Cements for CO₂Capture and Storage Wells

Abstract

Carbon capture and geological storage requires to complete wells with a cement which is CO2 resistant for at least several centuries. Conventional American Petroleum Institute (API) Class oil well cement which is based on Portland cement is thermodynamically unstable against CO2. The carbonation reactions include formation of CaCO3 from Portlandite and conversion of the strength-providing calcium silicate hydrates (C S H) into CaCO3 and silica gel. Laboratory studies demonstrate that CaCO3 crystallization often results in microcracks and that wet CO2 can leach CaCO3 from the cementitious matrix as Ca(HCO3)2, thus increasing porosity and further promoting advance of the carbonation front. Pozzolanic or chemical additives have not shown much improvement, except for a specific fly ash. Current field evidence from CO2 injection wells (Enhanced Oil Recovery (EOR), CO2 flooding) over a few decades is non-conclusive. Different from Portland cement, calcium aluminate phosphate cement produces hydrates such as apatite which are completely CO2 resistant. When exposed in Na2CO3 to temperatures up to 150 °C, specimens maintained their initial properties except for a decrease in strength. Field data from geothermal wells in Indonesia confirm its exceptional resistance against H2SO4 and CO2. Currently, this cement appears to present the best choice among the group of inorganic cements. Epoxy resins constitute a potentially alternative organic sealing system, e.g. as bottom plug. Their stability in CO2 atmosphere has been demonstrated in limited laboratory work and more experimental proof is required. Furthermore, expandable packers might be used as a mechanical barrier. Overall, a dual containment approach is recommended to reduce the safety risk. Apart from selecting a CO2-resistant cement, attention must also be given to mud displacement efficiency prior to pumping cement, and to the prevention of casing corrosion. Only a holistic approach can guarantee the safe sealing of CCS wells over geological time periods.