Sensitivity of chemical cement alteration - modeling the effect of parameter uncertainty and varying subsurface conditions

To ensure the safety of a CO2 storage site and containment of CO­2 in the subsurface, the integrity of wellbore materials must be maintained. Field and laboratory studies have shown CO2‐induced reactivity of wellbore cement, but these results have to be extrapolated to the extended time span of CO2 storage. Geochemical modeling provides a tool for the prediction of cement alteration; however, large uncertainties in input parameters exist and significant variation in subsurface conditions is expected. This asks for a systematic investigation of the sensitivity of modeled cement alteration towards these factors. In this paper we report PHREEQC simulations of CO2 diffusion into cement and subsequent chemical reactions. The sensitivity of cement alteration toward reaction rates, initial porosity, temperature/mineralogy and flow/no flow conditions were investigated. The base case model indicated that intact cement and tight interfaces between the reservoir and the cement would yield less than 1% porosity change after 300 days of diffusion. For porosity increase or degradation to occur at the cement interface, leaching/flow along the wellbore was required. The sensitivity scenarios yield CO2 penetration depths between 0.3 cm and 1.4 cm after 300 days of diffusion. The maximum was reached for the high porosity (fast diffusion) scenario that facilitates CO2 transport through the cement matrix. The minimum CO2 penetration was for enhanced calcium silicate hydrate (C‐S‐H) decalcification, which increases calcite precipitation, CO2 consumption, and hence decelerates CO2 penetration. This is related to high temperatures (and more crystalline C‐S‐H) or to higher kinetic rate constants used. © 2015 Society of Chemical Industry and John Wiley & Sons, Ltd