Carbon capture and storage (CCS) is a viable approach to reduce the level of carbon dioxide (CO2) emission into the atmosphere. Among different CCS challenges, the leakage of CO2 from injection wells has been identified as an environmental concern. Bentonite could provide effective sealing for plugging and abandoning such wells. However, very little is currently known about the suitability of this material in the presence of CO2. This paper provides a modelling approach to study the application of bentonite as a sealing material for CCS wells by studying the displacement mechanism of CO2 in water-saturated bentonite.
A homogeneous numerical flow simulation model was built with bentonite characteristics (porosity, permeability) in the presence of CO2 and water. The base model was examined in four scenarios to investigate the effects of CO2 transport parameters (diffusivity and permeability) in typical bentonite to be used to seal CCS wells. In the first and second scenarios, the bentonite permeability and diffusivity effects were investigated by monitoring CO2 (leakage) flux, saturation, and concentration through the bentonite plug. Further, a sensitivity analysis on permeability was conducted to identify the permeability effects in the model. Also, this study investigates the effect of the temperature with/without considering gas diffusion.
By monitoring the average gas saturation as a parameter to identify leakage over a period of 100 years, the results show that bentonite plugs can significantly decrease the amount of CO2 leakage from the well. According to the results, the inclusion of gas diffusivity in the model leads to a smoother water-gas displacement front through the bentonite plug. It is also shown that despite the considerable effects of diffusivity on CO2 saturation, CO2 migration flux is dominantly controlled by permeability.