Aquifer selection for CO₂ sequestration based on mechanical properties evaluation

The long-term geological sequestration of carbon dioxide (CO₂) in underground formations (deep saline aquifers) is the most economically viable option to decrease the emissions of greenhouse gas in the atmosphere which are the main contributing factors for global warming. The injection of CO₂ in carbonate aquifers dissolves some of the calcite rock due to the formation of carbonic acid as a result of the interaction between CO₂ and brine. This rock dissolution may affect the rock integrity and in turn will affect the rock mechanical properties. The effect of CO₂ on the rock mechanical properties is a key parameter to be studied to assess the aquifer performance in the process of geological sequestration and to get safe and effective long-term storage. The main objective of this study is to address the impact of geological sequestration of CO₂ on the mechanical properties of carbonate aquifer and cap rocks. In addition, the effect of the storage time on these properties are investigated. Moreover, the effect of CO₂ sequestration on rocks with different mechanical properties are studied, and the good candidate carbonate rocks for geologic sequestration are identified. In this study the CO₂ was injected and soaked with the brine with the core at high pressure and high temperature (2000 psi and 100°C) to simulate the actual downhole conditions The carbonate cores were analyzed for mechanical properties using indirect tensile strength, unconfined compression, and acoustics testing machines. Results show that CO₂ sequestration affected the mechanical properties of the carbonate rocks as well as the cap rocks. Long time soaking of CO₂ in brine allowed for the formation of enough carbonic acid to react with the cores and this greatly impacted the rock mechanical and acoustic properties. The significant impact of CO₂ storage was noted on Khuff limestone and the good candidate among the carbonate rocks studied here for geological sequestration of CO₂ is found to be Indiana limestone.