Treated Ceramic Microsphere-Cement Lightweight Composites for Geothermal Cementing Systems

Abstract

The compressive strength and water permeability of high temperature lightweight cementing materials containing sillimanite based hollow microspheres as a a filler can be improved by treating the surfaces of the microspheres with a Ca(OH)2 saturated solution at temperatures up to 200°C. The precipitation of an epitaxial layer formed by an interaction between a hot calcium hydroxide solution and the surface of the sphere plays an essential role in developing favorable bonding characteristics at the interfaces and in promoting the hydration of the cement matrix. The properties of the composites include a slurry density of <1.2 g/cc at 25°C, a 300°C-24 hour compressive strength of >1015 psi(7.0 MPa), a water permeability of <10/-4 Darcy, and a build density of >1.0 g/cc. It was observed that tobermorite truscottite transformation and anorthite formation during autoclave exposure for 180 days in brine at 300°C results in slight strength reductions and increased water permeability.