4D synchrotron imaging of CO2 transport in reservoir rocks and caprocks: Informing safe CO2 sequestration

Tracking CO₂ transport in subsurface rock during geological carbon sequestration has received significant attention. Although X-ray computed tomography (XCT) imaging techniques, visualization and quantification of CO₂ behavior has been undertaken by previous researchers, little attention has been given to the nature of CO₂ transport in a reservoir-caprock interface with well-developed pores in the reservoir and micro-fractures in the caprock. As the transition of two different pore systems, this complex system is crucial in controlling the sequestration safety case. We have used advanced time-lapse synchrotron imaging at in-situ pressure and temperature conditions to image the CO₂ transport process in such an experimental system, for both gaseous and supercritical phases.

Dynamic XCT images of fluids and the pore- and fracture-system in the mudstone and sandstone couplet were acquired at high resolution (effective voxel size 1.625 µm). Strain maps following high-speed gaseous and supercritical CO₂ (ScCO₂) flooding were modelled using a digital volume correlation (DVC) method to reveal the hydro-mechanical effects. The results suggest that under the influence of brine, high-speed gaseous CO₂ will not increase the total pore-throat volume and can even cause a reduction in permeability in sandstone due to fines migration induced by CO₂ flooding. Clay behavior, notably dispersion in brine, migration of fines and swelling induced by CO₂, plays a notable role in the opening and closing of pores/fracture. Fluid breakthrough occurred at 10.6 MPa during high-speed ScCO₂ injection. The significant fracturing effect of high-speed ScCO₂ resulted in the connection of natural fractures in the mudstone with newly developed secondary branches and also created larger pore space in the sandstone, leading to an increase in porosity by approximately 2.8 times in the mudstone and 1.6 times in the sandstone. Additionally, there was an approximate increase of 2.6 times and 8.6 times in the permeability of mudstone and sandstone, respectively, after CO₂ phase transition. The concentrated strains around the main fracture in the mudstone and web-like strains around the boundary of granular minerals in sandstone show the different modes of action of ScCO₂ when passing through a reservoir rock and caprock system. This work is of practical significance in improving understanding of CO₂-fluid-rock interaction in a complex reservoir-caprock system.