not have a great impact on the pumping time of the slurry.
Additionally, most retarders have a strong dispersing effect on the slurry. This creates problems for slurries with high water content, or foam. However, this new technology displays a thickening effect if used alone and can enhance the properties of other additives such as dispersing or fluid-loss agents. The ability of the slurry to build gel strength rapidly, sometimes referred to as transition time, should also be noted. Slurries using HR-800™ have demonstrated no adverse impact on transition times even with the ability to impart some added viscosity to the slurry.
Thickening Time Response of HR-800™
Thickening time data for HR-800™ cement retarder in a Capitol H cement mixed at 16.4 lb/gal.
The new retarder imparts advantageous properties on the set cement as well. Lignins and sugars are well known to cause delayed compressive strength development in cement. Testing has shown that the HR-800™ retarder can enhance early strengths rather than inhibiting strength development. The material has been tested over a wide range of slurry densities and shows good slurry stability between 8 and 18 lb/gal, including typical water-extended slurries, pozzolanic slurries, foam slurries, slurries with hollow spheres, densified slurries, etc. It does seem to perform better in API-grade cements than in construction-grade cements but is still competent to retard construction- grade cement slurries.
The retarder displays numerous beneficial physical properties as well. It is a free-flowing white powder with low dusting tendencies. The hygroscopic nature of the material is very low and does not tend to alter the performance over time due to water absorption. One common issue among operators is the shelf life of additives when in liquid form. In order to assess the shelf life of HR-800™ a 50% active aqueous solution was prepared and tested over a period of several months. Little variation was seen in pump time over the course of a four-month period.
Temperature sensitivity of HR-800™, HR-5, and HR-4 retarders, showing the thickening time to reach 70 Bc. The slurries were 100% bwc Norcem G cement, 4.91 gal/sk fresh water, NF-6 defoamer, and 0.25% bwc retarder.
When a retarder is needed to lengthen the time required to place cement downhole, HR-800™ is a quality retarder that offers consistent performance and has no cost premium. The product offers wide availability, excellent consistency, and exhibits impressive performance when used with cement slurries. It is also accepted for North Sea operations and is cost-competitive with typical lignin retarders.
The technology shows nearly linear retardation response with concentration rather than the somewhat erratic "S-curve" responses seen in lignins. And, contrary to what is seen with most other retarders offered to the industry, strength development is unhindered and is sometimes even enhanced by HR-800™.
Finally, the new retarder has been field-proven. In almost one hundred jobs run during a recent three-month period, HR-800™ has proven to be an extremely consistent performer for operators who used it in their cementing operations.