Strength Retrogression in Cements under High Temperature Conditions

Abstract

Cement designs for high-temperature geothermal applications have typically included 35 to 40% additional crystalline silica to help prevent loss of compressive strength and an increase in permeability. This was based on research performed on Portland-cement systems, which indicated that at temperatures above 230°F, additional silica was required to provide a high-strength stable crystalline structure. This standard has been used by the industry for many years, both in geothermal-well applications and high-temperature oil- and gas-well applications. New research, however, has shown that 40% additional silica can be inadequate to provide a high-strength, low-permeability cement at temperatures typical for geothermal-well conditions of around 500°F or higher. This research also indicates that larger amounts of silica might be required to provide long-term strength stability in cements that are typically used in geothermal-well applications.

Preliminary results of this research are provided, including strength- and permeability-test results on cements cured at temperatures from 500 to 650°F, as well as a discussion on the associated crystalline phases found in these samples. In addition, a discussion of the practical ramifications, ongoing research, and additional research needed in this area is included.