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

To ensure the safety of a CO 2 storage site and containment of CO 2 in the subsurface, the integrity of wellbore materials must be maintained. Field and laboratory studies have shown CO 2 ‐induced reactivity of wellbore cement, but these results have to be extrapolated to the extended time span of CO 2 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 CO 2 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 CO 2 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 CO 2 transport through the cement matrix. The minimum CO 2 penetration was for enhanced calcium silicate hydrate (C‐S‐H) decalcification, which increases calcite precipitation, CO 2 consumption, and hence decelerates CO 2 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