A Study of CO₂ Storage in Bimodal Carbonate Aquifer Rocks: Challenges and Enhancement through Foaming

In carbonate rock reservoirs, the presence of dual-pore systems, characterized by two distinct pore size distributions, plays a crucial role in gas storage in saline aquifers. However, comprehensive research on the impact of bimodal porosity on CO₂ storage in such reservoirs is lacking. This study explores CO₂ storage efficiency in carbonate aquifer rocks with bimodal porosity. Petrographic examination, capillary pressure measurements, and nuclear magnetic resonance (NMR) T₂ profiling revealed two distinct and interconnected pore systems with equal effective porosity proportions. While both systems facilitated fluid flow, the micropores had a high capillary entry pressure (>100 psi). Coreflood experiments showed gas displacement efficiencies below 50%, primarily in larger pores, with only 49-58% of these large pores (20-28% of total pores) storing injected gas through residual trapping after water imbibition. To address this challenge, CO₂ was foamed using a foaming agent to enhance viscous forces and overcome capillary forces. This resulted in significant improvements, including a 31% increase in displacement efficiency and a 28% increase in residual gas. NMR analysis revealed effective redirection of CO₂ into smaller pores when foam was applied. In another experiment on a sample with a different pore geometry, the use of nanoparticles to strengthen foam increased sweep efficiency by up to 117%, with a corresponding increase of up to 169% in residual trapping. While the use of foam entails additional operational expenses, we contend that the economic viability of foam technology for CO₂ storage hinges on a rigorous cost-benefit analysis that weighs increased storage capacity/security against chemical costs. Such analysis should also consider the time, cost, and uncertainty associated with developing additional storage sites. Furthermore, improved site selection criteria in carbonate rocks are needed, including a predictive model for T₂ cutoff to identify aquifers with excessive micropores and avoid them during site selection. While fractured porosity is not within the scope of this study, it is expected that foam can equally improve sweep and trapping efficiencies in naturally fractured carbonate rocks.