Storing CO2 in geological formations is gaining greater importance as various companies start transitioning towards a carbon neutral future. CO2 storage in depleted hydrocarbon reservoirs and saline aquifers is considered an effective and secure option to reduce atmospheric CO2. Once underground, four different mechanisms keep the supercritical CO2 securely stored. The mechanisms, in increasing order of storage security are, 1. Structural/stratigraphic trapping, 2. Residual trapping, 3. Solubility trapping, and 4. Mineral trapping. Optimization of injector design to increase the amount of CO2 trapped in one of the more secure mechanisms is desirable.
Structural trapping is the most dominant and least secure trapping mechanism for CO2 storage. Any opportunity to move structurally trapped CO2 into one of the other trapping mechanisms is preferable from the standpoint of storage security. Mechanistic models are used to study ways to improve amount of CO2 trapped by certain mechanisms. Residual trapping is affected by several factors including path traveled from perforation to the top of the structure. Similarly, solubility trapping is influenced by several factors including the amount of contact CO2 has with water. Injected CO2, due to buoyancy, rapidly rises to the top of the structure. There is potential to increase residual trapping and solubility trapping by optimizing the injector design to increase volume of reservoir contacted by CO2.
Mechanistic modeling study shows that residual trapped and solubility trapped CO2 volume can be increased by optimizing injector design. There is up to 50% improvement observed in both trapping mechanisms depending on the reservoir characteristics and injector design. Interestingly, lower permeability reservoirs are more sensitive to injector design compared to higher permeability reservoirs. Of the injector designs studied, horizontal injectors placed at the bottom of the structure show the most improvement in both residual and solubility trapping mechanisms. Pressure of the reservoir also influences trapping mechanisms. At higher reservoir pressures, density difference between CO2 and water is smaller. This affects how CO2 plume migrates in the reservoir.