Technical Committee Drilling Technology
Eignung von Tonerdezementen für die Bohrlochzementierung unter korrosiven Bedingungen, insbesondere bei CO₂-Einpressung – Literaturstudie
Calcium aluminate cements, also called aluminate cements, are special cements based
on calcium aluminates. In construction (especially in dry-mix mortars), they are
valued for their remarkably higher acid and corrosion resistance compared to Portland
cement. Their high temperature resistance especially qualifies them for refractory
cements which are used e.g. as liners in cement rotary kilns. The worldwide production
of calcium aluminate cements adds up to about 3 million tons.
In oil well cementing, calcium aluminate cements have first been used at the beginning
of the 90ies, in fact initially for geothermal wells. Applications in permafrost soils
as well as fire-flooding wells came later. The reason for the use in geothermal wells
were severe problems with API Portland cements affected by hot, carbonic acid rich
formation brines. With support from the companies Unocal and Halliburton, the
Brookhaven National Laboratory (USA) developed a special cement based on 60
parts by weight of calcium aluminate cement and 40 parts ASTM class F fly ash. This
binder is notably CO2 resistant when mixed with a 25 wt. % Na-polyphosphate solution.
Laboratory tests and field applications conclude that this cement is resistant to
CO2 corrosion at temperatures up to 300 °C for over 20 years. In 1997, for the first
time numerous geothermal wells have been cemented using this cement. They are
intact to this day.
When this cement hardens at room temperature, amorphous reaction products are formed and the reaction is incomplete. However, at higher temperatures hydroxyapatite, boehmite, Na-P-zeolite, analcime or katoite are formed as crystalline reaction products. The CO2 resistance of this cement is due to specific chemical reactions. In contact with hot alkaline carbonate solutions, analcime converts to cancrinite, and CO2 is incorporated into hydroxyapatite. During this conversion, the compressive strength is slightly reduced, whereas porosity and permeability increase. After the conversion is completed, these properties stabilize and the cement maintains its durability. Calcium carbonate is not formed, or only in very small amounts. Common additives such as retarders (e.g. citric acid), fluid loss additives (e.g. cationic starch derivatives), latex dispersions, lightweight aggregates (hollow spheres), rubber powder and mineral or carbon fibers are applicable. Filtration control at temperatures above 100 °C is still an unresolved technological problem. Also, there is no experience with salt cement slurries. So far, only the resistance of hardened cement samples stored in brine was investigated. Halliburton company markets this cement under the brand-name ThermaLock®. To date, more than 1000 tons have been successfully pumped in the field in US, Indonesia, Japan and Kuwait.