Effects of CO2 on Geothermal Cements

Abstract

The cements used to complete geothermal wells are required to withstand temperatures in excess of 300oC. To avoid ongoing strength degradation at these elevated temperatures, silica flour additions of ~40% BWOC have been routinely used. This normally leads to the formation of the low Ca/Si ratio phases of tobermorite (Ca/Si = 0.83) at 150oC and xonotlite (Ca/Si =1) above 200oC. These phases provide good compressive strengths and have low permeability. However, when exposed to fluids containing high levels of CO2 these phases have been shown to rapidly carbonate and become porous, forming aragonite and calcite depending on the temperature. If the level of dissolved CO2 is high enough so the fluid becomes acidic, then the carbonated layer will readily dissolve leading to corrosion and eventual loss of the protective cement sheath. By using lower additions of silica, the calcium silicate hydrates that form (Ca/Si ? 1.5) expand on carbonation and provide a carbonation sheath that offers some protection to corrosi ve fluids. When fresh cement slurries are cured in the high CO2 fluids they rapidly give rise to a carbonated layer which effectively reduces the availability of calcium so low Ca/Si ratio phases form, such as tobermorite, which offer no protection to carbonation. Ceramic cenospheres used as lightening agents for cement slurries, react with the hydrating cement to give pockets of low Ca/Si phases that readily carbonate, opening the matrix to further attack and rapid carbonation throughout what becomes a porous binder. Addition of additives containing aluminium such as slag does give rise to calcium aluminosilicates, which while more carbonation resistant make poor binders. However, even these will ultimately carbonate under high CO2 conditions and if sufficient CO2 is present, dissolve. The challenge for the industry is to develop a whole new cementing system that is not based around calcium that can be used to complete geothermal wells in a high CO2 acidic environment.