Influence of Chemical, Mechanical, and Transport Processes on Wellbore Leakage from Geologic CO2 Storage Reservoirs

Innovative method for CO2 fixation and storage

The concentration of CO₂ in Earth’s atmosphere has been gradually increasing since the Industrial Revolution, primarily as a result of the use of fossil fuels as energy sources. Although coal and oil have been vital to the development of modern civilization, it is now recognized that atmospheric CO₂ levels must be reduced to avoid the serious effects of climate change, including natural disasters. Consequently, there is currently significant interest in developing suitable methods for the fixation of CO₂ in the air and in exhaust gases. The present work demonstrates a simple yet innovative approach to the chemical fixation of extremely low and very high CO₂ concentrations in air, such as might result from industrial sources. This process is based on the use of aqueous solutions of the water-soluble compounds NaOH and CaCl₂, which react with CO₂ to produce the harmless solids CaCO₃ (limestone) and NaCl (salt) via intermediates such as NaHCO₃ and Na₂CO₃. The NaCl generated in this process can be converted back to NaOH via electrolysis, during which H₂ (which can be used as a clean energy source) and Cl₂ are produced simultaneously. Additionally, sea water contains both NaCl and CaCl₂ and so could provide a ready supply of these two compounds. This system provides a safe, inexpensive approach to simultaneous CO₂ fixation and storage.

Alteration of Fractured Foamed Cement Exposed to CO2-Saturated Water: Implications for Well Integrity

Geologic CO₂ storage (GCS) is a method to mitigate the adverse impact of global climate change. Potential leakage of CO₂ from fractured cement at the wellbore poses a risk to the feasibility of GCS. Foamed cement is widely applied in deepwater wells where fragile geologic formations cannot support the weight of conventional cement. Thus, it is critical to know whether fractures in foamed cement self-seal in a similar manner as conventional cement systems. This study is the first to investigate the changes in physical and chemical attributes of foamed cement under dynamic flow conditions using CO₂-saturated water. Self-sealing of fractures in the cement was observed at a solution flow rate of 0.1 mL/min and a pressure of 6.9 MPa. The formation of CaCO₃ precipitates in pore spaces and fractures led to a decrease in permeability by 1 order of magnitude. The extents of self-sealing in foamed cement samples, specifically the 20 and 30% air volume formulations, were similar to that of conventional cements. We attribute this to the greater alteration depth in the foamed cement, which compensated for the reduced availability of Portlandite and higher initial porosity. The results can be used to evaluate the risk of leakage associated with foamed cement.