Abstract: Sequestration in abandoned petroleum fields has the potential to reduce atmospheric emissions of CO₂ if adopted on a large scale. When CO₂ comes in contact with brine in the sequestration formation, it will form carbonic acid. The acid may damage cement that is used to construct and abandon wells in the sequestration formation, allowing the wells to leak. CO₂ release from a sequestration formation through abandoned wells to the vadose zone and then from the vadose zone into a residence could cause exposure to high levels of CO₂. This study presents the results of two types of experiments that were conducted in order to understand how CO₂ will affect well cements. This dissertation also presents a hypothetical risk assessment that examines the risks that a sequestration site may pose. The experiments examine how well cements react when exposed to carbonated brines at 20° and 50°C. Month-long flow-through experiments were conducted on samples made from Class H neat paste and Class H cement containing 6% bentonite under conditions that simulated sandstone (pH 2.4 and 3.7) and limestone (pH 5) sequestration formations. Year-long batch experiments were conducted on composite samples made from Class H well cement and either sandstone or limestone in order to determine how carbonated brine affects the cement-stone interface. The results of the flow-through experiments showed that calcium-containing phases in cement may be completely degraded and the formation of the calcium carbonate layer acts to slow, but not stop, degradation. The results of the batch experiments showed that carbonated brines can degrade cement and damage the integrity of the cement-stone interface. The damage to the cement at the cement-stone interface caused an order-of-magnitude increase in permeability in the sandstone-cement samples. The risk assessment identifies two potential screening levels. The assessment then uses a semianalytical wellfield model coupled with analytical models of diffusion through the vadose zone and foundation walls to establish CO ₂ concentrations in residential basements. The results show CO ₂ levels may not reach either screening level unless the value of the air-exchange rate in the houses is small.