The Effect of CO2 Exposure and Curing Time on the Cement-Rock Interaction in Geothermal Wells

Abstract

The current study is focused on the interactions between well cements and one of the most common types of rocks found in geothermal environments, under hydrothermal conditions. Holes were drilled in blocks of ignimbrite rock and slurries of API class G cement with 0, 20 and 40% added silica flour (SF), were poured into the cavities. The whole assemblage was autoclaved under typical geothermal conditions at 150°C for 84 days. The results show that chemical reactions occur in the intertransition zone (ITZ) which are dependent on both carbon dioxide (CO2) exposure and the curing time. By comparing results with those of only 28 days curing it is clear that ongoing reactions continue to occur but at a slower rate than in the first 28 days. When exposed to CO2, the cement-rock interaction is very dependent on the amount of added quartz. When 20% of SF is added to the cement mix, the CO2 presence seems to exacerbate the reaction between the cement and the rock, when compared with neat cement. On the other hand, 40% addition of SF seems to slow the migration of Ca2+ and OH- ions and formation of cementing compounds in the rock, compared to 0% and 20% SF addition. Work is ongoing in order to investigate the effect of these and other variables such as drilling fluid, temperature, rock type and cement mixture.