Modelling of wellbore cement alteration due to CO[formula omitted]-rich brine interaction in a large-scale autoclave experiment

Abstract

The objective of the present study is to investigate wellbore cement alteration due to CO2-rich brine interaction under reservoir conditions. Hence, an experiment made with a unique large-scale autoclave system has been numerically simulated. Results from that experiment showed an alteration zone of 3 mm thickness at the bottom of the cemented casing, where the cement interacted with the CO2-rich brine. A reactive transport model has been made in order to understand the geochemical processes that occurred during the experiment and the impact of mineral changes on the cement porosity. The numerical model allowed to determine the diffusion coefficient of the cement by calibration using the alteration thickness. Results revealed an increase of porosity at the inner rim due to C-S-H and portlandite dissolution, followed by a drop of porosity due to mainly calcite, but also, C-A-S-H and M-S-H precipitation, and finally, an increase of porosity in the last mm due to portlandite dissolution. In the rest of the 2 m cement column, neither mineral nor porosity changes are predicted by the numerical model, indicating that the well integrity practically would not be affected for the period of time of exposure to dissolved CO2.