Effect of reducing irreducible water saturation in a near-well region on CO2 injectivity and storage capacity

Abstract

Fossil fuels currently supply 80% of the world’s energy demands and are primarily responsible for increasing carbon concentration in the atmosphere and the associated climate change. Achieving internationally discussed goals for reducing carbon emission will be either impossible or involve significant costs without implementing the CO2 capture and storage (CCS). The volumetric CO2-storage capacity of a brine aquifer is among the most critical factors for the economically-viable & large-scale geologic CO2-storage projects. The maximum sustainable injection rate or CO2 injectivity is another important criterion that depends on many reservoir properties. Based on previous experimental studies, we hypothesized that if the sweep efficiency of CO2 is improved by using suitable additives, then a higher end-maximum CO2 saturation can be achieved that, in turn, can increase the CO2 injectivity. In this regard, a strategy to enhance the CO2 injectivity was proposed by reducing the irreducible water saturation in a near-well region. In this study, the influence of reducing the irreducible water saturation and thus improving the relative permeability of CO2 on the CO2 injectivity and storage capacity is investigated in depth. A series of numerical simulations involving two test models – a hypothetical horizontal aquifer and a pilot test site in South Korea – were conducted to examine our assumptions with the concentration of surfactants confirmed from the previous experimental study. The results help to ascertain the applicability and effectiveness of the proposed injectivity-enhancing strategy. It was observed that the pre-injection of surfactant solution slows down the well-bottom-hole pressure and increases the total CO2 injection capacity, with the effect deemed proportional to the quantity of surfactant for a hypothetical flat aquifer. However, a nonlinear relationship between the surfactant quantity and the injectivity was found in the case of the pilot test site, which revealed that the surfactant pre-injection influences the migration of CO2-plume and solubility trapping as well.