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Monitoring the migration of pore pressure, deformation, and saturation plumes with effective tools is important for the storage and utilization of fluids in underground reservoirs, such as geological stores of carbon dioxide (CO2) and natural gas. Such tools would also verify the security of the fluid contained reservoir-caprock system. Utilizing the swelling strain attributed to pressure buildup and the adsorption of supercritical CO2 on clay minerals, we tracked the fluid plume in a natural clay-rich Tako sandstone at the laboratory core scale. The strain was measured by a high-resolution distributed fiber optic strain sensing (DFOSS) tool. The strain changes induced by CO2 adsorptions on clay minerals were significantly greater than those caused by pore pressure alone. The distribution of the swelling strain signals effectively captured the dynamic breakthrough of the CO2 plume from the high- to low-permeability regions in the Tako sandstone. Besides revealing the in situ deformation state, the measured strain changes can track the movement of the CO2 plume as it enters the clay-rich critical regions in the reservoir-caprock system. The present findings and potential future applications of DFOSS in the field are expected to enhance the monitoring and management of underground fluid reservoirs.

Plain Language Summary Carbon dioxide (CO2) sequestration in underground geological reservoirs is considered as a near-term solution to global warming. However, monitoring the migration of sequestered CO2 in deep reservoirs is inherently difficult. Here the well-known adsorption-induced swelling phenomenon in clay-rich rocks is considered as a naturally generated "saturation footprint" that tracks CO2 migration through reservoirs. When a CO2 plume migrates in a storage reservoir-caprock system, the CO2 can be adsorbed on/in the clay-rich components, leaving footprints of rock swelling. By tracking these footprints, we expect to monitor the migration of CO2 plumes. To test this idea at the laboratory scale, we measured the strain changes during dynamic CO2/brine displacements in a clay-rich rock using a high-resolution Rayleigh scattering-based distributed fiber optic strain sensing tool. The fluid distribution in the rock was then determined by X-ray computed tomography imaging. Large swelling strains and shrinkage strains were observed during CO2 drainage and brine reimbibition, respectively. Moreover, the distributed strain signals clearly revealed the breakthrough of the CO2 plume from the high- to low-permeability regions. The results suggest that the strain change, measured by the powerful distributed fiber optic strain sensing tool, not only reveals the in situ deformation state but also tracks the movement of the CO2 plume as it enters the clay-rich critical regions in a reservoir-caprock system.