The invasion of Portland wellbore cement-based porous matrix with CO2 rich geofluids (pH 2-5) enhances the leaching of calcium hydroxide (CH) and calcium silicate hydrate (CSH) increasing the porosity and precipitation of Si(Al)-anhydrous gel leading to strength loss and physical failure. The objective of this study is to investigate the pH variation of Olivine in various solutions at 900C and 95% RH representing subsurface fluid conditions and carbonation of olivine in geothermal wellbore cement exposed to harsh low pH environments. Also, olivine activity will be examined in high alkaline Portland cement. TEM was used for nanostructure characterization. 5% WOC olivine was added to cement slurry prepared and cured for 21 days at 900C and 95% RH. SEM/EDS and TEM, are performed on the cleaned and dried cement plugs. TEM shows branching randomly oriented chains, feather-like, and dendritic morphology pH chemical sensitivity indicates the dissolution of olivine in HCl and pH increase from 1.0 to 1.92. SEM and EDS indicate unreactive olivine in high alkaline Portland cement which is suggested to serve as a filler within cement and react in low pH milieus to form carbonates minimizing the carbonation of CHS and CH thus enhancing the wellbore cement.