Review: Approaches to research on CO2/brine two-phase migration in saline aquifers

Abstract

Understanding CO2/brine multiphase migration processes is critical for effectively evaluating potential storage capacity, ensuring storage security, and predicting the long-term fate of CO2 storage in saline aquifers. Success depends on the development and application of appropriate research methods. This paper accordingly reviews the progress made in research methods on CO2/brine two-phase migration. Due to intrinsic linkage between CO2 migration and trapping in saline aquifers, prediction of CO2/brine migration processes requires an accurate understanding of CO2 trapping mechanisms. Six recognized physical or geochemical mechanisms, including structural and stratigraphic trapping, residual gas trapping, hydrodynamic trapping, solubility trapping, local capillary trapping and mineral trapping, can impede or prevent CO2 migration according to different dominating variables, and consequently immobilize CO2 in brine formations at varying time and spatial scales. Laboratory experiments, field-scale monitoring and computational modeling are the main approaches in studies on CO2/brine multiphase migration. Different techniques have been designed and developed within each of these methods in terms of physical conditions and spatial scales of multiphase migration phenomena. Due to multi-scale characteristics of CO2/brine multiphase migration processes and complementary relationships among these methods and techniques, different research methods and techniques are often used in combination. Based on a systematic analysis of limitations and weaknesses, improvements are recommended which could potentially increase the accuracy, reliability and applicability of the approaches.