A study of methodologies for CO2 storage capacity estimation of saline aquifers

Abstract

Implementation of carbon dioxide capture and geological storage to achieve a reduction in greenhouse gas emissions requires knowledge of carbon dioxide storage capacity in various geological formations. The storage capacity estimated can be of different levels of certainty and cost, while storage capacity estimation can be conducted at different scales and resolutions. Five different types of trapping mechanisms in deep saline aquifers are discussed, namely structural and stratigraphic trapping, residual gas trapping, solubility trapping, mineral trapping and hydrodynamic trapping. All these trapping mechanisms occur at different time frames, which need to be taken into consideration when estimating the storage capacity. A number of parameters that affect the storage capacity are also discussed for a better understanding of estimating the storage capacity of carbon dioxide. These parameters include in situ pressure, injectivity, temperature, permeability and compressibility and may affect one another but ultimately affect the storage capacity of carbon dioxide. The different methodologies for estimating carbon dioxide storage capacity are critically reviewed, including mathematical models, dimensional analysis and analytical investigations. The studies conducted on the benefits of cyclic injections were inconclusive, while the injection period was found to be an important factor in storage capacity as part of the ratio of time elapsed to injection period, which was a key parameter in storage capacity, for different gravitational numbers. Some studies showed that for short-term storage (time elapsed is less than 60 times the injection period), medium valued gravitational numbers of about 4.58 were preferred, while for long-term storage (time elapsed is larger than 60 times the injection period), low gravitational numbers of about 0.045 were preferred. The gravitational number depends on the injection rate and it was found in the study that the optimum injection rate was very close to 6.56Mt/years. Meanwhile, several techniques were found to be able to increase the storage capacity of carbon dioxide in deep saline aquifers. These include using horizontal injection wells instead of vertical ones in the absence of chase brine injection or using vertical wells instead of horizontal ones in the presence of chase brine injection, introducing hydraulic fractures in the injection wells, producing brine from the formation to be transported to other aquifers, injection of chase brine on top of the injection of carbon dioxide and the injection of formation brine with carbon dioxide followed by injection of chase brine. These methods have shown to increase the storage capacity of carbon dioxide by 30–50% more than conventional methods of injecting carbon dioxide only without the additional injection features.