Geothermal environments are among the most difficult conditions for cements to survive. Normally accepted for high-temperature oil wells silica-modified Portland-based cement formulations are not durable in hostile geothermal environments failing to provide good zonal isolation and metal casing corrosion-protection. High-temperature well cement compositions based on calcium-aluminate cements have been designed to seal such wells. Two types of calcium-aluminate cement are of particular interest for geothermal applications. One is–chemical type, calcium-aluminate-phosphate cement (CaP) already used in the field and the other, alkali-activated calcium-aluminate type (thermal shock resistant cement, (TSRC), has been recently developed. The CaP cements were designed as CO2-resistant cements for use in mildly acidic (pH ~ 5.0) CO2-rich downhole environments. TSRC was formulated to withstand dry-heat – cold water cycles of more than 500°C. This chapter includes information and discussions of cement forming mechanisms, cements mechanical properties, resistance to mild and strong acids, cement-carbon steel bonding and self-recovery of mechanical strength and fractures closure after imposed damage. Performance of common high-temperature OPC-based composites is discussed for comparison.
Part of the book: Cement Based Materials