Air-foamed calcium aluminate phosphate cement for geothermal wells

Air-foamed low-density calcium aluminate phosphate (CaP) cement slurry was prepared by mixing it with chemical foaming reagent at room temperature without any pressure, followed by autoclaving at 200 °C. When the porosity, compressive strength, and water permeability of the autoclaved CaP foam cement made from a 1.22 g/cc slurry density was compared with those of N2 gas-foamed Class G cement made from a slurry of similar density under high pressure and hydrothermal temperature at 288 °C, the CaP cement revealed some advanced properties, such as a higher compressive strength and lower porosity. These advanced properties were due to the hybrid formation of three crystalline hydrothermal reaction products; hydroxyapatite, boehmite, and hydrogarnet phases. However, one shortcoming was an increase in water permeability because of the formation of an undesirable continuous porous structure caused by coalesced air bubble cells, suggesting that an appropriate lesser amount of foaming reagent be used to create a conformation in which fine discrete air-bubble cells are uniformly dispersed throughout the slurry. For non-foamed cement, three major factors contributed to protecting carbon steel against corrosion: (1) good adherence to steel, reflecting a high extent of coverage by the cement layer over the steel's surfaces; (2) retardation of cathodic corrosion reactions; and, (3) minimum conductivity of corrosive ionic electrolytes. However, adding an excessive amount of foaming reagent did not offer as effective corrosion protection as that of non-foamed cement.