Effect of CO₂sequestration period on the mechanical properties of carbonate aquifers

Geological sequestration of carbon dioxide (CO₂) in underground formations is the most promising way to decrease the greenhouse emissions into atmosphere. The understanding of long term effects of CO₂storage in carbonate aquifers is challenged by many uncertainties including geochemical effects of CO₂on carbonates and the coupled chemical-mechanical effects. The carbon dioxide dissolves in water forming bicarbonate which will dissociate to carbonic acid. This acid dissolves calcites in carbonate rocks. As a result, the mechanical properties of the aquifer changes through this interaction. It is therefore necessary to study the effect of carbon dioxide sequestration on the integrity of a given formation to get safe and effective long-term storage of CO₂sequestration in deep saline aquifers. To the best of our knowledge, previous works on CO₂sequestration have focused on the impact of CO₂injection on the petrophysical and mechanical properties of formation rocks but the injection occurred in a short period, which was not enough for carbon dioxide to react with formation brine and thus the chemical reaction that will impact the properties of the rocks. The main objective of this paper is to investigate the effect of CO₂storage on the mechanical properties of carbonate aquifer rocks that were exposed to soaking with CO₂. Furthermore, the effect of the duration of CO₂-brine contact time or solubility time on these properties was investigated. CO₂was stored in the cores at 2000 psi and 100°C for different time periods ranging from two weeks to three months. The cores were then analyzed for the mechanical properties using acoustics, unconfined compression, and indirect tensile strength testing machines. Results showed that storing CO₂for longer times changed the rock mechanical properties significantly. Samples in which CO₂was stored for longer times exhibited greater change in mechanical properties compared to those in which CO₂was stored for shorter time. The geochemical interaction between CO₂, brine, and rock is time dependent making changes in rock mechanical properties also time dependent.